principles of natural bodies, goes no farther than their more ftriking effe�ls. The fimilaritynbsp;of fome of thofe efiefls, and the diffimilarity ofnbsp;others, point out various particular properties ofnbsp;thofe principles, whence we are enabled to formnbsp;certain general rules, called laws of nature. There-fore it follows, that with refpeft to the elTential ornbsp;fimple ftate of thofe principles, we can only formnbsp;conjectures, or offer hypothefes; yet the more cir-cumfcribed nature of fotiie of them, renders ournbsp;hypothetical knowledge of their eifence more probable, and lefs equivocal, than that of other principles.nbsp;nbsp;nbsp;nbsp;'

Four of the latter fort have, on account of their �'wonderful effefts, and of their very extenfive in-'fluence, been fet apart for a more particular examination. Thefe are caloric, light, ekSlricity, andnbsp;VOL. Ill,nbsp;nbsp;nbsp;nbsp;'nbsp;nbsp;nbsp;nbsp;Bnbsp;nbsp;nbsp;nbsp;tnagnetijm.

inagnetijm. The principal properties of thofe natural agents, the more probable opinions which havenbsp;been entertained with refpebt to their elfence, andnbsp;the principal advantages which we derive therefrom, will form the c'ontents of the prefent^ ornbsp;third, part of thefe Elements; which, therefore,nbsp;will be divided into four fe�tions ; and each feftionnbsp;will be fubdivided into as many chapters as the nature of the fubjeft may feem to demand, confiftentnbsp;with perfpicuity and concifenefs.

SECTION I.

or CALORIC} OR, OF THE ELEMENT WHICH PRODUCES HEAT, FIRE, amp;C. nbsp;nbsp;nbsp;-

\ GENERAL idea of the element which pro-duces the fenfation of heat, amp;c, has been given in the preceding volume, wherein the naturenbsp;of the affinities of the various elements has beennbsp;concifely illuftrated. In the following pages wenbsp;mull unavoidably repeat fome of the particularsnbsp;which have been already mentioned �, but the repetition will be ffiort, and the advantage, in point ofnbsp;perfpicuity, will probably prove more than an adequate compenfation for the trouble of twice peru-fing a few paffages.

CHAPTER L the theory of heat ; OR-J THE GENERAL EFFECTSnbsp;OF A SUPPOSED CALORIFIC FLUID,

When we approach a common fire, we feel a fenfation which we call heating. Whennbsp;we recede from the lire, and approach a quantity ofnbsp;ice, we feel another fenfation, which we call cooling.nbsp;On a clofer examination it will appear that thefenbsp;words heating and cooling, ox heat and cold, are relative expreflions j for the very fame body may feelnbsp;cold to one perfon and hot to another or it maynbsp;feel both cold and hot to the fame perfon. Let,nbsp;for inftance, a perfon warm one of his hands nearnbsp;the fire, and cool the other hand in fnow; then letnbsp;him put both hands in water of a middling temperature, and the fame water will feel cold to one of hisnbsp;hands, and hot to the other.

It is impoffible to give a more precife definition of thofe fenfations, than what is conveyed by thenbsp;Common meaning of the words. But with refpedtnbsp;to the vifible effeds which are produced by thofenbsp;refpedive approximations, to a fire and to the ice.nbsp;Or to the different degrees of heating and cooling,

we may give a more decerminare anfwer j viz. we may fay, that all the effedts of heating may be reduced to an enlargement of the bulk, or to thenbsp;fcparation of the parts, of all forts of bodies; andnbsp;that, on the contrary, all the �ffefts of cooling maynbsp;be reduced to a contradlion of the bulk, or to anbsp;mutual approximation, of the parts, of all forts ofnbsp;bodies.

A human body, and every part of an animal body, a ftone, a piece of metal, a piece of glafs, or,nbsp;in foort, every other body, whether folid or fluid,nbsp;grows larger by heating, and fm aller by cooling ;nbsp;but different bodies are expanded more or lefs bynbsp;the fame degrees of heating, and are contradtednbsp;more or lefs by the fame degrees of cooling. Bodies are not only expanded differently by the famenbsp;degrees of heating, or contradled differently by thenbsp;fame degrees of cooling; but by thofe means theynbsp;do alfo acquire different forms. Thus a piece ofnbsp;ice heated to a certain degree, becomes fluid water jnbsp;by increafing the heat the water is increafed in itsnbsp;bulk,' and after a certain period the water becomesnbsp;an elaftic fluid ; viz. fleam. By continuing to in-creafe the heat, that fleam becomes continuallynbsp;larger and larger j nor do we know the limits ofnbsp;its expanfibility. The like effedls, in a contrarynbsp;order, are produced by cooling ; viz. a quantitynbsp;of fleam grows fmaller and fmaller, until it becomes liquid water, and at laft the water becomes anbsp;folid j viz. ice.

The converfe of the above-mentioned law has likewife been pretty well proved by means of experiments; namely, that if a certain fubftahce benbsp;compreffed into a narrower fpace,' a quantity ofnbsp;heat will come out of it, and will be communicated to the furrounding bodies; and, on the contrary, if a certain fubftance be expanded into anbsp;larger fpace, it will abforb a quantity of heat fromnbsp;the furrounding bodies; for thofe farrounding bodies will thereby be cooled. Thus, if you wetnbsp;your hand, and then expofe it to the ambient air,nbsp;the water, in the aft of expanding itfelf into vapour,nbsp;abforbs a quantity of heat from the hand, which isnbsp;thereby fenfibly cooled. If air that has been com-preffed by art in a ftrong veffel, be let out of itnbsp;through an aperture, that air, in the aft of expanding itfelf, will abforb a quantity of heat. If a piecenbsp;of metal be compreffed, heat will be produced. Ifnbsp;the fteam of water be condenfed, heat will benbsp;depofited on the bodies which are in contaft

with it.

The acceffion of heat, by placing the particles of matter farther from each other, diminifhes theirnbsp;mutual attraftion viz. the attraftion of aggregation, in confequence of which their attraftion fornbsp;other bodies ; viz. the attraftion of affinity, growsnbsp;Wronger ; hence, heating to a certain degree effeftsnbsp;dccompofitions and compofitions, which in generalnbsp;have been called comhujiions; but when the heatednbsp;fubftances have not fuch affinities, or when they

B nbsp;nbsp;nbsp;are

are not heated enough to render their affinities aftive, fo as to form decompofitions and new combinations, then the fubftances are not faid to havenbsp;undergone a combuftion, or to be burnt �, butnbsp;they are faid to be heated, or rarefied, or ignited ,�nbsp;(viz. rendered red hot) or foftened, or liquified, ornbsp;evaporized, amp;c. according as any of thofe effects isnbsp;produced or attended to.

Heat penetrates bodies of every fort; for whatever body is placed near a common fire, is expanded, or foftened, or ignited j or, in Ihort, it (hews fomenbsp;of the effedts of heating; and the fame thing isnbsp;true with refpedt to cooling; but this heating doesnbsp;not penetrate all forts of bodies with equal quick-nefs; it paffes through certain bodies quicker ornbsp;eafier, than through others; hence the former arenbsp;faid to be better ccnduhfcrs of heat, than the latter;nbsp;we are not however acquainted with any bodynbsp;which may be faid to be a perfedl noncondudtor ofnbsp;heat,�The fame thing may be underftood ofnbsp;cooling.

With refpedt to the communication of heat, it has been obferved, that if an heated body be placednbsp;amongfl: colder bodies, or heat be' produced bynbsp;certain bodies in certain proceffes amongfl: coldernbsp;bodies, that heat will gradually pafs from � thenbsp;former bodies to the latter, fo as to render thenbsp;former bodies lefs hot, and the latter, hotter, thannbsp;they were before; and as there is not a perfedtnbsp;nonconduaor of heat, therefore nothing can effectually

feftually prevent that expanfion, or that diftribu-tion, of heat; though it may be much obftrucled stid impeded by the interpolition of bad conducingnbsp;bodies.

So far the efFe�l is well known, and is daily proved by common experience. But there is another phenomenon attending the communication ofnbsp;heat, which is neither very obvious, nor fo eafilynbsp;obferved. This is, that in the diftribution of heatnbsp;amongft a variety of fubftances, fome bodies ab'nbsp;forb more of it than others, though they be allnbsp;placed exaftly in the fame fituation ; hence differentnbsp;bodies are faid to have different capacities for ab-forbing heat.�An example will eafily illuftrate thisnbsp;remarkable property.

If a pound of water heated to a certain degree, for inftance, to 6o degrees, be mixed with anothernbsp;pound of water which has been heated 120 degrees,nbsp;the 60 degrees of heat, which the latter has abovenbsp;the former, will be divided alike between thofenbsp;equal quantities of water ; viz. 30 degrees will benbsp;communicated to the former pound of water, andnbsp;the other 30 degrees of heat will remain with thenbsp;latter; hence the whole will appear to have 90nbsp;degrees of heat. Now, if a pound of water heatednbsp;to 60 degrees, be mixed with a pound of quicksilver, heated to 120 degrees, the mixture willnbsp;appear to have (not 90 degrees as above) butnbsp;only 62 degrees of heat; which Ihews, that of thenbsp;60 degrees of heat, which the mercury had more

B 4 nbsp;nbsp;nbsp;than

than the water, a greater portion muft have been abforbed by one of the two fluids than by thenbsp;other. In order to afcertain which of the two hasnbsp;abforbed the greateft quantity of heat, you neednbsp;only repeat the experiment with this difference; viz.nbsp;that the pound of water be heated to 120 degrees,nbsp;and the pound of mercury be heated to 60 degrees ;nbsp;for in this cafe 'the mixture will appear to havenbsp;the heat of 118 degrees 5 which plainly fliews,nbsp;upon the leaf: refledtion, that the water has anbsp;much greater capacity for abforbing heat, than thenbsp;quickfilver.

The above-mentioned particulars are the heads to which all the phenomena of heating and coolingnbsp;may be referred; and fo far we havei related fa�ts:nbsp;but if the caufe of thofe fads be demanded, wenbsp;muft then anfwer by means of fuppofitions or hy-pothefes.

Various hypothefes have at different times been offered by different philofophers in explanation ofnbsp;this fubjecl; but of all thofe hypothefes, none feemsnbsp;to be fo fatisfadory as the modern theory of heat,nbsp;which is as follows ;

ift. It is fuppofed that there exifts a very fubtile and elaftic fluid, difperfed throughout all the bodies of the univerfe, and capable of paffing, withnbsp;more or lefs facility, through them all: but thisnbsp;fluid cannot be exhibited by itfelf in an uncombinednbsp;ftatej for nothing will confine it; nor has it any

2�ly. DifFerent bodies have different affinities for' this, fluid, or they can abforb different quantities ofnbsp;itj juft as pieces of different woods can abforb different quantities of water; and thofe affinities arenbsp;increafed or diminiflied by a variety of caufes, fuchnbsp;as by combination with othyr fubftances, by com-preffion, by expanfion, See. hence this fluid, owingnbsp;to the conftant aftion of thofe caufes in the world,nbsp;is continually moving from one fet of bodies tonbsp;another.�Its tranfition, its accumulation on onenbsp;body, and its diminution on another body, givenbsp;motion to every particle of matter, and feem tonbsp;animate the whole j for every body is rarefied andnbsp;condenfed by the accumulation or diminution ofnbsp;this fluid. And every body is fufceptible of different ftates by its combination with a greater ornbsp;fmaller quantity of this fluid. Thus ice is a combination of folid water with a certain quantity ofnbsp;this elem.ent ; fluid water is combined with anbsp;greater quantity of it �, and vapour is a combinationnbsp;of water with a much greater quantity of this element.

3dly. This fuppofed, fubtile, and elaftic, Piuid, has been called elementary heat, or, Amply, thenbsp;caloric.

According to this hypothefis, combined caloric is that quantity of caloric which enters into combination With other bodies, and which quantity hasnbsp;heen faid to differ according to the nature of eachnbsp;particular body. Thus in the above-mentioned in-

ftance

ftance of a mixture of water and mercury, the water has been (liewn to contain more combined caloricnbsp;than the mercury. Free calorii is that portion of itnbsp;v/hich is not combined with a certain body, butnbsp;which is ready to pafs froth that body to other furrounding bodies ; and the quantity of it, (which isnbsp;meafured by the effeft it produces on thofe othernbsp;bodies; viz. by the quantity of cxpanfion, amp;c.)nbsp;is called the temperature of that body. The fenfa-tion which animals perceive by the communicationnbsp;of caloric to their bodies, is called heat, or heating 5nbsp;and the fcnfation which they perceive by the efcapenbsp;of the caloric from their bodies, is called cold, ornbsp;cooling. Therefore it appears, that cold is not anbsp;pofuive thing; for it is only the abfence or privation of caloric. When we touch a hot body, thenbsp;caloric paffes from that body into our hand, ornbsp;face, amp;c. expands that part, and excites in us anbsp;fenfation of heat. When we touch a cold body,nbsp;the caloric paffes from us to that body, and wenbsp;feel the fenfation of cold. By a cold or hot body,nbsp;it is meant only that a body is colder or hotternbsp;than our bodies, or a certain other body; fo thatnbsp;\Vhen our body touches a body of the fame temperature, then we feel neither heat nor cold, becaufe innbsp;that cafe there is no tranfition of caloric either way.

Thus we have briefly ftated a fummary of the phenomena v/hich fall under the common appellations of heat and cold, and have fubjoined thenbsp;moft plaufible hypothefis, which has been offered

for their explanation. .But it is now neceflary to examine the different parts of the fubjeft in a manner both more particular and more ufeful j viz. tonbsp;ftate the fads which have been afcertained with re-fpeft to the expanfion of different bodies, with re-fpeft to the rheafurements of that expanfion, withnbsp;refped to the different capacities of bodies for ab-forbing caloric j as alfo the fads relative to thenbsp;produftion, communication, and application ofnbsp;heat, amp;c.�This we fhall endeavour to do in thenbsp;following chapters, wherein we fhall add the theoretical explanation, agreeably to the above-mentioned hypothefis.

NE of^the moft general efFeds of heat, or of the free caloric, is a dilatation of bodies,nbsp;or an augmentation of their bulks. The contrarynbsp;efFeft is produced by cold; viz. by a diminutionnbsp;of the free caloric. It muff, however, be ob-ferved, that bodies of equal bulks, but of differentnbsp;kind, are not expanded alike by being heated tonbsp;the fame degree; nor are the increments of bulknbsp;in the fame body, always proportional to the quantities of heat which are communicated to it.�If anbsp;bar of iron and a bar of glals, of equal dimenfions,nbsp;be both heated to the fame degree, for inflance, bynbsp;plunging them in boiling water, the bar of ironnbsp;vill thereby be lengthened more than that of glals.nbsp;�If a given quantity of water, by being heated tonbsp;a. certain degree, be increafed in bulk one cubicnbsp;inch, the addition of double or treble that quantitynbsp;of heat will not increafe its bulk two or three cubicnbsp;inches rcipcdively ; tliercfore, the expanhons of

water are not proportional to the increments of heat.�This is alfo the cafe with moft other fub-ftances.

Wc muft now ftate the moft remarkable�fafts wl�.ich have been afcertained refpeifting the dilatation of particular fubftancesj fluid firft, and thennbsp;folid.

The only pracbicable method of meafuring the cxpanfions of fluids, is by inclofing them in certainnbsp;veflels, and by meafuring that part of fhe cavity ofnbsp;each veffel which is occupied by the particular fluidnbsp;�which fills it in different temperatures. It is evident that the fubftance of the veffel is alfo expanded by the heat, and of courfe its cavity is enlarged.nbsp;Therefore, when we find that the bulk of the fluidnbsp;is increafed, that apparent increm.ent is only thenbsp;difference between the enlarged capacity of thenbsp;veffel and the increafed bulk of the fluid. For thisnbsp;reafon thofe veffels muft be made of fuch fub-ftances as are lead expanfible by heat. Indeed glafsnbsp;is the fubftance which is univerfally ufed for fuchnbsp;purpofes, both on account of its little expanlibility,nbsp;and of its tranfparency, befides its having other remarkably ufeful properties.

A glafs veffel filled to a certain degree with a liquid, for the purpofe of fliewing the expaniionsnbsp;of that liquid in different temperatures,, or for thenbsp;purpofe of (hewing the temperature by the cor-r^fponding expanfion of that liquid, is called a thermometer i viz. a meafurcr of the temperature.

The fluids moftly ufed for thermometers, are either mercury or fpirit of wine, the latter of whichnbsp;is generally tinged red, by means of cochiheal, ornbsp;Brazil wood, amp;c. for the purpole of rendering itnbsp;more viflble ; hence they are denominated thenbsp;mercurial thermometer, and the Jprit thermometer.nbsp;Other fluids, on account of their clamminels, or ofnbsp;their great irregularity of expanfion, are not ufefulnbsp;for thermorrieters ¹.

The moft proper and the mod ufeful fliape for thermometers, is that of a long tube with a narrownbsp;bore, and with a globular cavity at one extremity.nbsp;See fig. I. of Plate XVIII. The cavity of thenbsp;bulb C, and part of the tube, as far, for inftance,nbsp;as A, is filled with the fluid, the reft of the tube isnbsp;either partly, or quite, exhaufted of air, and thenbsp;end B of the tube is hermetrically fealed; viz. per-fedlly clofed by melting the extremity of the tube atnbsp;the flame of a candle or lamp, urged by means ofnbsp;a blow pipe f.

Thermometers have alfo been made, with bars of metal, without any glafs or fluid. Thefe foew the temperature by th� expanfion of the bars, and are, therefore,nbsp;called metallic thermometers.�They will be noticed hereafter.

I On account of the narrownefs of their bore, it is im-poffible� to fill thermometers merely by pouring the fluid into their cavity. But in order to fill a thermometer, the

- nbsp;nbsp;nbsp;Of the Therniomeie}-, fj'c.nbsp;nbsp;nbsp;nbsp;15

When the bulb C is heated, the mercury, or the Ipiric of wine is expanded; and not being able tonbsp;extend itfelf any other way, all the increment ofnbsp;bulk is manifefted in the tube; viz. the furface Anbsp;of the fluid will rife confiderably into the tube. Onnbsp;the other hand, when the bulb C is cooled, thenbsp;fluid contradls, and its furface A defcends. It isnbsp;evident, that, ceteris paribus, the larger the bulbnbsp;is, in proportion to the diam.eter of the cavity of thenbsp;tube, or the narrower the latter is in proportion tonbsp;the former, the greater will the motion of the fur-bulb C mud be heated over the flame of a candle, and, immediately after, the aperture B mufr be turned downwards,nbsp;and muft be immerfed in the fluid ; for iiiftance, the rncr-cury ; by which means part of the bulb will be ^lied withnbsp;mercury; for the heat of the ca,ndle having rarefled, and of^nbsp;'Courfe expelled fome of the air from the cavity of the thermometer, when afterwards the bulb cools, and the air in itnbsp;is condenfed, the atmofphere preffing upon the furface ofnbsp;the mercury in the bafon, forces it into the tube and bulb.nbsp;This done, the bulb C is again heated over the flame of thenbsp;candle, until the mercury in it appears to boib and is thennbsp;immediately turned down, with the aperture B, into thenbsp;mercury; by which means more mercury will be forcednbsp;into the bulb. This operation muft be repeated until allnbsp;the air is removed from the bulb, and part of the tube, andnbsp;�ts place is occupied by the mercury. \Yhen this is done,

pipe.

face A be in the tube. But it muft be obferved, that when the bulb is very large, the thermometernbsp;will not eafily arrive at the precife temperature ofnbsp;any place, wherein gt;t may be fituated. Some per-fons, in order to give the bulb a greater furface, andnbsp;of courfe to render it more capable of readily attaining a given temperature, have made it notnbsp;globular, but cylindrical, (which lliape was adoptednbsp;by Fahrenheit) or fiat, or bell-like, amp;c.; but thofenbsp;fluapes are improper, becaufe they are liable to benbsp;altered by the varying gravity of the atmofphere,nbsp;confequently thole thermometers cannot be accurate.

If a thermometer be heated fuddenly, as when the bulb C is Immerged in hot water, the furface Anbsp;of the fluid in it will be feen to defeend a little,nbsp;and inflantly after wdll be feen to rife ; the reafonnbsp;of which is, that the heat of ^the water enlarges thenbsp;glafs firfl, and is then communicated to the fluid,nbsp;amp;c. - On the contrary, if the bulb of a thermometer be cooled fuddenly, the furface A of the fluidnbsp;will firfl rife a' litde, and then will defeend'; becaufenbsp;the cold contradls the glafs alone at nrft, and doesnbsp;afterwards contrail the fluid.

Ice is, melted by a certain invariable degree of temperature ; and water freezes at about the famenbsp;temperature; therefore, if the bulb C of a micrcu-rial thermometer be placed in ineking ice, or melting fnow, and a mark is made on the outfide of thenbsp;tube, even with the furface of the fluid, as at D ;

mark is called tht freezing feints though in fa�: it is the melting point of ice 5 the freezing point'ofnbsp;Water being: not fo confiant. If the bulb oCthenbsp;thermometer be placed in boiling water, and anbsp;mark be made on the glafs tube, even with thenbsp;furface of the fluid within, as at E, that mark ianbsp;called the boiling pm �, for in an open velTel, andnbsp;under the fame atmofpherical preflure, which is indicated by the barometer, water does conftantlynbsp;boil at the fame temperature, and an increafed firenbsp;will force it to evaporate fatter, but will not raife itsnbsp;temperature. Thofe points being afeertained, �fquot;nbsp;the length of the tube from D to E be divided intonbsp;any number of equal parts, thofe parts will be thenbsp;degrees of the thermomieter, or the degrees of heat,nbsp;indicated by tlie correfponding expanfions of thenbsp;fluid within tlie thermometer. And the fame degrees, or equal divifions, may be continued belownbsp;D and above E, in order to fhew the degrees ofnbsp;temperature below the freezing, and above the boil-point *.

Thofe two unalterable points' of temperature; ^12. the former where ice becomes water, and thenbsp;fecond where water becomes vapour, have beennbsp;Univerfally adopted by the various conftruftors ofnbsp;d'ermometers for the graduation of thofe inftru-

� fchs Report of the Comriiittee, appointed by the Royal Society, in the 67th vol. �fthe Philamp;fiphlCBl Tra;if-p. 8j6.

menls; bvrt the fpace between them has been divided differently by different perfons, and this difference gives the different nannes of thermometers, or rather of their graduations; fuch as Reaumur�snbsp;thermometer, Fahrenheit�s thermometer, (Fc. � Reaumur divides the fpace between the abovementionednbsp;two points, into 8o equal parts or degrees j placingnbsp;the o at freezing, and the 8oth degree at the boilingnbsp;point. Fahrenheit divides it into i8o degrees ornbsp;equal parts, but he places the o tnirty-two degreesnbsp;below the freezing point D ; fo that the freezingnbsp;point is at 32, and the boiling point E is at 212nbsp;degrees'¹.

Other perfons have adopted other dlvifions, which have been fuggefted by fuppofcd advantagesnbsp;or fanciful ideas.

Mod of thofe graduations are at prefent out of ufe 5 but they are to be met with in various, notnbsp;very recent, publications; I have dierefore thoughtnbsp;necefiary to fet them down in the following Table,nbsp;which contains j ift, the name of the perfon ornbsp;fociety that has ufed each particular divifion j 2dly,nbsp;the degree which has been placed, by each of them,nbsp;againft the freezing point; jdly, the degree whichnbsp;has been placed againd the boiling point; andnbsp;4thly, the number of degrees laying between thofenbsp;two points.

In Head of adding the word degrees to the number, a (mail � is ar.ne.xcd to it on the right-hand fide^ a little abovenbsp;the level of the number; thus 24-�' means 24 degrees-, 6o�.nbsp;means 60 degrees, he,

The Theory of Heat, Csfc,

Freezing point.	Boiling point.	degrees betweennbsp;the preceding twonbsp;points.
28	199 f	17� T
511	73	21 k
47 tV	6a	15 T3
130	0	150
1070	1510	440
73 i	141 �	i'JS f
0	34	34
34 �	25� T	284 T
0	163	163
St	47	38 f

DelaHire�i Thermometer, which flood in the Obfervatory atnbsp;Paris above 6o years, feemsnbsp;to be graduated thus - .

The ancient Thermometer of the Royal Society of LiOndon, feemsnbsp;to have had the o cprre-fponding with about thenbsp;88th degree of Fahren-' heit�s; and from that pointnbsp;the numerati�n afcendednbsp;and defcended; viz. --

Fo-Tukr�s o feems to have coincided with about the 53d or 54th degree of Fahrenheit�s, and from that pointnbsp;the numeration afcended andnbsp;defcended, thus -nbsp;nbsp;nbsp;nbsp;.

The Edinburgh Thermometer, formerly ufed*, feems tonbsp;have been graduated thus

Thei uiomcters

Thermometers have been made of a great variety of (hapes and fixes, fuitable to the different purpofesnbsp;for which they were intended.

Thermometers for fiiewing the temperature of the atmofphere, need not have their fcales muchnbsp;extended; it is more than fufficient if they go asnbsp;high as iso�. The lower degrees may be carriednbsp;down as low as may be fuppofed neceflary for thenbsp;cold of any particular climate. The m.ercurialnbsp;thermometer needs not be graduated lower than 40�nbsp;below o, becaufe at about that degree mercurynbsp;ceafes to be a fluid.

The rpirit thermometer may be graduated lower if neceflary.�I Ihall here juft mention, that, for rea-fons which will be noticed hereafter, if a mercurialnbsp;thermometer and a fpirit thermometer, be both graduated according to the above-mentioned direftions,nbsp;the two thermometers will not, in their ufual indications of the fame temperatures, point to the famenbsp;degrees.

The degrees of therrhometers may be delineated, on metal, or wood, of paper, or ivory, amp;c. but fuchnbsp;fubftances fhould be preferred for the fcales of iher-.roometers, as are not apt to be bent or fliortened, ornbsp;otherwife altered by the weather, efpecially vyhen thenbsp;roftruments are not defended by a glafs cafe, or by anbsp;box with a glafs face.

The bulb of the thermometer muft be clean and Colourlefs ^ fmee. coloured ftirfaces are apt to be par-

c 3 nbsp;nbsp;nbsp;tially

Tbe T^heory of Heat, amp;c.

tially heated by a ftrong light *. The ball of the thermometer ought not to be in conta6b with thenbsp;fubftance of the fcalcj left it fhould be influenced bynbsp;the temperature of that fubftance.

Thofe thermometers muft be fituated in the open air out of the houfe, and at fome diftance (at leafl;nbsp;a foot) from the wall, and where the light of the funnbsp;may not fall direftly upon them. Fig. 2. Platenbsp;XVIII. reprefents a therm.ometer of the moft ufualnbsp;lhape independent of the cafe.

For chemical purpofes, the bulbs and part of the tubes of the thermometers, muft- project fome waynbsp;below the fcales, in order that they may be placednbsp;in liquids, mixtures, amp;c.

For other purpofes, as for botanical obfervations, hot houfes, brewing man�fa�tories, baths, amp;c, thenbsp;thermometers muft be made longer, or fhorter, ornbsp;narrower, or particular diredtions muft be addednbsp;to the fcales, amp;c. ; but I fhall not take any farthernbsp;notice of thofe fludluating varieties of lhape only.

It is necelTary, however, to take notice of a fort of thermom,ete-rs which have been conftruclednbsp;for a particular purpofe �, namely, for lliewing the

� Take two equal thermometers, paint the bulb of one of them black, or of any dark colour, and expofe them bothnbsp;to the fun ; the mercury in the latter will rife ,feveral degrees higher than in the former. Even a llrong day light,nbsp;independent of the diredf rays of the Ian, wai affect themnbsp;differently.nbsp;nbsp;nbsp;nbsp;, ,

hiRheft

f^lghefl: degree of heat or of cold which has taken place during the abfence of the obferver; as fornbsp;inffance, in the courfe of the night, or in thenbsp;hotteft part of the day, or even during a wholenbsp;feafon.

Thermometers for this purpofc have been contrived differently by vapiotis ingenious perfons, as by Bernoulli, Kroft, Lord Charles Caveudifli, amp;cc,¹ ²nbsp;but the beft of them (which however is not withoutnbsp;faults, and of courfe is in need of improvements)nbsp;v/as contrived by Mr. James Six, and is defcribednbsp;in the 72d vol. of the Philofophical Tranfaftions.nbsp;F'g- 3- Plate XV PI I. exhibits this inftrument, butnbsp;diverted of the fcale and frame; � a his a. tube of thinnbsp;glafs, about 16 inches bng, and Tt: of an inch innbsp;� diameter; cdefgh, a fmailer tube with thenbsp;� inner diameter, about V�, joined to tlie larger atnbsp;� the upper end b, and bent down, firft on thenbsp;� left fide, and then, after d'efcenuing two inchesnbsp;below ab, upwards again on tl^e right, in the fe-� veral direftions'parallel to,'and onenbsp;� inch diflant from it. On the end of the fiimenbsp;tube at h, the ihner diameter is enlarged to halfnbsp;an inch from' k to i, which is two incnes nnbsp;This glafs is filled with highly reclified

� P' ^53�255, The Philoiophical TranfaSions, vo!. L. *��1 ''ol. L[, The Tranfaclions of the Edinburgh Society ;nbsp;and tht; Encyclopaedia Brit, article Thermo.::der.

� fpirits of wine, to within half an inch of the end *' i, excepting that part of the fmall tube from dnbsp;� to^, which is filled with mercury. From a viewnbsp;� of the inftrument in this flate, it will readily benbsp;conceived, that when the fpirit in the large tube,nbsp;which is the bulb of the thermometer, is expand-� ed by heat, the mercury in the fmall tube on thenbsp;� left fide will be prefifed down, and confequentlynbsp;caufe that on the right fide to rife ; on the con-� trary, w'hen the fpirit is condenfed by cold, thenbsp;� reverfe will happen, the mercury on the left fidenbsp;quot; will rife as that on the right fide defcends. Thenbsp;*' fcale, therefore, which is Fahrenheit�s, beginning.!nbsp;� with o, at the top of the left fide, has the degreesnbsp;� numbered downwards, wdiile that at the ricrhtnbsp;� fide, beginning with o at the bottom, afcends.

The divifions are afcertained, by placing this ther-� mometer with a good ftandard mercurial one in � water, gradually heating or cooling, and markingnbsp;� the divifions of the new fcale at every 5�. Thenbsp;� .method of Ihewing how high the mercury hadnbsp;� rifen in the obferver�s abfence, is effefted in thenbsp;� following m.anner. Within the fmall tube of thenbsp;� thermometer, above the furface of the mercurynbsp;� on either fide, immerled in the fpirit of wine, isnbsp;placed a fmall index, fo fitted as to pafs up andnbsp;� down as occafion unay require: that fOrface ofnbsp;� the mercury which rifes, carries up the indexnbsp;� with it, which index does not return with thenbsp;� mercury when it defcends j but, by reniaining

fixed, fliews diftin�lly, and very accurately, how fi'gh the mercury had rifen, and confequentlynbsp;what degree of heat or cold had happened.nbsp;Fig, 4. Plate XVIII. reprefents thefe indexesnbsp;drawn larger than the real ones, to render itmorqnbsp;diftinifl;; 0 is a fmall glafs tube | of an inch long,nbsp;hermetically fealed at each end, inclofing a piecenbsp;of fteel wire, nearly of the fame length ; at eachnbsp;end c d, is fixed a fhort piece of a tube of blacknbsp;glafs, of fuch a diameter as to pafs freely up andnbsp;down within the fmall tube of the thermometer.nbsp;The lower end, floating on the furface of thenbsp;mercury, is carried up with it when it rifes,nbsp;while the piece at the upper end, being of thenbsp;fame diameter, keeps the body of the indexnbsp;parallel to the Tides of the thermometrical tube.nbsp;From the upper end of the body of the index atnbsp;o, is drawn a fpring of glafs to the finenefs of anbsp;hair about of an inch in length, which being fetnbsp;a little oblique, prefles lightly againft the innernbsp;furtace of the tube, and prevents the index fromnbsp;following the mercury when it defeends, or beingnbsp;moved by the fpirit pafllng up or down, or bynbsp;any fudden motion given to the inftrument by tlienbsp;hand or otherwife; but at the fame time thenbsp;prefTure is fo adjufted, as to permit this index tonbsp;fie readily carried up by the furface of the rihngnbsp;mercury, and downwards whenever the inftru*nbsp;ment is to be redified for obfervation. To pre-

� vent the fpirit from evaporating, the tube at the � end i is clofely fealed.

� This inftruiTient in its frame muft be fecured � againfl the wall out �f doors, to prevent its be-ing flaaken by violent winds. Towards eveningnbsp;�^ T ufually vifit my thermometer, and fee at onenbsp;� view, by the index on the left fide, the cold ofnbsp;� the preceding night; and by that on the right,nbsp;the heat of the day. Thefe I minute down, andnbsp;then apply a fmall magnet to that part of the tubenbsp;againfl: which the indeoces rei% and move each ofnbsp;� them down to the furface of the m.ercury: thus,nbsp;without heating, cooling,, feparating, or at ailnbsp;� difturbing the mercury, or moving the inftru-ment, may this thermometer,without a touch, benbsp;immediately reflified for another obfervation.quot;

It might at firft fight be imagined that equal increments of heat would caufe fluids to expand

equably; viz. that if the heat be increafed gradually by one degree, two degrees, three degrees, amp;c. the fluid thus heated would expand its bulk bynbsp;a certain quantity, then by twice that quantity, threenbsp;times that quantity, and fo on. But this is not thenbsp;cale, and every fluid feems to follov; a particular lawnbsp;of expanfion.

Mercury feems to expand more equably than any other fluid '**. Yet its increments of,bulk are not

quite - * It will be necefliiry to fliew how this irregularity maynbsp;be afeertaiaed.�All the rcafoiiiiig and all the phenomena

l'he Theory of Heal) Sc. , nbsp;nbsp;nbsp;27

quite proportional to the increments of heat. With �ther fluids the irregularity of expanfion is verynbsp;confiderable.

One

arifing from the mixtures of fluids, have eftabliihed the propofition, that when equal quantities of the fame fort ofnbsp;Jluid) hut diferently heated^ are inixed together) the temper-a-ture of the mixture is a mean proportional between the temperatures of the feparate parcels. Now if a thermometerjnbsp;having (hewn the temperatures of the feparate parcels, onnbsp;being placed into the mixture, does alfo ftiew an arithmetical mean between the feparate temperatures, you may conclude that the liquor, of which the thermometer is made, expands equably through the degrees about that temperature,nbsp;and thus by trying fimilar experiments in different temperatures, the law of the expanfibility of that fluid will be afeer-tained.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;�

Thus, if the thermometer being placed fucceflively into two equal quantities of water, fhews 40� in one parcel, andnbsp;60� in the other ; mix the two parcels of water, and if thenbsp;thermometer, being placed in the mixture, (hews 50�, yonnbsp;tnay conclude that its fluid has expanded equably; if it fhewsnbsp;iefs than 50�, or more than 50�, you may conclude that itsnbsp;t^xpanfion is not fo great, or greater, amp;c.

Mr. De Luc has, with great care and afliduity, afeertain-ed the expanfibility of mercury, or rather the real quantities �f heat which are required to expand mercury arithmeti-cally. Thefe are exprefled in the following table, the firfl:nbsp;column of which, contains the degrees of Reaumur�s fcale,nbsp;horn five to five, vvhich are equal parts; the fecond fhewsnbsp;*he real quantities of heat which are required to raife thenbsp;triv-rcury to the correfponding degrees, where z is a fixtnbsp;hut unknown quantity; and the third column fhews thenbsp;differences of thofe quantities. (De Luc�s Rechcr. furiesnbsp;Modif. de 1�Atmofphere, 1772, p. 309.)

28 nbsp;nbsp;nbsp;The l�heory of Heat, Cfc.

One cubic inch of mer�ury, or one nneafurc whatever of it at 32� of temperature, when heated to the'

temperature

Mercurial	Real quantities
thermc- ineters.	of heat.
er 80quot; ,	24-80,00
75	z 75,28
70	z 70,56
�5	Z4-65,77
60	z 4-60,96
55	24-56,15
50	24-51,26
45	2-446,37
40	24-41,40
85.	2 4- 36,40
30	2-431,32
25	2-426,22
20	24-21,12
15	24-15,94
10	2 4-10,74
5	�-b 5,43
0

Real differences of hear corre-fponding to thenbsp;variatioiiS.pf thenbsp;inerciir. therm,nbsp;from 5 to 5 deg.

From the third column, it appears that the differences oi' heat rcqu'ffite to make equal and progreffive additions to thenbsp;bulk of the mercury, though not exactly equal, yet they acenbsp;not very far from the ratio of equality. Vv^ith other fluidsnbsp;the irregularities arc much greater ; as will appear frotn thenbsp;following table, which contains the expanfions of the principal flni'Js that have been ftibjcfted to fuch experiments, according io Mr Dc Luc�s obfervatiops.

tcrnperature of boiling water; viz. at 212�, wili be found increafed in bulk by the quantity 0,01836.nbsp;This fluid metal boils and becomes a vapour at .600�nbsp;of Fahrenheit�s thermometer, and it beconies anbsp;fblid at �40�; viz. 72� below melting ice. Belownbsp;that point; viz. �40�, it conti ads irregularly.nbsp;See Mr. Hutching�sExperimentsinrhe Philofophicainbsp;Tranfadions for 1783, Art. XX ; alfo Art. XXL

Spirit of wine boils at about 180�, and the purcS: probably never freezes. When brandy, or a mixturenbsp;of water and fpiric freezes, it is the water that becomes folid, but the fpirit will be found colledednbsp;together in one or more bubbles, in fome part ofnbsp;the ice.

The extenfr/e ufe and influence of water both in natural and artificial affairs, renders it neceffarynbsp;to be more particular with refped to its expanfi-bility, and to the elaflic force it acquires by beingnbsp;heated, amp;c.

The expanflbility of water forms a Angular devia-�^lon from an otherwife general law of nature ; for

In order to comprehend the meanirtg of this table, it mulf he underflood that different thermometers (each being fillednbsp;with a particular fluid, fuch as is mentioned at the top of thenbsp;lt;=tgt;lumns, and each being divided into 80 equ^il parts betweennbsp;^he freezing and the boiling water points) are placed withnbsp;bulbs in the fame vefl'el full of water, and th.�.t thenbsp;water is gradualiy heated. Then when the mercurial ih. r-rnornettr

JO nbsp;nbsp;nbsp;^he �Theory of Heaty ifc.

though every other fubftance, as far as we know, is continually expanded by heat and contrafted bynbsp;cold j yet water is expanded by heat from aboutnbsp;40� of Fahrenheit�s thermometer upwards ; butnbsp;below 40' its bulk is expanded by a farther de-creafe of heat, or increafe of cold (fee p. 86. vol. IL)nbsp;and, in fadt, ice is lighter than water, fo as to float

mometer is at 5�, 10�, 15�, amp;c. the furface^^of the fluids in the other thermometers will be found at the degrees whichnbsp;fiand on the fame levels ; for inftance, when the mercurial _nbsp;thermometer (lands at 40�, the water thermometer will benbsp;found to (land at 20�,5; the fpirit thermometer will benbsp;found to (land at 35�, . the oil thermometer at 39�,2, amp;c.

1�be �Theory of Heat, iSc. nbsp;nbsp;nbsp;3 ^

^pon it; the fpecific gravity of ice being to that of ^ater nearly as 7 to 8 j whereas the ice of oil isnbsp;heavier than fluid oil, and finks in it.

The bulk of water from the mofl: contraflied ftate of that fluid, W'hich is at 40�, increafes ,nbsp;continually; but the increafe is not very regular �,nbsp;for inftance, the increafe of bulk from 180� tonbsp;212�, is confiderably greater than from 40� to 7a�.

If the bulk of water at 40� be called i, its bulk at 212� will be 1,04785. After that degree of heat.nbsp;Water becomes vapour; viz, an elaflic fluid, 'andnbsp;the formation of this elaftic fluid on the fides of thenbsp;veflel within the W�ater, forms the bubbles, thenbsp;efcape of which conftitutes the- boiling. Beyondnbsp;that point the water cannot be heated f for all thenbsp;additional heat combines with the water, and rendersnbsp;it vapour, which is elaftic enough to overconne thenbsp;niean preflurs of the atmofphere. Water is at leaftnbsp;2000 times denfer than vapour. When waternbsp;IS caufed to boil on a common fire, the heat,nbsp;which is communicated, cannot at once convert allnbsp;the water into fteam; but if the quantity of water benbsp;fmall in proportion to the-quantity of heat whichnbsp;can be communicated in a given time, then th'econ-'^erfion of water into vapour is much more expeditious, and indeed it may be rendered inftantane-; in which cafe it produces a very great expan-hon Or an explolion. This happens with dreadfulnbsp;W'hen water fills upon large quantities ofnbsp;ted hot or fufed rqetals. Count Rumlord juftiy

attributes tlie vaft force of gunpowder, to the fud-den converfion into vapour of that quantity of water which naturally enters into the compofuion ofnbsp;that powder ¹.

At 212� of temperature, water can entirely overcome the mean preflure of the atmofphere. Beyond that degree the vapour is expanded farther and farther, or becomes more and more elaftic innbsp;proportion, as it is heated to a greater and greaternbsp;degree. But below 212� the water can in partnbsp;overcome the preffure of the atmofphere; and, innbsp;fadl, vapour is feen to proceed from a quantity ofnbsp;water long before its boiling.

The different powers pf overcoming the preffure of the atmofphere at different temperatures, havenbsp;been afcertained with fufficient accuracy withmnbsp;certain limits, principally by means of the following apparatus f.

A, fig. 5. Plate XVIII. is a veffel placed upon a fmall furnace B. It has three apertures; viz. 0,nbsp;to which the bent and open glafs tube otgr is-ficted; the hole s, to which a therm,ometer thnbsp;clofely adapted, and the tube x x, which has thenbsp;ftop cock b. The lower part mg% of .the glafs

See his very ingenious paper and oalculaticn in the Philofophical Tranfactions for 1797. Art. XII.

�jquot; Tills method was contrived and u(ed by the Chevalier de Eettancouit, See Dc Prony�s Archit. Hydraul. vol. I.

tube is filled with quickfilver. This vefiel A, when the ftop-cock is doled, has no communicationnbsp;with the external air; therefore it is evident that ifnbsp;the air be rarefied within the velTel, the externalnbsp;preffure of the atmofphere will prefs the mercurynbsp;down into the leg rg, and of courfe will force it tonbsp;rife into the leg ^nbsp;nbsp;nbsp;nbsp;; fo that the difference of per-

pendicukr altitude between the furfaces of the mercury in the two legs fhews the degree of rarefaftion within the velfel, or the difference of preffurq between the internal and the external air. (See whatnbsp;has been faid with refpedt to the gauges of the air-pump in vol. II. p. 485, and following.) On thenbsp;contrary, if the elafticity of the fluid within thenbsp;velfel be increafed, the mercury will be lowered innbsp;the leg kg, and will be Vaifed in the leg gr. Thennbsp;the difference of altitude between the furfaces ofnbsp;the mercury in the two legs lliews the force of thenbsp;daftic fluid within the velfel; fo that when thatnbsp;difference of perpendicular altitude is equal-to thenbsp;sdrual altitude of the mercury in the barometer, younbsp;tuay conclude that the force of the elaftic fluidnbsp;'''ithin the velfel is equal to theprelfure of the at-tnofphere, amp;c.

Mow, when fome water is placed in the velfel A, and is heated, the temperature of it is indicatednbsp;the thermometer th, the fcale of which projectsnbsp;out of the veflel j and the correfpondent force ofnbsp;Vapour is indicated by the elevation of thenbsp;III.nbsp;nbsp;nbsp;nbsp;Dnbsp;nbsp;nbsp;nbsp;mercury

34 nbsp;nbsp;nbsp;I'he 'theory of Heat, i^c.

As the vapour of water, in order to aflume the elaftic form, muft overcome, more or lefs effedlu-ally, the preffure of the atmofphere, fo the prelTurenbsp;of the atmofphere forms a manifefl: oppofition tonbsp;the reduction of water into vapour, or to its boiling.nbsp;Therefore, according as that preffure varies, viz.nbsp;as the barometer is high or low, fo the water requires a greater or lefs degree of heat, in order to

* With refpefi: to the refults of the experiments, I fhall only mention a few in the following table, from which thenbsp;force for the intermediate degrees of heat may be eafily conceived.�The firft column contains the temperature of thenbsp;water in degrees of Fahrenheit�s fcale; the fecond containsnbsp;the correfpondent altitudes of the mercury it fupports innbsp;inches and decimals.

Beyond the boiling point every additional 30� of heat nearly double the elafticity of fleam; fo that at the temperature of 242�, the elaflic force of fleam is equal to thenbsp;prcfi'urc of two atmofpheres ; at 272� it is equal to four at-mofpheres �, at 302� it is equal to eight atmofpheres, amp;c.

The Theory of Heat, i^c. nbsp;nbsp;nbsp;35

boil. Hence, upon mountains water boils at a lower temperature than on the plains *, and innbsp;clofed veffels, or under an additional preffure, waternbsp;can be heated to a. much greater degree than

212� t.

The Angular property of water, viz. that in cooling from the 40th degree downwards, it ex-

* The precKe degrees of heat (Fahrenheit�s thermometer) at which water will boil at different altitudes of the liarometer, both according to Mr. De Luc�s, and Sir G.nbsp;Shuckburg�s obfervations, are ftated in the following

t Water heated in a clofed ftrong veffel may be rendered red-hot; in which fiate it acquires a prodigious expan-five force, as alfo a diffolving power, fuch indeed as to dif-fulve even bones. It is from this property that a ftrongnbsp;fit for this experiment, and capable of being accurate-y clofedj has been called a dig/fer-, or from its inventornbsp;\^apin's digejig^^

pands, and becomes lighter and lighter, In proportion as it becomes colder and colder, is a moft ftriking inftance of the wifdonn of the Creator, andnbsp;is a circumftance of immenfe confequence to thenbsp;very exiftence of animal and vegetable life.

A quantity of fluid water is indifpenfably necefTary both to animals and to vegetables at all times ofnbsp;the year. When in winter the cold air freezes thenbsp;furface of water, that effedt feldom penetrates lowernbsp;than two or three feet; below that depth thenbsp;water continues fluid, and the cruft of ice itfelfnbsp;contributes to preferve its fluidity. The heat of thenbsp;earth which has been acquired during the fummer,nbsp;undoubtedly prevents the formation of ice beyondnbsp;a certain depth. But if water in cooling had continued to increafe in fpecific gravity, and had icenbsp;been.adtually heavier than water, the heat of thenbsp;earth would not have fufiiced to prevent the totalnbsp;freezing of all the waters of lakes, feas, amp;c. � For,�^�nbsp;fays an eloquent modern writery � as the particles ofnbsp;� water, on being cooled at the furface, would innbsp;� confequence of the increafe of their fpecific gra-� vity, on parting with a portion of their heat, im-� mediately defcend to the bottom, the greateftnbsp;� part of the heat accumulated during the fum-� mer in the earth, on which the water repofes,nbsp;� would be carried off and loft beforp the waternbsp;� began to freeze; and when ice was once formed,nbsp;� its thicknefs would increafe with great rapidity,nbsp;� and would continue increafing during the whole

The Theory of Heat, �f?f. nbsp;nbsp;nbsp;,37

quot; winter; and it feems very probable that in cli-� mates which are now temperate, the water in quot; the large lakes would be frozen to fuch a depthnbsp;quot; in the courfe of a fevere v/inter, that the heat ofnbsp;� the enfuing fummer would not be fufficient tonbsp;� thaw themj and fhould this once happen, thenbsp;� follov/ing winter v/ould hardly fail to change thenbsp;�� whole mafs of its waters to one folid body ofnbsp;� ice, which never more could recover its liquidnbsp;� form, but niuft remain immoveable till the endnbsp;� of time V�

It has been already remarked, that though ice melts at 32�, yet water does not freeze at 32�, butnbsp;it requires to be cooled fome degrees lower beforenbsp;it will acquire the folid form. This is alfo the cafenbsp;'With mercury ; viz. it bears to be cooled fome degrees below � 40�.

Several fads which have been obferved by various experimenters, have thrown a good deal of iight upon this fingular phenomenon, ver the realnbsp;caufe of it is far from beins clearlv underftood.

It has been frequently aflerted, that boiled water freezes fooner than unboiled water j and this may

true with refpe�t to impure waters, becaufe as a �Tiixture of fairs, or acids, or earths, or alkalies, amp;c.

renders water lefs liable to be frozen than pure water, the boiling may caufe a precipitation ornbsp;evaporation of the fait, earth, amp;c. and thus, bynbsp;leaving the water in a purer ftate, may render itnbsp;more capable of being frozen. But the boiling ofnbsp;pure (viz. diftilied) water, which feems to donbsp;nothing more than deprive it of air, certainly renders it lefs apt to freeze ; for water, thus deprivednbsp;of air has been often cooled feveral degrees belownbsp;32�j viz. even fo low as to 14�, before it wouldnbsp;freeze ¹.

When w�ater has been cooled as many degrees below aia� as it will bear without freezing, thenbsp;congelation in that cafe may be haftened by variousnbsp;means, ^viz. by a fudden (Iroke againft the fides of.nbsp;the veflel, by touching the furface of the water withnbsp;a piece of ice, by placing a largifh piece of metalnbsp;in contafl with the outfide of the veffel, amp;c. Thenbsp;water then will Ihoot into cryftals in a very ftrikingnbsp;manner, and will acquire the folid form, almoft in-ftantancouily ; the thermometer, which has its bulbnbsp;in it, riling at the fame time immediately to 3 2� f.

Mr. De Luc�s Idees fv^ la Meteoroligle, tom, II. p. 105. Yet in the atmofphere, water begins to freeze when thenbsp;temperature of the atmoiphere is very little below 32'�.

t For farther particulars refpefling this property of water, as alio for the conjectures which have been offerednbsp;in explanation of it, fee the Philofophlcal Tranfadiions,nbsp;vol. for 1788. Art. X.

The admixture of faline and feveral other fiib-ftances lower the freezing point of water; bur it is impoffible to examine the numerous fadls whichnbsp;may be obferved with refpedf to the infinite pofliblenbsp;combinations of water with other fubftances; wenbsp;fliall, however, flate the principal phenomena whichnbsp;relate to the congelation of fait-water or fea-water,nbsp;which is of more general confequence.

Sea-water, which at a mean may be faid to contain i-28th of its weight of fait¹, mull be cooled to about o� before it will freeze. A frigorific mixture of three parts of pounded ice, and two parts ofnbsp;common fait, which will lower the thermometer tonbsp;��4�!, is quite fufficient to freeze it.

A quantity of fea-vyater is never entirely congealed, a portion of it does always remain fluid ; and what is very remarkable, this fluid portion isnbsp;incomparably more full of fait, and more naufeous,nbsp;than the frozen part; fo that if the former be fepa-rated, the latter, on being melted, will be found tonbsp;contain much lefs fait. See, than It did before thenbsp;Congelation. If this water of the firft purificationnbsp;be again congealed, wlrich requires lefs cold, it will

alfo leave a, fluid portion, which will be found to contain a greater proportion of fait than the congealed part, and may be feparated from it, Thusnbsp;by repeatedly freezing the fame fea-water, and fe-parating the fluid from the congealed part in everynbsp;operation, you will at laft obtain the water perfedWynbsp;purified, and fit for drink and other purpofes. Seawater thus purified, by means of fix fuccefllve congelations, has been found perfedlly fweet, capable ofnbsp;diflblving fope, and fo nearly of the fpecific gravitynbsp;of rain-water, that the former was to the latter asnbsp;�7801 to 7800

Thefe fafts will eafily explain why fome navigators have found the ice of fea-water, when melted, not good for drink, whilft others have found it verynbsp;fweet and ufeful, as it proved to Captain Cook�snbsp;expedition near the fouth pole.

After the liqiiids, it will be neceffary to notice the cxpanfibility of the aerial fluids, and here wenbsp;niuft confine ourfelves almoft entirely to commonnbsp;air; for the various experiments which have beennbsp;made with other permanently elaftic fluids are bynbsp;no means conclufive ; excepting their havingnbsp;proved that hydrogen gas is expanded by meansnbsp;of heat confiderably more than common air.

The inftrument in which the expanfion of air is tried has been called manometer, but in truth it is

* See the' Chevalier Lorgna�s Experiments, concerning the purification of fea-water.

only an air-thermometer, and, though rather larger, it is not however unlike the common thermometer jnbsp;viz. it confifts of a tube five or fix feet long, havingnbsp;a bulb at one end and open at the other. The borenbsp;of the tube is about a 2Cth of an inch in diameter. Anbsp;fmall quantity of quickfilver is placed in fome part ofnbsp;the cavity of the tube, and the expanfion of the air ofnbsp;the bulb, when heated, forces the quictcfilver to movenbsp;towards the openend of the tube. The degree of heatnbsp;to which the manometer is expofed, is meafured bynbsp;means of a thermometer ; the quantity of expanfionnbsp;of the air is afcertained by gauging the manometer,nbsp;and making marks on the tube, which marks maynbsp;indicate parts of tlae cavity of the tube that are proportional to the capacity of the manometer, as fornbsp;inftance, looths or loooths, amp;c. See fig. 6.nbsp;PlateXVIII.

By placing the manometer horizontal or vertical, either with the bulb downwards or the bulb upwards, the air in it may be either left of the naturalnbsp;denfity, or it may be condenfed, or, laftly, it maynbsp;be rarefied ; for when the manometer is horizontal,nbsp;the quickfilver a b does neither prefs upon the airnbsp;of the manometer, nor on that of the atmofphere jnbsp;v^hen the bulb is downwards, the quickfilver a bnbsp;Ptefles upon the air of the manometer, and whennbsp;the bulb is upwards, the quickfilver a b preflesnbsp;and counterafts, in fome meafure, the,nbsp;ctavity of the atmofphere. Hence this prclTurenbsp;9'nd this rarefadtion of the air within the manometer

meter may be incrcafed to any required degree by increafing the quantity a h o� the quickfilver withinnbsp;the tube ; and thus the expanfibility of common} ornbsp;of condenfed, or of rarefied air may be tried.

The expanfion of air by the fame degrees of heatj differs according to its denfity, and to thenbsp;quantity of moifture it contains; nor are the increments of its bulk proportional to the degrees ofnbsp;temperature.

It appears from Col. Roy�s very numerous experiments¹, that looo parts of air, of the denfity of the common atmofphere, at o� of heat, becomenbsp;1484,21 at 212�, viz. are expanded 484,21 bynbsp;a 12� of heat.

lOoo parts of air, loaded with 2f atmofpheres, are expanded 434 of thofe parts, by 212� ofnbsp;heat.

1000 parts of air preffed only with 4 ths of an atmofphere, are expanded nearly 484 of thofe partsnbsp;by 212� of heat.

1000 parts of air preffed with 4 th of an atmofphere, are expanded about 141 parts by 180� of heat; viz. from the freezing to the boiling point.

it would leem, that the particles of air may be � fo far removed from each other, by the diminu-� tion of preffure, as to lofe a very great part ofnbsp;� their elaftic force.�

The abovementioned expanfions are by no means regular; viz. they are not proportional to thenbsp;number of degrees of heat. fhe maximum ofnbsp;cxpanfion takes place between 52� and 72�, andnbsp;the minimum is conftantly at the boiling point ofnbsp;water.

Moifc air expands vaftly more than dry air, efpe-cially when it approaches the boiling point of Water; fo that between 192� and 212� moift airnbsp;expands about 8 or 9 times as much as dry air innbsp;fimilar circumftances.

From all what has been faid with refpeft to the expanfion of fluids, it appears that on account ofnbsp;the great irregularity of the rate of expanfion, mercury and fpirit of wine are the only two fluids whichnbsp;can be ufed for thermometers; obfcrving that fomenbsp;compenfation muft be made in the fcale of the fpi-tit thermonneter, in order to make it correfpondnbsp;with the fcale of the mercurial thermometer. Butnbsp;the mercurial thermometer cannot indicate a temperature higher than 600�. Hence various ingenuous perfons have endeavoured to contrive inftru-�^ents capable of indicating the higher degrees ofnbsp;heat, which would be of great ufe in philofophy,nbsp;'^hemiftry, and various arcs * ; but the only ufeful

'S mentioned by De Magellan, in his Effay fur la du Feu elementaire-, 1780, that Mr. Achardnbsp;�u had contrived and executed a thermometer capable

contrivance of this fort was made by the late Mr. Wedgwood. This ingenious gentleman appliednbsp;to the meafuring of high degrees of heat, a Angularnbsp;property of argillaceous bodies, a property whichnbsp;obtains more or lefs in every kind of them, as far asnbsp;has been examined. This property is, that an argillaceous fubftance, when expofed to a fire, is di-miniilred in bulk by it, nor does the bulk increafenbsp;again after cooling; and this diminution of bulk isnbsp;proportionate to the degree of heat to which thenbsp;fiubftance has been expofed.

This property may feem to be a deviation from the general rule, viz. that heat expands all forts ofnbsp;bodies, and that a diminution of heat enables themnbsp;to contrafl their dimenfionsj but in this cafe itnbsp;muft be confidered that the clay-pieces contradl andnbsp;remain contradted, becaufe fome fubftance, viz.nbsp;water and an aeriform fluid, is feparated from themnbsp;by the action of the fire.

Mr. Wedgwood�s thermometer, or apparatus for meafuring the high degrees of heat, confifts of fmallnbsp;of meafuring the degrees of heat from 212� to a confiderable �nbsp;number beyond 600� 5 the tube and bulb of this thermometer are faid to confifl: of a femi-tranfparent porcelain, and tonbsp;be filled with a mixture of two parts of bifmuth, one of lead,nbsp;and one of tin, which metallic mixture melts at about thenbsp;poibt of boiling water, and is afterwards expanded by annbsp;increafe of heat. But I never faw one of thofe thermometers.

pieces of clay of a determined length, which are to be placed in the furnace, crucible, amp;c. whofe degreenbsp;of heat is to be afcertained, and of a gauge to mea-fure the contraded dimenfions of the clay-pieces,nbsp;after they have been expofed to the fire.

Fig. 7. Plate XVIII. reprefents the gauge, which is either of brafs or of porcelain. Fig. 8. reprefents.nbsp;a fedlion of the fame ; and the letters refer to thenbsp;like parts in both figures. EFHG is a fmooth flatnbsp;plate; AC, BD, are two rulers or flat pieces, anbsp;quarter of an inch thick, and fixed faft upon thenbsp;plate, fo as to form a converging canal ABCD,nbsp;whofe wndth at CD is three-fifths of the width atnbsp;AB. The whole length of the canal from AB tonbsp;CD, is divided into 240 equal parts, and the divi-fions are numbered from the wider end. It is evident that if a body, fo adjufted as to fit exadlly thenbsp;�wider end of this canal, be afterwards diminifhed innbsp;its bulk by the aflion of fire (as the thermometri-cal pieces, which will be defcribed in the next paragraph) it may then be pafled further in the canal,nbsp;and more fo, according as the diminution isnbsp;greater.

As different fpecimens of clay are contraAed differently hy the like degrees of heat' Mr. Wedgwood endeavourednbsp;to find a fort of argillaceous fubftance, which might be

.46 nbsp;nbsp;nbsp;'The Theory 'of Heat, i�c'.

nearly as much in diameter as they are in length. When one of thefe pieces is to be ufed, it is propernbsp;to meafure it firft by placing it in the gauge at AB ;nbsp;for fometimes thofe pieces are a fev/ degrees largernbsp;or fmaller than the diftance A B, which excefs ornbsp;defedl being afcertained, muft afterwards be allowednbsp;for. P reprefents one of thefe pieces fet in thenbsp;gauge for meafure,ment.

The piece is then placed into the furnace, or crucible*, and if it be taken out cither at the end of the operation, or at any period, and, when grown cold,nbsp;be meafured by Aiding it as far as it will go, intonbsp;the canal of the gauge, the number of divifions

conftant in its contra�ing property. After a variety of trials he found that two parts of tb� Cornwall porcelain clay,nbsp;and one part of the earth of alum, formed a fubftance pof-felTed of the defied property.

The alum earth is prepared by diflblving the alum in water, precipitating with a folution of fixed alkali, andnbsp;wafhing the earth repeatedly with large quantities of boiling water.

The two earths muft be well mixed together, then the pafte is formed into a cylinder by means of a mould, and cutnbsp;to the proper length. They are alfo expofed to a low fire,nbsp;viz. barely red-hot, in order to give them fome hardnefs.

* In certain cafes it will be proper not to expofe the ther-mometrical piece by itfelf, but to place it in a fmall crucible or cafe of crucible earth, and expofe it with the cafe to thenbsp;fire; which prevents the adhefion of extraneous matter to thenbsp;piece, kc.

againft the place where it flops will fliew the contraled dimenfions of the piece, and of coiirfe the fltgree of heat to which it has been expofed. Itnbsp;'vill be found that thefe pieces will go very littlenbsp;beyond o in the canal, if they have been expofed tonbsp;a vifible red heat �, will go to 27� if they have beennbsp;expofed to the heat in which copper inelts; tonbsp;about 90quot; ,if expofed to the welding heat of iron;nbsp;about 160� if expofed to the greateft heat that cannbsp;he produced with charred pit-coal in a weh con-ftrufted common air-furnace, amp;c.

The fame thermometrical piece which has been ufed before, may be ufed again for .higher degreesnbsp;of heat, but not for lower degrees.

It is now neceflary to fhew the correfpondence between the fcale of this, and the fcale of Fahrenheit�s mercurial thermometer.

As the mercurial thermometer cannot Ihew 2. temperature higher than 600�, and Wedg-�wood�s thermometer cannot fliew a temperaturenbsp;Wer than red heat, which is by feveral degreesnbsp;higher than 600�, therefore it was neceflary tonbsp;Contrive a meafure for the intermediate degrees,nbsp;^nd which might reach fome degrees below 600�,nbsp;^�^d fome degrees above the temperature of a rednbsp;heat. IVtr. Wedgwood chofe a piece of filver, thenbsp;^^panfion of which meafured in a gauge made fornbsp;the purpofe, fimilar to the gauge, fig. 7. might indicate the degrees of temperature between the twonbsp;thermometers i with this inftrument he firft Ibund

the correfpondence between the degrees of Fahrenheit�s fcale and the laft-mentioned gauge, by placing them alternately in water of the temperature of 50�,nbsp;and in boiling water. Then he found the correfpondence between the degrees of the gauge ofnbsp;the filver piece, and that of the earthen thermome-trical pieces, by placing them both at the famenbsp;time into different and higher degrees of heat;nbsp;laftiy, by computation from thofe refults, he determined the correfpondence between the degrees ofnbsp;Fahrenheit�s fcale and thofe of his own thermome-trical gauge

It was found that one degree of Wedgwood�s thermometer is equal to 130� of Fahrenheit�s;nbsp;and that the o of Wedgwood�s coincides with thenbsp;1077,�5 of Fahrenheit�s ; from which data a com-parifon of the two thermometers may be made, ornbsp;rather of the imaginary extenfions of their twonbsp;fcales ; for, in faft, Fahrenheit�s thermometer cannot fhew higher than 600� ; and Wedgwood�s

* If it be afked why could not the abovementioned gauge with the filver piece be ufed, in chemical and other pro-ceffes, for meafuring the interrnediate degrees between thenbsp;thermometers of Fahrenheit and Wedgwood ? The anfwernbsp;is, that the piece of filver, after being expanded by heat,nbsp;does not remain in that flate, bat is contradled in cooling;nbsp;therefore it muft be meafured whilft hot; and for this pur-pofe the gauge itfelf muft be adlually expofed to the famenbsp;degree of heat; which is attended with very great inconveniences.

reach near fo low. It is likewife to be observed that the degrees of Wedgwood�s fcale are fuppofed to fliew equal increments of heat; whereasnbsp;in truth we do not know whether the clay thermo-metrical pieces contraft in proportion to the increments of heat; which fhews that, though this is thenbsp;beft known thermometer for meafuring the highernbsp;degrees of heat, yet an improvement of the fame,nbsp;or Ibme other more manageable and more accuratenbsp;contrivance, is highly defirable.

Upon the whole it appears, that the fpirit thermometer enables us to meafure the degrees of heat as low as has ever been experienced, either naturally, or by artificial cooling j that the mercurialnbsp;thermometer enables us to meafure ^he heat fromnbsp;� 40� to 600�; and that Wedgwood�s thermometer enables us to meafure from a red heat upnbsp;to the farther extent of that fcale, viz. to its a40thnbsp;degree, which is reckoned equivalent to 32277� ofnbsp;Fahrenheit�s fcale¹.

The following Table contains the correfpon-dence between Fahrenheit�s and Wedgwood�s thermometer j and it exhibits a confiderable number peculiar effedts or phenomena, which have beennbsp;found to take place at particular degrees of heat.

Extremity of the fcale of Wedgwood�s thermometer -nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;. -

Greateft heat of an air furnace 8 inches in diameter, which neither meltednbsp;nor foftened Nankeen porcelain - 21877

Stone ware, or pots de gres, hskt�. in 14337 Worcefter porcelain vitrifies *nbsp;nbsp;nbsp;nbsp;- 13297

The furface of polilhed fteel acquires a pale ftraw colour*; and is thenbsp;beft heat to which hardened bladesnbsp;of pen-knives and other tools withnbsp;a fine edge, muft be brought downnbsp;for a proper temper. Alfo bifmuthnbsp;melts, and likewife a mixture of 4nbsp;parts of lead and one of tin melts -Tin melts -j-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-

Heat, to which the hardened blades of razors Ihould be brought down fornbsp;a fine temper �nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-

^ mixture of 3 parts of tin and 2 of Lad melts ; alfo a mixture of 2 partsnbsp;of tin and one of bifmuth melts

jv ^ intermediate degrees of heat between 460� and � produce upon fteel the fuccefiive (hades of colour beween palenbsp;nbsp;nbsp;nbsp;jggp colour.

f bare (Mr. Cavendifti fays in the Philofophical E2nbsp;nbsp;nbsp;nbsp;� Tranfadtioijs,

A compound of equal parts of tin and bif-muth melts - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;_

Nitric acid boils ^ nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;'nbsp;nbsp;nbsp;nbsp;-

A faturated folution of fait boils f -Water boils (the barometer being at 30 inches) ; alfo a compound of 5 parts ofnbsp;bifmuth, 3 parts of tin, and 2 parts ofnbsp;lead, melts -nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;,

Heat fit for the hatching of hen�s eggs The moft ufual heat for a pleafant bath -Heat of the interior bath of Edinburgh -

� Tranfaflions, vol.73d, p.313) formerly kept a thermometer � in melted tin and lead, till they became folid; the ther-mometer remained perfectly ftationary, from the time thenbsp;� metal began to harden round the fides of the pot till itnbsp;� was entirely folid; but I could not perceive it to fink atnbsp;all below that point, and rife up to it when the metal be-� gan to harden.�

* Concerning the freezing point of this acid, fee the Phil. Tranf. vol. Art. XilL.

A mixture of alkohol nbsp;nbsp;nbsp;andnbsp;nbsp;nbsp;nbsp;waternbsp;nbsp;nbsp;nbsp;in equal

Mercury congeals ¹ ² nbsp;nbsp;nbsp;,nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;�39

* In alTuming the folid form, mercury contradls its bulk ''�regularly. Upon the whole it feeras to contradl aboutnbsp;�rd of its bulk ; but it is impoffible to fay how much morenbsp;'tttiay contract by a greater cold. Mercury may be cooled

brne degrees below its freezing point, before it affumes the ^ol'd form; but as foon as it begins to harden, the thermo-rifes to its freezing point, which, from a variety of thenbsp;accurate experiments made by Mr. Hutchins at Hud-s Bay, appeared ujxin his thermometers to be �40. � Butnbsp;(Mr. Cavendifhobferves in the Phil. 1'ranf. vol. 73d, p. 321)nbsp;'t appeared from the examination of this thermometer,nbsp;af²- ²nbsp;nbsp;nbsp;nbsp;^

��^r It came home, that �40 thereon anfwers to � 38�^ � b ^ ^^^rtnometer adjufted in the manner recommendednbsp;� all ��'^'uittee of the Royal Society} it follows, thatnbsp;^be experiments agree in Ihewing that the true point atnbsp;�3nbsp;nbsp;nbsp;nbsp;�nbsp;nbsp;nbsp;nbsp;� which

Solids are expanded by heat much lefs than fluids i and, indeed, there feerps to be that lawnbsp;in nature, namely, that in general bodies of lefsnbsp;denfity are expanded more than other bodies ofnbsp;greater denfity. This law, however, has feveralnbsp;exceptions, efpecially amongft the folids. Thus hydrogen gas is expanded, more than common air jnbsp;common air more than fpirit of wine j fpirit ofnbsp;wine more than water; water more than mercury jnbsp;mercury more than iron j but iron lefs than tin^,nbsp;though tin is fpecifically lighter than iron.

The inftruments for meafuring the expanfions of folids, have been called fyromters, (viz. meafurersnbsp;of fire) ; and they have been made of a great varietynbsp;of (hapes. The whole confifts of a machine capablenbsp;of rendering the fraall expanfions of folids apparentnbsp;to the obferver, and of an apparatus fit to heat thenbsp;Dodics under examination to a determined degree.nbsp;The mod; ufual, and, indeed, the moft eligible, modenbsp;of heating the bodies, is to place them in water,nbsp;wherein a thermometer is placed, and to heat thenbsp;water by means of lamps. The fmall expanfionsnbsp;� which quickfilver freezes, is.�38� f, or in whoje numbers,nbsp;� 39� below nothing.�

In becoming folid, mercury fometimes ihoots into chry-ftals or longitudinal filaments like pins. Congealed raer-; cury poflefles confiderable dudlility; but it does not feemnbsp;to be perfectly malleable. See the Phil, Tranf. vol. 73d,nbsp;Part II.

of the heated folids have been rendered vifible, ift, by rnultiplying wheels, or by leavers, or by finenbsp;fcrews, which render a fmall motion communicatednbsp;to one end of the mechanifm produdtive of a greatnbsp;movement at the other end j and 2dly, by magnifying the fmall expanfion through microfcopes;nbsp;which, upon the whole, feems to be by far the modnbsp;certain and the mod manageable method ; for withnbsp;wheels and pinions, and even with levers or fcrews,nbsp;there is always fome equivocal motion, arifingnbsp;from the loofe connexion of teeth and pinions,nbsp;or from the drefs and bending of other parts ¹.

Muffchembroek feems to have been the firft inventor of the pyrometer. See his Pyrometer, with alterations andnbsp;improvements by Defagulier, in Defagulier�s Exp. Phil,nbsp;vpl. I. p, ^21.

Mr. ElUcot contrived a different mechanifm, which is defcribed in the Phil. Tranf. ISl. 443. With that pyro-meter he determined the proportional expanhons of fevennbsp;metallic lubftances, produced by the fan^e increafe of temperature. They are a¹ follows:

Gold, Silver. Brafs, Copper. Iron. Steel, Lead, 73nbsp;nbsp;nbsp;nbsp;�03nbsp;nbsp;nbsp;nbsp;95 S9nbsp;nbsp;nbsp;nbsp;¹49

A. very fitnple pyrometer, more ufeful for fhewing the expanfion of metals in a courfe of leisures, than for accuracynbsp;meafurements, is defcribed by Fergufon in the fupple-�¹'ent to his le�tures.

Mr, Smeaton contrived a pyrometer, which is vaftly E¹� pwior to any that had been conftrufted before, either in this

The beft inftrument of the kind, in point of accuracy, is undoubtedly Mr. Rarafden�s pyrometer, of whofe conftruftion I Ihall now give a (liort ideajnbsp;referring the reader for farther particulars to thenbsp;original and particular defcription in the Philofo-phical Tranfa�fions. ,

The metallic bar, whofe expanfion is to be mea-fured, and which may be even five feet long, is placed in a copper trough little longer than five feet;nbsp;and this is placed over.12 fpirit lamps, the flamesnbsp;of which can heat the water of that trough fully tonbsp;the boiling point, and of courfe heat the bar whichnbsp;is plunged in it.

Two other wooden troughs, alfo full of water, are placed parallel to, and at a little diftance from,nbsp;the copper one. Each of thefe contains a call ironnbsp;prifmatic bar^ to the ends of one of thofe barsnbsp;two microfcopes are fattened in an horizontal ficua-tion, perpendicularly to the bars. One of thofe microfcopes is furnifhed with a micrometer, or me-country or elfewhere, and with it he determined the expan-fibility of feveral fubftances to a confiderable degree of accuracy. See the Phil. Tranf. vol. 48th. See alfo De Luc�snbsp;pyrometer in the Phil. Tranf. vol. 68th. But all thofenbsp;contrivances muft be confidered as inferior to a pyrometernbsp;which was contrived by the late very ingenious Mr. Jefleynbsp;Ramfden; of which an accurate and minute defcription isnbsp;given by General Roy, in the 75th vol. of the Philofophicalnbsp;Tfanfa�tions,

chanifm,

' chanifm, to meafure the magnified image of an ob-the other microfcope has a fimple mark. Now, without entering into any particular detail,nbsp;�'^hich would not be proper, previous to the de-fcription of microfcopes, I fliall only fay, that thenbsp;parts of the microfcopes, as alfo the proper marksnbsp;for meafurement, are fo feparated and difpofed,nbsp;partly upon the ends of the caft-iron rods, and partlynbsp;^pon the ends of the rod under examination, thatnbsp;if any of them be lengthened or fhortened, that alteration is clearly perceived through the microfcopes, and may be meafiired by means of thenbsp;micrometer. It follows, that if the temperaturenbsp;of the caft-iron prifmatic bars be kept unalter-rnbsp;ed, v/hilft that of the bar under examination is in-creafed, then the increafe of length, which is mea-fured by the micromoter, mull: be attributed to thatnbsp;bar only j and quot;by this means the expanfions of fevennbsp;fubftances were afeertained *.

quot;Ihe following table Ihews in parts of an inch bo'v much a foot length of different fubftances isnbsp;expanded by 18o� of heat, between the freezingnbsp;2nd boiling point of water. To the firft feven fub-ftances (having been examined in Mr. Ramfden�s,nbsp;^bich is by far the moft accurate, pyrometer), Inbsp;b^ve added the expanfions, for a fingle degree ofnbsp;The reft were determined by Mr. Smeaton

v�Uf fc Pha. T,^.

Standard

Fahrenheit's fcafe.

' By 1� By iSo�quot;*

Standard brafs fcale, fup-pofed to be Hamburgh

brafs nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;. 0,0001237	0,0222646
Englilh plate brafs, in form
of a rod - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;- 0,0001262	0,0227136
Englifh plate brafs, in form
of a trough - nbsp;nbsp;nbsp;- 0,0001263	0,0227386
Steel rod - nbsp;nbsp;nbsp;,nbsp;nbsp;nbsp;nbsp;- 0,0000763	ogt;oid7368
Caft iron prifm - nbsp;nbsp;nbsp;- 0,0000740	0,0133126
Glafs tube* nbsp;nbsp;nbsp;,nbsp;nbsp;nbsp;nbsp;- 0,0000517	0,0093138
Solid glafs rod * - nbsp;nbsp;nbsp;- 0,0000539	0,0096944
White glafs barorneter tube *	0,0100
Martial regulus of antimony	0,0130
Bliftered fteel r nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	0,0138
Hard fteel nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;,	0,0147
Iron	0,0151
Bifmuth - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;,	0,0167
Copper hammered - , nbsp;nbsp;nbsp;-	0,0204
Copper 8 parts with tin one	0,0218
Caft brafs nbsp;nbsp;nbsp;- .nbsp;nbsp;nbsp;nbsp;,nbsp;nbsp;nbsp;nbsp;,	0,0225
Brafs 16 parts with tin one	0,0229
Brafs wire nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;_	0,0232

* Sometimes glafs tubes are extended more than folid glafs rods; their dilatation, however, is not conftant; fornbsp;tubes of different diameters, or of different forts of glafs,nbsp;are expanded differently by the lik? degrees of heat. Phil,nbsp;'Tranf. vol. 67th, p. 665,

S|)cculurp

^he �theory of Heai, ifc.	59
Speculum metal ^ nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	0,023 a
Spelter folder, viz. brafs 2 parts, zinc one	0,0247
Fine pewter - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	0,0274
Grain tin nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;�	0,0298
Soft folder, viz. lead 2 parts, tin one	0,0301
Zinc 8 parts with tin one, a little ham-r
mered nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	0,0323
Lead - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	0,0344
Zinc or fpelter	ogt;o35J
Zinc hammered half an inch per foot -	0,0373

Wood is not much expanded longitudinally, viz, in the direftion of its fibres, by heat; and this is particularly the cafe with deal and other ftraight grainednbsp;wood. Probably upon the whole, the longitudinalnbsp;expanfion of wood is lefs than that �f glafs. It hasnbsp;been obferved, that very dry and feafoned wood, ifnbsp;t'ot expofed to a very high or to a very low temperature, will expand in length pretty regularly jnbsp;Otherwife its expanfion by heat and contraftion bynbsp;cold are very irregular; for they ffeem to depend partlynbsp;�pon the heat, and partly upon the moifture, whichnbsp;the Wood acquires in certain circqmftances, and isnbsp;deprived of in others*.

The expanfion and contradlion of every fubflance heat and cold, and the continual change ofnbsp;temperature, to which all the furface of the earth.

Dr. Ritenhoufe�s Experiments in the Tranfaftions the American Philofophical Society.

with

with whatever thereon exifts, is fubje^j fhsw that every particle of matter, every fibre of our bodies,nbsp;metals, bricks, ftones, fluids, amp;c. are all in continual motion, though thofe movements may appearnbsp;very fmall to our fenfes. Hence it is faid, that heatnbsp;is the univerfal mover, and feenns to animate thenbsp;whole.

Some ufeful advantages in mechanics are derived from the knowledge of the various expanfibilities ofnbsp;bodies. Thus well-conftrufted clocks would benbsp;fubjedl to a confiderable imperfeftion, were not remedies pointed out by thofe various expanfibilities.nbsp;To deferibe the mechanifm of clocks, watches, �i'c.nbsp;would be foreign to the fubjeft of this work ; but Inbsp;fliall endeavour briefly to flievv the advantage whichnbsp;clock-making derives therefrom.

It is evident that when the pendulum of a clock grows longer, the time of vibration is lengthened,nbsp;and of courfe the clock goes flower. The contrarynbsp;happens when the pendulum grows fliorter. Nownbsp;ri variety of contrivances to obviate this defedt'ofnbsp;clocks, amp;c. have been made by divers ingeniousnbsp;artifts, fuch as Graham, Harrifon, Ellicot, Ber-thoud. Gumming, Mudge, amp;c. which contrivancesnbsp;are all deriv^ed from the expanfibilities of bodies.nbsp;The following two or three inftances in the limplefl:nbsp;mode of conltruction, will give a fufficient idea ofnbsp;the application.

A wooden rod for the pendulum of a clock is certainly better than a common metallic rod i the ex-

panfion of the former being much lefs than that of the latter. A glafs rod or tube is alfo preferable tonbsp;ttietal; but the elFedts of the expanfibility of thenbsp;glals tube may be entirely removed by this means�nbsp;viz. by filling part of the tube with mercury.nbsp;Suppofe, for inftance, that AB, fig. 9. Plate XVIII.nbsp;is fuch a tube fufpended at Aj and filled with mercurynbsp;from B to C ; and let D be the centre of ofcillationnbsp;of the tube. Now it is evident, that when the tubenbsp;is extended by heat, the diftance of the point Dnbsp;from A is increafed ; for inftance, D comes to d ;nbsp;but the fame heat which extends the tube, rarefiesnbsp;the mercury alfo, .and extends its furface C to c jnbsp;hence on account of this elevation, the centre ofnbsp;ofcillation of the mercury B r, which before ftood atnbsp;E� is raifed to e, and of courfe the centre of ofcil-iation of both the tube and the quickfilver together remains in the fame place,- the centre ofnbsp;ofcillation of the one being raifed as much as thatnbsp;�f the other is deprefled.

The fame remedy may be obtained by a combi-tiation of bars of different metals, (which form ''^hat is called a compound pendulum); and the following is the fimpleft, though not the moft correft,nbsp;^ttd the moft commodious, conftruftion. Fig. 10.nbsp;^i^te XVIII, reprefents a pendulum, confifting of

is the moft expanfible by heat. CD is a lever P'liried at B and C to the extremities of the bars,nbsp;^od fufpen(jjj^g ^ vveight or bob E. Now, if the

two bars were expanded alike by heat, it is evident that the diftance of the weight E, from the point ofnbsp;fufpenfion A, would be increafed; but as AC isnbsp;expanded more than AB, when the extremity Bnbsp;comes to b, the extremity C comes lower down,nbsp;viz. to c; hence the lever CD is placed in the fitu-ation cbTifo that the extremity D rem.ains in thenbsp;fame place it was before, and of courfe the diftancenbsp;EA remains unaltered. Jn thofe cafes the lengthsnbsp;of the rods, levers, or (in the above inftance of thenbsp;glafs tube) the quantity of mercury muft be calculated and adjufted according to the quantities ofnbsp;expanfions, lengths of the pendulums, amp;c.

Other contrivances, but'upon the fame principle of different expanfibility, have been applied to the

Heat, or, according to the theory, caloric, cannot be accumulated and detained in anynbsp;particular place or body, but it continually tendsnbsp;to expand itfeif over the adjacent bodies, till theynbsp;are brought to the fame temperature. Thus, if anbsp;piece of red-hot iron be placed amongft other piecesnbsp;tgt;f ftone, metal, amp;c. in a colder ftate j the heat willnbsp;communicated from the iron to the other bo-j to the neareft firft, and then to thofe that arenbsp;^ore remote ; fo that by degrees the iron lofes partnbsp;its heat, and the other bodies acquire it, until,nbsp;all attain the fame temperature; for though thenbsp;caloric will pafs through certain bodies eafier thannbsp;through others; yet there is no body which cannbsp;confine it effeftually.

bodies, is proportionate to the quantity of matter of each body refpeftively. And on the contrary, ifnbsp;they be placed in a lower temperature, the quantities of caloric which they lofe, are proportionate tonbsp;their quantities of matter refpedlively. Thus, if twonbsp;leaden balls of equal weights be placed in boilingnbsp;water, they will be heated alike, or equal quantitiesnbsp;of caloric will be communicated to them. If theynbsp;be placed in cold water, they will be deprived ofnbsp;equal quantities of caloric. When a pound ofnbsp;water is mixed with another pound of water hotternbsp;than the former, the excefs of heat which one ofnbsp;them has above the other will be divided into two ;nbsp;viz. it will be diftributed equably amongft the twonbsp;pounds of water. If two pounds of water be mixednbsp;with one pound of hotter water, the excefs of heatnbsp;will be divided into three parts, viz.' it will benbsp;diftributed equably amongft the three pounds ofnbsp;water: hence we derive an eafy rule for determining the temperature of a mixture of homogeneousnbsp;bodies, which pofleffed different known temperatures before the mixture. The rule is as follows:

Multiply the weight of each parcel or body by its peculiar temperature ; add the produSls together, andnbsp;divide the fum of thofe produbls by the fum of thenbsp;weights; the (quotient is the temperature of the mixture.

Thus, if three pounds of water, whofe temperature is 40�, be mixed with 9 pounds of watet, whofe temperature is 100% the temperature of the mixture will be 85% for 3 multiplied by 40quot; gives 120%

Of the 'Capacify of Bodies for Caloric, (fc. 6$

and 9 multiplied by ioo% gives 900�. The fum of thofe two produfts is 1020�, which being dividednbsp;the fum of the pounds, viz. by 12, quotes 85� inbsp;^nd fuch is the temperature of the mixture¹.

Again, if 2 pounds of mercury at 40�, be mixed '''ith 4 pounds of mercury at 60% and 4 pounds

Several circumftances, of which the following are the elTential, muft be attended to in the performance ofnbsp;thofe experiments.

1. The thermometers muft have fmall balls, and (hould fo fenfible as to indicate at leaft quarters of each degree |nbsp;the quantities of the bodies concerned �iould be prettynbsp;otherwife the dipping of the thermometer in the mix-^¹�6 introduces a third fubftance, namely, the mercury ofnbsp;thermometer, which will alter the refult confiderably.nbsp;refults however may be calculated by confidering thenbsp;and the thermometer, as two of the bodies concernednbsp;^ forming the mixture.

'^^ughom-; therefore it will be proper to take the tem-J^tature of the bottom, of the middle, and of the Upper part '�f the mixture ; for a mean of thofe three will give the tem-^ tatute of the whole mixture.

The foundation of the rule is very evident; for fince heat or caloric expands equably amongft homogeneous bodies that are in contadlj it followsnbsp;that the temperature of the mixture mufl: be a meannbsp;of the feparate temperatures of the parcels, and this ^nbsp;rule does only afcertain that mean ; for the fum ofnbsp;all the degrees of heat is divided by the fum of thenbsp;weights of all the, parcels. See page 231. vol. II.

When bodies of different fort of matter are placed in a higher temperature, or, in other wordsgt;nbsp;when heat is communicated to bodies of differentnbsp;nature, but of the fame weight and equally ex-pofed, it has been found that fome of them abforb,nbsp;or combine with, a greater quantity of caloric than lt;nbsp;others; hence the former are faid to have a greaternbsp;capacity for caloric than the latter ; and the proportional quantity of caloric which each body abforbs�^nbsp;is called the fpecific caloric of that body ¹. Eachnbsp;particular body, as far as has been tried, has beennbsp;found to have a particular capacity for caloric¹nbsp;Thus, if a pound of mercury, and a pound of anothefnbsp;metal, be placed in a higher temperature, and it be �

For if a body A can abforb, for inftance, 3 times a� much caloric as B; it is evident that, in a natural ftate, Anbsp;contains 3 times as much caloric as B, when both appear cf ^nbsp;the fame temperature.nbsp;nbsp;nbsp;nbsp;, j

found that the other metal abforbs twice as much Caloric as the mercury j then the fpecific caloric ofnbsp;that other metal is to the fpecific caloric of mer-t^tiry, as two to one.

This proportional quantity of caloric, which one body abforbs more or lefs than another body, hasnbsp;alfo been called latent heat; but the expreilion isnbsp;Evidently improper; for though one body A has,nbsp;inftance, twice as much caloric as another bodynbsp;^ of equal weight, yet A has a double capacity, ornbsp;has an affinity for caloric as ftrong again as B ; irtnbsp;^onfequence of which the caloric is detained withnbsp;^qual power ; nor can it be communicated from Anbsp;t� B ; in which cafe only it would aft as heat, viz,nbsp;�quot;'ould expand B, or if B be a living animal*nbsp;quot;'ould excite in it the fenfation pf heat.

It is now neceffary to fhew by what means the fpecific caloric of bodies is to be afeertained. � Fornbsp;this- purpofe,

Take two bodies of equal weights, and whofe capaci-an permanent; for inftance, A and B, one of �^hich at leaf is a fluid; let them acquire different tem~-P^fatures, and afeertain thofe temperatures by the ther~nbsp;^ometer. Then put the Jolid into the fluid, or mix thenbsp;fluids together very expeditioufty, and immediatelynbsp;place the thermometer in the mixture, and. afeertainnbsp;temperature. Now thefpecifle caloric of the body Anbsp;^nbsp;nbsp;nbsp;nbsp;fpecifle caloric of the body B, as the difference

the temperature of^ previous to the mixture, ihe temperature of the mixture, ii to the differencenbsp;F %nbsp;nbsp;nbsp;nbsp;between

Gt Of the Capacity of Bodies for Caloric, iBc. between the temperature of A previous to the mixture:

And the temperature of the mixture {^obferve the precautions of the note in fage 65.) For inftance,

Therefore the fpecific caloric of A is to the fpecific calorie of B, as 40 to 30 ; or as 4 to 3.

If the fame two bodies be heated differently, the refuk will give precifely the fame proportion ofnbsp;differences, and of courfe the fame proportion ofnbsp;fpecific calorics. Thus,

It is evident that if the two bodies had had equal degrees of affinity, or had had equal capacities fornbsp;caloric; (both thofe expreffions meaning the unknown caufe of the fame perceivable effect) thenbsp;temperature of the mixture v/ould have been 75quot;,nbsp;viz. an arithmetical mean between the temperaturesnbsp;of A and B ; fo that, in the firfl example, A wouldnbsp;have loll 35�} and B would have acquired 35� ofnbsp;heat. But, fince the temperature of the mixturenbsp;was 80�, it appears that the 70 degrees of heat,nbsp;w'hich A had more than B, mull have been diltri-buted fo as to increafe the temperature of B bynbsp;40�, and to leffen the temperature of A by 30�.nbsp;Now, when a quantity of heat is communicated to anbsp;� ^nbsp;nbsp;nbsp;nbsp;body.

that portion of it, which is abforbed by the ^ody, does not raife its temperature, or is that portion which, added to the increment of temperature,nbsp;becomes equal to the original quantity of heat; fornbsp;inftance, if io� of heat be communicated to a body,nbsp;and the temperature of that body is thereby raifednbsp;4�gt; it is evident that fix degrees of heat have beennbsp;abforbed, amp;c. therefore, in the above-mentionednbsp;cafe, the portion of caloric abforbed by B, muftnbsp;have am.ountcd to 30�, fince its temperature wasnbsp;raifed 40�, and the quandty retained by A miifl:nbsp;have amounted to 40�, fmce its temperature wasnbsp;lefl�ned 30�.

The fame reafoning, rmtatis mutandis, may be applied to the fecond cafe, and in general to all fuchnbsp;like cafes.

By this means the fpecific caloric of various fubftances have been determined with as muchnbsp;Ptecifion as the difficulty of performing fuch experiments, efpecially with elaftic fluids, and thenbsp;fluctuating quality of the articles, will admit of.

In the following lift, the fpecific caloric of water called one, or unity, and that of every other fiib-ftance is exprefled in proportion to this unity;nbsp;fl�r inftance, the fpecific caloric of fpermaceti oilnbsp;piSoo, viz. the half of that of water; 5-tenthsnbsp;the half of a unit; which means that anbsp;^^�^ntity of fpermaceti oil can abforb the half ofnbsp;Quantity of caloric which an equal quantity of

70 nbsp;nbsp;nbsp;Of the Capacity of Bodies for Caloric, amp;c.

the fpecific caloric of the ruft of iron is 0,250, viz. a quarter of that of water; which means that innbsp;equal circumftances the ruft of iron will abforb I ofnbsp;that quantity which an equal weight of water cannbsp;abforb ; and fo of the reft *.

* This table has been collecfteci from the experiments of the principal labourers in this field of curious inquiry. quot;1 onbsp;each article the initial of the name of the gentleman bynbsp;whom it was determined, is fubjoined, viz. C, for Crawford; K, for Kirwan; and L, for Lavoifier and Laplace,nbsp;conjointly.

The fpecific caloric of water in the ftate of vapour is fuppofed, by Mr. Pidlet, to be about 8 \ times greater thannbsp;that of the fame water in a boiling ftate. But the volumenbsp;of the vapour (he fuppofes) is about i8co times greaternbsp;than that of boiling water. There is therefore 21? time�nbsp;more caloric in any given volume of boiling Water, than innbsp;an equal volume of vapour. Phil, Mag. vol. VI. p. 244'nbsp;Other .perfons have reckoned it much higher, and othersnbsp;lower. In fliort, the fpecific caloric of fteqm iS not yc*-known with fufiicient certainty.

S�lutlOft

Of the Cafacity of Bodies for Caloric, Sc, 71

Solution of fal ammoniac, or muriate of ammoniac, fait 10, water 15 gt; K -Azotic gas, C

Muriatic acid, fmoking, fpecific gravity 1,122, K � -Solution of alum, fait 100, water 445, K Solution of nitre, fait i, water 8, K -C -nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;_

The capacities of bodies for caloric are pretty permanent, as long as the bodies remain in the famenbsp;ftate with refpeft to confiftency.

Almoft all bodies in nature have been found capable of esifting in three different ftates, viz. the fofid, the liquid, and the aeriform, vaporous, ornbsp;clafcic ftate of fluidity. In each of thofe ftates thenbsp;capacity of the body for caloric, or its fpecific caloric, is different from what it is in the other ftates.nbsp;It is leaft in the folid and greateft in the ftate ofnbsp;elaftic fluid.

If more caloric be communicated to a folid body than what its capacity will bear, and if that excefsnbsp;of caloric be equivalent to the capacity of that bodynbsp;when in a liquid ftate j then that folid will benbsp;liquified by it. The fame thing muft be under-ftood of the conyerfion of a liquid into the aeriformnbsp;ftate

The reverfe of this propofifion is likevrife true^ viz. if fo much caloric be abftradled from a liquid,nbsp;as to leave in it that quantity only which its capacitynbsp;when in a folid ftate can' hold ; then that liquid willnbsp;be rendered folid or congealed. The followingnbsp;example will illuftrate this theory.,

When a piece of ice, of a temperature lower than jcquot;, is placed in a higher temperature, thenbsp;temperature of the ice is raifed as, high as 32.�,nbsp;and there it remains j for all the furplus of heatnbsp;is abforbed by that external quantity of ice which isnbsp;converted into liquid water. When a quantity of

is placed in, a higher temperature, the temperature of the water is gradually raifed as high as 212�, (under a mean or more t)fual preflure of the at-rnofphere) but it cannot be raifed higher, becaulcnbsp;^11 the furplus of heat is abforbed by that quantitynbsp;of water which is converted into fteam. On thenbsp;other hand, when fteam is converted into water.nbsp;Caloric is feparated from it, and when water isnbsp;converted into ice, caloric is alfo feparated fforqnbsp;it *.

It has been found, that when water at 172� is mixed with an equal weight of water at 32�, thenbsp;temperature of the mixture is 102�, agreeably tonbsp;what has been faid above. But when a quantity ofnbsp;water at 172� is mixed with an equal weightnbsp;of ice or fnow at 3 2�, the temperature of the mixture is 32�. Whence it is juftly inferred, thatnbsp;''''^ter in a liquid ftate pontains 140� (viz. 172*

* If a quantity of water, with a thermometer in it, be placed in a cold or freezing mixture, the water often rema nnbsp;fluid, when the thermometer fliews, that its temperature inbsp;30� or 28�, or even lower. At laft it freezes very quick y,nbsp;efpeclally on giving a little ftroke or agitation to the ve ,nbsp;at the fame time the thermometer immediately ri es tonbsp;3^�, which feems to indicate that tne water cannot e ynbsp;P^tt with its caloric, even W'hen placed in a lower tempenbsp;tare, q caufe of that impediment is not known.

In fhort, it appears that a certain quantity, of caloric is indifpenfably peceffary to keep a body in a ftate of vapour or elaftic fluid, that a fmaller quantity is indifpenfably neceflary to keep it in a liquidnbsp;ftate, and that a quantity Hill flnaller of caloric exiftsnbsp;in a folid and in any temperature, as low however asnbsp;a certain limit. This limit, or the point of totalnbsp;privation of caloric, has been deduced by calculation from the preceding refults, and upon a probable fuppofition.

Before we endeavour to explain the nature of that limit, it will be neceffary to make a few ufefulnbsp;remarks with refped to the formation of the preceding table of the fpecific caloric of bodies.

The rule for finding the fpecific caloric of bodies (page 67.) direds to mix bodies, whofe capacitiesnbsp;are permanent during the operation; the reafon of

* Owing to the difficulty of performing this experiment, and particularly of afcertaining the firft temperature of thenbsp;mixture, the quantity, or tlie degrees of caloric which fluidnbsp;water contains more than an equal quantity of ice, has beennbsp;fiated differently by different authors. Dr. Crawfordnbsp;reckons it 172�. Dr. Leffie fays to have been found bynbsp;Dr. Black, equal to I47^ Lavoifier reckons it 167�,nbsp;ProfeiTor Wikke' found it equal to 130�. Bergman found itnbsp;equal to 129�,6, viz. almoft the fame as Wikke. In fomenbsp;books I find it fiated at 162�, in others at 140�.

^�Wch is, that when bodies have their capacities altered by a change of form, the temperature of the tnlxture muft vary according to that unknownnbsp;change of capacity, amp;c. Therefore the proportionnbsp;between the fpecific caloric of water and of ice,nbsp;Cannot be determined by mixing equal quantities ofnbsp;^ater and pounded ice or fnow. But it may benbsp;determined by employmg a third fubftance, viz. bynbsp;determining in the firft place, the proportion between the fpecific caloric of that other fubftance andnbsp;ice, in a temperature lower than 32.�, and makingnbsp;the fpecific caloric of the other fubftance i. Secondly, by determining the proportion between thenbsp;fpecific caloric of that other fubftance and water innbsp;a temperature higher than 32�, ftill reckoning tlienbsp;fpecific caloric of the other fubftance i; and laftly,nbsp;by comparing the fpecific caloric of water with thatnbsp;tif ice ¹.

This firft operation (hews, that the fpecific caloric of ice to that of diaphoretic antimony, as 8 to 2, or 4 to i.

When in fuch experiments a fluid fubftance can be ufed with a folid, the operation is undoubtedlynbsp;preferable to the ufe of two folids ; yet in certainnbsp;cafes powders, or comminuted folids, give a to-lerably ufeful refult.

Experiments of this nature with aerial fluids are extremely difficult, principally owing to the fmallnbsp;weight of thofe fluids, and to the difiiciiity of mixing them with fiifficient expedition. The operationnbsp;likewife requires particular inftruments�^.

The total privation of caloric, or the lowefl: degree to which a thermometer with Fahrenheit�s fcale would defcend, if it were fituated in a place totally deftitute of caloric, has been deduced from thenbsp;above-mentioned proportion between the fpecificnbsp;caloric of water and that of ice, and from thenbsp;known quantity of caloric which water at ^,2� contains more than ice at 32�. The fame thing mightnbsp;be deduced from the fpecific caloric of any othernbsp;'fubftance in its two ftates of exiftence, amp;c.

For fince the quantity of caloric which water contains more than ice, is 140% and fince the capacity of water is to that of ice as 10 to 9, it follow'^

This fecond operation flrews, that the fpecific caloric of diaphoretic antimony is to that of water as 22,5 to 100 ; ornbsp;as I to 4,4444, See. , Therefore the fpecific caloric of waternbsp;is to that of ice as 4,444, See. to 4; or as 10 to 9.

* See Dr. Crawford�s Experiments and Obfervations ofi Animal Heat, the fecond edition.

that 140� is the difference between all the caloric contained in ice, and all that which is contained innbsp;t^'ater, when both are at the temperature of 32quot;-A-lfo, that this quantity is one-tenth part of thenbsp;�whole caloric which is contained in water when thenbsp;temperature of that fluid is' 32�. Therefore thenbsp;whole quantity of caloric is 10 times 140% ornbsp;1400� viz. 1368� below o of Fahrenheit�s fcale;nbsp;fo that in a place totally deftitute of caloric, thatnbsp;thermometer would defeend to �1368�, providednbsp;the fluid of that inftrument were capable of it f.

The fuppofition upon which this determination is eftablifhed, is that the capacity of ice is permanentnbsp;at any temperature below 32�.

Meflleurs Lavoifier and Laplace contrived a different method of determining the fpecific caloric of bodies. According to their method, a body heated

to a certain degree is placed contiguous to a quart-.tity of ice, and its fpecific caloric is determined by the quantity of ice which that body is capable ofnbsp;melting. Thus, if of two bodies, A and B, ofnbsp;equal weight and equally heated, placed fucceffivelynbsp;in a certain quantity of ice, A be found to havenbsp;melted three times as much ice as B, then the fpecific caloric of A will be to the fpecific caloric of B,nbsp;as 3 to I. '

For this purpofe the above-mentioned gentlemen contrived an apparatus, which they called the calo-rimtery and of which, fig. ii. Plate XVIII. exhibits a fection. It confifts of a vefiel Handing onnbsp;three legs, of which two are feen in the figure, andnbsp;having three divifions, viz. an interior one /��,nbsp;�which is formed of iron wire, a middle divifion hhbb,nbsp;and an external one a a. a a. The body fubj^fted tonbsp;experiment is placed within the wire divifion ��/�,nbsp;and this divifion is covered with a particular covernbsp;FIG. The other two divifions and the cover HGnbsp;are filled with ice, and a large cover alfo filled withnbsp;ice goes over the whole inftrument, its edge beingnbsp;fitted to the external grove of the inftrument. Thenbsp;ice of the external divifion ferves to proteft the icenbsp;of the middle divifion from the heat of the at-mofphere ; hence the ice of the middle divifion cannbsp;only be melted by the heat of the body, which isnbsp;placed in the wire divifion. The water of the icenbsp;thus melted, paffes through the grate m m, andnbsp;through a ftrainer placed a little below j then comes

out of the tube dx, and is received in a veflei placed Under it. This is the quantity of water which indicates the fpecific caloric of the body under trial.nbsp;1'he tube T^, with the ftop-cock r, ferves to drainnbsp;^he water from the ice in the external divifion *.

If it be required to determine the fpecific caloric of a folid body, its temperature muft be railed ; fornbsp;example, to 212�; it muft then be placed into thenbsp;Calorimeter, and fufFered to remain there till itsnbsp;temperature be reduced to 32�. Then by weighingnbsp;the Water which has flowed out of the tube dx, thenbsp;quantity of ice diflblved during the cooling may benbsp;determined. According to Lavoifier�s determination, one pound of water at 167% will diffolve anbsp;pound of ice; therefore, to determine the fpecificnbsp;Caloric of the body, the quantity of ice diffolvednbsp;tUuft be divided by the produft of the weight of thenbsp;body (expreffed in pounds and decimals) multiplied by the number of degrees above 32�, to whichnbsp;chat body has been raifed previous to the expe-C'ment. The quotient indicates the quantity ofnbsp;which a pound of that body can difiblve innbsp;cooling 1�^ jf quotient be multiplied by 167�,nbsp;product will fhew the quantity of ice which anbsp;cooV^ cgt;f that body heated to 167% �'an diffolve innbsp;� mg down to 32�. This will be the value of itsnbsp;caloric.

Otherwife, having the original temperature of the body, and the quantity of ice melted by thenbsp;fame, fay, as that temperature is to that quantity ofnbsp;ice, fo is 167�, to a fourth proportional, viz, to thenbsp;quantity of ice which would have been melted if thenbsp;original temperature of the body had been 167�.nbsp;Laftly, the above-mentioned fourth proportionalnbsp;muft be divided by the weight of the body (ex-prefled in pounds and decimals) and the quotientnbsp;will exprefs how much ice one pound of that bodynbsp;can diflblve. This is the value of its fpecific caloric.

� If the body be a fluid, it muft be put into fome vefTel, the fpecific caloric of which has beennbsp;previoufly determined. The procefs is the fame asnbsp;that defcribed in the preceding two paragraphs 5 butnbsp;care muft be taken to deduft from the quantity ofnbsp;ice diflblved, that which arifes from the cooling ofnbsp;the veiTel.

� If the quantity of caloric which difengages it' felf from the combination of feveral fubftances benbsp;required, they muft be all reduced to the tem.pera-ture of 32�, they .are then to be mixed together ionbsp;the interior part of the calorimeter, and to be lef^nbsp;there till they return to the term 32�. Th� quantitynbsp;of ice diflblved will indicate the quantity of calori�nbsp;difengaged during the combination.

� When bodies in a ftate of combuftion, or living animals, are fubjefted to trial, the operation is

o/ the Capacity of Bodies for Caloric^ i^c. the calorimeter gt; that this air, when it arrives, fliallnbsp;be at the temperature of 3^��nbsp;nbsp;nbsp;nbsp;b b^ the

fame when it ifiues from it, in order to avoid error �n the refult: for this purpofe, when it enters andnbsp;ifilies from the veflel, it mull, be made to pafs through

By means of the calorimeter, Meflieurs Lavoificr ^ud Laplace determined the fpecific caloric of fe-'^eral fubftances, of which mention has been madenbsp;in the table of page 70, and to which the followingnbsp;three curious refults will be added ¹.

The combuftion of one pound of phofphorus requires r| pound, or 27648 cubic inches, of oxygen gas, and forms a | pounds of concrete phofphoricnbsp;y acid. The caloric difengaged in this combuftionnbsp;and furnilhed by the oxygen gas, caufes 100 poundsnbsp;of ice to diflolve, and confequently excites 13532''�nbsp;heat. Hence it refults that one cubic foot of oxygennbsp;�it Can furnilh caloric enough to excite above 876�nbsp;tgt;f heat, and of diffolving 6,25 pounds of ice.

The combuftion of one pound, or 249081 cubic inches of hydrogen gas, requires 104448 cubicnbsp;inches of oxygen air, diflblves 29536 pounds of ice,nbsp;^�nd forms 61440 grains of water; which fhews thatnbsp;feme caloric remains in the water; for otherwifenbsp;that quantity of oxygen air would melt more ice.

The combuftion of one pound of charcoal required ^7396 cubic inches of oxygen air, diflblved.96 \

pounds of ice, and formed 47358 cubic inches �f carbonic acid gas, which weighed 32914 grains;nbsp;This alfo fhews that a confiderable quantity of caloric enters into combination with, or is employednbsp;to form, the carbonic aeid gas.

I fhall juft obferve, with refpe�t to the calorimeter, that notwithftanding its great utility, yet both its conftruflion and its ufe are not free from ob-jeftions.

In conclufion it feems, as all the fads tend to prove, that caloric is a real fubftance, perhaps th�nbsp;only real fluid and the general folvent of all othernbsp;bodies ; for any other body, as far as we are able tonbsp;try, becomes a fluid by combining with a fuffleientnbsp;quantity of caloric.

It enters into combination or mixes with the particles of all bodies, and produces the efl�ds which other combinations are wont to produce, viz. it enlarges their bulk; is expelled by comprefTion; it fe-parates other fubftances which have lefs affinity thannbsp;caloric for a given body, and diminifhes their attraction of aggregation; it mixes in greater quantitiesnbsp;with fome bodies than with others; and it palTesnbsp;through fome bodies eafier than through others.nbsp;When caloric is expelled from a chemical combination, the bulk of the mixture is lefs than that of thenbsp;fum of the ingredients; and, on the contrary, whennbsp;the compound is greater in bulk than the fum of thenbsp;ingredients, cold is produced, viz, caloric is abforb-ed, and of courfe is feparated from the contiguous

The heating, or the addition of heat to a body, has not been found to increafe its weight ¹. Thennbsp;af Caloric be matter, it will naturally be aflced, whynbsp;^oes it not poffefs weight or gravity like othernbsp;otatter ? In anfwer to this queftion, Mr. Tillocknbsp;^�tgenioufly obferves, that the fpecific gravity of bodies is diminifhed by heating, viz. by the comrnuni-^ation of caloric, fince they are increafed in bulk jnbsp;that the addition of heat to a body in air produces the fame effedl that a piece of cork would donbsp;if it were annexed to a piece of gold in v/ater, viz.nbsp;Icflen its gravity, becaufe cork, though poffeffed ofnbsp;gravity, is lighter than water j and calorie maynbsp;likewife be poflefled of gravity, though it be lighternbsp;^han air. He imagines that if the experiment werenbsp;Performed in vacuo, the increafe of abfolute weightnbsp;V the addition of heat to a given body, might benbsp;perceived f.

See Count Rumford�s Paper on the weight afcribed to in the Philofophical Tranfadlions for 1799.nbsp;t Philofophical Magazine, N� XXXIV.

[ 86 ]

Having treated fufEeiently of the theory of heat in the preceding chapters, it is now nc-ceffary to examine the fubjefl; in a manner rnorcnbsp;popular and more generally ufeful.

Heat and cold are relative terms. The fam^ temperature is cold with refpect to a higher tem^nbsp;perature, and hot with refpeft to a lower tempera'nbsp;ture. But in common language the more ufual, oi'nbsp;the mean temperature of the country, is confidere^lnbsp;as the limit of heat and cold ; below that limit v/Cnbsp;ufuaily call it cold; above that limit we callnbsp;hot.

The mean temperature of the fame country fubjeft to a very trifling variation from yearnbsp;year; but the mean, temperature of different coui^'nbsp;tries differs confiderably; nor is that differencenbsp;portional merely to the latitude of the country, f*quot;nbsp;depends alfo upon the fituation of the land ornbsp;water, upon the vicinity of large continents, hig*'nbsp;mountains, or woods, or extenfive feas, amp;c.nbsp;nbsp;nbsp;nbsp;,

A.11 the heat we experience in the world is derived from three fources, viz. ift, from the (un ; zdly�nbsp;from comprei�ion, under which denomination wenbsp;comprife coUifion and friclion ; and 3dlygt; from thenbsp;decompofition and compofition of bodies, whichnbsp;comprehends combuftions, fermentations, amp;c.

I. The direft rays of the fun on the fame fpot of the furface of th� earth are more or lefs hot according to the time of the year, clearnefs of the at-rtiofphere, ftate of the wind, and the color, togethernbsp;with the quality of the fpot. On this ifland, and innbsp;the hotceft time of the fummer, the diredl rays ofnbsp;the fun feldom raife the thermometer fo high asnbsp;110�. In other climates, efpecially within the tropics, they raife the thermome'ter conliderably higher,nbsp;viz. 20, or 30, or, as it is faid, even 40 degreesnbsp;higher than 11 o. But we muft not believe the idlenbsp;ftories of their melting lead, or even of their fettingnbsp;hre to gunpowder.

It is not on account of the fun�s being nearer or farther from us, that we receive much more heat atnbsp;Qtie time of the year than at another } for the difference of its diftance is too fmall to produce anynbsp;fenfible effed: but we receive more heat in fum-*^er than in winter, ift, becaufe the fun is nearernbsp;to our vertex, or to the zenith, and its rays pafs anbsp;Shorter way through the atmofphere, and are ofnbsp;t^ourfe intercepted by it lefs than in winter. Seenbsp;h�' 12� Plate XVIII. where AB reprefents the fur-of the earth, FGH the atmofphere, D and E

two fituatio6s of the fun, and C a particular fpot on the furface of the earth. It is evident that HC isnbsp;greater than GC, viz. that the rays of the fun pafsnbsp;through a fmaller part of the atmofphere, when thenbsp;fun is at D than when it is at E. Tlfis aifo fnewsnbsp;why the rays of the fun are, upon the whoE, hotternbsp;at about noon than when the fun is juft rifen or nearnbsp;fetting: 3dly, we receive more heat when the funnbsp;is higher, becaufe then a greater quantity of its raysnbsp;fall upon any given portion of the furface of thenbsp;earth, than when the fun is lower and its rays comenbsp;in a more oblique direftion: and ^dly, becaufe irinbsp;the furntner the fun remains longer above the ho^nbsp;rizon than in winter, and of courfe the furface ofnbsp;the earth is expofed longer to its rays.

It might at firft fight be expedled, that in general 'the hotteft time of the day would be at noon, viz.nbsp;when the fun is higheft j and that the hotteft timenbsp;of the year would be when the fun is at the fummernbsp;folftice. But this is not the cafe^ for the hotteftnbsp;part of the day, when no accidental circumftancenbsp;intervenes, is always fome time in the afternoon, a,ndnbsp;nearer to the noon in winter than in, lummer. Innbsp;the laft-mentioned feafon, in this climate, the hotteft time of the day (I mean not of the direft raysnbsp;of the fun, but of the air in the lhade) is at aboutnbsp;3 o�clock, or rather a little before. The hotteftnbsp;time of the year in this country generally is in July,nbsp;viz. after the folftice. The reafon of this is, thatnbsp;though the rays of the fun give more heat when the

fun

Of the ProduSiion, amp;c. of Heat and Cold. 89 fun is higher, and of'courfe at Q. o�clock they muftnbsp;lefs heat than at neon ; yet the earth, and thenbsp;sir fontiguous to it, are hotter at cv/o, becaufe theynbsp;retain a confidcrable portion of the heat acquirednbsp;before that time ; fo that as long as they acquire atnbsp;^�ny particular time a greater quantity of heat thannbsp;they lofe of what they had previpufly acquired,nbsp;their temperature muft continue to increafe. Thenbsp;fame thing, mutatis mutandis, mud be underftoodnbsp;'*'fith refpeft to the communication of cold, or privation of heat.

The earth acquires heat in the day-time, and lofes it during the night. |n fummer the lofs ofnbsp;heat during the night is lefs than the acquifition ofnbsp;it during the day �, therefore that excefs of heat isnbsp;gradually communicated frorn the furface to thenbsp;ruore internal parts of the earth. In winter the lofsnbsp;�^f heat during the night is greater than the acquifi-tion of it during the day; therefore cold is graduallynbsp;communicated from the furface to the more internalnbsp;parts of the earth. But when the above-mendone^nbsp;fummer heat has penetrated a certain way, thenbsp;'Winter cold begins to counterad it; and when thenbsp;Sold has penetrated a certain way, the next fummernbsp;beat begins again to counteradl itj fo that belownbsp;that certain depth, there is no alteration of temperature at any time of the year; unlefs you comenbsp;^be vicinity of feme, volcano, or near any par-^�sular combination of fermenting minerals; which

part

This is the reafon why in deep pits, and even �within 50 or 60 feet of the furface of the earth, thenbsp;thermometer makes little or no variation throughout the year. Thofe pits feem to us warm innbsp;�winter and cool in fummer 5 for as they remain al-�ways at the fame temperature, that temperature isnbsp;in faft warmer than that of the external air in winter,,nbsp;and cooler in fummer. This like wife fhews whynbsp;the waters of deep wells feem cool in fummer andnbsp;warm in winter. Indeed fo nearly uniform is theirnbsp;temperature, that we may from them afcertain thenbsp;mean temperature of a country ; viz. draw a pail,nbsp;of water out of a deep well (for inftance, of abovenbsp;50 feet) efpecially a w'ell that does not containnbsp;much water, and immedmtely place the thermometer in it; the degree to which that thermometer isnbsp;raifed, is the mean temperature of that countrynbsp;wherein the experiment is performed, or it differsnbsp;very little from it ¹.

In the cave of the Obfervatory at Paris, about 90 feet below the furface of the ground, the thermometer ftands at55�f, its variations not amounting to one degree. The me^an of the greateft colds

Of the ProduBiofii i^c. of Heat and Cold. 91

and the greateft heats oblerved at Paris during 5� years, is 54�,5¹ ^{2 3 4}.

In London the mean temperature is 50�.t But in procefs of time'the mean temperature of a country is liable to changes, owing to cultivation andnbsp;to a variety of other caufes. See Mann�s Papers,nbsp;Phil. Mag. vol. IV. and V.

The following general axioms have been formed by L. Cotte, refpe�ling the thermometer, from an examination of various meteorological obfer-vations made during 30 years �.

� The thermometer rifes to its extreme height oftener in the temperate zones, than in the torridnbsp;� zone.

It appears from the Journal of the Royal Society, which flates two ohfervations of the thermometer for every daynbsp;throughout the j^ear, that the general mean is 5n�,5 gt; ih?nbsp;mean for each fingle year being fometimes as low as aboutnbsp;48�, and at other times as high as about 52�. But, as itnbsp;appears from the regifter of Six�s thermometer, which hasnbsp;�f late years been inferted in the Journal of the Royal So-ciety, the mean between the greateft colds of the night, andnbsp;the greateft heats of the day, is 49,97 ; and from the obfer-t�afions made in Marlborough-ftreet, by the lateLord Charlesnbsp;Cavendifli, the mean between the greateft heats of the day,nbsp;^ndthe greateft colds of the night, is 49�, 196.

� It changes very little between the tropics j iti variations, like thofe of the barometer, are greaternbsp;the more one proceeds from the equator towardsnbsp;the poles.

� Moifture has a peculiar influence on it, if followed by a wind, which dilTipates jt.

� The greatell heat, and the greatefl; cold, take place about fix weeks after the northern or fouthernnbsp;folftice.

� The greatefl; heat in the fun and the lhade fel-dom takes place on the fame day.

� The heat decreafes with far more rapidity from September and Oftober, than it is increafed fronnnbsp;July to September.

It is not true, that a very cold winter is the prognoftic of a very hot fummer.�

The fituation of the thermometer, for afcertain-ing the temperature of the ambient air at different times, is not a matter of indifference. That itnbsp;fhouid be placed out of the houfe, at a little diftancenbsp;from the wall of the houfe, and where no ftream of

hot

hot air from kitctens, amp;c. may affeft it, is dear and obvious ; but there have been obferved certainnbsp;peculiarities of local heat, which often render thenbsp;indications of a fingle thermometer, doubtful ornbsp;equivocal; hence the fureft method would be tonbsp;employ two or thfee thermometers fituated at different heights from the furface of the earth, andnbsp;to take a mean of their contemporaneous indications;

In Ihoft, it has been obferVed that thermometers, fituated at different altitudes, are differently affedledjnbsp;and, what is more remarkable, that in the nightnbsp;time, efpecially when the air is ftill, and the fltynbsp;jfcrfeftly free from clouds, the thermometer, dolenbsp;to the furface of the earth, indicates a greater degree of cold chan at a higher fituadon. A confi-derable number of obfervations have been madenbsp;'''ith refpeft to this peculiarity of temperatures; butnbsp;do not as yet enable us to form any generalnbsp;It can only be faid, that this difference ofnbsp;partial temperatures, which does not amount tonbsp;^any degrees, may be owing to evaporation ; andnbsp;P'^rhaps, as Mr. Six conjeftures, to the coolnefsnbsp;''^hich the dews or vapours may acquire in theirnbsp;defeent*.

Mr. Wilfon�s Paper in the Philofophical Tranf-foj. nbsp;nbsp;nbsp;2nd Mr. Six�s curious papers

This however muft be imderftood of no great altitudes; for on great elevations, fuch as mountains, the difference of temperature is very remarkable ; fuch indeed, that in every climate, even innbsp;the torrid zone, there are mountains which, beyondnbsp;a certain altitude, are tovered with everlafting ice;nbsp;owing to their being fo far from the body of thenbsp;earth,- as not to participate of the general ftock ofnbsp;heat which the whole body of the earth receivesnbsp;from the fun. Another concurring caufe is, thatnbsp;mountains are greatly cxpofed to winds, efpeciallynbsp;to thofe which rife from the plains below,, andnbsp;which muft occafion a confiderable refrigeration, innbsp;confequence, as Dr. Darwin juftly obferves, of thenbsp;expanfion of the afcending air ¹.

The line of, congelation, beyond which no flqid water is to be found, is more or lefs diftant fromnbsp;the plane furface of the earth, according to the difference of latitude.

It appears from the obfervations of Mr. Bougner and others, that in the middle of the torrid zone thenbsp;line of congelation lies at about the height of 15600nbsp;feet; and near the tropics, or the entrance to thenbsp;temperate zones, it lies at the height of aboutnbsp;13428 feet. On the ifland of Teneriffe, in lat. 28¹nbsp;north, the line of congelation is at the altitude ofnbsp;about 10000 feet. It is about 6740 feet highnbsp;Auvergne, lat. 45� north. It feems to be about

5800 in latitudes between 51� and 54� north. In lat. 8q� north, lord Mulgrave found the line ofnbsp;congelation at the altitude of about 1200 feet abovenbsp;the level of the fea; wlience, as General Roy ob-ferves, we may conclude, that the furface of thenbsp;earth at the Pole itfeif, is for ever covered v,'ithnbsp;ftow.

Before we conclude the account of the principal fource of heat, viz. the folar heat, it may be ne-ceflary juft to mention, that no. fenfible heat isnbsp;known to be derived from any other celeftial body.nbsp;The moon indeed, on account of the great light itnbsp;refle�ts on the earth, might be expefted to communicate fome degree of heat; but though tliatnbsp;light, concentrated by a large concave mirror, hasnbsp;been thrown upon the moft fenfible thermometers,nbsp;yet I am not certain that it ever affected them. Anbsp;�feat many calculations have been made concerningnbsp;*-he proportion between the light which we receivenbsp;dir��tly from the fun, and that which is reflefted tonbsp;os from the moon ; from which it appears that thenbsp;latter is feveral hundred times lefs denfe than thenbsp;former j and the heat of both is fuppofed to be innbsp;'�he fame proportion.

ll* Compreffion, or collifion (which is a fudden ^onapreffion) is the fecond, and the more generallynbsp;foorce of heat, and the communication ofnbsp;heat thus produced to combuftible matters,nbsp;P ^luces that decompofition which is commonly

eaUed^,,.

Wood nibbed againft -wood, or againft any hard body j metal rubbed againft metal, or againft anynbsp;other body; in fhort, foHd bodies rubbed or knocked againft each other, are thereby heated, often fonbsp;far as to become red-hot.

By this means heat may be produced where there is no oxygen air whatever, fo that in thofe cafes itnbsp;cannot be derived from the decompofition of thatnbsp;air. This has niade feveral perfons fufpeft thatnbsp;heat is not the effed of a peculiar fubftance callednbsp;caloric, but that it is only a peculiar movement ofnbsp;the particles of bodies. It muft however be confi-dered, that there is no fridion which does not produce compreffion, viz. a contradion of the bulk ofnbsp;the bodies concerned, at leaft for a time*; andnbsp;therefore that the caloric is forced out of the bodiesnbsp;themfelves, and being communicated to the fur-rounding bodies, produces the iifual figns of heat.

What very much corroborates this aflertion is, that fubftances which are not compreffible, are notnbsp;heated by mechanical force; thus a flint will onlynbsp;be broken, but a piece of foft metal will be heated;nbsp;by the ftrokes of a hammer. Thus alfo you maynbsp;place any weight upon a quantity of water, without -altering its temperature, becaufe the compreflibility

* Woods and other foft fubftances are viftbly contraded by fridion. Metallic bodies are alfo contraded by rubbing,nbsp;rolling, or hammering ; for their fpecific gravities are thereby increafed.

of water is almoft nothing ; but if you place an additional weight upon a quantity of . air, the bulk of that air will b'e contracled, and its temperature willnbsp;he raifed. .

By about 15 or 20 fitiart and quick flrokes of ^ hammer on the end of an iron rod of about anbsp;Quarter of an inch in diameter, placed upon annbsp;^fivil, an expert blackfmith will render that end ofnbsp;the rod vifibly red hot. But the produiSlion ofnbsp;t^ivid red fparks from the flrriking of a piece of fteelnbsp;againft the edge of a flint,*is a phenomenon not lefsnbsp;Curious, Thofe fparks, if let fall upon a flheet ofnbsp;paper, will be found to be particles of the fteelnbsp;partly oxygenated. They are fcraped off by thenbsp;hint, and of courfe compreffed fo as to be heated, amp;c.

III. The third developement of heat arifes from '^I�iture, from compqfition and from decompofitionnbsp;tif bodies.

�^cids on being mixed with water, fpirit of '^ine on being mixed with water, and a greatnbsp;'Variety of other bodies, on being mixed, becomenbsp;tnore or lefs hot. It is not every mixture that be-^'^^cshotj but it has been remarked, that when-a mixture of two or more bodies is attendednbsp;p'^�^h heat, the bulk of that mixture is lefs than thenbsp;the bulks of the feparate ingredients; viz. anbsp;^'^prefliQjj Qj. concentration takes place, which' isnbsp;^^o^panied with a developement of caloric.nbsp;Subftances, whether animal or vegetable, undernbsp;III,nbsp;nbsp;nbsp;nbsp;Hnbsp;nbsp;nbsp;nbsp;fermentation,

fermentatiorij viz. decompofing fubftancesj are al-ways attended with heat. That fort of decompofi-ticn of combuftible fubftances, or of oxygen air, which produces a rednefs, amp;c. is comrhonly callednbsp;fire ; and is gradually propagated from one part tonbsp;another by its own adtion. Thus, when the above-mentioned ignited particles of fteel are receivednbsp;upon a fubftance of eafy decompofition, fuch a�nbsp;tinder, the touching parts of the tinder are heatednbsp;and decompofed by that heat, their component particles then, attradling the oxygen of the air, difen-gage the caloric of that fluid, and this caloric heatsnbsp;and decompofes the contiguous particles of thenbsp;tinder, which alfo decompofe more air; and thusnbsp;the combuftion proceeds and continues as long asnbsp;there are combuftible fubftances and oxygen airnbsp;ready for decompofition.

It is evident that the contaft of a fubftance actually burning is not abfolutely neceffary for communicating the combuftion to other combuftible bodies; it being only necelfary to heat thofe combuftible bodies to a certain degree j and heat isnbsp;communicable without the aftual contadl of the ignited body.

Sometimes combuftion is communicated from * burning body to another, wliich is not fo near as tonbsp;be heated fufficiently by if. Thus, when a tallo'Vnbsp;candle juft blown out is fituated within a certainnbsp;diftance of the flame of another lighted candle, andnbsp;in fuch a diredtion as that the ftream of fmoke or

vapou^quot;

Vapour, which proceeds from the former, may pafs through the flame of the latter; it frequently happens that the former is thereby lighted. But itnbsp;mufl: be obferved, that in this and other fimilarnbsp;cafes, the ftream of fmoke and vapour is a real trainnbsp;of combuftible matter, which is inflamed, and burnsnbsp;progreffively from the flame of the lighted candlenbsp;to the wick of the other.

A variety of ceconomical regulations, the efta-bliflied cuftoms of the greateft part of the human fpecies, the operations of different arts, the comforts and even the aftual exiftence of human life,nbsp;require an artificial fupply of heat; and the greateftnbsp;part of that heat can only be obtained from thenbsp;burning of combuftible bodies.

The combuftibles, or the fuel for common fires, a-te either wood or pit-coals; for all the other com-buftible fubftances are neither plentiful, nor can theynbsp;he advantageoufly ufed. Wood for burning is be-�^ome rather fcarce almofl: all over Europe; coals arenbsp;^'ot to be found in every country, and even wherenbsp;found their mines muft be exhaufted in time,nbsp;^hefe confiderations fuggeft the propriety of ufingnbsp;V'lth care and oeconomy thofe two fpecies of fuel,nbsp;proper management a great deal of wafte maynbsp;prevented, without diminifhing the advantagesnbsp;''^^ich are derived from the ufe of fires.

1. nbsp;nbsp;nbsp;The materials for the conftriidlion of fireplaces ought to be bad conduftors of heat, viz,nbsp;earthen ware or ftone, rather than metal; butnbsp;where metal cannot be avoided, then the metallicnbsp;parts ought to be furrour.ded by bricks or other badnbsp;conduftors of heat, excepting where the heat maynbsp;be required to be tranfmitted.

2. nbsp;nbsp;nbsp;The draught of air necelTary for the com-buftion ought to be juft fufficient, and not toonbsp;much. The ftream of it muft be conveyed in fuchnbsp;a dire�lion as not to interfere with the veffels, ornbsp;people, amp;c. that are to be heated by the fiie.�Itnbsp;has been found that in a furnace where ftrong heatnbsp;is required to be produced, and where bellows arenbsp;ufed, a large quantity of air thrown in with littlenbsp;velocity, is more ufeful than a fmaller quantitynbsp;which is thrown in with greater velocity.

3. nbsp;nbsp;nbsp;When heat is to be conveyed through tubes,nbsp;paflages, amp;c. care muft be had to furround thofenbsp;tubes with bad condudtors of heat.

4. nbsp;nbsp;nbsp;In the conftruftion of fire places, furnaces,nbsp;ovens, amp;c. and in the management of heat, it muftnbsp;be likewife remarked, that heat pafles through certain bodies, is refledted by others, and is refradlednbsp;(viz. its courfc is bent) in paffing through others ¹.nbsp;Thofe properties v/ill be briefly explained in thenbsp;fequel.

For the particular conftrudlion of Kitchen fire-places, fee Count Rumforcl�s Effays.

Of the f�iids the metallic fubftances are the beft ^ondu�lors of heat, next to them are fome compadlnbsp;ftones. The other earthy bodies conduct lefs andnbsp;icfs, in proportion as they are lefs compaft in theirnbsp;texture, and more mixed with water or oleaginousnbsp;fubftances. Coals, and other combuftible minerals,nbsp;are very bad conduftors of heat. Wood, and othernbsp;'Vegetable parts, and fuch bodies as are formed ofnbsp;them, viz. paper, ropes, amp;c. are fo bad condudlorsnbsp;of heat, that you may fafely hold a piece of any ofnbsp;them that is actually burning within lefs than annbsp;inch diftance from your fingers. Charcoal and charcoal duft, are vpry good non condutftors of heat,nbsp;and on that account very fit to be placed roundnbsp;tubes, partions, amp;c. wherein heat is to be retained.

Fluids leem to be exceedingly bad conduftors, if not perfect non-conductors, of heat. In fhort,nbsp;heat feems to be propagated through fluids merelynbsp;confequence of the internal motion of their par-ticles. Whatever p ermits or promotes that motion,nbsp;^Contributes to the propagation of heat; � whatevernbsp;ooftru�ts that motion, retards the propagation ofnbsp;^^^t through fluids. The particles of air whichnbsp;oorne in contact with an heated body, being therebynbsp;^^^ted and rarefied, become fpecifically lighternbsp;the furroundins; air, and of courfe afeend;nbsp;^ir then comes in contadt with the heated

body, by the air, to a diftance from itj but if that motion of the air be obftrudted, as by the interpofi-tion of partitions, of papers, woo], cotton, furs,nbsp;and the like; then that communication of heat isnbsp;thereby prevented more or lefs, in proportion tonbsp;the obftruiStion to the motion. It is principally onnbsp;this account that furs, feathers, eider down, cotton,nbsp;and the like, form warm coverings, viz. becaiife,nbsp;by preventing in a great meafure the motion of thenbsp;air between their filaments^ prevent at the famenbsp;time the diflipation of hear.

The like obfervations are applicable to water, and perhaps to all other fluids. When a vefiel fullnbsp;of water is placed upon the fire, the particles ofnbsp;�water that are clofe to the bottom of the veflTel arenbsp;firfl: heated and ratified, viz. become fpecificallynbsp;lighter; hence they afcend, and other colder particles take their place j thefe are heated next, andnbsp;likewife rife, amp;c. This is the caufe of the Inteftinenbsp;motion of water whilft heating. If the fire be applied to the upper part of the water, the lower waternbsp;will not thereby be heated j for heated and rarefiednbsp;water will not defcend.

Count Rumford confined a piece of ice at the bottom of a pretty tall glafs velTel full of water nearnbsp;the boiling point, and noted the time it required tonbsp;melt the ice. He then repeated the experiment,nbsp;with this difference, viz. that a fimiiar piece of icenbsp;was placed on the furface of the hot water. It wasnbsp;found that the ice melted more than eight times

ftower -when boiling hot water flood on its furface, than when the ice was fufFered to fwim on the fur-of the hot water. This very remarkable phe-tiornenon is eafily explained on the already mentioned property of fluids j viz. when the ice isnbsp;Swimming on the furface of the hot water, the particles of the latter that are contiguous to the formernbsp;being cooled, defcend, and other hot particles ofnbsp;'vatcr take their place, which give to the ice part ofnbsp;their heat and defcend, and fo on j but when thenbsp;ice is at the bottom, the particles of water thatnbsp;firft come in contaft with it, are cooled, and are rendered fpecifically heavier, in confequence of whichnbsp;they remain in their place, and no motion will takenbsp;place within the water *.

AB, fig. 13. Plate XVIII. reprefents a glafs '^^fiel, like a thermometer veflfel, but larger, andnbsp;at top ; the diameter of the cavity of the tubenbsp;being about a quarter of an inch. Fill fuch a veflelnbsp;Water till within about an inch of the top, andnbsp;with the water fome powders that may havenbsp;their fpecific gravity nearly equal to that of water,nbsp;as to remain fufpended in it (powder of tranfpa-rent yellow amber anfwers very well); for the motionnbsp;the Water will be rendered manifefl by the motionnbsp;the particles of the powder. If the bulb B be gentlynbsp;^ated, a current of warm water will be leen to rifenbsp;'Ount Rumford�s 7th Effay.

along one fide of the tube, and another current of colder water will be feen to defcend along the othernbsp;fide of the tube.

Whatever obftruds the free motion of the particles of the fluid, does alfo obftruft the propagation of heat through it. Thus, water thickened by^ a mixture of flarch and other fubftances, or impeded in its motion by wool,' cotton, eidernbsp;down, amp;:c. cannot be heated nearly fo fooh as clearnbsp;water. Hence -it appears why apples and fomenbsp;other fruit are difficultly heated or cooled �, namely,nbsp;becaufe they confift almofl: entirely of minute ve-ficles full of liquor, confequendy the liquor cannotnbsp;move from one part of tlie fruit to the other.

An emanation of heat proceeds from an heated body, when placed in a colder temperature, andnbsp;expands itfelf in every direftion, provided it be notnbsp;prevented by the interpofition of particular fubftances. Separate parcels of that emanation arenbsp;called rays of heat-, not becaufe that emanationnbsp;(as far as we know) confifts of feparate ftreams ;nbsp;but merely for the conveniency of explanation.

The rays of heat are not the fame thing as the rays of lightj for if they were the fame thing, thennbsp;a certain quantity of heat ought to be conftantlynbsp;accompanied with the fame quantity of light inbsp;whereas we find that feveral fubftances give out anbsp;good deal of light without any fenfible heat, andnbsp;others give out a confiderafile degree of heat withoutnbsp;any light. But a very ftrong confirmation of theirnbsp;'nbsp;nbsp;nbsp;nbsp;being

being two feparate powers of nature, is afforded by Herfchel�s late difcoveries, the principal ofnbsp;�'vhich will be mentioned in the feqiiel.

The rays of heat vdrich come either from the fun, from any other hot body, proceed in ftraightnbsp;bnes .all round the body, as long as they do notnbsp;�^eet with any oppofition, viz. any body that hindersnbsp;^�leir progrefs. When they do meet with anynbsp;body, then, according to the quality and figure ofnbsp;body, they are either refledted, viz. turnednbsp;backwards, or they are abforbed, or they are tranf-�^itted through the body. In general, tbofe threenbsp;^ffcfts take place at the fame time, viz. the rays ofnbsp;beat are- partly reflefted, partly abforbed, and partlynbsp;^^�anfmittedj by the fame body ; but every bodynbsp;P'�oduces fome one of thole effecis ftronger thannbsp;^be others.

The fame obfervations m,ay be made with re-|Pe6t to light, viz. the rays of light do alfo proceed ftraight lines from the luminous body in everynbsp;'^'fedtion, as long as they do not meet with anynbsp;body which either refiedts, or ablbrbs, or tranfmitsnbsp;, But not all the effedts produced by a givennbsp;upon the rays of light, are the fame as thofenbsp;^bich are produced by the fame body upon the raysnbsp;'^bbeat j for inftance, a plate of metal which is im-

lous to light, will in great meafure tranfniit a plate of glafs will tranfmit light in greaternbsp;th?n heat 5 and fo forth.

The

The furfaces of all bodies refleft in greater orlefs quantity the rays of heat which happen to fall uponnbsp;them, Polifhed furfaces, efpecially of metallicnbsp;bodies, refleft them moft. It has been found thatnbsp;the angle of incidence is equal to that of refledion;nbsp;or, in other words, the angle which the rays of heatnbsp;falling upon any point of a given furface form withnbsp;the perpendicular to the furface at that point, isnbsp;equal to the angle which the fame rays, after re-fledlion, form with the fam.e perpendicular �, viz. thenbsp;heat which proceeding from the hot body A, fig.nbsp;16. Plate XVIII. palfes through the hole at G, andnbsp;impinges at B, upon the refledting furface E F, isnbsp;refledled in the direction B C, forming the anglenbsp;of incidence, G B D, with the perpendicular B D,nbsp;equal to the angle of refledtion D B C j and, in fadt,nbsp;the thermometer, fituated any where in the diredtionnbsp;EC, will be affedled by the reflected heat.

Hence it follows, that when the refledling furface is not uniform, viz. not polifhed, but rough and uneveninbsp;then the heat is fcattered in various diredtions jnbsp;w'hen the furface is flat and polifhed, the ftream ofnbsp;refledled heat is equal to the incident ftream; wliennbsp;the furface is convex, the heat is refledted diverg-ingly i and, laftly, when the furface is concave, thenbsp;heat is refledted convergingly, viz. towards a narrownbsp;fpace, called a focus, beyond which the rays ofnbsp;heat having crofTed each other, proceed divergenbsp;ingly. Thus, in fig. 18. Plate XVIII. the heat,nbsp;which proceeding from the hot body A, falls upoo

the Concave reflefting furface BC, will be reflefted towards EF ; viz. the rays which fall upon the partnbsp;tiearer to Bj will be reflefled in the direftion BF,nbsp;^^d thofe which fall upon the part nearer to C willnbsp;hs refledted in the diredlion CE fo that all thenbsp;rays of refiedted heat pafs through a fmall fpace ornbsp;focus at D, where they crofs, and afterwards pro-t^ced divergingly towards EF. It is hardly neceffarynbsp;toobferve that the thermometer will be afFedled morenbsp;'^hen fituated at D, than in any other part of thenbsp;double cone B CD E F. That thofe rays of heatnbsp;do adlually crofs each other at D, is eafily proved ;

if by interpofing a fcrecn G, you intercept ^^�0 upper part of the incident rays of heat, then thenbsp;^^^trnometer will be afFedled by the refledled heatnbsp;in the diredlion CE; and if, inftead of that, younbsp;hitercept the lower part of the incident rays bynbsp;*^^ans ofthe fcreen H, then the thermometer willnbsp;affedled by the refledled heat only in the direc-.

doriBF.

'f'he application of refledted heat is pretty well ^�^derftood in ceconomical affairs, and may benbsp;J'^pted to a great variety of purpofes. Thus everynbsp;�^y knows the refiedting power of tin plates in

are commonly called Dutch ovens, and in ^0 fire-fcreens. The refledling power of thenbsp;of fire-places, of ovens, of walls in gardens,nbsp;likewife well known.

^ ^ritercepdng property, or the abforption of by different bodies, depends upon the colour

of the bedy, upon its capacity for heat, upon its conducting power, and upon the fmoothnefs ofnbsp;roughnefs of its furface.

Upon the w'holc, bodies of the darkeft colour, greateft capacity for caloric, and rougheft furfaces,nbsp;abioxb moil heat. If various thermometers havingnbsp;their bulbs painted each with a different colour, benbsp;expofed to a fire, or to the fun, or to a lighted candle,, they will be unequally affecfled¹.

If equal weights of water, and of mercury, be, cateris faribus, expofed before a fire, the,water willnbsp;abforb more heat than the mercury j hence its temperature will be raifed flower than that of thenbsp;mercury -j'.

The tranfmiffion of heat through bodies is alfo attended with remarkable phenomena.

Upon the whole, it feems that in palling obliquely from a thinner into a thicker medium, the rays of heatnbsp;are bent towards the perpendicular to the boundingnbsp;furface j and inpaffing from adenfer into a thinner medium, they are bent from that perpendicular. (Thisnbsp;bending, in pafling through, iscalled refraction) Thusnbsp;if a ftream of heat, proceeding from a body A, fig. 19¹nbsp;Plate XVIII. impinges upon the furface CD of ^nbsp;piece of glafs CE.DF, its courfe will be bent intonbsp;the direction BG, and in going out of the glafs G,

See the Phil. 1'ranf. vol. LXX. laft article, f See Dr. Martine�s Effay on the Heating and CoolinSnbsp;i�)f Bodies.

follows from this, that accoi'ding to the figure of the refrafliing body, the ftream of heat may be re-^'�^fted, either irregularly, or in a diredtion parallelnbsp;its incident diredtion, of in a diverging diredlion,nbsp;laftly, in a converging diredlion ; and in thenbsp;^^tter cafe the fmallefl: {pace into which the refradlednbsp;are colledted, (beyond which they proceed di-^^tgingly) is called the focus of the refradted heat ;

a thermometer, fituated in that place, will be ^ffedVed more than in any other part of the refradled

�ream.

the thermometei' be placed out of the refledted but very near the focus either of refledtednbsp;of refracted rays, the temperature of that ther-ll^ometer will not thereby be raifed ; for the rays ofnbsp;do not deviate from their diredlion as Ions: asnbsp;^^y are not oppofed; but if a piece of wood ornbsp;or, in fhort, of any irregular fubftance, ca-^'^�0 of obftrudling the palTage of the heat, be,nbsp;focus, then the above-mentionednbsp;||�^*'naometer will be affedled on the vicinity of thatnbsp;for in that cafe the fays of heat are refledtednbsp;foattered about. Hence alfo clear w'ater placed'nbsp;^nbsp;nbsp;nbsp;nbsp;^ fold focus will be heated either not at all, or

The

feader muft not wonder that the refle�ing and re-��o properties of differently fhapcd bodies, fuch as concave

The rays of light are likewife refrafted in going through diaphanous bodied. But it has been clearlynbsp;proved by Dr. Herfchelj that the rays �f heat arcnbsp;refrafted differently frona the rays of light; fo thatnbsp;though they are often emitted together, as frqm thenbsp;fun or a common fire, yet they feem to be two dif-tindl powers.

If the fun�s ,rays, AB, fig. 15. Plate XVIIB which come into a dark room through a hole at Agt;nbsp;be received upon a triangular glafs prifm C, thatnbsp;ftream which confifts of the rays of heat as well asnbsp;of the rays of light, wdll not only be bent by thenbsp;prifm, but will be likewife difperfed into the ob'nbsp;long figure DF, which is about as broad as the holenbsp;at A, and longer or Ihorter in proportion to thenbsp;diftance from the prifm; for it may be receivednbsp;upon a table or a fereen ficuated at any diftance fromnbsp;the prifm. This oblong figure confifts of light andnbsp;heat difpofed in the following manner. From D tonbsp;E the figure confifts of the luminous rays exhibitingnbsp;the vivid colours of the rain-bow, with the violc*'nbsp;next to D, and the red next to E. The rays whichnbsp;produce the heat are extended from D to F, Dfnbsp;being t� DE, as 21 to 12. Thofe rays of

cave and convex reflectors, lenfes, prifms, amp;c. are flightly mentioned in this chapter; for they will be partial'nbsp;larly deferibed in the next Section, which treats of ligh^'nbsp;and where the different fliapes of bodies are more eflentiah/nbsp;concerned.

cannot be feen, but they are manifefted by the thermometer ; for a thermometer placed any where in extenfion DF, will be affedled by the heat?nbsp;but it will not be affected equally in every part ofnbsp;that oblong figure. The greateft heat is at g, viz.nbsp;^ little beyond the red light, and where there is nonbsp;1'ght at all. From that point the heat decreafes bothnbsp;'''ays. It appears therefore that the rays of heat arenbsp;tUUch more extended by their refrangibility than thofenbsp;uf light; for we find heat not only with the lightnbsp;brom D to E, but alfo from E to F, where therenbsp;appears no light whatever *.

It is owing to this refrangibility of the rays of beat, that if they be received upon a convex glafsnbsp;they will be collefted into a narrow fpace ornbsp;at a certain diftance from the lens, ^bisnbsp;*ucus has all the properties of the focus of refieclednbsp;of heat; which have been concifely mentionednbsp;^bove. Ift fimilar circumftances the rays of lightnbsp;alfo collecled into a focus ; but, owing to thenbsp;'lifference between the mean refrangibility of thenbsp;of light and thofe of heat, the focus of heat isnbsp;^ bittle farther than the focus of light, from the fur-of the lens

moft pow'erfiil effects of heat are produced ^y fuch a focus of reflected or refrafted heat. Thenbsp;property of convex lenfes and concave

' nbsp;nbsp;nbsp;^800, Part III. p, 43S.

fpeculums, are fo commonly known, that they are generally called burning glajfes. With a doubknbsp;convex lens of notquot; more than an inch in diameter,nbsp;the heat of the fun�s rays may be colleded fufficient-ly to fet fire to tinder, paper, gun-powderi wood, amp;c.nbsp;but with large ienfes, or large concave mirrors, andnbsp;a clear fun, the moft refraftory metals are fpeedilynbsp;fufed.

� Ceteris taribus, the flhorter is the di fiance of the focus of the lens, or of a fpeculum, from its furface,nbsp;the more a�tive its power will be. Speculums havenbsp;been conftructed within this forty or fifty years,nbsp;having a focus fo long as to inflame combuftibles atnbsp;the diftance of 200 feet and upwards * ; but we findnbsp;in hiftory a few accounts of their having aded atnbsp;much greater diflances f.

The moft powerful burning glaffes have been made in France and in England. Mr. Trudaine�s

� Buffon, of the French Academy, formed a burning refledor, confiftlng of 168 fmall plane refledors, whichnbsp;were difpofed in a hollow fegment of a fphere, fo as all tonbsp;refled the light and heat of the fun to the fame place. Withnbsp;this inftrument he could fire wood at the diftance of 209 .nbsp;feet. Burning lenfes have alfo been made of two fhells ofnbsp;glafs, like two watch giafies, cemented with their edges towards each other in a proper franjo, and encloftng fpirit ofnbsp;wine or water. Globular or nearly globular glafs decanters,nbsp;filled with wat�r, are well known to ad like burning glaflb*nbsp;when expofed to the fun.'

lens

in France meafured four feet in diameter, ^hen its focus was fiiprtened by the interpofitionnbsp;^f a fmaller lens, the effeft prodigioufly great.

about a minute�s time it not only melted all ^Ofts of metallic fubftances, but it vitrified earthen-'^are, flate, pumice ftone, afiies of vegetables, amp;c.

Mr. Parker�s lens in London, which coll a great ^^al of trouble, time, and expence, and which, I amnbsp;forry to fay, is no longer in this country, was a verynbsp;Extraordinary inftrument of the kind. It was anbsp;double convex lens of flint glafs, 3 feet in diameter,nbsp;3 inches thick in the middle, and weighing 212nbsp;pounds. When fet in its frame, it exhibited a clearnbsp;Surface of 32 | inches in diameter. Its focal diftancenbsp;6 feet and 8 inches; but in performing experiments, that focus was generally fhortened bynbsp;'�ke interpofition of a fecond and much fmallernbsp;lens.

Whilft it remained in London, this extraordinary kurning glafs was ufed by various fcientific perfons,nbsp;^ great number of experiments, the refults of

Subltances

Cauk, or Terra ponderofa -A topaz, or cryfolite -An oriental emerald -Cryftal pebble nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-

AB and CD, fig. 17. Plate XVIII. reprefent ^Wo concave metallic refleftors, about 10 inches, ornbsp;'^ore, in diameter, and fituated facing each other atnbsp;diftance of about 15 feet. Suppofe the focusnbsp;AB to be at E, 18 inches diftant from its fur-^�ce, as alfo the focus of CD to be at F, 18 inchesnbsp;diftant from the furface CD. The operator maynbsp;enabled to fituate the fpeculums exactly facingnbsp;^ach other, by placing a lighted candle in the focusnbsp;^f one of them, and then moving the other untilnbsp;the reflefted image of the candle in the focus ofnbsp;this other is found by trial (viz. by receiving it uponnbsp;^ final] piece of paper) to fall in the diredlion of thenbsp;h^cus and centre of the firft fpeculum *.

f�lace a piece of iron almoft red-hot at E, viz. in the focus of the fpeculum AB 5 then that part ofnbsp;heat, which, proceeding from the iron, fallsnbsp;^pon the furface AB, is refle�led by it in a parallelnbsp;^'J'eflion to CD, from which it is reflefted again

th ^ rnentioned the above determinate dimenfions of ^'^fledtors, diftance, amp;c. in order to fhew what will an-. e purpofe; but I need hardly add, that with ftnaller ornbsp;nearer or farther from each other, thenbsp;notnbsp;nbsp;nbsp;nbsp;proportionately more or lefs apparent. It is

convergingly to the focus Fj fo that if the bulh of a thermometer be placed at F, the temperaturenbsp;of that thermometer will be raifed; If you put ^nbsp;fheet of paper upon, or in iFort fcreen, either oi�nbsp;the fpeculums, the thermometer will defcend to it^nbsp;ufual temperature. Remove the paper, and thenbsp;thermometer will rife again.

if, inftead of the thermometer, you place a fmall quantity of gun-powder upon a piece of paper, ofnbsp;upon any other convenient Hand, at the focus F}nbsp;and inftead of the piece of iron, you place a burn'nbsp;ing charcoal at the other focus E : then, if yoUnbsp;render the charcoal vividly red-hot by blowing uponnbsp;it with a pair of bellows, the gun-powder will benbsp;fired off at F by the reflcded heat.

If, inftead of'the charcoal and the gun-powdet) you fituate a thermom.eter at F, and a piece of icenbsp;at E, the temperature of tlie thermometer willnbsp;thereby be lowered. Cover the furface of eithe�nbsp;refleftor, and the thermometer will rife. Uncovernbsp;the relleftor, and the thermometer will be lowerednbsp;again, amp;c.

The relult of this laft experiment has been fup' pofed to militate againft the commonly receivednbsp;theory of heat, which has been explained in the pt^'nbsp;ceding pages �, imagining that the cold which prO'nbsp;ceedsfrom the ice is refledled by the fpeculums toth^nbsp;thermometer, and that, of courfe, cold is fomethin^^nbsp;pofitive. But, in my opinion, the true caufe

phenomenon is, that the heat of the thermometer is refledted upon the ice, in the fame manner as the ^eat of the charcoal, 'in the preceding experiment,

If, inftead of the thermometer, a burning charcoal placed at F, no perfon will hefitate to fay, thatnbsp;the heat of the charcoal is remedied upon the ice;nbsp;^tid there is no realonwhatever for concluding thatnbsp;the fame thing does not happen when the thermo-ttieter is at F.

The heat of a body, fituated amongft other bodies, paffes from the former to the latter, untilnbsp;they all acquire the fame temperature, and thatnbsp;paffage is more expeditious in proportion � as thenbsp;difference of temperature is greater. Alfo, if thenbsp;colder bodies be not of an equal temperature, thenbsp;beat of the firft-mentioned body will efcape quickernbsp;from that fide of it, which is oppofed to the coldeftnbsp;cgt;f the furrounding bodies j and if a fereen be inter-Pofed between thole two bodies, then th� lofs ofnbsp;beat will be lefs expeditious. Now upon the leaftnbsp;^�efleftionj it will appear, that the experiment withnbsp;^be thermometer and the ice is a fimilar cafe, ex-cepting that the heat, or caloric, inftead of proceed-diredlly from the former to the latter, is refleiftednbsp;�^d concentrated by the refleftors.

The artificial produflion of cold is by no means fr^ eafy 33 jjig production of heat; but it muft atnbsp;^be farne time be confeffed, that the former is not fonbsp;^frbntialiy neceffary as the latter.

I 3 nbsp;nbsp;nbsp;The

The methods of cooling known, and moftly practiced wherever the heat of the climate or the habits of life render it defirable, are, ift, by ventilation;nbsp;2dly, by employing the natural temperature ofnbsp;caves, wells, and the like, when that temperaturenbsp;is below the temperature of the external air?nbsp;3dly, by evaporation j 4thly, by the ufe of ice,nbsp;�where ice can be had j and, 5thly, by the folutionnbsp;of certain falts.

There is however another operation, which produces cold j but the difficulty of the performance renders it imprafticable in commion affairs. Thisnbsp;is by the expanfion of compreffed air.

I. nbsp;nbsp;nbsp;The effedts of ventilation (fuch as is producednbsp;either by means of a judicious difpofition of doors,nbsp;paffages, amp;c. or by means of fans, bellows, andnbsp;ventilators) are fo commonly known, that little neednbsp;be faid concerning them. By ventilation the heatednbsp;air which furrounds animal bodies is removed, andnbsp;the fenfible or infenfible perfpiration is more effectually diffipated; hence a few degrtes of cold arenbsp;produced upon the animal bodies when their temperature is above the adtual temperature of the air;nbsp;but mere ventilation produces no effedl upon a thermometer, or upon a body which is of the fame temperature with the ambient air.

II. nbsp;nbsp;nbsp;In moft of the warm countries it is commonlynbsp;praftifed to cool fruit, wines, amp;c. by keeping themnbsp;a certain time in deep caves, cellars, and wells j ofnbsp;�0 place them in water juft raifed from deep wells.

' nbsp;nbsp;nbsp;� This

This method produces a very moderate refrigeration, for it has been already mentioned chat the temperature of thofe wells, caves, amp;c.does not differnbsp;ttiuch from the mean temperature of the country.nbsp;Tet certain it is that fruit and wines thus cooled innbsp;fornmer, prove very pleafant. But a more effentialnbsp;^^tiefit is derived from the ufe of caves and deepnbsp;''^ells in warm countries; which is, that meat, fifh,nbsp;tiutter, and other things, are preferved free fromnbsp;t^orruption confiderably longer in thofe places thannbsp;in the open air above ground.

III. The cooling by means of evaporation is proportionate to the quicknefs of the evaporation :nbsp;therefore thofe fluids, which evaporate quickeft,nbsp;produce the greateft refrigeration ; and whatevernbsp;promotes the evaporation, fuch as ventilation, a diynbsp;of the air, amp;c. does likewife increafe the refrigeration.

In Warm climates the apartments of the opulent often rendered pleafandy cool by fprinklingnbsp;^^�ter on the floors, on the tops of the houfes, andnbsp;^Ipecially upon the curtains of windows and doors;nbsp;taking care to renew the fprinkling as foon as thenbsp;lormer is evaporated.

In encampments, in travelling through hot coun-Of nbsp;nbsp;nbsp;fjQ ocher refrigeration can be ufed,

generally praftifed to wrap up bottles and jars ^ liquor, in two or three folds of wet linen,

care to wet the linen coverings in proportion as the .water evaporates from them. By this means thenbsp;water or other liquor within the bottles, is cooled anbsp;few degrees.

But the cold which the evaporation of water produces, is inferior to that which can be produced by the evaporation of fpirit of wine, and vaftly lefsnbsp;than that which the evaporation of ether produces}nbsp;ether being fo very evaporable, that were it not fornbsp;the ufual prefTure of the atmofphere, it would onlynbsp;exift in an aeriform date} and fuch is the cafenbsp;under the exhaufted receiver of an air-pump.

In order to try the degree of refrigeration produced by the e vaporation of different fluids, I held up a naked thermometer, (viz. a thermometer thenbsp;bulb of which was not in cornacl with the metalnbsp;of the fcale) and poured upon its bulb a ftream ofnbsp;fome particular fluid, which iffued out of the capillary aperture of a tube; taking care to throw juftnbsp;' fluid enough to fupply the wafte by evaporation-By this means, when the temperature of the airnbsp;was 64�, I found that the evaporation of waternbsp;cooled the thermometer 8�, viz. brought it dov/nnbsp;to f� j the evaporation of fjjirit of wine cooled irnbsp;16�, viz. brought it down to 48�; and the evaporation of ether cooled it 54�, vizi brought it downnbsp;to 10�: but by the ufe of the beft purified ful-phuric ether, when the temperature of the air was

Of the 'Prcduclion, isc. of Heat and Cold. lai sbout 56'', I brought the thermometer down to

Some years ago I contrived a very fmall apparatus for freezing a fmall quantity of water, viz. about 10 grains, in every climate. The whole apparatusnbsp;contained in a box 4 f inches long, 1 inchesnbsp;broad, and 1 f deep. This apparatus, and thenbsp;uaanner of ufing it, is reprefented in Plate XVIILnbsp;bg- 20. EFG is a common phial with a glafs ftop-ple, and full uf ether; ED is a glafs tube with anbsp;capillary aperture at D, and having fome threadnbsp;�Wound round the other extremity for the purpofenbsp;of fitting the neck of the bottle when the experiment is to be performed. AB, is a glafs tube aboutnbsp;4 inches long, and about one-fifth of an inch in diameter, hermetically clofed at B. Into this tube anbsp;fiender wire H is introduced, the lower extremitynbsp;which is lhaped into a fpiral, and ferves to drawnbsp;�ut the ice when formed. When a little water, CB,nbsp;tgt;ut into the tube, I hold the tube by its uppernbsp;P^rt with the fingers of the left hand, and keep itnbsp;continually but gently turning round its axis, firftnbsp;c�ne way and then the other; whilft with the rightnbsp;band I hold the phial in fuch a manner as to difeft

* The cooling produced by the evaporation of other need not be mentioned ; their effea being generallynbsp;*a.ermecliate between the effect of water and that of fpiritnbsp;uf wins;_ ggg Paper in the Philofophical Tranfaftions,

the dream of ether, which comes out of the capillary aperture D, towards the outfide of the tube, a littlenbsp;above the furface of t,he water within. The dreamnbsp;of ether fhould be fuch as that a drop' of ether maynbsp;now and then, (for inftance every lo feconds) failnbsp;from the under part of the thermometer. By con-tinifing this operation during 2 or 3 minutes^ thenbsp;water CB will be frozen, and may be drawn out ofnbsp;the tube in one hard lump of ice. When this isnbsp;done, the phial is turned with its aperture upwards,nbsp;the Ihort tube ED is removed, the ftopple is placednbsp;in its dead, and the remaining ether is preferved fornbsp;other trials. -

If ether be placed in an open vedel, together with a thermometer under the receiver of an air- pump ;nbsp;on exhauding tiie receiver, a very great degree ofnbsp;cold will be produced. A mixture of fulphuric andnbsp;muriatic ether will produce (by the exhaudion) coldnbsp;enough to freeze quickfilver.

IV. The application of ice to the outfide of vedels full of liquors or other fubdances, is thenbsp;mod obvious way of producing refrigeration. Butnbsp;ice cannot be procured in all countries j and whennbsp;it can be procured, which is in the winter feafon, itnbsp;mud be preferved for the dimmer, at wiiich timenbsp;it is modly wanted.

Ice, or fnow, well rammed clofe together, is preferved in refervoirs, or ke-houfes^ which are generally made jud below the ground in fome flielterednbsp;place, wherein the ice melts very gradually.

this climate an ice houfe of 20 feet in diameter, and ^bout 20 deepi properly filled with ice, will be foundnbsp;to quot;Contain fome of it even after two years.

A very remarkable manufaftory of ice is praftiled the Eaft Indies at Allahabad, Mootegil, and Calcutta, which places lie between 23� � and 25� f ofnbsp;^orth latitude. The following is a Ihort accountnbsp;�Pf the procefs, which is deferibed at large by Sirnbsp;Robert Barker in the 65th volume of the Philo*

Boiled fofc water is poured into fliallow and porous pans, which are fituated towards the evening in ihallow pits, the bottoms of which are ftrewednbsp;with- fugar canes or dried ftems of corn. In thenbsp;coiirfe of the night, and efpecially towards the morning, a cruft of ice is formed in the pans, and is col-Rdted by the ice-makers. This cruft of ice differsnbsp;thicknefs according to the temperature of the airnbsp;and other circumftances favourable to evaporation ;nbsp;for a great part of the effedt is undoubtedly due tonbsp;evaporation through the pores of the pans, fincenbsp;�hofe countries the thermometer was never ob-^^^ved to fink fo low as 32�.

Re by itfelf cannot communicate a greater de-of cold than itfelf poffeffes; but by the admix-of common and other falts, or of acids, it may . '^^ufed to produce a much greater degree of re-quot;^Seration.

ice P''�portion of common fait and pounded ^ fnow, which produces the greateft cold, is

varioufly fcated; but 3 parts,, by weight, of common fait to 8 of ice, or � of fait to a of ice, is the neareft, and is capable of lowering the thermometer to �4�.

The degrees to which the thermometer is brought down by the mixture of ice and other fubftances, arenbsp;as follows, the materials before the mixture beingnbsp;at 32quot;.

V. The refrigeration which nhay be produced merely by the folution of falts, is very great j indeednbsp;as great as may be produced by any known means-But for the knowledge of its greateft efFedts, we are

The quantity of thofe acids cannot be.ftated with accuracy, on account of their differing in ftrength. In general? about equal quantities of diluted acid and of ice maybenbsp;The acid, when very flrong, may be diluted with half

indebted to Mr, Richard Walker of Oxford j for 'vhat was known before was, that the folution ofnbsp;nitre in water produced a degree of refrigerationnbsp;fufficiently ufeful in hot climates; which folution hasnbsp;long been in ufe, as it is at prefent ufed in the Eaft-Indies; alfo, that the folutition of fal ammoniacnbsp;produced cold fufficient to lower the thermometernbsp;t-o about 32�, the folutions of fome other falts be-^ng known to produce a very few degrees of refrigeration ; whereas Mr. Walker difcovered fucknbsp;faline folutions, and fuch modes of employing them,nbsp;as to freeze even quickfilver in the middle ofnbsp;fummer *.

For this purpofe the folution of the faline fub-ftance is made by putting the proper quantity of fait and of water in an open vefi'el, in the middle ofnbsp;''^hich another velTel with the wine, cream, or othernbsp;�Materials to be cooled, is fituated. The cold isnbsp;produced only whilft the fait is diflblving; viz. thenbsp;^3-loric of the annexed bodies is abforbed by the fa-1'ite fubftance, whofe capacity is increafed by itsnbsp;being converted from a folid into a fluid ftate.

The cold produced is greater in proportion as the temperature of the materials w'as lower previous tonbsp;his Papers in the Philofophical Tranfaftions, vol.nbsp;y^^rs 1787, 1788,1789,1795, and for the yeariSoi;nbsp;punlication, entitled, Account of the R./gt;markah!enbsp;in the Produiiion rf Artificial Cold. Oxford,

�*796.

the

the making of the mixture. ThereforCj when a very great degree of cold is to be produced, viz. fuch as tonbsp;freeze quickfilver, then a folution of falts in waternbsp;mull be firft ufed for cooling the materials neceflarynbsp;for a fecond mixtre, which of courfe will be able tonbsp;produce a greater degree of cold; and by the likenbsp;means the materials for a third nhixture may benbsp;cooled, amp;c.

The following lift contains the moft powerful mixtures of falts and water, or acid, for the pro-duftion of cold. The materials are fuppofed to benbsp;at the temperature of 50� before the mixture, andnbsp;annexed to each mixture is the degree of cold produced, or the degree to which the thermometer isnbsp;brought down. The falts muft be powdered verynbsp;fine, and dry; but not fo as to have loft the waternbsp;of cryftallization.

N. B. Some of the fubftances in the following lift are exprefled by their old names, they being fonbsp;exprefled by Mr. Walker j but from what has beennbsp;faid in vol. 2, the reader may e^fily recoiled!:, thatnbsp;fal ammoniac is muriate of ammoniac, Glauber fairnbsp;is fulphat of foda, and vitriolic acid is fulphuricnbsp;acid.

Of the TroduSliorp, of Heat and Cold. 127

^ Sal ammoniac 5, nitre 5, water 16 Sal ammoniac 5, nitre 5, Glauber�snbsp;fait 8, water 16

* Nitrous ammoniac i, water i -Nitrous ammoniac I, falfodae i, water i t Glauber fait 3, diluted nitrous acid 2nbsp;Glauber fait 6, fal ammoniac 4, nitre 2,'nbsp;diluted nitrous acid 4nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-

Glauber fait 6, nitrous ammoniac 5, diluted nitrous acid 4nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-

Phofphorated foda 9, dilvited nitrous acid 4nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;-

Phofphorated foda 9, nitrous ammoniac 65 diluted nitrous acid 4 -t Glauber fait 8, marine acid 3 -� -t Glauber fait 5, diluted vitriolic acid 4 j * Muriate of lime 5, water 4 - -

T'hefe falts may be recovered by evaporation to dry-and may be ufed again and again repeatedly, t Thefe falts may be recovered by diftillatioir and cry-^^'1'zation,

J, ^ following extrads from Mr. Walker�s account of ^pcriments on Cold, may be of ufe to the reader.

fays he, � frequently frozen quickfilver by ^ 'Xing together at 0�, three drams of ground ice, with 'nbsp;�Itams of nitrous acid,

128 Of the production-, of Heat and Cold.

The laft-mentioned method of producing cold, is by the expanfion of air. But this method, whichnbsp;is cftablifhed by a fufBcient variety of fads, hasnbsp;not, however, be�n applied to any oeconom.icalnbsp;iifes.

It has been found that whenever air is comprefled in any veffel, heat is produced, the degree of whichnbsp;is proportionate to the quicknefs of operation andnbsp;the quantity of compreflion. On the contrary^nbsp;when air is expanded, a degree of cold is produc'nbsp;ed, which is proportionate to the quicknefs andnbsp;quantity of expanfion.

A thermometer placed under the receiver of aD air-pump, is lowered a few degrees, by expeditknbsp;oufly rarefying the air of the receiver.

� If it be required to make it perfedlly folid and hard, 3 � mixture of equal parts �f the diluted vitriolic acid andnbsp;� nitrous acid fhould be ufed with the powdered ice; butnbsp;� then the materials ftiould not be lefs than � lo� beforenbsp;� mixing.

� If a kill greater cold be required than a mixture of ths*-� kind can give, which is about �y�, the diluted vitrioh'^ � acid alone Ihould be ufed with fnow or powdered ice, andnbsp;� the temperature at which the materials are to be mix^nbsp;� not lefs than-�20�.�

If Glauber�s fait be added in cfyftals unpounded to doubl� aqua fortis, or diluted nitrous acid, even at a warm tempet�'nbsp;ture, the cold produced will be fuliicient to freeze waternbsp;cream.

In the fame veffel the air may be alternately rarefied and condenfed by the ufe of a proper engine, the thermometer in it will be' lowered in thenbsp;former cafe, and raifed in the latter *.

If the comprelfed air in the refervoir of an air-gon, be difeharged on the bulb of a thermometer, rhe mercuiy in the tube of the thermometer willnbsp;^ofeend a few degreesf.

In the Chemnicenfian mines in Hungary, the air in a large veffel is compreffed by a column of waternbsp;260 feet high, when the ftop-cock, which gives exitnbsp;to that air, is opened, the air rulhes out with greatnbsp;violence, and its expanfion produces a furprifingnbsp;degree of cold; for the moifture is precipitatednbsp;from it in the form of fnow, and icicles adhere to thenbsp;nofel of the ftop-cock

Having deferibed the different means of pro-^ncing heat and cold, I lhall conclude this Sedtion ^pon Heat or Caloric, by briefly mentioning an in-ftance of the infinite wifdom of nature in the appli-

1 See the defeription at large of the machine, and the ^eount of the phenomenon in the 52d vol. of the Philolb-^ Tranfadtions.nbsp;nbsp;nbsp;nbsp;'

cation of proper means for the prefervation of the animal creation in great heats or colds.

The range of temperature in which a human being can live with comfortj is trifling indeed.nbsp;Natives of different climates can fuffer withoutnbsp;uneafinefs different, but not very different, degreesnbsp;of temperature. In this climate wc can live withnbsp;comfort between the temperatures of 60� and 70quot;,nbsp;viz. a range of about iolt;�. When the temperature .nbsp;of the air is below 60quot;, mofl people would be gladnbsp;to approach the fire j above 70�, moft people complain of the heat.

But nature has made ample provifion for obviating the pernicious effedts of a hidden increafe of great heat or cold. It is a dilpofition for generatingnbsp;cold in the former, and heat in the latter cafe, atnbsp;leaft to a certain degree and for a certain time ; andnbsp;this is eff��led by the natural change of capacity innbsp;fome of the component parts of the animal body,nbsp;thus, for inftance, when the body is very warm,nbsp;its perfpiration is increafed, and the fluid, whichnbsp;'becomes vapour, having its capacity for caloric increafed, contributes to cool the body; and a fimilarnbsp;cffefl is produced by the change of the capacity ofnbsp;other animal fluids, amp;c.

In faft, animals of various fpecies, and even human beings, have frequently been expofed to ex-ceffive degrees of heat or cold for a certain time, without receiving any material injury} and'witJiout

having had the natural temperature raifed or lowered by more than a very few degrees. Thus men havenbsp;been expofed to a temperature where quickfilvernbsp;�Would freeze, and tin the other hand, to a temperature above that of boiling water ¹.

See the experiments in ah h�at�d room, in the Philofo-phical Tranfaftions, vol. for the year 1775lt;

SECTION ir.

The fubjeft pf the prefent fe�lion is fo very extenfive, that a full inveftigation of all itsnbsp;branches, both in theory and in practice, would fillnbsp;up feveral fuch volumes as the prefent. A ufefulnbsp;and competent explanation of its principles is whatnbsp;can be expeited in the prefent work; and this wenbsp;lhall endeavour to render as comprehenfive as thenbsp;nature of the fubjefl; feems to admit of. Withnbsp;this objeft in yiew we fliall take little notice ofnbsp;what is merely hypothetical or controverted j wenbsp;lhall however refer the inquifidve reader to thofenbsp;works which treat more at large of thofe particularnbsp;branches. With refpeft to the ufeful part of thenbsp;fubjeft, we lhall endeavour to explain the principles chiefly; for when thefe are well underftood, anbsp;very moderate degree of ingenuity on the part ofnbsp;the ftudent, will enable him to apply them eithernbsp;for the explanation of new fads, or for the improvement of particular branches.

TH E difFerence, in the day time, of what we perceive when our eyes are open and whennbsp;Our eyes are fhut, is produced by what is callednbsp;iight. The privatipn of light, as when our eyes arcnbsp;fhut, is called darknejs. It is this light that informsnbsp;us of the prelence of objefts which are not nearnbsp;enough to touch our bodies, or which do not affeftnbsp;S'Uy of our other fenfes- Hence the blind mudnbsp;judge of the prefence of particular objects, by thenbsp;found, or by the fmell, or by the touch, amp;c. butnbsp;not by the means of light. In Ihort, light does notnbsp;f^nfibly alfedt any other part of our frames, befides

have no certain knowledge with refpe�l to nature of light. 'A variety of conjeflures havenbsp;made, and a variety of hypochefes have beennbsp;offered concerning it; but of thofe hypothefes twonbsp;deferve to be mentioned.

Was fuppofed by Defcartes, Huyghens, and that a very fubtile fluid is difperfed through-� or fills up, the univerfe; that the luminousnbsp;^ fuch as the fun, a candle, a fire, amp;c. put

that fluid, not in a progreflive, but in a certain vibratory motion ; and that this motion, being communicated to the nerves of our eyes, renders the luminous bodies perceptible to us; fomewhat like the efFeft of a founding body upon the air, whichnbsp;puts the air in a certain vibratory motion, and thisnbsp;motion being communicated to our organs ofnbsp;hearing, excites in us the fenfation of found.

Newton and his followers fuppofed that light is a real emanation from luminous bodies; viz. that anbsp;fubtile fluid, confifting of certain peculiar particlesnbsp;of matter, proceeds from the luminous bodies, andnbsp;by entering our eyes, excites in us the fenfation ofnbsp;light, or the perception of the luminous objedts.

A variety of fa6l:s and confiderations feem to place Newton�s hypothefis on a bafb of greatefl:nbsp;probability.

Admitting then Newton�s hypothefis, feveral con-fequences, which are naturally deduced from it, demand a particular explanation ; viz. this emanation, this light, muft confifl of particles; thofe particlesnbsp;muft have a very minute, but determined, fize; theynbsp;muft be at a certain diftance from each other, m.uftnbsp;move with a certain velocity, and muft have a certain momentum.

Several remarkable difcoveries made in aftro- ' nomy and in other branches of natural philofophy,nbsp;enable us to determine the above-mentioned fize,nbsp;diftance, velocity, amp;c. of the particles of light, notnbsp;with abfolute precifion, but within certain limi^

probability. Previous to the ftatetnent of thole �Iriantitiesj it will be neceffary briefly to mention thenbsp;principal fafts upon which the determinations of

Ip a fmall hole be made in a fcreen, and the fcreen placed before our eyes, at about the diflrance ofnbsp;5 or 6 feet; and if a luminous body, for inftance,

^ red-hot coal, be repeatedly paflTed by the hole on other fide of the fcreen, we muft naturally per-r^eive the hole luminous at intervals. But if the in-'�^I'val, or the time during which the coal is not be-Pore the hole, be lefs than the tenth part of a iecond,nbsp;^lien the hole will appear to us conftantly luminous,nbsp;Cxaftly as if the red-hot coal were held fleadilynbsp;efore it. This fliews that the impreffion ofnbsp;^^nt upon our eyes continues a certain time, viz.

appearance of an objedl remains upon our eyes ^^ttain time after the removal of the objeft, or after

is for this reafon, that if a ftick with a lighted ^^iremity be turned round in a circle before ournbsp;and if the revolution be quick enough, wenbsp;not a fucceffion of light along the circum-^nce of the circle, but vve imagine to fee an un-^^^*^Ptcd circle of light.

the tnuft be remarked, that the duration of �tnpreHjon of light upon our eyes, is longer ornbsp;according as the objeft is more or lefs lu-5gt; Viz. according as the impreffion is ftrongernbsp;^aker; hence, if the above-mentioned experi-�� ^nbsp;nbsp;nbsp;nbsp;ment

ment be performed with a flick whofe extremity U barely red-hot, the revolution muft be rnadenbsp;quicker j but if that extremity be very vivid, thejinbsp;the revolution needs not be fo quick, in order tonbsp;reprefent an uninterrupted circle.

The impreflion is fometimes fp ftrong, that the eye does not eafily recover its tranquillity, evertnbsp;after feveral minutes; and the very fhape of the luminous objeft reipains a certain time in it. Thus,nbsp;after having looked for a Ihort time at the fun, or atnbsp;the bright fire of a large furnace, the eye remainsnbsp;dazzled, fo as to render the appearance of other ob-jefts defe�live or confufed for a very confiderabknbsp;time.

It has been obferved by aftronomers, that the eclipfes of the fatellites of the planet of Jupiter, ap'nbsp;pear to take place fooner than the time determinednbsp;by the tables of their motion, when that planet isnbsp;nearer to us; and that thole eclipfes appear to takenbsp;place later when that planet is farther from us*nbsp;Hence it is conjedlured, that light moves pro'nbsp;greflively and equably, viz. that it employs ^nbsp;certain time in percurring a certain fpace j andnbsp;this conjecture is corroborated by other aftronp'nbsp;mical obfervations, which, as well as the above'nbsp;mentioned appearance of the fatellites of Jupice'��nbsp;will be explained in a lubfequent part of thelhnbsp;elements. But we m.uft not omit to mention in thi^nbsp;place, that from the difference between the nearednbsp;and fartheft diftances of Jupiter from us, and fro'^nbsp;the difference of time between the apparent and rh^l

iabuiar times of the eclipfes of its fatellites at thofe two ftations of the planet; it has been computed,nbsp;that light moves at the aftonifhing rate of, at leaft,nbsp;t64'3oQ ixiiies per fecond, or, we may fay, 170000nbsp;P^t fecond; fo that in moving from the fun to us,nbsp;%bt employs about 8 | minutes; whereas, if anbsp;'^^nnon ball could continue to move with the famenbsp;''Velocity with which it firft comes out of the can-hotij (viz. at the rate of about one-eighth part of anbsp;'^ile per fecond) it would employ 32 years in goingnbsp;from the earth to the fun.

rom fome imperfedt experiments made by J'^wing the focus of a concave mirror on the extre-^ delicate beam nicely fufpended, bynbsp;means a flight motion was given to the beam,nbsp;^^Was deduced that the light thus collected, had anbsp;^ ^i^omentum. Now, from the weight of thenbsp;and from the motion w'hich was communicated

cared to it by the impulfe of light (if that was the real caufe of its motion); alfo from the above-mentioned velocity of light, it was calculated¹, thatnbsp;the matter contained in the light which was thrownnbsp;upon the end of the above-mentioned beam duringnbsp;one fecond of time, and which was collefted fromnbsp;a refledting furface of about 4fquare feet, amountednbsp;to no more than one twelve hundred millionth partnbsp;of a grain f.

Now, from the above-mentioned fafls, as alfo from the common, obvious, and daily experience,nbsp;we may draw the following conclufions :

1. nbsp;nbsp;nbsp;Since every phyfical point of a luminous objedtnbsp;may be feen from every point of an immenfe fphe-rical fpace which furrounds it, when no opaque bodynbsp;interferes, it follows, that the ftreams of light whichnbsp;proceed from all the points of vifibie objedls, andnbsp;move in all manner of diredtions, is paft all conception. If this be alledged as an objedlion to Newton�s theory, the leaft refiedtion will ihew, that itnbsp;offers an objedlion equally great, if not greater, to.nbsp;the other hypothefis. But the following conlidera-tions will fmooth the difficulty with refpedt tonbsp;Newton�s hypothefis.

See the mai�ner of making fuch computations in the fitft volume of thele Elements. Chap. IV.

iight remains a certain time upon our eyes, and the cafe of the red-hot charcoal) it has beennbsp;^^wn to remain about one-tenth part of a fecond;nbsp;fuppofe it to remain only during the loodthnbsp;of a fecond j then it is evident, that if 150nbsp;l^^tticles cf light be emitted from a Angle point of anbsp;^ famous body, as from a point of the furface of thenbsp;f i thofe particles will be more than fufficient tonbsp;Our eyes an uninterrupted vilion of that point;nbsp;ftill thofe particles, on account of their immenfenbsp;may be more than loco miles diftantnbsp;One another, and of courfe leave room enoughnbsp;�Millions of other particles to pafs in all di-

The vvafte of the matter of a luminous body, from the emiffion of light, confidering thenbsp;^ ^�tenefs of its particles, is very trifling, evennbsp;^ 'quot;^fpedt to the fun, which has been the greatnbsp;of light during fo many centuries. Dr.

� dent

dent upon a fquare foot and half of futface, in fecond of time, ought to be no more than thenbsp;� twelve hundred millionth part of a grain. B'J¹'nbsp;� the denfity of light at the furface of the funnbsp;� greater than at the earth, in the proportionnbsp;� 450CO to i¹j there ought, therefore, to ilfti�nbsp;� from one fquare foot of the lun�s furface in onenbsp;fecond of time, in, order to fupply the wafte b/nbsp;� light, one-forty thoufandth part of a grain 0^nbsp;matter, that is, a little, more than two grains in �nbsp;� day, or about 4752000 grains, which is aboutnbsp;670 pounds avoirdupois, in 6000 yearsf.�

4. nbsp;nbsp;nbsp;On account of the motion of light, it is eviden'^nbsp;that if a luminous body were fuddenly placed in thenbsp;heavens, at, for inhance, the fame diftance that thenbsp;fun is from us, we could not poffibly fee it befof^nbsp;the lapfe of 8 J minutes. Alfo, when we behold ^nbsp;celeftial objedt, we do not fee it exadtly in the placenbsp;where it adlually ftands; but we fee it in the placenbsp;where it ftood fome time before.

5. nbsp;nbsp;nbsp;Light moves in ftraight lines as long as itnbsp;through the fame uniform fubftance, or through ^nbsp;vacuum.

6. nbsp;nbsp;nbsp;If we direct our eyes towards certain polilhe^nbsp;furfaces, we frequently fee in them the appearance^nbsp;of objects which are fituatedin places quite dilFeren'^

See the ill vol. of thefe Elements, p. 62. f Hill, of Difc. on Vifion, Light, and Colours, p. 39��

^�Om thofe in which we fee them. Thus an eye C, fig. 14. Plate XVni. dire�led towards thenbsp;and polifhed furface, of which A B is thenbsp;will perceive the exadt figure, colour,nbsp;of a body which aftually Hands at D; butnbsp;'''hich will appear as if it Hood at E ; it is,nbsp;^^^fefore, evident that the light which proceedsnbsp;D, falls upon the furface AB, and thence itnbsp;in another diredtion, FC, to the eve at C.

is called the refle^ingfurface or mirreur, be ^ ogure flat or otherwile fhaped. The light thusnbsp;back, viz. F C, is called the rcflcBed light-,nbsp;^^^^reas the light from the objedi: D to the refledlingnbsp;is called the incident lipht. The anglenbsp;�^bthe incident light makes with the pcrpendicu-the refledling furfirce at,the point of inci-^3 viz. the angle D F G, is called the angle of

water, A B D C, fo as to caft a the Tnbsp;nbsp;nbsp;nbsp;cgt;f the veifel upon the bottom,

to form a ftraight line EBG, but will be foinewhere elfe, as at F, and FBG will fof^nbsp;angle at the furface of the water, whi'^^�

proves beyond a doubt, that the light which ceeds from the candle is refradted, viz. bent, atnbsp;furface of the water. The angle which the incid^^^nbsp;light G B makes with the perpendicular to thenbsp;face at'B, viz. the angle G B K is the angle ofnbsp;dence; the angle which the refracted light mak^*nbsp;with the fame perpendicular produced, viz.nbsp;angle C B F, is called the angle of refradiion.nbsp;fome authors call the angle G BI the angle of in^'nbsp;dence, and F B A the angle of refradlion, viz.nbsp;angles which the incident and the refraded lig^*'nbsp;make with the furface A B I.

8. Light is likewife bent not only by pa0i��^ through, but by paffing within a Ihort diftancenbsp;bodies. This lort of bending is called infledliol^nbsp;light.

An indefinitely finall quantity of light, whi^*^ neither diverging nor converging, is called a rdJnbsp;light. The quantity of light which comes ff^*^nbsp;a luminous point in a diverging conical manf�

Thofe bodies, fuch as water, glafs, dc. throd' w�hich light will pafs, or through which out

can perceive obje�ls fituated on the other fide� called tranfparent bodies. All tranfparent bodie��nbsp;alfo a vacuum, are called mediums in optics.

bodies which obftruft the paflage of light, or through which nothing can be feen, are callednbsp;opaque bodies.

The fcience esioptics comprehends whatever be-longs to light and vifiqn, but fome authors confine *t merely to the explanation of direfl; vifion, viz.nbsp;quot;'hen the light comes directly from the objed: to thenbsp;That branch of optics, which treats of rejected light is called Catoptrics; and that whichnbsp;^teats of refraded light is called Dioptrics.

It is upon the reflecfion and refradion of light, that the whole fcience of optics principally depends;

if the rays of light were neither refiexible nor re-hangible, we fhould be deprived of telefcopes, mi-^tofeopes, fpedacles, and all other optical inftru-quot;tents ; as alfo gf the greatefi part of the nioft urefu| admirable phenomena of vifion.

L � 144 ]

OF the rays of light which proceed from a ln' minoHs body ¹, thofe which fall upon th^nbsp;furfaces of almoft all bodies, whether tranfparent o'!nbsp;opaque, folid or fluid, are more or lefs, but nevefnbsp;entirely, reflededf.

WheP

A luminous body, in this place, means any vifibl^ objeft, whether it be vifible by the emiffion of original lig^^�nbsp;like the fun, a candle, amp;c. or by refleded light, likenbsp;moon, a tree'in the day time, amp;c.

t Of the light which falls upon the furface of mercuff� not above three quarters are reflected ; and probably th^�^^nbsp;is no fubftance which refledls light fo well as mercury.

The quantity of light which is refledled from a giveri furface, varies with the angle of incidence; and a gre3�_nbsp;quantity of light is refledled when the angle of incidencenbsp;great, than when it is fmall. Thus of the light of the ¹nbsp;which falls upon the furface of fmooth water, anbsp;quantity is refledled foon after fun-rife, or before fun-fe¹^�-��'�

than at noon. But the increafe of refledled light, wU¹� Tncreafe of the angle of incidence, is not equally

When the reflefting furface is flat, or of a regu-^ar figure, then the diredlion of the refle�led rays be traced by the method which will be explained i^i the fequel; but when the refledling fur-^^ce is irregular, then the light is fcattered in va-�quot;lous and uncertain direftlons.

The rays, which proceeding from any Angle lumi-^ous point of an objedV, fall upon a given reflefting furface (or upon any furface) are innumerable j

by innumerable fpefliators placed in different ^^�^uations, and direfting their eyes towards the re-

*=gt;� 2.. Plate XIX. fends out rays of light fpheri-or in all direftions. �f thofe rays the portion B falls upon a plane reflefting furface, ofnbsp;'''1'ich A B reprefents the feftion j and are thencenbsp;^^^lt;fi:edlt;to the places D, K, L, M, amp;c.

is evident that thofe rays fall upon the furface * quot;'ith different angles of incidence ; but if younbsp;any one of thofe rays and its refleftion, younbsp;^^d that the angle of incidence is conftantly

light is an emanation, and falls upon bodies gt; fince an oblique impulfe is more eafilynbsp;Worknbsp;nbsp;nbsp;nbsp;^ diredt one, amp;c. See the firft volume of this

equal to the angle �f refleftion ; and this is a fundamental law in catoptrics; viz. the angle COH is equal to the angle HOD; C I G is equal tonbsp;GIL, CEN is equal to NEM, and fo on; OHgt;nbsp;I G, E N, being perpendiculars to the furface.

Another invariable law. is, that the angle of incidence and that of refleftion of the fame ray, lay in one and the fame plain, which is perpendicular tonbsp;the reflefting furface. The ray which falls perpendicularly upon a reflefting furface, (like C A) i*nbsp;reflefted back along the fame line, for in that cafenbsp;the angle with the perpendicular vaniflies.

It is evident that the rays of light which come from a luminous point, muft fall divergingly uponnbsp;any given furface; yet w hen the objeft is valtiynbsp;diftant, the divergency of the rays becomes infenfi-ble ; and, in that cafe, they are called parallel rays-Thus the rays of the fun, of the moon, of the ftars�nbsp;amp;c. are reckoned parallel rays. When the lumi'nbsp;nous point is pretry near, then the rays are fenfiblynbsp;diverging. The rays which come from differentnbsp;points of. the objeft to one point of a furface, arcnbsp;evidently converging rays.

In the following pages we fliall take notice not 0^ all, but of a few only of, the rays which proceednbsp;from certain luminous points of objefts; k bein?nbsp;evident that the intermediate or adjoining rays,nbsp;moftly regulated.by fimilar laws.

When an eye, as E, fig. 3. Plate XIX. viequot;'* an objeft as CD, or AB, dire�lly, fome of the ray��nbsp;which proceed from every perceivable point of

enter the eye, and the whole quantity of light which thus enters the eye, is clrcumfcrlbednbsp;% the rays which proceed from the extreme pointsnbsp;the objeft, viz. C E and DE, or A E and B E,nbsp;quot;I'he angle which thofe extreme rays form at thtnbsp;viz. the angle C E D, or A E B, is called thenbsp;angle, and it is from the fize of that angle, �nbsp;that we principally judge of the diftance of a knownnbsp;Thus, fiippoling that the obiefis CD andnbsp;are equal, or that they reprefent the very famenbsp;t^bje�l fucceffively fituated at different diftances ; itnbsp;evident that the farther the objeft is from the eye,nbsp;'�be fmaller wnll the vifual angle be.

It muft likewife be obferved that, the diftance ^tween the eye and the obiedt remaining the fame,nbsp;by any means the rays of light are bent fo as tonbsp;^filarge the vifual angle, then the objed will appearnbsp;�^'^Ser (or it is faid to be magnified) ; and on thenbsp;'�^titrary, if the vifual angle be diminifhed, then thenbsp;appear fmaller, in which cafe it is faid tonbsp;^ tiiminifhed.

thefe particulars, which have been mention-^tth refpeft to the above direft view, are likewile with refpedl to the view refteded by any regu-^ �'^fleding furface.

^ ^bjed placed before it. Draw the extreme ray? ^ftual quot;'bich, forming their angles of incidencenbsp;their refpedive angles of refledion, maynbsp;j, 2nbsp;nbsp;nbsp;nbsp;come

come to the eye at F ¹ j and the objedt v/iil appear as if it flood at I K, viz. as far behind the refleftingnbsp;furface as it adlually flands before it; or it will ap'nbsp;pear as it would by direft view to an eye at C, vi�¹nbsp;an eye fituated as far as to make the diftance C Pnbsp;equal to DF j which is owing to the lines DC, GPnbsp;forming an angle at C equal to the angle formed bynbsp;the lines DF, GF at F. That thofe angles arenbsp;equal is eafily proved by drawing the perpendicularsnbsp;IE and L N to the reflecting furface; for thenbsp;angle of incidence ADI is equal to the angle ofnbsp;refledion IDF, and equal to the angle EDC f�nbsp;therefore the angle IDF is equal to EDC, and thenbsp;angle F D G equal to C D G, llnce the whole anglenbsp;J D G is equal to the whole angle E D G; eachnbsp;being a right angle. By the like reafoning it willnbsp;appear that the angle F G D is equal to the angl^nbsp;C G D ; whence it follows, that the triangles DGCnbsp;and D G F, having two angles of the one equal tonbsp;two angles of the other, and a correfpondent fide,nbsp;viz. D G, common, are equal in every relpedt J ;

It is ufelefs to take notice of thofe rays, which, coming from the fame points A and B, fall upon the reft of the re'nbsp;fieiSilig furface, becaufe thofe rays cannot be reflected toth^nbsp;eye. The rays which come from other points of the objeiSnbsp;between A and B, and fall upon the furface D G, are aHnbsp;included between the extreme rays AD, D F, and Bnbsp;GF.

is to be obferved, however, that an objedt 'Viewed by refleflion from a flat furface, as by an eyenbsp;Fj does not appear fo bright as if it were viewednbsp;^'quot;ona C by a direfl view, becaufe fome light is loft bynbsp;refledions even from the beft refleding furface �,nbsp;^i�ich is owing to the pores, irregularities, amp;c� ofnbsp;^i^ofc furfaces. What has been faid of the inclina-of the extreme rays A D and B G, is evidentlynbsp;applicable to all other rays incident upon a planenbsp;'^^H^'ding furface, viz. that on account of the per-P^fidiculars IE, LN, amp;c. being parallel to eachnbsp;the incident rays will be refleded with thenbsp;inclination to each other as they had beforenbsp;incidence on the furface, viz. they will benbsp;l^��'^llel after refledion, if they were parallel beforenbsp;they will be diverging or converging after re-^'-bon, according as they were diverging or con-'^^'�ging before, and at the fame angle.

*Phe refledions from concave or convex refleding ^�^es, produce very different effeds, becaufe thenbsp;^^*�pendiculars to the different points of a curvenbsp;are not parallel to each other. Thus, fup-that two parallel rays, as AB, CD, fig. 5.nbsp;XlX. fall upon a fpherical convex furfacenbsp;gt; draw the perpendiculars to the furface at thenbsp;P'^'^its of incidence B, D, and thofe perpendiculars

(EBj FD*) mufi: diverge from each other, and of courfe the refle�led rays, KB, LD muft likewifenbsp;diverge from each other; for if a line at D, viz-M D, were parallel to E B, then the reflected raynbsp;ND would be parallel to the refleftcd ray KB; andnbsp;therefore the real reflefted ray D L, diverging fromnbsp;D N, muft alfo diverge from K B.

A fimilar reafoning applied to the refledlion of incident parallel rays, from a fpherical concave for-face, will prove that they muft be reftected con-vergingly. Thus the parallel rays A B, C D, fig. 6gt;nbsp;Plate XiX. are refledled in a converging mannernbsp;from the concave furface FIBDI; in ccnfequencenbsp;of which they muft meet in fome point where theynbsp;crofs each other, after which they proceed diverg'nbsp;ingly, like the refisfled rays B F, D F, which meetnbsp;at F, crofs each other, amp;c.

This explanation, which we have applied to parallel rays only, may be eafily extended to al^nbsp;forts of incident rays, viz. to thofe which come di'nbsp;vergingly as well as convergingly; the general laquot;'^nbsp;being as follows :

All forts of rays of light, viz. whether paralllt;^^� diverging,, or converging, which fall upon a fph^'

* The perpendicular to a curve furface at any point, ** perpendicular to a plain furface touching the curve lurfaC�nbsp;at that point. The perpendiculars to any points of a fphericdnbsp;concave,or convex furface, do all meet at the centre of fp^^'nbsp;ricity, viz. of the fphere of which the given furface is

, ricW

Catoptrics, or of RefleBed Light. 151 rically convex furface, are reflefted' in a more

diverging manner, viz. the reflefled rays pro-^ced lefs inclined to each other than the incident

All forts of rays, which fall upon a fpherically Concave refleftlng furface, are reflefted more con-''crgingiy^

or lefs divergingly, viz. the refleded will be more inclined to each other thannbsp;incident rays. But when the refleded rays meetnbsp;and crofs each other, then beyond that point or focusnbsp;^�^ey proceed divergingly.

Nowithas been faid in page 147, that if the angle, ^hich the rays, that proceed from the extreme pointsnbsp;an objed, form at the eye, is by any means di-*^inifhed, that objed will appear� fmaller, and vicenbsp;; therefore it follows, that an objed feeh bynbsp;��cfledion from a convex furface, mull appearnbsp;frnaller than if it were refleded from a flat furface.nbsp;that an objed feen by refledion from a con-furface, muft appear larger than if it were receded from a flat furface, to an eye fituated nearernbsp;the refleding furface than the focus of the re-�cded rays; but it will appear fmaller and invertednbsp;an eye fituated farther than that focus; for as thenbsp;Crofs each other at the focus, the upper ray,nbsp;fig. 6. will become the lower FG beyondnbsp;^ focus F, and the lower D F will become thenbsp;^Pper FK.

�wifli to exhibit them upon paper, for the fake of illuftration, he may eafily perform the neceffarynbsp;operatious, viz. he muft firft of all draw the curvenbsp;line which exhibits a fedlion of the refleftins furface;nbsp;fecondly, he draws the incident rays, whether converging, parallel, or diverging; thirdly, he de-fcribes the perpendiculars at the various points ofnbsp;incidence �, and laftly, he drawls the refiefted rays,nbsp;always making the angle of refledtion equal to thenbsp;angle of incidence. I fhall therefore enumerate thenbsp;properties of the ipherically concave and convexnbsp;reflefling furfaces, without any farther explanatiorrnbsp;of'thofe properties; but previous to this it will benbsp;proper briefly to remove a difficulty which frequently occurs to the learners of optics.

In fpeaking of parallel rays, it is not to be imagined, that all the rays which come from all the points of an objeft, and fall upon the eye or uponnbsp;any refledling furface, are parallel to each other;nbsp;but it mull be underftood of thofe rays only whichnbsp;proceed from one phyfical point. For inftance, letnbsp;us examine the rays which come from three pointsnbsp;only of the fun, and which enter the pupil of annbsp;eye. See fig. 7. Plate XIX. The rays whichnbsp;proceed from the point A, in truth form a cone,nbsp;the bafe of which is the pupil of the eye at D ; andnbsp;its height is from us to the fun ; hence, the variousnbsp;rays which form that cone are faid to be parallel,nbsp;becaufe their inclination to each other is infenfible�nbsp;and the fame thing muft be underftood of the rays

proceed from the point B, or from the point ^ 5 but if we take a ray from the point A, andnbsp;another from the point C, then thofe rays form anbsp;f^nfible angle at the eye; and it is from this anglenbsp;^b)C that we judge of the apparent fize of the fun.nbsp;quot;^be meafure of that angle is about 32 minutes.nbsp;This figure likewife fhews, that the larger thenbsp;is, the brighter will the objeft appear; becaufenbsp;��be larger the pupil is, the greater number of raysnbsp;quot;'ill receive from any Angle point of the objeft.nbsp;Since in nature an objeft which is near appearsnbsp;^^'�ger and brighter than a fimilar objeft fituatednbsp;birther from the obferver; therefore, whenever thenbsp;appearance of a given objeft is rendered larger andnbsp;^�'�gbter, we always imagine that the objedt is nearernbsp;than it really is.

'b'be objedts refledted from fuch a furface appear ^^quot;'ays fmaller than natural, always eredt, and al-as if they were behind the reflecting furface.

^ '�'be objedts never appear exadtly of the true �PP. If the objedt be a right line, or a plain fur-� me image or appearance of it will be a curvenbsp;� Or curve furface, becaufe the different pointsnbsp;objedt are not equally diflant from the re-

very diftant objedls, are reflefted diverging]}-, and their divergency is Inch, that if they be producednbsp;behind the refledting furface, they will meet at thenbsp;diftance of half-the radius ei convexity ^ that pointnbsp;is. called the virtual focus of thofe rays, and thenbsp;frincifal focus of the refledor.

Diverging incident rays, viz. fuch as come from near and fmall objefts, have their virtual focusnbsp;nearer to the refieding furface than half the radius.

When the incident rays are converging, if the diftance of the luminous point from the refleftingnbsp;furface be lefs tiian half the radius of convexity?nbsp;the reflected rays will have a real focus before th^nbsp;furface: otherwife they will have a virtual focusnbsp;behind it.

The reflefting furface muft be underftood to be a fmall portion of a large fphere ; .for, ftridtiy fpeak-ing, the rays refleded from a convex furface, can'nbsp;not have a common virtual focus, and the muld'nbsp;plicity of their foci increafes with the fize of ch^nbsp;fpherical portion. This property will l^e renderednbsp;more apparent by what will be faid in the follovV'nbsp;ing paragraphs concerning the concave refledtor.

be a Tingle point) then all the rays which fall upon bie concave refieftor will be refleflied fo as to meetnbsp;that fame point, or centre, or focus.

Rays which come from any other point, cannot be all refle6led to one and the fame focus. Thusnbsp;fig. 8. Plate XIX. AB reprefents the concavenbsp;�'^fleclor, Q^is the objedt or radiant point, and////nbsp;� 0 0 reprefent the various foci of the refledednbsp;^^ys, which form the two curve lines fff 000.nbsp;quot;bfiofe curve lines are called cauflicks, or cauflicsnbsp;^tfleClion *.

The rays which are refleded from the middle-part of the refie�lor, viz. from that part which more diredly oppofite to the radiant point, (asnbsp;part CD) meet or have their foci pretty nearnbsp;each other, and the narrow fpace, within whichnbsp;meet, is confidered as the focus of thofe inci-rays. On this account the concave refledorsnbsp;'''bich are commonly made for optical or other phi-j^bophical purpofes, generally are fmall portions ofnbsp;fpherical furfaces �, for whether the refledor isnbsp;refled light or heat to a particular place, as at F,nbsp;^be portions C A and D B will be quite ufelefs.

Such cauflicks may be feen upon the furface of milk, �Pon any opaque whitifh mixture of liquors contained innbsp;^hite china-cup, or upon the bottom of a fnuff-box, whofenbsp;�s well polifhed, when the light of a candle, or of thenbsp;gt; ^tof a remote window Ihines upon it.� Dr. Smith�snbsp;B. 1. Chap. II.

Therefore

Therefore in the following paragraphs, By a concav* refieftor, muft be underftood a fmall portion of ^nbsp;large ^herical furface.

The radiant point, or luminous point, from which the incident rays proceed, is alfo called a focus�nbsp;like the point in which reflefled rays meet but thenbsp;former is denominated the focus of incident raySynbsp;whilft the latter is called iht focus of reflelied rays.

� A line which is fuppofed to pafs through the centre of the refleflor, and through the centre ofnbsp;the fphere, of which that refledtor is a part, is callednbsp;the axis of the refleftor.

When the incident rays are parallel (viz. when the focus of incident rays is very remote) then thenbsp;focus of the reflefted rays is before the reflector at .nbsp;the diftasige of half the radius of concavity, fromnbsp;the refle�ting furface, and in the middle of that ra-dius to which the incident rays are parallel. Thisnbsp;diftance is called the focal diftance. Such a focusnbsp;of refleded rays, viz. when its diftance is equal tonbsp;half the radius of concavity, is called the principalnbsp;focus of that refledor.

The nearer the focus of incident rays comes to the furface of the refledor, the farther will the focusnbsp;of refleded rays recede from that furface; in Ihort,nbsp;thofe foci move in contrary diredions (1). When

the (l.) Of the following three quantities, viz. the diftancenbsp;of the focus of incident rays, the diftance of the focus of re-

Catoptrics, or of Reflected Light. 157

diftance of the focus of incident rays is equai half the radius of concavity, then the rays will

rays, and the radius of concavity or convexity, when are given the third may be found from the followingnbsp;^Jialogy^ which applies to convex as well as to concave fphe-*^'^al reflectors.

*' The diftance of. the focus of incident rays from the P''*ncipal focus, half the radius of the refledtor, and the dif-^3nce between the principal focus and the focus of reflectednbsp;will be in continual proportion.

Suppofe the refledlor to be concave, and the rays to ^�'^erge from a focus, the diftance of which from the furface

^ Let the radius of the refledtor � r �, we have, by the Preceding ru\c,d^S:L::l:Jl

� The focus of reflefted rays is in this cafe between the P�^'flcipal focus and the centre of the refledtor; wherefore.

This folution extends to all cafes of foci formed by from a fpherical furface, by changing the fign ofnbsp;� '''hen the refl�dlor is convex, and of d, when � the raysnbsp;�iVerge to a point, the diftance of which from the furface

, nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;----- o nbsp;nbsp;nbsp;-------�---------,

be refiefted parallel to each other j and when the above-mentioned diftance is lefs than half the radiusnbsp;of concavity^ then the refiefted rays will not meet innbsp;a focus before the refiedlor; but they will proceednbsp;divergingly, viz, their virtual focus will be behindnbsp;the refledor.

When an objecl, as O B, fig. 5. Plate XlX-is fituated before a concave refledor A R, the rays which depart from any point, as O, and fall uponnbsp;the refledor, are thereby refleded in a convergingnbsp;manner, fo as to crofs each other at I; alfo the raysnbsp;which proceed from any other point, as B, are like-wife refleded to a focus or point M ; and the likenbsp;thing mull be underftood of the intermediate points-Now becaufe thofe foci are fituated nearly at thenbsp;fame proportional diftances from each other, as thenbsp;correfpondent radiant points are in the objed OB gt;nbsp;therefore it is faid, that an image of the objed i*nbsp;formed before the refledor; but it muft not benbsp;imagined that a fpedator fituated on one fide, aSnbsp;at C, can fee the image IM j for though the raysnbsp;of light meet and crofs at I M, yet they proceednbsp;ftraight on beyond that place, and, of courfe, canno^

fhould be 10 inches from the furface, the focal length rC' quired will be -�^ � � - =nbsp;nbsp;nbsp;nbsp;^ in the prefent cale

to the eye at C. The meaning then of an irnage being formed at IM, is that if a folid opaquenbsp;^^bftance, as a fiat piece of paper, be placed at IM,nbsp;�hen the image of the objeefi: Will be formed on thatnbsp;of the paper, which faces the refleftor, and thisnbsp;*^age will be feen by an eye at C, becaufe in thatnbsp;the rays of light are obftru6led in their direftnbsp;^'^^'rfe. No image, or a very indiftin�l one, will benbsp;^*^trned, if the paper be placed nearer or farther �nbsp;the refleftor than the proper place.

�^Ifo, if an eye be fituated before the refledlor, as �, the refle�led rays of light will come to itnbsp;the fame inclination as if the objeft flood atnbsp;^ but in an inverted pofition; hence it is faid,nbsp;an inverted image is formed before the re-

likewife fhews what is meant by the ex-of an image being formed behind a re-; namely, that the reflefled rays come to ^ ^ye with the fame inclination as if the objeft it-^ f Were fituated behind the refleflor.

. ^hen an eye views an objed; direftly, the quan-y of light which enters the eye from any fingle l^'ot of the objed, is a pencil whofe bafe is equalnbsp;thnbsp;nbsp;nbsp;nbsp;ot aperture of the eye, and the fame is

^ ^afe when the objed: is viewed by refiedtion from ^^plane mirror; but when the mirror is concave,nbsp;of^p ^^cotint of the inclination which the raysnbsp;'�ht fuffer towards each other, a greater quan-y of light from each fingle point of the objed:,

enters the pupil. This is clearly fhevvn by fig. loy ir, and 12. Plate XIX. the firft of which rep re-fents a direct view, the fecond a view by the reflection from a flat refledtor, and the third a view bynbsp;reflection from a concave refleCtor; of the fame luminous point A.

Hence it is, that an image formed by reflection from a concave refleCtor, may appear a great dea^nbsp;brighter than the objeCt itfelf.

The image of an objeCt, formed by reflection from a fpherical furface, is never exaCtly like thenbsp;original objeCt. Thus the image of a rtraight linenbsp;is not a ftraight line, but a conic feCtion ; and thenbsp;kind of the curve is determined by the diftance ofnbsp;tlie object.

The intenfity of the light, or of the heat of the fun, which is produced by the collected rays in thenbsp;focus of a concave fpherical refleCtor, is faid to benbsp;as the fquare of the diameter of the refleCtor. di-reCtly, and as the principal focal diftance inverfelf*nbsp;Thus, if two reflectors, A and B, have the famenbsp;radius of concavity, but the diameter of A is ^nbsp;inches, and that of B is 18 inches, then the intenfi!^/nbsp;of light or heat at the focus of A, is to that at thenbsp;focus of B, as I to 9. This psoportion muft not?nbsp;however, be confidered as exaCt.

A concave mirror, about a foot in diameter, is fituated behind a partition FD, fig. 13. Plate XIX.nbsp;^tid a hole either circular or oblong, of about fevennbsp;�oches in length, is made in the partition. An in-'^^rted objed; for inftance, a flower, is placed atnbsp;behind the partition, and is illuminated bynbsp;^eans of lamps laterally fituated; alfo a pot or ftandnbsp;placed at D, before the partition. Now thenbsp;^iftance of the partition, flower, amp;c. mufl; be fuchnbsp;to form the refleded image of the flower juftnbsp;^�'^er the pot or ftand D. Then an eye fituated atnbsp;and looking ftraight through the hole in thenbsp;Partition, will perceive an image of the flower at I;nbsp;^'^d when the light is properly managed, viz. thatnbsp;extraneous light interferes, the illufion is fo great,nbsp;the fpedator will frequently extend his hand tonbsp;^��afp what he thinks to be a real flower at I.

Hcfleding furfaces have been made of various ^^Pes, fuch as cylindrical, conical, amp;c. but thenbsp;Wufe that can he made of them is to furprizenbsp;People gy fhewing them a regular figure refledednbsp;^*'orn an original deformed objed; the principle ofnbsp;may be eafily comprehended. For inftance,nbsp;you place a regular objed before an irregularnbsp;^^fledor, the refleded image muft evidently benbsp;^^formed; therefore, if the objed, fuch as anbsp;P'dure, amp;c. be drawn purpoi'ely deformed, accord-to certain rules (which may be eafily derivednbsp;''OL. HI,nbsp;nbsp;nbsp;nbsp;cither

cither from a due confideration of the form of the refleftor, or by trials) then the reflected image willnbsp;appear regular. Such deformed figures (called ana-morphofes) are fold by the opticians, together with,nbsp;a cylindrical or conical refle�lor ¹.

There is one fhape, however, for a concave re-fleftor, which is fuperior to all others, and that is the parabolical; for, as may be eafily deducednbsp;from the Elements of Conic Seftions, when raysnbsp;fall upon a parabolic concave refleftor, parallel to itsnbsp;axis, they are all reflefted to one and the famenbsp;point, namely, to the focus of the parabola, withoutnbsp;,m ing thofe cauftic curves which are produced bynbsp;fpherical concave refleftors. But the mechanicalnbsp;difficulty of forming a well polifhed parabolic re-flelt;3:or is very great, and indeed there is no certainnbsp;known methbd of forming it.

The reflexion of light from polifhed furfaces of almofl; all bodies, takes place not only when the incident rays, which proceed from the objeO:, pafsnbsp;through the air, and fall upon the filrface of thenbsp;liquid or folid; but likewife, when the rays travelnbsp;through the liquid or folid itfelf. Thus, let a fpecknbsp;A, fig- 14- Plate XIX. be in a lump of glafS)nbsp;bcde, an eye fituated at F will fee the fpeck i'�

For the methods of drawing thofe diftorted figures, Dr. Smith�s Optics, B. II. chap. 12. Prieftley�s Hift�^'^1^

of Vlfion, Light, and Colours, Part II. Sedt. V. as moll other writers on Optics.

direction FG ; its incident light AG, being re-flefted by the furface BD ; and this refieftion becomes very ftrong or total, when the angle of incidence, A G H, exceeds 40�. The fame thing ^akes place in water, and other tranfparent bodies*. Of this more will be faid in thepext chapter.

A common flat refleftor, or looking glafs, con-fifts of a flat polifhed plate of glafs, to one fide of '''hich a plate of tin foil is made to adhere by meansnbsp;quickfilver. In confequence of this conftrudtionnbsp;looking glafs makes a double refledion of everynbsp;cbjed, viz. one from the upper furface, which isnbsp;*^116 weakeft, and another from the under furface,nbsp;'''hich is contiguous to the tin foil. When a perfonnbsp;ftands juft before the glafs, the two refledions coincide, and he perceiyes one image; but if he ftandsnbsp;cblique, as at A, fig. 15. Plate XIX. and viewsnbsp;like refledion D, of an objed B C, fituated on thenbsp;ether fide, he will then perceive two images, viz.nbsp;etie caufed by the upper, and the other caufed bynbsp;^ke lower furface of the glafs EF. If the objednbsp;^ C be very luminous, fuch as a lighted candlegt;nbsp;��ken 4he eye at A will perceive a great fucceffion ofnbsp;Candles at D, gradually decreafing in fplendourj thenbsp;caufe of which phenomenon is, that the ftrong re-^edion from the under furface of the, glafs is again

When a ray of light thus paffing through a medium is tffleded by its furface, that refledion will be ftronger thenbsp;the other medium is which furrounds the former.

M 2 nbsp;nbsp;nbsp;refleded

There is no fubftance fo perfed�ly tranfparent, but what contains home fmall opaque or reflectingnbsp;particles, v/hich fcatter part of the light thatnbsp;would other wife entirely pafs through. This is thenbsp;reafon why w� fee the direftion of the light, which,nbsp;entering through a fmall hole, paffes through the airnbsp;of a room, viz. on account of the refle�ting particles of fubftances that float in the air. Hence wenbsp;fee light in a room out of the real direftionnbsp;the rays which come from the aperture, or window, amp;c.

When light falls upon a body, and is thence re-fle�led, it is fuppofed that the refledtion takes place not exadlly at the furface of the refledting body�nbsp;but at a little diftance from it. One of the proofsnbsp;of this fuppofition is, that bodies which are madenbsp;fmooth by art, reflect light regularly ; though their �nbsp;furfaces, when narrowly examined by means ofnbsp;a magnifier, will be found full of fcratches andnbsp;droles.

We fliall conclude this chapter W�ith the defcrip-tion of a pradtical method of meafuring the angles of incidence and refledlion, and 'a method of mea-furing the quantity of light which is loft by reflection.

There are feveral ways of meafuring the angles of incidence and refledtion, but the follow'ing is onenbsp;of the eafieft. ILet ACB, fig. 16. Plate XIX. be ^

Semicircle, divided into twice 90 degrees. AB re-Piefents the fedion of a flat refleftor. Cover the Surface of this refledtor with paper, excepting a verynbsp;Small circular fpot as at D. Place the femicirclenbsp;perpendicularly upon the rcfledldr, and with itsnbsp;�Centre in the middle of the uncovered fpot D of thenbsp;^^fledlor. This done, fix a pin or other fmall ob-clofe to the edge of the femicircle, for inftance,nbsp;Ej the 50th degree �, then move your eye alongnbsp;fide AFC of the femicircle, and ybu will per-'^^ive the objeft E reflefted by the refledtor D, onlynbsp;''��hen the eye is at F, viz. at the 50th degree ofnbsp;'�he quadrant AFC; whence it appears, that thenbsp;^Ugle of refleftion, CDF, is equal to tlie angle ofnbsp;'ticidence E D C.nbsp;nbsp;nbsp;nbsp;.

Bouguer's methods of meafurlng the quantity of |'�ht loft by refledtion is defcribed by Dr. Prieftleynbsp;the following manner. � He placed a miiTor,nbsp;or refledting furface B, fig. 17. Plate XIX.nbsp;on -which the experiment was to be made, trulynbsp;'ipright; and having taken two tablets, of precisely the fame colour, or of an equal degree ofnbsp;quot;'hitenefs, he placed them exadtly parallel to onenbsp;soother, at E and D, and threw light upon them,nbsp;hy means of a lamp or candle P, placed in a,nbsp;^'ght line between them. He then placed him-mlf fo that, with his eye at A, he could fee thenbsp;tablet E, and the image D, refledted from thenbsp;mirror B, at the fame time; making them, asnbsp;tt Were, to touch one another. He then moved

the candle along; the line E D, fo as to throw more or lefs light upon either of them, till henbsp;could perceive no difference in the ftrength ofnbsp;the light that came to his eye from them. Afternbsp;this he had nothing more to do than to meafurenbsp;the diftances EP and DP ; for the fquares ofnbsp;thofe diftances expreffed the degree in which thenbsp;refleftion of the mirror diminilhed the quantitynbsp;of light. It is evident that if the mirror refledednbsp;all the rays it received, the candle P muft havenbsp;been placed at C, at an equal diftance from eachnbsp;of the tablets,' in order to make them appearnbsp;equally illuminated: but becaufe much of thenbsp;light Is loft in refledion, they can only be madenbsp;to appear equklly bright, by placing the candlenbsp;nearer to the tablet D, which is feen by refledionnbsp;only.

To find how much light is loft by oblique refledion, he took two equally poliftied plates, D and E, fig. i8. Plate XIX. and caufed themnbsp;to be enlightened by the candle P; and whilenbsp;one of them, D, was feen at A, by reflexionnbsp;from B, placed in a pofition oblique to the eye?nbsp;the other, E, was fo placed, as to appear conti'nbsp;guous to itj and removing the plate E, till thenbsp;light which it refleded was no'ftronger than tha^^nbsp;which came from the image of D, feen by refledion at B, he eftimated the quantity of ligh*^nbsp;that was loft by this oblique refledion, by the

Catcptrics, or of ReJ�eSed Light. 167 � fquares of the diftances of the two objefts, fromnbsp;quot; the candle.nbsp;nbsp;nbsp;nbsp;'

� I need . not add that, in thefe experiments, all foreign light was excluded, that his eye wasnbsp;' lhaded, and that every other pi ecaution was ob-' ferved, in order to make his conclufions un-queftionable¹.�

Notwithftanding all thofe precautions, it mull acknowledged that the above-mentioned methodnbsp;meafuring the light loft by refle�lion is by nonbsp;�^eans very accurate; nor do I know of any othernbsp;objedlionable. The principal fources of inac-^uracy are, the difficulty of determining, by thenbsp;�l^'^^gment of the eye, when two objedts appearnbsp;^^i^ally bright, and the want of an accurate expe-^�'^ental proof to confirm the propofition, that lightnbsp;decreafes in proportion of the fquares of thenbsp;'iiftances from the luminous or radiant point.

[ I�8 ]

^ I'' H E objeft of this Chapter is to Rate and to explain the various effeds which aiife fromnbsp;the refradion of light throiigb tranlparenc mediums.

When a ray of light pafTes from one medium into another, in a diredion perpendicular to thenbsp;contiguous furfaces, or to the jundion' of the twonbsp;mediums, then that ray proceeds ftraight on, without any deviation from the ftraight line.

But v;hen the ray pafles from one medium into another medium of different denfity, in a direction oblique to their contiguous flirfaces j then thatnbsp;ray will be bent, fo .as to form a right lined angle atnbsp;the jundion of the two mediums; for the diredionnbsp;of the ray through either of the mediums is rectilinear, as long as the medium is of a uniform denlt;nbsp;fity j but if the medium be continually varying ii^nbsp;denfity, like the air of the atmofphere from thenbsp;earth upwards; then the ray of light in pafTmgnbsp;through it will be continually bent, viz. it will forPinbsp;a curve line,

When

When a ray of light paffes from a thinner into a ^enfer medium *, ornbsp;nbsp;nbsp;nbsp;verf a, if a perpendicular

drawn to the j-unftion of the two mediums at �^at point, through which the ray paffes j then thenbsp;^^gle which that ray makes with the above-men-tioned perpendicular in the thinner mediuni, is ge-^^tally larger than that which it makes with thenbsp;^arne perpendicular in the denfer medium.

This is otherwife ufually expreffed, by faying, in paffing from a thinner into a denfer me-the angle of incidence is generally larger thannbsp;angle of refraftion, and vice verf a.

Now it has been obferved, that in the paffage of ^'Nique light through the fame two mediums, thenbsp;of the angle of refradtion always bears thenbsp;^arne proportion (either accurately or nearly fo) tonbsp;fine of the angle of incidence. Alfo in paffingnbsp;��^^ough any two-other mediums, the fine of thenbsp;of refraftion likewife bears a certain propor-� (either conftantly the fame, or nearly fo) to thenbsp;of the angle of incidence} but the ratio ofnbsp;l^ofe two'fines in the latter two mediums, is dif-^^tit from the ratio of the tw'o fines in the formernbsp;rnediums. All this will be illuftrated by thenbsp;�^^*^wing explanation of fig. i. Plate XX.

Let FGX Z be a quantity of water. B repre-a narrow tube, through which the fun fhines.

^ ^ot all the light incident upon a tranfparent body paffes it, but a portion is always reflected from its furtace.

and, on account of the oblique fituation of the tube� the fun�s light miift fall obliquely upon the water atnbsp;C. Then that light will not pafs through the waternbsp;along the line C Z, which is in the fame ftraight di'nbsp;region with B C \ but it will pafs in the dire�lion CDrnbsp;(which may be clearly perceived, efpecially if thenbsp;water be not very clean) making the angle of rC'nbsp;frafHon D C E, with the line ACE, (which isnbsp;perpendicular to the furface of the water, or to thenbsp;boundary of the two mediums, viz. water and air)nbsp;iefs than the angle of incidence A C B.

Otherwife, fuppofe that various objedls, for in' fiance pebbles, be placed below the water, and thatnbsp;an obferver at P, looks through the inclined tubenbsp;B � then the obferver will perceive, not the pebblenbsp;Z, hut the pebble D j whereas, if the water werenbsp;drained off, then he would perceive the pebblenbsp;and not the pebble D.

. If a circle FHE be defcribed about the centre or point of incidence C, in the fame plain with th^nbsp;lines B C, CD; and from the interfedlion Hnbsp;the circle with the incident ray, a perpendicularnbsp;Id K be dropped on the line A E; then HK is thenbsp;line of the angle B C A. Alfo, if from the inter'nbsp;foeflion I of the circle with the refracled ray, a per'nbsp;pendicular IL be dropped on the fame line A�gt;nbsp;then IL will be the fine of the angle D C E.

Now it has been found that the fine IL is alway* nearly three-fourths of the fine HK, let that fine

Mj then the fine of the angle of incidence MCA, '''ill be OR ; and the angle of refradtion, or of thenbsp;5t^g!e in water, will be YCE, whofe fine is YQ;nbsp;Y will, as above, be nearly three-fourths ofnbsp;fine OR,

is evident that when the incident ray comes ^long the line A C, the angle of incidence, as wellnbsp;Its fine, vanilhes or becomes nothing; confe*nbsp;Suently the angle of refradion and its fine mufl:nbsp;'^^tn�h too, viz. the ray of light mufl: proceednbsp;^'quot;^'ght along the line ACE. Hence it is faid,nbsp;there is no refradion when the rays of lightnbsp;a medium in a diredion perpendicular to itsnbsp;^'^tface.

^ow if, inflead of water, FGXZ be fuppofed i�e glafs, every thing elfe remaining as before,nbsp;the difference of refult will be, that the rationbsp;fine of refradion to that of incidence is (notnbsp;�t vv'as in the cafe of water, viz. 3 to 4, but;)nbsp;�'^arly as 2 to 3 5 viz. the angles of refradionnbsp;''^dl be refpedively fmaller when FGXZ is glafs,nbsp;when it is water. And if, inftead of water ornbsp;F G X Z were a diamond, then the angle ofnbsp;quot;quot;fradion would be fmaller Hill; viz. the fine of thenbsp;in the diamond would be to the fine of thenbsp;^^'�le in air, nearly as a to 5.

confiderable variety of tranfparent fubftances been thus examined with refped to their re-^ tgt;ve properties. Their peculiar refradive powersnbsp;''dl be dated in the fequel.

In the above-mentioned example; the light inftancCj of the fun) which pafhng through die tub^nbsp;Bj falls upon the water at C, is not only bent, bi'^nbsp;alfo enlarged in a fedoral manner, and its enlargement-is in -the plain CI EB. It is alfo remarkablenbsp;that the refra�led, and e'nlarged or difperfed ligb'��nbsp;is not of one uniform colour, but appears tinge�snbsp;with the colours of the rain-bow.

In fig. 2. Plate XX. which is intended to illuf' trate this v/onderful property, IC is fuppofednbsp;be a fmall beam of folar light, which paffes throughnbsp;the air, and enters a refrafting medium atnbsp;Through that m,edium the beam of light will b^nbsp;Ipread in the fe�loral fhape, vCr, which is calk^nbsp;the angle of dijperfim, or diffipation, and which is ilt;^'nbsp;felf divided into fmalicr fedtors or different colour��nbsp;viz. next to the upper fine Cr, the light appealnbsp;red, and thence it gradually degenerates into orang^�nbsp;yellow, green, blue, indigo, and lafrly,' viole�^�nbsp;which is nearefc to the lower line or boundai'ynbsp;ex�. . �

Now a line C ?;z, through the middle of the angl^ vCr, is the m'e.an diredlion of the refracted ligb*-�nbsp;and m e is its fine, or the fine of the mean anglenbsp;refradtion ; w'hereas vftLVi^rd are the fines ofnbsp;extrem.es, of which vf is called the fine of the .nbsp;refrangible colour, and r d the fme of the leaf refrang}^^^nbsp;colour.

This feparation of the white or colourlefs I'o^*' into various colours. Induced Sir Ifaac Newto'^

conclude, that white light'confifts, or is' a mlx-^^rc, of different coloured rays, which being differently refrangible, are of courfe, feparated by the *�cfra6ling medium. We fnall prefently treat of thenbsp;dumber and other properties of thofe colours. Butnbsp;iTiufl; for the prefent be remarked, that throughnbsp;��^e fame medium, the. angle of diffipation is always-P��Oportionate to the mean angle of refradtion, andnbsp;Courfe when the mean angle of refradlion is verynbsp;f'^allj thgfj rhe angle of diffipation mull be muchnbsp;f'^aller, in which cafe the different colours cannotnbsp;diftinguifhed : but when the angle of incidence,nbsp;confequently the mean angle of refradtion, isnbsp;^ctifiderably larger, then the angle of diffipationnbsp;alfo be fo large, as to exhibit the different co-'o�rs.

different refradtive mediums have different quot;Perfive powers; for inftance, the angle of inci-cnce I C H remaining the fame, not only thenbsp;angle of refradlion mCE, will vary accordingnbsp;refraftive medium ABDG is water, or glafs,nbsp;diamond, amp;c. but the angle of diffipation -z; C rnbsp;^dl alfo vary. And in fbme refradling mediumsnbsp;�^ean angle of refradtion is larger, whilft thenbsp;. cf diffipation is fmaller j and in other refradt-0 rnediums the mean angle of refradtion is fmaller,nbsp;^ the angle of diffipation is larger* In fliort,nbsp;knowledge of the mean refradtive power of anbsp;�t'en lubftance will not enable us to determine itsnbsp;Pctfive power, and vice verja.

�74 Dioptrics, or of Refra5fsd Light,

Heat -or an increafe of temperature generally if' creafes, but not much, the refraftive power of traO'nbsp;Iparent bodies, efpecially of fluids.

The following table contains the nrean refraftiof from air into the following mediums. The firft column contains the fubftances; the fecond cxprelfe*nbsp;the fine of the angle of incidence, that of refra�tiofnbsp;being reckoned one or unity; and the third colurnfnbsp;expreflfes the difperfive powers in proportional nutr�'nbsp;bers, that of water being reckoned ico. Thf*nbsp;the fine of incidence is' to that of refraftion froifnbsp;air into flint-glafs, as 1,5998 to i, or as 1,6 tonbsp;And the difperfive power of the fame glafs isnbsp;that of water, as 227 to 100 *.

* The various articles of this table have been from the experiments of Nev/ton, Euler, Zeiher, Haukf�^^�nbsp;Martin, Rochon, and others. A vaft number of othernbsp;fiances might have been added, fuch as folutions ofnbsp;decoffions or infufions of woods, amp;c. but thefe havenbsp;omitted principally on account of their indefinite andnbsp;tuating quality. See Diflilled Finegar in the Table. ^nbsp;An idea of the real quantity of the difperfivequot; powernbsp;flint-glafs may be derived from the following particularsnbsp;The fine of the angle of incidence is to the fine of the arignbsp;of refradfion of the lead refrangible or red rays from airnbsp;flint-glais, as 1,5889 to i; and the fine of the angle ofnbsp;dence to that of refradtion, of the moft refrangible ornbsp;rays, as 1,6107 to I. Of this more hereafter.

Dioptrics, or of Refrailed Light, nbsp;nbsp;nbsp;177

^pon the whole it appears, that the denfer bo-viz. thofe of greater fpecific gravity, refradl bend the light more than thofe which are lelsnbsp;excepting (as Sir Ifaac Newton exprelTes itf)nbsp;unftuous and fulphureous bodies refradl morenbsp;others of the fame denfity.

There is no fubftance that has an intermediate power betw'een air and rain, or diftilled

This refrai�ion of the air is determined by that of �^'Tiofphere obferved by aftronomers. For if light pafs

denfer and denfer, the fum of all the refradtions will be to the fino-Ie refradfion v/hich it would have fufFered in

�'J�niediatcly out of the firfl: medium into the laft. thenbsp;nbsp;nbsp;nbsp;the whole refraffion of light, in palling through

heffer in palling, at the fame obliquity, out of a va� ��tirnediateiy into air of equal denlity with that whichnbsp;� Cowell part of the atmofphere.� Newton�s Optics^nbsp;Fart III. Prop. 10.

water. The refraftive property of the diamond greater than that of any other known fubftance¹'nbsp;Spirituous liquors have a greater refraftive pow^'¹nbsp;in proportion to their ftrength. Spirit of turpentinenbsp;is the moft refradfive of the fluids.

It is now neceflary to examine the coloured light itfelf, viz. the various colours into which whitenbsp;light (like the folar light, candle light, amp;c.) is divided by refraclion. And fince this diviflon, ofnbsp;the angle of difperfion in a given medium is proportionate to the angle of incidence j therefore, i'�nbsp;order to examine with more accuracy the differentnbsp;colours, amp;c. it will be neceflary to let the lightnbsp;the fun enter through an hole in a dark room, andnbsp;to let it fall upon a refracting medium at a grealt;^nbsp;angle of incidence. For this purpofe, glafs prifm¹nbsp;have been found to be the moft ufeful.

This property induced Sir Ifaac Newton to conje¹^' ture that the diamond is a fubftance of an unduous qualit/�nbsp;like oils, refins, amp;c. meaning of a combuftible qualit)�nbsp;which was fome years after adually verified by experiment^'nbsp;Optics, B. II. Part III. Prop. lO. But the fame con¹nbsp;jedure, deduced from other principles, is mentioned ^1nbsp;Boetius, previous to Newton. He obferves, that watet/nbsp;fubftances will adhere to other watery fubftances, but notnbsp;oleaginous bodies, and that oleaginous bodies will adherenbsp;other oleaginous bodies; then adds, � Qiiod itaquenbsp;� quse ignese natars eft, adamant! facile-jungi poffit, fign^��'nbsp;� eft id propter materise fimilitudinem fieri, ac adama

3* Plate XX. reprefents a triangular glafs P'^lrn, viz. a lump of glafs having two trian-S*^lar and parallel bafes ABC, DFE, and threenbsp;parallelepipedal fides*. AD, CE, BE, arenbsp;angles of th� prifm. A line which paffesnbsp;^Itro�gh the centres of the bafes is called the axis,nbsp;a beam of light paffes through the prifm, bynbsp;�^'^tering at one of its parallelepipedal fides andnbsp;going out at another, then the angle formed bynbsp;*'^ofenbsp;nbsp;nbsp;nbsp;is called the refraSiing angle of tht

fig. 4. Plate XX. reprefents part of the |ltutter of the window of a room, wherein nonbsp;�nt enters, excepting what comes through thenbsp;' ^1- C. If this light, fuppofing it to be the lightnbsp;fun, be received upon a fcreen at any diftancenbsp;the hole, as at F, an image of the fun, or�anbsp;^''quot;cellar luminous fpot, will be formed upon thenbsp;which is larger in diameter than the holenbsp;rrums are alio made hollow, (viz. the Tides are formednbsp;. plates of glafs) and are filled with different fluids,nbsp;^^oider to determine the refraddive power of thofe fluids,nbsp;� flat plates are often made to move between two folidnbsp;^ oales; by which means the refrafting angle of the

Briflns in general are frequently furhifhed with metal P� 3nd pins at their bafes, as at A B, Fig. 6. 1'late XX. by

at C, and that principally on account of the diamet^^ of the fun; for the rays of light which depart frof**nbsp;the various points of the fun�s furface, and pa^*nbsp;through the holcjOiuft crofs each other at that place�nbsp;and mufl proceed divergingly, or in a conical foriAnbsp;within the room.

Place a glal's prifm DOE before the hole, that the light may pafs through it in a dirediofnbsp;perpendicular to the axis of the prifm; and infteadnbsp;of going ftraight from E to P', the light whichnbsp;comes through the hole will, by paffing through thenbsp;prifm, be bent and difperfed in fuch a manner as tonbsp;form a coloured fpe�lrum G H upon a fcreen, whichnbsp;may be fituated at any dihance from the prifm, butnbsp;below the ftraight diredlion C F. The angle F E ^nbsp;made by the ftraight direftion, and the mean di'nbsp;reftion of the refrafled light, is called the angle 4nbsp;deviation.

The fpedtrum G H (moft beautiful to the eye) is about five times as long as its breadth, and i*nbsp;terminated by femicircular ends. The highe^^nbsp;part G is of a beautiful red colour, which, bfnbsp;infenfible fliades, degenerates into an orange, theOnbsp;, a yellow, a green, a blue, an indigo, and a violet�nbsp;which is the colour next to PI, viz. at the loweftnbsp;part of the fpe�trum.

From thofe denominations it appears that the colours of the above-mentioned fpeftrum are feven;nbsp;but an unprejudiced fpedtator will find it difficultnbsp;determine their number. Sir Ifaac Newton reckone�i

lorne of the feven*. Nollet thought there was ^^^afon to conclude that the orange, the green, andnbsp;indigo, are the three fimple, or homogeneousnbsp;^^iours -j-. Some perfons have acknowledged fivenbsp;i^*^^fiiitive colours. Others, obferving that all fortsnbsp;dolours may apparently be formed by mixturesnbsp;yello w, and blue, in due proportions, havenbsp;^�^itted thofe only as primitive, homogeneous, ornbsp;'^��^otnpofed colours. However, certain fads andnbsp;lervations, which will be mentioned in the fequel,nbsp;'^ery much in favour of Newton�s theory.nbsp;Various methods have been tried for the purpofenbsp;��^ndering the colours of the fpedlrum diftinft or

a fcreen at a little diftance (for inftance fix within the room, let the rniddlemoft partnbsp;'�i^at ligiit pafs through a fimilar hole in thenbsp;^^0; the objeft of which is to prevent in greatnbsp;'^^0 the indefinite light or penumbra on the

* his Optics, he,^. de Phyjtque^ tome V. p, 388. See alfoD�AIeiji-Mathem. tome iH. p. 3^3. Rochon�s Recher.nbsp;^ Rature 4e la Lumlere des Etoiles fixes.

N 3 nbsp;nbsp;nbsp;fidesi

fldes of the fpedrinr. Let that light fall perpendi' cularly upon a convex lens, at the diftance of aboH''nbsp;3 0 feet, by which means an image well deflne�^nbsp;of the fun wiU be formed upon a fereen placed at ¹nbsp;proper diftance from the lens: but if a prifm b^nbsp;placed clofe to the lens, fo that the light, after havin�nbsp;paired through the lens, may pafs through, and b^nbsp;refrafted by, the prifm j then a coloured fpedruf^nbsp;will be formed upon the fereen, fig. i2. Plate XXgt;

The long fides of this fpedtrum are very well d^' fined. Its narrow terminations are femicircul^i'�nbsp;and its whole length confifts of circular colourednbsp;images of the fun, which are intermixed with eacbnbsp;other, cfpecially about the middle or axis of th^nbsp;fpeclrum ; yet the moft predominant coloursnbsp;more diflinguilhable from each other, efpecialiynbsp;towards the fides of the fpeflrum, fo that the'¹'nbsp;boundaries may be marked with tolerablenbsp;curacy ¹.

Fig. 7. Plate XX, reprefents fuch a fpedtrub'� and the lines F M, ha, dc, fe, bg, Sic. are dra�'¹'¹'nbsp;through the centres of the principal circles beloflo'nbsp;ing to the feven principal colours. The fpace^gt;nbsp;wlfich thofe feveral colours occupy, are not

For this purpofe th� prifm and the lens mull be formed, and as free from veins, bubbles, fcratches, �re-poffiblc. Every other part of the operation muil abbnbsp;coiiduamp;d with great accuracy, excluding every othernbsp;from the room, amp;c.

^the length of the fpeftrum, from I to M, be di-'^'ded into 360 equal parts, then the red colour will found to occupy the fpace M.'P ba, the length ofnbsp;'^hich, M a, is equal to 45 of thofe parts; the lengthnbsp;� of the orange, �bdc, will be found equal to 27nbsp;thofe parts; the length f ^ of the yellow equalnbsp;48 ; that of the green to 60, that of the bluenbsp;that of the indigo to 40, and, laftly, thenbsp;^^ogth 71 of the violet, lmn\, equal to 80 ofnbsp;^^ofe parts.

It is very remarkable that by thofe divlfions, the line I M is divided very nearlynbsp;a mufical chord. Let the fide IM of the fpec-he produced, fo as to make M X equal tonbsp;. then /X will be found to be S-qths of IX ;

^ will be found to be 5-6ths of I X ; ^ X to be ? 4Chs, eX to be 2-3ds, rX to be 3-5ths, 0X tonbsp;^ 9-16ths, and MX to be one half of I X : fonbsp;^ ^t if I ^ were a mufical firing, like a violinnbsp;and expreffed a certain mufical tone, for in-ance C, then the length 7 X would exprefs D, ornbsp;^ focond ; i X would exprefs E flat, or flat third ;nbsp;^ ''^ould exprefs F, or fourth; would exprefsnbsp;Or fifth, f Xi would exprefs A, or fixth lharp ;nbsp;^ '''Quid exprefs B flat, or flat leventh; and MXnbsp;^^prefs C, or the oflave. But it muft benbsp;^��ked, that the divifions of the colours of thenbsp;$nd'^ ^ cannot be obtained with great accuracy gt;nbsp;if they could always be obtamed precifelynbsp;^be above, which are exaftly as were originally

� 84 Dioptrics, or of RefraS�ed Light.

given by Sir Ifaac Newton *; yet the arrangernet*^ of the mufical notes, correfpondent with thofe di'nbsp;vifions, is by no means regular j a flat third with ^nbsp;fharp fixth and a flat feventh being inadmiffible in annbsp;odlave of mufica:! notes.

It is evident that white light confifls of coloured rays, which have different but peculiar refrangibih'nbsp;ties; the red being the leaft, and the violet the tnoftnbsp;refrangible f. The following experiments will iUn'nbsp;ftrate and confirm this theory.

It has been laid above, that the fine of incidence is the fine of refraffion from air into glafs, nearly as 3 to 2 gt;nbsp;therefore by inverting the analogy, the fine of incidence isnbsp;the fine of refradion from glafs into air, as 2 to 3. Now,nbsp;the prifm in fig. 4. be turned round its axis, fo that thfnbsp;beam of light C O may fall perpendicularly upon the 1quot;�^�nbsp;P O, then that beam will not fuffer any refradion from ^nbsp;to E; but as it fails obliquely upon the fide DE ofnbsp;prifm, it will, on going out from the glafs into the air atnbsp;fuffer a refradion. Now when the fine of the angle of

cidence of the beam COE, upon the fide D E, was eq^^ to $0, Sir Ifaac Newton found that the fine of refradi^*^nbsp;for the red rays, or extreme part E G of the fpedruif�nbsp;was 77, and the fine of refradion for the violet orextren�^nbsp;part � H of the fpedrum, was 78. Therefore, div'ilt;ldSnbsp;the difference between 77 and 78, in the lame proportionnbsp;the fpedrum AGFiM, fig. 7. is divided, he obtainednbsp;following fines fcf the boundaries of the feven different nn'nbsp;lours, viz. 77, 77 i, 77nbsp;nbsp;nbsp;nbsp;77f, 77nbsp;nbsp;nbsp;nbsp;771, 77^, 7^�

that is, the fines of the red rays are between 77 and 77 thofe of the orange rays are betv/een 77 I, and 77

After having received a beam of light upon a Pdfm SVT, Fig. 5. Plate XX. place, at fomenbsp;^�ftance from the prifm, two fereens or boards, PQ^nbsp;P each perforated with a fmall hole X, x j andnbsp;^^yond the fereen pq place a fecond prifin svt, iiinbsp;fituation indicated by the figure. The refraftednbsp;^%ht will form the ufual fpedtrum upon the fereennbsp;Now, if by turning the prifm SVT gentlynbsp;its axis, you let the rays of the different co-^^Urs pafs fucceffively through the two holes 'S.,x,nbsp;through the prifm svt, you will perceive anbsp;^��cular image of the fun upon the wall or fereennbsp;changing colour according to the ray whichnbsp;Produ ces it, and likewife changing place; viz. whennbsp;image is red, its place will be, for inftance, Z; butnbsp;Hen yellow, its place will be higher than Z ; whennbsp;its place will be higher ftill, and fo on; fornbsp;yellow rays are more refrangible, viz. are bentnbsp;^ore by the prifm than the red, the green morenbsp;*lian the yellow, amp;c.

If the light which has been refradled and difperfed ^ prifm, be received again upon another prifmnbsp;F5g_ which miift be fituated in a diredionnbsp;P^^'pendicular to that of the former; the fpedrum

then the fecond prifm AB ought to expand the fpe�trum in breadth, fo as to form the quadrilateralnbsp;broad figure Z Y � ; but inftead of that we findnbsp;that the colours and their breadths remain unaltered ; the fpedlrum has only been removed fromnbsp;the original fituation MN, by the refraftive powernbsp;of the prifm, and the violet rays have been removednbsp;nroft, viz. from M to Z, becaufe they are moftnbsp;refrangible, the red rays have been Removed leaftgt;nbsp;viz. from N to Y, becaufe they are leaft refrangible, and the other colours come in order betweenquot;nbsp;thofe extremes.

If the refrafted and difperfed beam of folar light be received upon a concave refledfor CD, Fig. 8.nbsp;Plate XX. the differently coloured rays will henbsp;reflefted to a focus A, where they will form a whitenbsp;or colouriefs image of the fun j but if any of thenbsp;colours be flopped ,by interpofing a wire or fbmenbsp;other opaque and flender body between the prifmnbsp;and the refleftor, as at B, then the image A willnbsp;become coloured with fome mixt colour. Thisnbsp;proves that white light confifts of coloured rays intermixed in a certain proportion ; and that by ^nbsp;mixture of the rays of the feven primary coloursnbsp;in that due proportion, white light is produced.nbsp;Therefore white arifes from a certain mixture ofnbsp;colours, and blacknefs arifes from a ftoppage ornbsp;abforption of all colours. Beyond the focus A, thenbsp;rays are feparated again, and the image' js co�nbsp;loured.

Ifgt; when a fpe^lrum is formed by the light which paired through a prifm upon a fcreen, a fmalinbsp;^ole be made through the fcreen, and the rays ofnbsp;colour only be permitted to pafs through it onnbsp;other fide of the fcreen; then v.�hatever is viewednbsp;in that homogeneous light will appear of that par-ticular colour. Thus, if the red light only hasnbsp;P^lamp;d through the hole, then blood, or grafs, ornbsp;amp;c. viewed in that light behind the fcreen,nbsp;'^ill all appear red, excepting that the blood willnbsp;appear a ftronger red chan the grafs and the milk. Ifnbsp;blue light only has been tranfmicted throughnbsp;hole, then the above-mentioned three fub-ftances will all appear blue j and the like thingnbsp;be underftood with refpefl; to the other co-

If this homogeneal light behind the fcreen be re-^eived upon another prifm, it will be refrafted, viz.

but not difperfed by it, fo that it will form ^ circular fpot of one uniform colour upon thenbsp;^^reen.

If two holes at about a foot diftance from each �ther be made in the fhutter of a dark room, andnbsp;prifins, viz. one be placed to receive the lightnbsp;^ach of chofe holes, two fpciSrums will therebynbsp;formed upon the fcreen ; and by turning thenbsp;Prifnas gently round their axes, the fpeflrtirns maybenbsp;^^ufed to fall one upon the other. Let the yellownbsp;^f One fpeflrum fall upon the blue of the other,nbsp;at that place the mixture of thofe two colours

will produce a green. Let a fmall hole be made exadtiy at that place, and that green colour will pafsnbsp;through it behind the fcreen, and will form a o-reeitnbsp;circular image upon another fcrcen placed to receive it. Now, if exactly behind the perforationnbsp;of the firft fcreen you fix the refrafting angle of anbsp;pvlfm, then the image upon the fecond fcreen willnbsp;not only be movied from its place, but will appearnbsp;oblong with a yellow border on one extremity, andnbsp;a blue border on the other extremity j becaufe thatnbsp;fpot or image confifts of two primitive colours ofnbsp;different refrangibilities. The fame thing mufl benbsp;underftood of any other colour f ormed from a mixture of two primitive prifmatic colours j for anynbsp;two of thofe colours will form, or rather look like,nbsp;an intermediate colour; thus red and yellow formnbsp;an orange, blue and violet form an indigo, amp;c.

If the fame experiment be performed with one fblar fpedlrum, viz. a fingle prifmatic colour benbsp;permitted to pafs through a hole in the fcreen, andnbsp;then be received upon another fcreen, the imagenbsp;will be of the fame colour, for inftance, green, andnbsp;circular. Now, by placing a prifm behind thenbsp;perforation of the firft fcreen, the green image willnbsp;be moved from its place, bur will not be elongated,nbsp;nor altered in colour, becaufe that image conftfts otnbsp;one uniform primitive colour *.

This fhews, that though green may be formed two colours, or any other prifmatic colour maynbsp;formed from two other colours, yet each of thofcnbsp;colours in the prifmatic fpeftrum is a primitive,nbsp;^i^iform or homogeneous colour.

When a perfon looks at any objeft through a P'ifm, that objeft, tfpecially if it be a white one,nbsp;and well illuminated, will appear bordered withnbsp;Colours at top and bottom ; the reafon of which is,nbsp;that the colours of the light which comes from thenbsp;are refrafled by the prifm, and more or leisnbsp;^'^tording to their different refrangibilities: hencenbsp;Only the whole image will appear in a place dif-^^'tent from the real diredlion j but the blue indigonbsp;3td violet colours will be removed more than tlicnbsp;orange, amp;c. Thus, if an eye O, Fig. 9.

XX. looks through the prifm P, at a piece -vvhife paper A B, that paper, which would ap-white, and at its real place without the prifm,nbsp;when the prifm is interpofed, appear elevatednbsp;the place CE, alfo elongated, and terminatednbsp;. ^ Coloured fringes at top and bottom; the bluenbsp;and violet being at top, and the red, orange,nbsp;yellow at the bottom j for, in truth, the prifm,nbsp;fjnbsp;nbsp;nbsp;nbsp;the different colours differently, forms

higheft, the indigo next, then the blue, allnbsp;nbsp;nbsp;nbsp;yellovy, the orange, and lowefl of

thofe images are intermixed towards tke middle ghim, where of courle the paper appears white; btiCnbsp;they begin to be lefs mixed towards im and ghjnbsp;where of courfe the colours begin to appear, amp;c.

It is necelTary to mention in this place an obfet', vation concerning the reflection of light, whichnbsp;could not have been well explained previous to thenbsp;theory of the different refrangibilities of colourednbsp;rays; for this purpofe we mult alfo premife a ufefulnbsp;practical method of tracing a ray of light throughnbsp;a prifm, or in general through any refracting medium.

l,et H IK, Fig. to. Plate XX. reprefent � glafs prifm, whofe angle at H is equal to 60% andnbsp;A B a ray of light, which coming through a narrov'^nbsp;tube A, falls upon the fide of the prifm at B. Dra'^^

1 .BG perpendicular to the furface of the prifm B, then ABL will be the angle of incidence, whichnbsp;we lliall fuppofe equal to 38�. Find in the trigonometrical tables, the fine of 38'�, which is 61566�nbsp;then, becaufe the fine of incidence is to that'ofnbsp;fradion from air into glafs, as 3 to 2.¹, fay, as 3

In tliis place wc have adopted the ratio of 3 to 2, ,the fake of avoiding fra�tior.�; but it is evident that anfnbsp;other ratio may be iifed ; and in pradice the ratio of the flt;n�nbsp;of incidence to that of refradion for any particular l�bflanc�nbsp;muft be taken from the table in page 176; and whennbsp;curacy is required, the ratio for any particular colourednbsp;ot light may be deduced from the difperlive property otnbsp;fubftance in queftion, which is obtained from the fame tabl^�nbsp;and from what has been faid in page 184.

into the thinner medium, but wi l be reflected froni the furface of the denfer medium. And the moftnbsp;refrangible rays, viz. the violet, indigo, amp;c. Vv'ili benbsp;found to be the mofl; reflexible, and vice verfa.-^nbsp;The following illuflration will fnew the meaningnbsp;and the reafon of the alTertion.

Let a beam of light, coming from an hole at hi Fig. II. Plate XX. fall perpendicularly, or nearlynbsp;fo, upon the fide E D of a glafs prifm, in which cafenbsp;the beam of light will not be refradted, but willnbsp;proceed ftraight into the glafs, and will fall uponnbsp;the furface or boundary GD, between the glafsnbsp;prifm and the air. Now if the angle of inclinationnbsp;B C D be fmaller than about 49�, or, which comesnbsp;to the fame thing, if the angle of incidence O C Bnbsp;be greater than about 41�, then the beam of lightnbsp;ABC, inftead of pafling out of the prifm at C intonbsp;tiie air, will be reflected towards C F. Suppoff�nbsp;for, inftance, that the angle of incidence B C O 1*nbsp;56quot;, its fine will be 82920; then fay, as 2 is tonbsp;fo is 82920 to a fourth proportional, viz. tonbsp;124380, which exceeds the radius or fine of ^nbsp;rilt;^ht angle j therefore the angle of refradtion PC0nbsp;muft be greater than a right angle, viz. the beam olnbsp;light cannot come out of the furface G D, but moftnbsp;be refledted towards F, and that is the cafe when-ever the fine of refraction exceeds the radius.

By turning the prilm gently round its axis, will be found that whilft the angle B C O isnbsp;dian 41% all the light will pafs out of the

C ; but by increafing that angle ftill farther, you '''ill find that the violet rays will begin to be refleftednbsp;firft, whilft the others pafs through, then the indigonbsp;i'^ys will be reflefted, amp;c. and laft of all the rednbsp;i'ays will be reflefted ; becaufe the ratio of the finenbsp;incidence to that of refradlion is greateft for thenbsp;''^�olet rays, and leaft for the red rays.

The different refraftive, and different dlfperfive powers of the various tranfparent mediums, whichnbsp;firft fight might be confidered as an imperfedtionnbsp;great obftrudlion to the improvement of pradlr-^^1 optics, have, on the contrary, been the meansnbsp;improving certain optical inflruments to a verynbsp;'^iinfiderable degree. The immediate applicationnbsp;the principles to the conftrudfion of thofe in-^��nients will be explained in a fubfequent chap-but we fhall in the prelent chapter en-^eavour to explain the principles upon which it

'depends.

The principal ufe of rnofl of the optical inftru-�^^nts is to render objefts more perceptible to our than they are without their affiflance; andnbsp;end is in general obtained by bending (viz.nbsp;the rays of light, fo that a greaternbsp;5'^antity of thofe, which iffue from any given ob-may enter our eyes, and may alfo form anbsp;��ger vifual angle. But by the bending, or thenbsp;light is feparated into colourad rays;nbsp;erefore the objefts which are viewed by any re-light, mull appear coloured irregularly andnbsp;III.nbsp;nbsp;nbsp;nbsp;onbsp;nbsp;nbsp;nbsp;differently

diiFerently from what they really are. And thisj ftricElly fpeaking, is aftually the cafe; yet when thenbsp;light is not much refrafted, the difperfion or fep^'nbsp;ration of colours is fo trifling, that the eye take�nbsp;no notice of it, or fuffers it without inconvenience�nbsp;But when the light is much refrafted, then thenbsp;difperfion of colours becomes hurtful and uii'nbsp;pleafant.

Now a method has been contrived (from the various refradtive and difperfive powers of tranfp^' rent mediums, and gt; efpecially of crown glafs anlt;inbsp;Hint glafs) of preventing the difperfion at -the lamenbsp;time that the rays are bent or refrafted. The fol'nbsp;lowing paragraphs will fhew in what manner thisnbsp;effedl can be produced by a combination of I'C'nbsp;fradting mediums.

The reader is requefted to recolledf, ift, tha^^ different tranfparent mediums have � different rCquot;nbsp;fradlive powers; adly, that they have different diknbsp;perfivc powers; and 3dly, that in the fame m^'nbsp;dium the angle of difperfion becomes largernbsp;fmaller, according as the angle of refradlion isnbsp;creafed or diminilhed.

Let ABC, fig. 13. Plate xx. be a pri^gt;^ of crown glafs, and D E a beam of light fallif^nbsp;upon it, which, by paffing through the priftOjnbsp;dilperfed into the coloured pencil F G I. ^nbsp;another prifm, in every refpedt equal to th�nbsp;former, were placed clofe to it, but in a contra''^nbsp;pofition,, fuch as is indicated by the dotted reprefcquot;�'

tation B K C j then this fecond prifm would undd 'vhat has been done by the firft, viz. the rays fepa-^ated and bent by the firll, would be colledled andnbsp;bent the other way by the fecond ; fo that the beamnbsp;of light would emerge out of the fecond prifm innbsp;^ direftion parallel to DEj and without being al-*^ered in colour. But if the fecond prifm have anbsp;different refraftive and a different difperfive power,nbsp;^hen, notwithftanding the equality of the figure dfnbsp;the prifms, the beam of ligltt, after having paffednbsp;through both, would emerge both bent and coloured, becaufe the fecond prifm cannot exadllynbsp;oounteradt the effedl of the firfl.

Now, by altering the refradlive angle of the fecond prifm, viz, by tnaking it larger or fmaller,nbsp;the angle of refradion, and of courfe the angle ofnbsp;difperfion,' may be increafed or diminifhed. If thenbsp;Soantity of difperfion in the fecond prifin be rendered equal to the angle of difperfion in the' firftnbsp;Brifm, then the ray of light will emerge withoutnbsp;alteration of colour, but its dircftion will benbsp;tuclined to its original diredion D E, by as muchnbsp;the refradion of one prifm exceeds that of thenbsp;other. This is called the achromatic refraction ¹.

Hence we have the achromatic�telefcope,y\%. atelelcope quot;'hich does not alter the natural colours of the obje�s thatnbsp;feen through it; �whereas other telefcopes with glalTesnbsp;E^nerally introduce the prifmatic colours, efpecially aboutnbsp;the edge of the field of view.

196 Dioptrics, or of Refracted Light.

On the contrary, if the refraftion of one prifitt be rendered equal to that of the other prifm, thennbsp;the beam of light will emerge with its mean direction parallel to DE, but it will be coloured, ornbsp;feparated, by as much as the difperfive power ofnbsp;one prifm exceeds that of the other prifm.

The attentive reader may eafily connprehend, that either of the above-mentioned effeds may alfonbsp;be produced by a combination of three or morenbsp;prifms *.

Having explained the principal properties of the regular reffading mediums, it might perhaps benbsp;expeded that an account of thofe fubftances whichnbsp;have a double, or multiple refrading power�

* A beam of folar light, refraded by paffing through a prifm of crown glafs, the refrading angle of which is go��nbsp;when the ray of mean refrangibility paffes, or enters andnbsp;emerges, at an equal diftance froVn the vertex of the prifm,nbsp;the angle of dillipation will be about 39'.

If a prifm of flint glafs, whofe refrading angle is 23*5 40', be adapted in an inverted pofltion, as in fig. I4�nbsp;Plate XX. to a prifm of crown glafs, whofe refradingnbsp;angle is 25�; a beam of folar light will emerge at A, withnbsp;its mean diredion parallel to DE, viz. it will pafs ftraight�nbsp;but it will be coloured or difperfed.

But if to thofe you add a third prifm of crown glafs, ths refrading angle of which is 10% as in fig. 15. Plate xJt-then the emergent beam of folar light will deviate by aboutnbsp;5�gt; 37 s ftom the courfe of its incident part DE, but it vvi^nbsp;not be altered in colour, viz. it will be white, as it W�*

. nbsp;nbsp;nbsp;amoo�

among which the Ifland cryftal is the moft diftin-guiflied, would be fubjoined: but the equivocal na-bire of thofe fubftances, the unknown caufe of their ^ffedts, and the little ufe which is made of them,nbsp;iiave induced me to employ the following pages fornbsp;niore ufeful purpofes, and to refer the inquifitivenbsp;deader to the works of other authors *.

With refpedl to the prifmatic colours feparately '^onfidered, it may be obvioufly obferved, that fomenbsp;them affedl our eyes more powerfully thannbsp;others ;' or in other words, that objeds in generalnbsp;Riay be feen much better in fome of them than innbsp;others; yet the precife order with refped to theirnbsp;Peculiar illuminating power, cannot be determinednbsp;Without a conliderable number of accurate obferva-tions.

For this purpofe a prifmatic colour mull be fe-P^rated from the folar fpedrum, viz. by permitting 't to pals through a hole in the fcreen, upon whichnbsp;^^e Ipedrum is projeded by the prifm, and objedtsnbsp;be viewed in that homogeneous light behindnbsp;the fcreen, then the fame objeds mull be viewednbsp;�'t another homogeneous light, and fo on.

F)r. Herfchel, who, as farasi know, has made the recent experiments upon this fubjed, afternbsp;. ^ account of thofe experiments, exprelfes himfelfnbsp;the followin

From thefe obfervations, which agree uncom� monly well, with refpe�t to the illuminatingnbsp;power affigned to each colour, we may conclude,nbsp;that the red-making rays are very far from havingnbsp;it in any eminent degree. The orange poflefsnbsp;� more of it than the red j and the yellow rays il-luminate objefts ftill more perfeftly. The maxi-mum of illumination lies in the brighteft yellow,nbsp;� or paleft green. The green itfelf is nearlynbsp;� equally bright with the yellow; but, from thenbsp;� full deep green, the illuminating power decreafesnbsp;� very fenfibly. That of the blue is nearly uponnbsp;� a par with that of the red �, the indigo has muchnbsp;� lefs than the blue ; and the violet is very de-� ficient¹.�

I {hall conclude this Chapter on Refraflion, by obferving, that befides the method in the precedingnbsp;page, viz. by the prifms fimply ufed, the refraftivenbsp;powers of tranfparent m.ediums may alfo be deter-mined by other means, as by meafuring the focalnbsp;diftances of lenfes when they tranfmit the rays ofnbsp;different colours fuccelTively ; by employing othernbsp;inftruments in eonjundlion with the prifms, amp;c.-j-

Philofophical Tranfaftions for 1800, page 267. f See the deferiptions of thofe methods in Prieftley�snbsp;Hiftory of Villon, amp;c. Period V. Se�l. VIII. Chap. II¹nbsp;Martin�s Optics ; Rochon�r Rscuellde Mem. fur la Mecafnnbsp;et la Phyftque; Mem. fur la Mefure de la Dlfprfion, et denbsp;(a RefraSiian, amp;c.

[ *99 1

The inflection of light, the colours of THin transparent bodies, and of coloursnbsp;In general.

Light, in pairing within a certain dlftance of the furface of bodies, is bent fo as to formnbsp;apparently a reftilinear angle at that place. Thusnbsp;a fmall hole be made in the Ihuttef of a windownbsp;a darkened room, and the light of the fun benbsp;Permitted to pafs through it, the image of,the fun,nbsp;white fpot which is formed upon a fcreen placednbsp;receive that light in the room, will be found tonbsp;larger than it ought to be if right-lined raysnbsp;Proceeded from the various points of the fun�s fur-and paffed through the hole to the fcreen;nbsp;it appears that they are bent at the hole ; fornbsp;^'�^^rwife the image would be fmaller than experi-Ihews it to be.

a folid opaque body, fuch as a hair, a flender ^*re, l3g placed in the ftream of light withinnbsp;^ ^ roorm, the fize of the lhadow of that body willnbsp;^ found different from what it ought to be if thenbsp;of light were not bent in pafling by it. This

bending of the rays, of light by paffing not through� but near the furface of a body, is called thenbsp;tion of light. It has alfo been called diffra�iion.

The phenomena, which relate to this fubjeifhj not appearing to be reducible to one general priU'nbsp;ciple, were particularly examined under a con-liderable variety of circumftances by Sir IfaaCnbsp;Newton j yer his obfervations were not quite cot'nbsp;reft i nor was his hypothetical explanation verynbsp;plaufible.

Subfequent experiments and obfervations feeU^ to reduce the phenomena of infleftion to a fingl^nbsp;principle, namely, to the attraftion of bodies towards light, which attraftion becomes confpicuousnbsp;when the rays of light pafs within a certain diftaricenbsp;of their furfaces. Befides their being bent, the raysnbsp;of light are likewife feparated into colours by thenbsp;vicinity of bodies, and this produces the fingulafnbsp;phenomenon of the coloured fringes that accompany the infleftions. But previous to the application of the hypothefis, it will be proper to defcribcnbsp;the principal phenomena of the infleftion of light�nbsp;and for this purpofe I fliall prefer the experimentsnbsp;of a recent anonimous writer, which appear tonbsp;have been inftituted with judgment and accuracy *.

� In fig. 16. Plate xx. let � X be the hole (about the 50th part of an inch) of the light�snbsp;paflage into a darkened room, and let X A, X B,nbsp;lines drawn from each external oppofite edge onnbsp;fide of the folar dife. to each external oppofitenbsp;on the contrary fide of the hole, croffing onenbsp;another; X C D will reprefent the beam of lightnbsp;^^�^er its paflage through the hole^ at all diftancesnbsp;^herefrom, confiderably larger than the penumbralnbsp;�one E A B.

� At feven feet from the hole the breadth of tlw; ^�am was patts of an inch. If the light hadnbsp;�ot been bent, that breadth could not have exceed-4-6-. Hence it muft be concluded, that the lightnbsp;laeing attrafted by the fides of the hole, is infle^led,nbsp;of courfe caufed to proceed more diverginglynbsp;'�^an otherwife it would have done.

^iVith a hole one-tenth of an inch wide, or the centre of the beam was compofed of thenbsp;derife diredl light of the fun, unchanged in itsnbsp;� paflage j but farther therefrom, towards the bordersnbsp;^he beam, this light began to decreafe in denfity,nbsp;gradually decayed more and more in the ap-Proaches nearer and nearer to the borders, becomingnbsp;laft confiderably diluted and evanefeent, and ren-dering the edge of the beam ill-defined and in-diftindt.

With a fimaller hole than the laft, the central �'flic light entirely difappeared, and with a hole yetnbsp;than this, the external edges of the beam

became more eondenfed and better defined; and the whole beam of light became, as before defcrib'nbsp;ed, of more uniform denfity in all its parts. Withnbsp;a hole fmaller than any of the foregoing, aboutnbsp;T-s-s- of an inch wide, various colours began tonbsp;appear in the beam, the central parts of whichnbsp;became now, in their turn, more diluted than thenbsp;reft, the external parts denfer than tliefe,' and bordered with tinges of yellow and red light on thenbsp;very edge or margin of the beam.

� All thefe appearances are to be aferibed to the fame attraftions of the edges of the holes, and ofnbsp;the different parts of the edges. Thefe, when thenbsp;hole is large, affeft only the parts of the light pallingnbsp;neareft to them ; when the hole is reduced, thefnbsp;attradl and dilate the whole of the paffmg lightsnbsp;when the hole is yet more confiderably diminilhed,nbsp;they adfc, not only each part upon the light paffmgnbsp;neareft to each, but each part alfo upon the lightsnbsp;paffmg neareft to each oppofite part of the edgcgt;nbsp;condenfing by diminifhiiig the attradlion and diffu-fjon of the light on the edges of the beam,nbsp;rendering the whole more equably and uniforrnl/nbsp;divergent, and thefe at laft, when the hole is innbsp;moft reduced ftate of about .5-*-^ part of an inchnbsp;wide, by their various adlions produce colours mnbsp;the paffmg light.

�In the beam of folar light paffing through the fmall hole -j-V part of an inch wide, I obferved the

flairs to be confiderably broader, as they ought to be fii this divergent light, than the bodies themfelves;nbsp;flut as �ach of thefc bodies exercifes upon the lightnbsp;Pafiing by it, the fame attractions by which thenbsp;fight is bent in pafTing through the hole, I concluded that a part of the light would be in everynbsp;Cafe bent, in paffing by, towards the body intonbsp;^he fliadow, and illuminate it and diminilh itsnbsp;fifeadth.

Acrofs a beam of folar light, admitted into a ^ark chamber through a fmall hole in a thin piecenbsp;cflead, nearly -5-V of an inch wide, I interpofed anbsp;f'air of a man�s head, and receiving the beam on anbsp;fcfeen or (heet of white paper at a diftance, andnbsp;'''ith an obliquity convenient for the purpofe, Inbsp;^'ited the following appearances.

� At the termination of what may be confidered and therefore may be called, a fliadow, whofenbsp;��itenfity or darknefs was not confiderable, the fol-^'i^ing orders and diftinftions of colours-appeared.

and neareft to the dark or black parts of the ffiadow might be feen a diluted blue, clianging intonbsp;^ fireadth of white light, followed by breadths ofnbsp;I'ellownbsp;nbsp;nbsp;nbsp;'Pq thefe fucceeded an interval of

'I'futed fliade, then breadths of diluted violet, blue, uted green, yellow, red; then green, diluted

the lafl: evanefcent and requiring accommodation circumftances to produce, and attention to perceivenbsp;them,

*' When the diftance of obfervation from the hai�' was very fmali, and before the firft bright ftreaknbsp;light began to appear, the fhadow of the hairnbsp;dlftinft and well-defined, and of intenfe blacknefs-At a greater diftance, this fhadow appeared tonbsp;divided by a parallel line of light throughoutnbsp;whole length, into two parts, and refembled ^nbsp;double fhadow, or the fhadows of two hairs, b^*-was by no means of the fame degree of blackne^nbsp;as was the fingle flradow obferved clofe to the hai*'*nbsp;At ftill greater diftances, it increafed in breadth aOlt;^nbsp;diminifhed in blacknefs, whilft the tranfverfe diquot;nbsp;menfions of the dividing line of light increafednbsp;the fame time, until, at a confiderable diftance froHinbsp;the hair, this intermediate band or line of light bC'nbsp;gan to put on the appearance of colours onnbsp;edges, and to affume, on both fides externally, caft�nbsp;of yellowifh and reddifh light. By further increal^nbsp;of diftance, this apparent fhadow, thefe dark intervals became more diluted, and of nearly the fam�nbsp;colour throughout, the line of light more and morenbsp;dilFufed, and was at laft extinguifhed by the extremenbsp;diffufion and ultimate invi^fibility of the light tha*^nbsp;produced it.

Of the IpfleElion of Light, tfc. 205 the light paffing next in order of dlftance by thenbsp;confiderable changes alfo are produced.

� The (hadow that firft appeared clofe to the is perfeftly and truly a lhadow, being produced by the interception of the paffing light bynbsp;the hair.

� This lhadowj however, quickly ceafes to appear, the rays of light neareft to it on both fides of t^e hair being bent into it at confiderable anglesnbsp;infieflion and difperfion, and croffing, illumi-Uating and extinguiffiing it.

�� The rays of light are not only bent, they 3re alfo diftributed or divided into different raysnbsp;uf different colours, in angles of difperfion greaternbsp;the diftances are lefs, and lefs as the diftancesnbsp;'^te greater, in fuch a manner, that of different colours at the fame diftance, the purples, blues,nbsp;B*quot;eens, yellows, and reds, are bent towards thenbsp;lgt;ody j the purples moft, each of the others in duenbsp;fucceffion lefs, and the reds leaft, according to thenbsp;^��der of their ftatement, and of colours of thenbsp;I'^rne forts at different diftances, the nearer morenbsp;^han the more remote, and the more remote lefsnbsp;*^han the nearer. So various, however, are thenbsp;hendings of different colours at different diftances,nbsp;ftiat in certain diftind portions of light, and at dif-^�^rent diftances of obfervation, the more remotenbsp;�ud the nearer rays of different colours containednbsp;'''ithin each of thofe portions or diviftons* of thenbsp;light, become varioufly intermingled with each.

other, and by their various intermixtures, form of thefe divifions into particoloured fringes, win¹nbsp;the rays of different divifions, never mixing wi^^nbsp;thofe of other divifions, the intervals of the diviho'^¹nbsp;are preferved, and become the dark intervals whkl'nbsp;leparate the fringes.�

I have tranfcribed the above paffages princip^l^l^ to give the reader an idea of the inflection of light ¹�¹nbsp;a few eafy and confpicuous experiments; but the^^nbsp;are not all the phenomena of inflection, nor is th^nbsp;fame explanation entirely new or applicable to the^¹nbsp;all. Befides Newton, various experimentsnbsp;made relative to the inflection of light by Maralflhnbsp;Grimaldi, Delifle, Mairan, Du Tour, Mufchei¹'nbsp;broeck, and others; an account of which experimei¹^�nbsp;the reader may fee in Prieftley�s Hiftory of Vif¹�¹�'nbsp;Light, and Colours¹.

Thin plates of tranfparent bodies, efpecially whe** they are not of an uniform thicknefs, frequentlynbsp;hibit the principal prifmatic colours (viz. fuchnbsp;lours as are exhibited by the refraction of lig^''nbsp;through a prifm) either in rings, or zones,nbsp;mixt.nbsp;nbsp;nbsp;nbsp;^

All the phenomena which have been obferv^ relative to thefe colours, are by no means reco**'nbsp;cliable to any known and determinate laws �, thei^nbsp;fore all the obfervations ftvould be fingly and

Confidered by whoever wifhes to inveftigate the nature of thofe phenomena, or to difcover new fafts. fhall however only give the following fummary

Account. nbsp;nbsp;nbsp;*

The colours that are feen on the bubbles of im-P'Jte water, and efpecially of a folution of foap, are this fort. Thofe bubbles are thin veficles ornbsp;^Irns of the folution, and are continually varying innbsp;^hicknefs.

When two flat glafl�s, and efpecially when one '''hich is a little convex, and a flat one, are gentlynbsp;PreflTed together, coloured rings are frequently vi-fible about the point or points of contaft; whichnbsp;have been fuppofed to be produced by the thin filmnbsp;of air that remains between the glalTes, and which isnbsp;of various thicknefs j yet thofe colours are vifiblcnbsp;^ven when the glaffes are under the exhaufted receiver of the air-pump.

Metallic plates, glaffes,-amp;c. flightly moiftened 'vith mofl liquors, frequently exhibit fuch colours.nbsp;'Thin plates of talk, or Mufcovy glafs, do the

The caufe .of thofe phenom.ena has been attrl-huted by Newton to certain difpofitions of the rays light, which he called fits of eajy travjmijftcn andnbsp;'fi eaJy refieSliont � a flrange hypothefis. Othernbsp;Pcrfons have attributed it to refraftion or to re-fleftion only: the duke de Chaulnes attributednbsp;fomc of thofe cffefts to the infledtion of the rays

In order to give my readers a competent idea of thofe phenomena, I fliall lubjoin a few of the moftnbsp;ftriking fads.

Sir Ifaac Newton took the objed glaffes of two telefcopes, one a plano-convex for a telefcope ofnbsp;14, and the other a double convex for a telefcopenbsp;of about 50 feet, laid the latter upon the flat fidenbsp;of the former, and prefled them gently againft eachnbsp;other. Circles of colours immediately appearednbsp;about the point of contad, which increafed fonbsp;number and in fize when the prefllire was increafed�nbsp;and vice verfci. The colours appear more vividnbsp;nearer to the central fpot, which is black and co-lourlefs, and more dilute in proportion as the/nbsp;recede from it. When the glaffes were very nruchnbsp;preffed againfl: each other. Sir Ifaac found the co-Joured circles to be of unequal breadths, as innbsp;fig. 17. Plate XX. where the letters h, c, d, e, ��nbsp;amp;c. indicate the colours in the followino- order whichnbsp;commences from the centre a. � Black, blue, white�nbsp;yellow, red j violet, blue, green, yellow, red; purple�nbsp;blue, green, yellow, red; green, red; greenifli bine�nbsp;red; greenifli blue, pale red; greenifli blue, redifhnbsp;white f.

See Prieftley�s Hiftory of Vifion, Light, and Colour^� P. VI. Sed. V. Alfo, New Obfervations concerningnbsp;Colours of thin tranfparent Bodies, amp;c. London iSoo¹

� The Abb� Mazeas obferved, that if the fur-faces of fiat pieces of glafs be tranfparent, and well polifhed, fuch as are ufed for mirrors, and thenbsp;prefTure be as equal as poffible on every part of thenbsp;two furfaces, a refiftance will foon be perceived,nbsp;when one of them is made to Aide over the other,nbsp;fometimes towards the middle, and fometimes towardsnbsp;the edges; but wherever the refiftance is felt, two ornbsp;three very fine curve lines will be perceived, fomenbsp;of pale red, and others of a faint green. Continuing the fridion, thefe red and green lines increafenbsp;in number at the place of contaft, the colours beingnbsp;fometimes mixed without any order, and fometimesnbsp;difpofed in a regular manner. In the laft cafe, thenbsp;coloured lines are generally concentric circles, ornbsp;ellipfes, or rather ovals, more or lefs elongated, asnbsp;the furfaces are more or lefs united. Thefe figuresnbsp;will not fail to appear if the glalTes be well wipednbsp;and warmed before the fridion.

� When the colours are formed, the glafles adhere with confiderable force, and would always continue fo, without any change In the colours. In the centre of all thofe ovals, the longer diameternbsp;of which generally exceeds ten lines, there appearsnbsp;a fmall plate of the fame figure, exadly like a platenbsp;of gold, interpofed between the glafles; and in thenbsp;centre of it there is often a dark fpot, which abforbsnbsp;all the rays of light, except the violet; for this colournbsp;appears very vivid through a prifm.

� If the glaffes be'feparated fuddenly, either by Aiding them horizontally one over another, or bynbsp;the adion of fire, the colours will appear immediately upon their being put together, without thenbsp;leaft fridion.

� Beginning by the Aighteft touch, and increafing the preAure by infenfible degrees, there firft appearsnbsp;an oval plate of a faint red, and in the centre of itnbsp;a fpot of light green, which enlarges by the preAure,nbsp;and becomes a green oval, with a red fpot in thenbsp;centre; and this enlarging in its turn, difcovers anbsp;green jTpot in its centre. Thus the red and thenbsp;green fucceed one another in turns, afiuming different Atades, and having other colours mixed withnbsp;them.

quot; The greateft difference between thefe colours exhibited between plane furfaces and thofe by curvenbsp;ones is, that, in the former cafe, preffure alonenbsp;will not produce them, except in the cafe above-mentioned. With whatever force he compreffednbsp;them, hi^ attempts to produce the colours were in.nbsp;vain, without-.previous fridion. But the reafon ofnbsp;this plainly was, that without Aiding one of thenbsp;glaffes over the other, they could not be brought tonbsp;approach near enough for the purpofe,

� At firft the Abbe Mazeas had no doubt but that thefe colours were owing to a thin plate of airnbsp;between the glaffes, to which Newton has aferibednbsp;them j but the remarkable difference in the circum-ftances attending thefe produced by the flat plates,

thofe produced by the obje�l glaffes of Newton, Convinced him that the air was not the caufe ofnbsp;^bis appearance. The colours of the flat platesnbsp;vaniflied at the approach of flame, but thofe ofnbsp;*^be objed: glafles did not. He even heated thefenbsp;*^'ll that which was next to the flame was crackednbsp;by the heat, before he could obferve the leaft dilatation of the colqured rings. This difference wasnbsp;not owing to the plane glafles being lefs comprelfednbsp;than the convex ones; for though the former werenbsp;comprefled ever fo much by a pair of forceps,nbsp;it did not in the leaft hinder the effed: of thenbsp;flame

The coloured circles, fuch as have been mentioned above, feen by refleded light, are much ttiore vivid and diftind; than thofe feen by tranf-ttiitted light.

The rings feen by refledion generally are differently coloured from thofe made by tranfmitted iignt. White in the latter cafe is oppofed to blacknbsp;*ti the former, red to blue, yellow to violet, andnbsp;�teen to a compound of red and green.

the brightnefs of the colours is thereby much dimt* nllhed. Alfo the rings contradl in number andnbsp;breadth.

Newton�s theory, principally eftablifhed upon the above-mentioned phenomena of thin plates, has longnbsp;been thought to afford a fufficient explanation of thenbsp;various colours which are exhibited t� our eyes bynbsp;the different bodies of the world.

According to that theory, all colours are fuppofed to exifl in the light of luminous bodies only, fuchnbsp;as the fun, a candle, amp;c. and that light, falling in-ceffantly upon different bodies, is feparated into itsnbsp;primitive colours, fome of which are abforbed,nbsp;whilft others are incefiantly refledted j fo that thenbsp;bodies which appear red to us, are fuch as abfoi'bnbsp;all the other colours of white light, and refleft thenbsp;red rays only. Thofe which appear green to ttsnbsp;have the property of abforbing all the coloured raysgt;nbsp;excepting the green. Thofe bodies which appealnbsp;not of any primitive colour, have been fuppofe'inbsp;to reflect fuch of the primitive colours, andnbsp;fuch proportion as to produce their mixt cO'nbsp;lours, amp;c.

So far the hypothefis feems to be warranted by fome of the experiments that have been mentionednbsp;in the preceding chapter ; but the next ftep is 1^^�nbsp;evident, even to the eyes of fancy. It was fuppo^b*^nbsp;that the particles of all bodies confift of very thi^^nbsp;and tranfparent plates, or laminae, which refieftnbsp;tranfmit one colour of another, according to the�*'

thicknefs gt;

�^^iickncfs i that the thinner produce the more vivid colours ; and that the colours of fotne plates varynbsp;according as the eye changes pofition, whilfl; thofenbsp;�f others are fteady and uniform.

A dole examination and application of this doc-^^rine to a , variety of phenomena, which have been obferved by various ingenious perfons, efpecially ofnbsp;the prefent age, render this theory of colours doubt-ftil in almoft all its parts. In the firft place, it maynbsp;be doubted whether there really are only fevennbsp;diftin�t primitive colours, or an indefinite numbernbsp;of them, which are perhaps produced by fome un.nbsp;known modifications of white light. The breadthsnbsp;3nd the gradations of the fuppofed feven primitivenbsp;Colours in the prifmatic fpedbrurn, are the greatefbnbsp;foundation for tlie above-mentioned doubt. With,nbsp;^�-cfped; to the thin tranfparent plates, of which allnbsp;bodies are fuppofed to confift, we are greatly innbsp;^ant of experimental confirmation j and even if wenbsp;^ere fure of their exiftence, it would be difficultnbsp;�^bereby to explain how are the fixed and unchangeable colours produced by them in allnbsp;dire�tions. Such doubts may be feen in all thenbsp;tUodern writers on optics, to whofe works, whichnbsp;^*�0 principally to be found in Tranfaftions of Societies, Journals, amp;c. I lhall refer the inquifitivenbsp;��cader, who may wiffi to be farther informed on thenbsp;ffibjeft, or to extend our knowledge of nature;nbsp;^hilftl fubjoin fome more remarkable fads relativenbsp;^0 colours.

p 3 nbsp;nbsp;nbsp;The

The changes of colour in the fame body, which are produced either by pofition, or by a change ofnbsp;quality in the body itfelf, n�ver fail to ftrike thenbsp;obferver with admiration and pleafure.

With refped to the change by pofition, it has been obferved that certain folids, and efpeciallynbsp;certain fluids, appear of one colour by refle�ednbsp;light, and of another colour by tranfmitted light?nbsp;the reafon of which is, that if they reflcd all the raysnbsp;of one or of certain primitive colours, the light,nbsp;which, by pafling through them, comes to our eyes,nbsp;muft exhibit other colours; for it has been deprivednbsp;of thofe colours which have been refleded from thenbsp;anterior part.

JVlr, Boyle obferves, that if an infufion of lignum nefioriticum be put in a glafs globe, and be expofednbsp;to a ftrong light, it will be as colourlefs as purenbsp;water? but if it be carried into a place a littlenbsp;fliaded, it will be a mod beautiful green. In anbsp;place ftill more fliaded, it will incline to red, and innbsp;a very fliady place, or in an opaque veflel, it willnbsp;be green again. If it be held diredly between thenbsp;light and the eye, it will appear tinged (exceptingnbsp;the very top of it, where a fky coloured circle fomc-times appears) almoft of a golden colour, exceptnbsp;the infufion be too ftrong, in which cafe it willnbsp;dark or rcddifli, and requires to be diluted withnbsp;water. But if it be held from the light, fo thatnbsp;the eye be between the light and the phial, itnbsp;will appear of a deep lovely blue colour, as will

The changes of colour which are produced by mixtures, by boiling,-heating, pounding, amp;c. arenbsp;fo common, and fo remarkable as to come withinnbsp;everybody�s notice; but the reafon of fuch changes .nbsp;has been differently accounted for by different phi-lofophers. One of the moft plaufible theories is,nbsp;that an attenuation of the particles of a given bodynbsp;changes from the violet, or from fome colour nearernbsp;to the violet, to fome other colour nearer to thenbsp;red, in the order of prifmatic colours and t/icenbsp;�verfa, the thickning of the particles changes thenbsp;colours in the contrary order ¹.

If to the diluted fyrup of violets you a^d fome drops of acid, the liquor becomes red; add a fmallnbsp;quantity of carbonated pot-afh, and it becomesnbsp;green.

To a folution of fulphate of copper add a few drops of ammoniac, and the liquor becomes blue jnbsp;add a little nitric acid, and the blue colour va-nifhes.

A vaft number of fuch changes is obferved in chemiftry, and are ftated in almoft all the chemicalnbsp;works f.

See Delaval on Colours; alfo fee his Paper on Colours, in the fecond volume of Memoirs of the Manchefter Philo�nbsp;fophical Society.

The refrangibility and reflexibility of light are the properties upon which the conftruftionnbsp;of the moft ufeful and moft furprifing optical in-ftruments depends. Of the various parts of thenbsp;mechanifms, thofe which depend upon reflection,nbsp;j viz. the mirrors, have been fufflciently explained innbsp;the preceding pages. It is now neceflary to enumerate, to defcribe, and to explain the adions ofnbsp;thofe which depend upon refraction.nbsp;nbsp;nbsp;nbsp;�

The principal fhapes of the latter are comprehended under three generic names, viz. flat plates, prijms, and lenjes. The flat plates need no particular defcription j the prifms have been defcribednbsp;in the preceding chapter j the lenfes will be defcribed in the prefent.

A thin piece of glafs, or of any other tranfparent inedium, having at lead one concave or convexnbsp;fpherical furface, is called a lens. The differentnbsp;forms of lenfes and their peculiar appellations arenbsp;^lerived from the figure of their furfaces, which may

he. cither flat, or convex, or concave, or mixt: hence we have flx forts of lenfes, fe�tions of which,nbsp;are exhibited in fig. 18, Plate XX. and in thenbsp;order in which they are nominated, viz. the plano-conveit, the plano-concave, the double convex, thenbsp;double concave, the concavo-convex, and the menijcus,nbsp;which is likewife a concavo-convex, but differsnbsp;from the preceding by its having the radius of thenbsp;convexity fmaller than that of the concavity, innbsp;confequence of which 'its edges are fliarp, and itsnbsp;fedtion refembles the nev/ moon.

The middle point of each of thofe ienfes, when the lenfes ar^ thin, is called its centre. A ftraightnbsp;line pafTing through that centre, and perpendicularnbsp;to both furfaces of the lens, as the line H G isnbsp;called its axis. The points C, D, on the furfacesnbsp;of the lenfes, where the axis cuts the furfaces, arenbsp;called the vertexes of the lens. But when the lensnbsp;is pretty thick, and its furfaces of unequal curvatures, then the centre of the lens is nearer to onenbsp;vertex than to the other, by as much as the radiusnbsp;of curvature of the former furface is lefs than thatnbsp;of the other.

It is evident, on the leafl: refleftlon, that the axis mufl: pafs through the centres of convexity or concavity, viz. of the fpheres of which the furface ornbsp;furfaces of the lens are a portion; unlefs the lens benbsp;irregularly formed, as in fig. 19, which would benbsp;ufelefs for optical inftruments, and which is frequently found to be the cafe in praftice.

VOL. lu, nbsp;nbsp;nbsp;p 5nbsp;nbsp;nbsp;nbsp;When

When a ray of light falls perpendicularly upon the vertex of a lens, viz. coincides with the axisnbsp;HC, it muft evidently pafs ftraighc through thnnbsp;lens without fuffcring any refradion (fee pagenbsp;i68 amp; �7� ); but when it falls obliquely upon it,nbsp;then it muft emerge out of the lens in a dircdionnbsp;inclined tpits former diredion. Thus of the rays ofnbsp;light, which, ifiuing fiom the luminous point A,nbsp;fig. 20, Piate XX. fall upon the lens BE, the raynbsp;A C, which proceeds in the diieftlon of the axisnbsp;of the lens, muft pafs ftraight through it; but thenbsp;ray xV B, falling obliquely upon the furface of thenbsp;lens, muft be refraded, viz. bent j and if the lensnbsp;be a plano-convex, or double convex, that ray muftnbsp;be bent inwardly, viz. towards the axis (as maynbsp;be traced by nneans of the method deferibed innbsp;page 150.); confequendy it muft cut the axis atnbsp;fotne point F. Now that point F is called thenbsp;refraSed focus of that ray, or rather of the raysnbsp;A B, A E, amp;c. which fall upon the lens at equalnbsp;diftances from the axis A C j it being evident thatnbsp;they muft all meet and crofs at the fame point F jnbsp;whereas the point A is called the radiant point, ornbsp;fhe focus of incident rays j and both thofe points, innbsp;reference to each othei-, are called the conjugate foci.

if the lens be a concave one, as in fig; 11, PI. XX. then the oblique rays AB, AE, amp;c. will be bentnbsp;outwardly, viz. from the axis : in which cafe ifnbsp;you fuppofe thofe refradted rays to be continuednbsp;backwards until they meet the axis, as at F gt; thennbsp;gt;! � . . . '

that point F is called the virtual focus of the rtr fraSied rays, it being in faft the centre of divergency of the rays. In this cafe the conjugate focinbsp;are both on the fame fide of the lens, viz. the realnbsp;focus A of incident rays, and the virtual focus F ofnbsp;the refrafted rays B G, D O, ES.

If the reader will give himfelf the trouble of tracing the progrefs of all the rays which proceednbsp;from a luminous point, and fall upon the furfacenbsp;of a lens, be it convex or concave, after the manner which is mentioned in page 190, he will findnbsp;that the rays will not all meet at one and the famenbsp;focus, if it be a convex lens; nor will they have anbsp;common virtual focus, if it be a concave lens; butnbsp;thofe rays which are more diftant from the axisnbsp;after the refraftion, meet fooner than thofe whichnbsp;are nearer to the axis ; and this effe�l is greater innbsp;proportion as the furfaces of the lens are farthernbsp;from each other, and confift of larger fpherical feg-*nents. Hence a glafs globe renders the above-mentioned effedl very confpicuous; and hence arenbsp;*he lenfes made as thin as poffible j but in all cafes,nbsp;^ Ens which confifts of fpherical furfaces, does nevernbsp;fefraft the rays which fall from a luminous point,nbsp;to one focus. The rays which fall upon the edgenbsp;tgt;f the lens, have their refrafted focus not only nearernbsp;to the lens, but alfo fartheft from the axis, viz, onnbsp;one fide of it. Lines drawn through the refraftednbsp;foci of the rays which belong to one luminous ornbsp;radiant point, form two curves, which make an

angle with each other at � the axis, or principal focus, and are called caujlics by refrablion; whichnbsp;are real in convex lenfes, but imaginary in concavenbsp;lenfes.

When the lenfes are thin and their fphericity not very great, thofe cauftics are fo trifling that the eyenbsp;does not perceive them ; but lenfes that are prettynbsp;thick and of great convexity, produce a confidera-fale aberration of the rays, and an evident diftor-tion of the objedl, to an eye that looks throughnbsp;them*.

An experimental proof of this aberration may be had in the following manner cover one fide ofnbsp;a glafs globe or thick lens with a circular piece ofnbsp;brown paper, having a row of equidiftant pin-holesnbsp;in its diameter. Let the light which paflTes throughnbsp;thofe holes, and through the lens, fall upon a piecenbsp;of white paper held perpendicular to the rays ofnbsp;light, and you will find that when the paper is heldnbsp;near to the globe or lens, the fpots of light upon itnbsp;are at equal diftances from one another fucceflively�nbsp;but if the paper be gradually withdrawn from thenbsp;lens, the intervals between the exterior fpots groW

* In order to avoid this aberration, which arifes from the fpherical figure of the lens, other figures have been dc'nbsp;termined, which might refradl the light without forrninSnbsp;thofe caufiics; but the pradtical difficulty of giving thenbsp;lenfes any other figure^ befides the fpherical, is fo verynbsp;great, as not'to be attempted by any of the opticians.

left

On the other hand, if the fame operation be ^performed with a thick concave lens, the intervals 'nbsp;between the exterior fpots will be found to grownbsp;larger titan the interior, amp;c.

Befides the above-mentioned aberration, which arifes from the figure, lenfes are fubjedi to a muchnbsp;greater imperfedlion, which arifes from the difihrentnbsp;refrangibility of the coloured rays; fo that thoughnbsp;the rays which proceed from a luminous point, andnbsp;fall upon a fpherical lens at equal diftances from thenbsp;axis, would meet or have their focus at the verynbsp;fame point, in confequence of a particular figurenbsp;of the lens; yet as the refraftion feparates the lightnbsp;into coloured rays, the violet rays being the moftnbsp;refrangible, will form their focus nearer than thenbsp;blue, and fo on i the red rays being the lead refrangible, will form their focus fartheft from thenbsp;lens.nbsp;nbsp;nbsp;nbsp;*

When the lens is thin, and its convexity or concavity not very great, this feparation ofnbsp;Colours paffes unperceived ; but with a thick lensnbsp;�f great convexity, or when the imperfedtions ofnbsp;ooe lens are magnified by another lens, as in te-lefcopes and other compound optical inftruments,nbsp;the feparation of the coloured rays, efpecially towards the edge, where the refradlion is ftrongeff, isnbsp;fo very manifeft as greatly to obftrudt the effedls

This imperfeftion of lenfes was confidered as unfurmountable by the great Newton, and all othernbsp;philofophers of his time; but the fubfequent dif-covery of the different difperfive power of differentnbsp;tranfparent mediums about the middle of the laftnbsp;century, made fome philofophers entertain hopesnbsp;of remedying that imperfeftion of lenfes; and foonnbsp;after the late ingenious Mr. J. Dollond, after anbsp;variety of trials and confiderations, accomplifhednbsp;it . in the conftru�lion of what is now commonlynbsp;known under the name of achromatic (viz. co-lourlefs) telefcope j the objeft lens of which is compounded of glaffes of different difperfive powers�nbsp;fo well proportioned, as not to feparate the lightnbsp;into its primitive colours : hence the objedts,nbsp;which are feen through it, appear of their naturalnbsp;colours.

Achromatic lenfes for telefcopes have been made by a combination not only of glaffes, but alfo ofnbsp;glaffes and fluids of different difperfive powers ¹ inbsp;yet the comimon pradlice is confined merely ttgt;nbsp;glaffes, which are upon the whole the moft manageable and the moft durable fubftances.

See a paper entitled, The Principles and Jpplication of a New Method of confruSiing Achromatic Telefcopes, bynbsp;Robert Blair, M. D. in the firft volume of Nicholfon�snbsp;Journal of Natural Philofophy, amp;c.

Achromatic lentes are formed either of two lenfes or of three lenfes, fixed in a common cellnbsp;cloi'e to one another. Fig. 22, Plate XX. exhibitsnbsp;a feftion of one of the former, and fig. 23, exhibitsnbsp;a ledtion of one of the latter fort.

In figure 22, AB is a double convex lens of crown glafs, and C D is a concavo-convex lensnbsp;of flint glafs.

In fig. 23, AOB and EF are two double convex lenfes of crown glafs, the internal lens C D being a double concave of flint glafs.

The principle upon which thofe achromatic lenfes are conftrufled may be eafily derived fromnbsp;what has been faid in page 195? where it has beennbsp;fhewn, that by a combination of two or three prifmsnbsp;of different forts of glafs, the light which paflTesnbsp;through them may be refraded without difperfion;nbsp;for after the fame manner the thicknefles and curvatures of the lenfes may be fo proportioned as tonbsp;produce a fimilar effect. In fad, if we examine anbsp;very fmall portion of fuch a compound lens, viz.nbsp;the portion which is contained between the linesnbsp;gi, bm, fig. 22, it will appear that the two portions of external lenfes, niuft ad like two prifms ofnbsp;crowm glafs, whofe bales are towards the commonnbsp;axis of the lenfes; whereas the portion of the middlenbsp;concave lens ads like a prifm of flint glafs placednbsp;in contrary diredion, viz. with its vertex towardsnbsp;the axis.

determining the curvatwres neccffary for the furfaces of thofe compound lenfes, which are dependingnbsp;upon the refradive and difperfive properties of thenbsp;glaffes, which properties vary greatly; but thenbsp;difBcuky of determinjng the real difperfive as wellnbsp;as the real refraftive power of a particular fpecimennbsp;of glafs, which are feldom uniform throughout thenbsp;fpecimen, and tlie difficulty of forming the component lenfes exadly of the computed thicknefs andnbsp;curvature, render thofe calculations of little ufe innbsp;pradice. At moil: they ferve as approximations,nbsp;which mud be improved and correded by adualnbsp;trials with different glaffes and different grindingnbsp;tools¹.

The following two formulas for the conftrudion of triple object achromatic lenfes of telefcopes, are taken fromnbsp;Euler�s Dioptrics, vol. I. p. 335.

The radii of the furfaces, commencing with the one next to the objed, are as follows ;

An excellent achromatic lens for a telefcope, ^ade by Dollorid, and confifting of thre� lenfes, asnbsp;fig. 22, being examined, was found to hav� thenbsp;*'adii of the curVatur�s of its fix furfaces i, 2,354,5,.

6, of the following dimenfions in inches and decimals. Another fimilar lens was allb examinedinbsp;^nd the radii, amp;c. were as in the laft column.

A double convex nbsp;nbsp;nbsp;fnbsp;nbsp;nbsp;nbsp;ill furfacenbsp;nbsp;nbsp;nbsp;=:nbsp;nbsp;nbsp;nbsp;0,2829 x P�

of crown glafs nbsp;nbsp;nbsp;\nbsp;nbsp;nbsp;nbsp;2d furfacenbsp;nbsp;nbsp;nbsp;=:nbsp;nbsp;nbsp;nbsp;2,0729 X P.

A double concave nbsp;nbsp;nbsp;fnbsp;nbsp;nbsp;nbsp;ill furfacenbsp;nbsp;nbsp;nbsp;=:nbsp;nbsp;nbsp;nbsp;� 2,1459 Xnbsp;nbsp;nbsp;nbsp;f*�

of flint glafs nbsp;nbsp;nbsp;^nbsp;nbsp;nbsp;nbsp;2d furfacenbsp;nbsp;nbsp;nbsp;=:nbsp;nbsp;nbsp;nbsp;0,2955 Xnbsp;nbsp;nbsp;nbsp;Pi

A double convex nbsp;nbsp;nbsp;fnbsp;nbsp;nbsp;nbsp;ill furfacenbsp;nbsp;nbsp;nbsp;=nbsp;nbsp;nbsp;nbsp;0,5938 x P.

of crown glafs nbsp;nbsp;nbsp;\nbsp;nbsp;nbsp;nbsp;2d furfacenbsp;nbsp;nbsp;nbsp;=:nbsp;nbsp;nbsp;nbsp;2,5006 X P�

The principal focus of the firft ofthofe achroma-*' tic lenfes was diftant from its furface about 4^nbsp;inches. That of the fecond achromatic lens wasnbsp;diftant about 46,3 inches.

Notvvithftanding the aberrations mentioned in the preceding .p^g^s, when glafs lenfes are not verfnbsp;thick, they are reckoned to have a determined focusnbsp;of refrafted rays for fuch rays as come from a finglenbsp;radiant point, and the diftance of that focus fromnbsp;the f�rface of the lens is called the focal difiance ofnbsp;theje rays. Even a globe is faid to have fuch anbsp;focus, meaning, however, of the middlemofl: partnbsp;of the globe.

It is now neceflary to deferibe the method of determining that focal diftance for the various fortsnbsp;of lenfes. This determination indeed may be obtained in all cafes from the general method de-feribed in page 190; but it will be ufeful to ftatenbsp;the principal refults of fuch inveftigations for thenbsp;more common lenfes, and likewile to give fomenbsp;more expeditious and pretty accurate rules for finding the foci of all forts of lenfes. We muft however prefix a fliort explanation of the aftion ofnbsp;plates.

In the firft place, it muft be recolleded, that if ^ ray of light, however incident upon a refralt;ftiu�nbsp;plate, like the glafs plate AB, fig. 24, Plate XX-pafles through it, the emergent part C D of thatnbsp;ray will always be parallel to the incident part Eh�nbsp;as long as th� furfaces of the refracting plate are

parallel to each otherj and the plate is furrounded by the fame uniform medium, be it air, or water,nbsp;amp;c.} for though the ray is bent in going into thenbsp;plate at the firft furface, it is evident that it muft benbsp;bent as much the contrary way in going out at thenbsp;other furface of the plate.

If an objedt in a refradbing medium be viewed by an eye fituated in another medium of different re-frangibility, the boundary of the two mediums beingnbsp;a plain furface, the vifual angle may be enlarged ornbsp;diminifhed, and of courfe the apparent fize of thenbsp;objedt may be enlarged or diminifhed ; viz. it willnbsp;be enlarged if the eye be in the lefs refradlive medium, and vice verfa.

Thus an objedt, AB, fig. 25gt; Plate XX. in �Water, will appear magnified to an eye at C, viz,nbsp;fituated in the air which is contiguous to the water jnbsp;becaule the external rays A F, B G, by falling obliquely upon the furface FG, are bent, and caufed tonbsp;fubtend a larger angle at the eye. And the fame thingnbsp;roufl be underftood of the intermediate rays, excepting DE, which, falling perpendicularly upon thenbsp;furface, is not bent by it.

From what has been faid of a flat plate, it may be cafily underftood, that about the middle of the furface of every lens there is a point, upon which if anbsp;fay falls and paffes through the lens, the emergentnbsp;part will be parallel to the incident; for the point ofnbsp;incidence and the point of emergence may benbsp;fituated fo that if two planes touch the furfaces

at thofe points, they may be parallel to each otlifer^ That ray, or part of a pencil of light, which thusnbsp;paffes through the lens, without being bent, isnbsp;called the axis of that pencil, and that axis always�nbsp;paffes through the centre of the lens.

When rays of light fall upon the fame lens with different inclinations, it is evident that after the re-fra�lion, they muft have their foci at different dif-tances from the lens ; for inftance, the fame inwardnbsp;bending, or refradlion, will incline towards eachnbsp;other, much fooner thofe rays which are alreadynbsp;much inclined, tharr thofe which are already lefs inclined or diverging-

When rays of light come parallel to each other, as thofe which come from a point of the fun's fur-face, or from any other diffant point, and fall perpendicularly, or nearly fo, Upon the furface of anbsp;lens ; then the focus of thofe rays after refraftion isnbsp;called the 'principal focus of that lens, and its diftancenbsp;from the lens, the principal focal diftance of thatnbsp;lens.

The principal focus of a lens may be found out either experimentally, which is by far the moft expeditious method, or by computation.

In a plano-convex, double convex, or menifeus,-the principal focus is real j in the other lenfes the focus is virtual. In order to find out the principalnbsp;focus of any of the former, place the lens before thenbsp;fun, fo that its beams may fall perpendicularlynbsp;upon it i then meafure the diftance at which thefaysnbsp;3nbsp;nbsp;nbsp;nbsp;arc

are colIe(5ted in a white, round, and well defined fpot, upon a piece of white paper, which for this purpofenbsp;muft be placed nearer or farther, amp;c, and that dif-tance is the principal focal diftance in queftion. In-ftead of the fun, fome ocher diftant luminous objcdtnbsp;will anfwer as welf

In order to find the virtual focus of any cpncave, plano-concave, or concavo-convex lens, cut a circular hole in a piece of black paper, and flick it onnbsp;the lens, fo that the centre of the hole may be in ornbsp;very near the middle �f the lens; alfo draw a circlenbsp;on a piece of fliff paper or card, whofe diameter isnbsp;juft double the diameter of the above-mentionednbsp;hole upon the lens; then hold the lens thus prepared perpendicular to the fun beams, and move thenbsp;card with the circle backwards and forwards on thenbsp;other fide of the lens, until the rays, which paffingnbsp;through the lens fall upon the card, may form anbsp;fpot upon it exaflly equal to the circle on the card;nbsp;and the diftance between the card and the lens isnbsp;the principal virtual focal diftatice in queftion; fornbsp;if ftraight lines be drawn frotri the edge of the fpotnbsp;un the card, and along the edge of the hole on thenbsp;^ens, they will meet at a point as diftant from thenbsp;other fide of the lens as the card is from the firft:nbsp;fide.

The principal focal diftance of a plano-convex glafs lens is very nearly equal to the diameter of itsnbsp;curvature. But for a double and equally convexnbsp;lens, that diftance is nearly equal to the radius of

2^0 nbsp;nbsp;nbsp;Of Lenjes, and of theif Effe^ls.

curvature. In a plano-concave, the diftance of the principal virtual focus from the lens is nearlynbsp;equal to the diameter of the curvature. In a doublenbsp;and equally concave lens, that diftance is equal tonbsp;the radius of concavity ¹.

The following two rules are demonftrated in Dr. Smith�s Optics, B. II. Chap. III. Gravefande�s Elementsnbsp;of Natural Philofophy, B. V. Schol, to Chap. IX. andnbsp;other works upon Optics.

I. To find the focus of parallel rays falling perpendicularly, or nearly fo, upon any given lens.

Let E, fig. I� 2, and 3, Plate XXI. be the centre of the lens, R and r the centres of its furfaces, (viz. ofnbsp;their fphericities) R r its axis, ^ E G a line parallelnbsp;to the incident ray upon the furface B, whofe centrenbsp;is R. Parallel to ^ E draw a feinidiameter B R, in whichnbsp;produced, let V be the focus of the rays after their firftnbsp;refraSion at the furface B ; and joining V r, let it cut ^Enbsp;produced in G j und G will be the focus of the rays that 'nbsp;emerge from the lens,

]I. The focus of incident rays upon a fingle furface, fphere or lens, being given, it is required to find the focus of the emergent rays.

Let any point Q_, fig. 4 and 5, Plate XXI. be the focps of incident rays upon a fpherical furface, lens ornbsp;ijphere, whofe centre is E, and let other rays comenbsp;parallel to the line Q_E q, the contrary way to thenbsp;given rays, and after refraftion let them belong to thenbsp;focus F (viz. mark the principal focus F) ; then takingnbsp;E � equal to E F, fay as Q^F to F E, fo E � to fq ; andnbsp;placing � y the contrary way from � to that of FQ_from F�nbsp;the point a will be the focus of the refraded rays, withon'^

It will be necefiary to obferve once for ever, that the diredlions of a given incident and refrafted raynbsp;are the fame when the latter is called the incident,nbsp;and the former is called the refrafted diredlion ; fornbsp;tnftance, if the incident rays are parallel, the redrafted focus will be at a certain diftance from thenbsp;lens; now if a luminous objeft be fituated at thatnbsp;diftance, then the refrafted rays will proceed parallel. The like thing muft be underftood of anynbsp;two conjugate foci of a lens; for if either of themnbsp;is called the radiant, the other will be the refraftednbsp;focus.

When the incident rays, inftead of being parallel, are either converging or diverging, then you maynbsp;determine whether the focus is nearer or farthernbsp;from the lens than the principal focus, by confi-derino whether the lens be concave or convex, amp;c.

'quot;'hich fo far needs no particular explanation �, but the precife diftance of the focus in thofe cafes may benbsp;^afily determined from the fecond rule in the notenbsp;l^elow.

We have hitherto taken notice of the progrefs of ^ fingle pencil of rays (viz. fuch as comes from a

^ny fenfible error j provided the point Q_ be not fo remote ftom the axis, nor the furface fo broad, as to caufe any of thenbsp;rays to fall too obliquely upon them.

When the rays do not come from a point, nor do they come parallel, but come Gonvergingly, then their virtualnbsp;focus muft be confidered as the radiant point.

fingle point) through a lens; but the application ot the ferne reafoning to the various points pf an objednbsp;is fo very eafy, that a flight illuftration is fufficient tonbsp;render it manifefl:.

Let Egt;E, fig. 6, Plate XXL be an objeft, AB a double convex lens, whofe centre is C; and letnbsp;us examine the pencils of rays which come fromnbsp;three points only of the objedt, fince the fituationnbsp;of the intermediate pencils is evidently comprehended between thofe thr^e. Now of all the raysnbsp;which proceed from each of thofe points, that whichnbsp;paffes through the centre C of the lens muff (fromnbsp;what has been faid in page 227) proceed in a ftraightnbsp;diredion¹; fo that DC I, FCH, and ECG, arenbsp;ftraight lines; fecondly, the focus of the rays DBA,nbsp;after refraftion, muft be fomewhere in the axis ornbsp;ftraight line D C I5 alfo that of the middle pencil,nbsp;FB A muft be fomewhere in F CH, and the focusnbsp;of the third pencil muft be in E C G. Thirdly, thenbsp;refrafted focus of each pencil muft be on the contrary fide of the axis of the lens, to what its incidentnbsp;pr radiant focus is j for inftance, the refrafled focusnbsp;I is below the axis of the lens, whilft its incident ornbsp;radiant focus D is above it; and the refradted

The incident and refradted parts of that ray, properly fpeaking, are not in one ftraight line, but they are parallelnbsp;to each other j yet when the lens is not remarkably thick,nbsp;they may be fafely confidered as one ftraight line, the difference being infenfible.

focus G is above the axis, whilft its radiant point � IS below it: the confequence of which is, that if th�nbsp;objedl D E be fufficicntly luminous, and a piece ofnbsp;white paper, or other flat and opaque body, be fitu^nbsp;.ated at G I, an image of the objeft D E will benbsp;formed upon it, but in an inverted pofition. ifnbsp;the opaque body be removed, then no image willnbsp;be feen by a fpeftator fltuated on one fide j for thenbsp;tays of light, though they meet at their fefpedbiv�nbsp;foci in I H G, yet they proceed divergingly beyondnbsp;that place through the air or other tranfparent body,nbsp;and none come to the lateral fpeftator. If the papernbsp;be fituated nearer or farther from the lens than th�nbsp;place G I, then an imperfeft image, or no image atnbsp;all, will be formed upon ic, becaufe the rays of thenbsp;refpedlive pencils do not meet at any other place.

From what has been faid above with refpeft to the conjugate foci of the fame pencil, it will benbsp;clearly deduced, that if the objeft D E be broughtnbsp;nearer to the lens, the refraded foci, or the imagenbsp;GHI, will be formed farther from the lens, andnbsp;vice verja. And from this it follows, that (fince thenbsp;ingles D C E, G C I, formed at the centre of thenbsp;iens by the axes of the two extreme pencils, arenbsp;equal*) when the diftance of the objeft from thenbsp;lens is equal to that of the image from the lens,nbsp;then the fize of the image is equal to that of the

objeft; when the former diftance is kfs than that of the latter, then the image is larger than the ob-jedj and when the former diftance is longer thannbsp;the latter, then the image is fmaller than thenbsp;objecl.

With refpeft to the brightnefs of that image it muft be confidered, that of the innumerable raysnbsp;which are inceffantly emitted in every diredtion fromnbsp;each point, for inftance D, of the objeft, a confi-derabk number, viz. DAB, falls upon the lens,andnbsp;are converged to a fingle point I; therefore thatnbsp;point muft be more or lefs bright in proportion asnbsp;the furface of the lens is larger or fmaller. Hencenbsp;alfo a very remarkable property of thofe lenfes isnbsp;eaftly comprehended, which is, that when an imagenbsp;GHI, is thus formed, if you cover part of the lens,nbsp;be it the middlemoft or fome lateral part of it, thenbsp;image IG will not thereby be rendered partly invi-fible�the whole image will be feen as well as before,nbsp;but it will appear lefs bright than before; for if wenbsp;confider each indefinite part of the lens, we maynbsp;cafily perceive that rays of light from every pointnbsp;the objedb muft pafs through that part, and muft meetnbsp;at their refpedive foci in GHI.

The above explanation of the progrefs of various pencils through a convex lens, may, mutatis mU'nbsp;tandis, without much difficulty be adapted to explain the aftion of concave lenfes.

Upon the whole, it muft be recolledted, that by the asftion of a double convex lens, or plano-convex

and menifciis, the rays of light v/hich pafs through them are made to incline more towards eachnbsp;other, or to proceed lefs divergingly than theynbsp;did before, excepting after their meeting or foci jnbsp;for beyond that place they proceed divergingly.nbsp;On the contrary, by the adlion of double concavenbsp;plano-concave, and concavo-convex lenfes, thenbsp;rays of light which pafs through them are made tonbsp;diverge more, or to proceed lefs inclined towardsnbsp;each other, than they did before their entrance into

We fhall now deferibe a few eafy experiments, which prove in a familiar manner moft of the above-mentioned properties of lenfes.

Take an hollow globe of glafs, or globular decanter ; make an hole of about a quarter of the globe�s diameter in a piece of brown paper, andnbsp;ftick the,paper on one fide of the globe or decanter; fill the veffel with water, then prefent it withnbsp;the covered fide to the fun, and the rays which pafsnbsp;through the hole in the paper and through the water,nbsp;�will be colledled to a focus or round and well defined fpot, on the other fide of the globe, which maynbsp;be received upon a piece of paper; and its diftancenbsp;fiom the globe being meafured, will be found equalnbsp;to half the diameter of the globe. If the, experiment be repeated with the empty globe, no focusnbsp;will be formed (fee page 226). If, inftead of water,nbsp;the globe be filled with fpirit of turpentine, the focalnbsp;diftance will be found (horter than when water isnbsp;ufed, the refraftive property of that Ipirit being

greater

greater than that of water. If the experiment he tried with a folid globe of glafs, the diftance of thenbsp;focus from the neareft part of its furfaee will be equalnbsp;to one quarter of the globe�s diameter.

The reafon why one fide of the globe muft be covered with a perforated paper, is, that when allnbsp;the globe is expofed to the light, the rays whichnbsp;fall upon the more external places will have theirnbsp;refrafted foci nearer to the globe than thofe whichnbsp;fall nearer to die axis, all which foci form the cau'nbsp;flics by refraflion (fee page 220) which furyes mafnbsp;be obferved in the following manner ;

When the light of the fun, or of a candle, amp;c. is refrafled through a globe, b.e It of foljd glafs or anbsp;globular decanter filled with water, fpirit of turpentine, amp;c, and falls upon a table cloth, or upon anbsp;piece of white paper held parallel and very near tonbsp;the axis of the light, the luminous figure therebynbsp;formed, is bounded by two bright curves, which arenbsp;the above-mentioned cauftics; which, as they recede from the globe, approach each other and thenbsp;axis of the pencil, until they touch it, and there formnbsp;a fharp angle, whofe vertex is the principal focus ofnbsp;the pencil.

Having covered either fide pf a convex lens with paper, in which there are feveral fmall holes rnadenbsp;with a pin, and having expofed the lens direflly tonbsp;the fun, the rays which pafs through the holes willnbsp;appear like fo many white fpots upon a paper heldnbsp;pretty clofe behind the glafs; and thele fpots willnbsp;come clofer together as the paper is gradually drawn

tgt;ack. from the lens, until at laft they all unite in one fpot, which is the focus. This Ihews the reafon whynbsp;^hat fpot is fo very bright, and why it burns with fonbsp;*^uch power as experience (hews it to do. By inclining the lens a little, that focal fpot will not benbsp;fenfibly altered. If the paper be removed ftill farther from the lens, the fpots will feparate again, nornbsp;^'ill they ever meet again; for beyond the focusnbsp;the rays proceed divergingly.

If the above-mentioned experiment be tried with 5 concave lens, the fpots will never meet, but theynbsp;'vill be found to recede continually from each othersnbsp;in proportion as the white paper is removed fromnbsp;the lens.

�� Having found the focal diftance, E F, fig. J, XXL of a convex glafs lens, fix it flat againftnbsp;�t rnoderatc hole made in a thin board C E, andnbsp;PLce it upright upon a long table or floor. Throughnbsp;the point C, direflly under the middle of the glafs,nbsp;^�quot;aw a long Hire AB, perpendicular to the board, in ,nbsp;''''hich meafure the principal focal diftance of the lensnbsp;h'Om C to F, and from F to I, I to 11, .II to HI,

moving

moving the candle to II and the paper to the ray� will be united here alio 1 and likewife when th^nbsp;candle and paper are removed to 111 and IV andnbsp;I, amp;c. and the efFeci will be the fame if the pap^fnbsp;and candle be tranfpofed into each other�s places-It appears that f q decreafes in the fame proportionnbsp;as FQ_increafes, and the contrary.

� Things remaining as they were, when a fecond candle is placed on either fide of the firft at thenbsp;lame diftance from the glafs, the union of its raysnbsp;will make another image upon the paper q on thenbsp;contrary fide of the axis QJi q j and the diftancenbsp;between the two images will be found to bear thenbsp;fame proportion to the diftance between the can'nbsp;dies, as the diftance of the images from the glafsnbsp;bears to the diftance of the candles from the glafs-Theft obfervations confirm the reafon why thenbsp;image of a fingle candle is inverted upon the paper 5nbsp;and why its magnitude is altered when its place isnbsp;altered. Becatift wliat has been obftrved ofnbsp;candles is applicable to any tv/o points of the faut^nbsp;candle

If in cither of the above-mentioned experiments part of the lens be covered with fome opaque body�nbsp;fuch as a brov/n piece of paper, the whole imag^nbsp;of the fingle candle, or of the two candlesj willnbsp;feen exadtiy as when the whole lens is uncovered�nbsp;excepting that it will appear lefs bright in prop�'''nbsp;tion to the covered furface of the lens.nbsp;nbsp;nbsp;nbsp;'

OUR perception of objects produced by the aftion of light, is called vifion. The organnbsp;which receives the impreffion of that aftion, andnbsp;through which the perception is communicated tonbsp;the fenforium, is the eye^ of which t!ie human beingnbsp;has two, and of which no animal, that we know of,nbsp;has lefs than two.

Of the five external fenfes of our body, the conftruftion of the eye feems to be beft underwood ; yet this goes no farther than to Ihew that anbsp;pifture or image of the. objefts we perceive, isnbsp;painted within the eye, whilft the objefls are beforenbsp;and that the eye is conftrufted fo as to adaptnbsp;^ffelf to the formation of that image in differentnbsp;*^gt;rcuniftances j but we do not pretend to explainnbsp;^he manner in which the perception of that imagenbsp;communicated to the fenforium through thenbsp;^crve, upon a projeftion of which that image isnbsp;formed. R Jg neceffary, for the purpofe of explaining how that image is formed, amp;c. to defcribenbsp;the wonderful qonftruftion and action of the eye j

but In order to render its aftion as well as th� defeription of its parts more intelligible to -thenbsp;tiovice, we lliall prefix, a fhort defeription of thenbsp;principle of wnat is called a camera chfeura] or darknbsp;chamber.

If a fmall hole be made in the fide of a darkened room or box, and bright objecls, fuch as trees, animals, amp;c. illuminated by the dlreft raysnbsp;of the fun, happen to be out of the ropm, and orinbsp;that fide of the room where the hole is, then an inverted image of thofe objefts will be feen eithernbsp;upon the oppofitc w�all, or upon a fereen, vtdthinnbsp;the room; but that image is Imperfedl and in-diftinft. Vv^hen the hole is very fmall, as about anbsp;tenth of an inch in diameter, few rays can pafsnbsp;through it, and of courfe the image is faint. Whennbsp;the hole is confiderably larger, the various raysnbsp;which belong to each luminous or radiant poih�^nbsp;come divergingly through the hole, are fcatterednbsp;upon the wall or fereen, and great indiftindlionnbsp;enfues. In either cafe, the rays are more or lefsnbsp;infledled by the fides of the aperture.

If a convex lens be applied to the hole, then a beautiful image of the external objedls in their truenbsp;colours, but inverted, will be painted upon the fereennbsp;within the room; but the fereen muft be placed atnbsp;the focal diftance of the lens, and that focal diftancenbsp;changes according as the diftance of the externalnbsp;objfdls from the room is altered ¹: hence not all

the external objeds can be well defined and properly depided upon the fcreen for as they are at unequal diftances, the focal diftances within thenbsp;room mull likewife be unequal, and of courfe thenbsp;fcreen may be fituated properly for fome of them,nbsp;but not for them all; yet when the external objeds . are not very near, the fame fituation of thenbsp;fcreen will do for them all fufficiently to rendernbsp;them difcernible, and to form a plealant pidure ofnbsp;the whole.

In the latter cafe, when a convex lens is fituated at the hole of the dark chamber, various advantagesnbsp;are obtained by it. In the firft .placcj the pencilsnbsp;of light being refraded by the lens, are caufed tonbsp;converge to their relpedive foci; hence the holenbsp;and the lens may be large, in confequence of whichnbsp;a great quantity of light from every Angle point ofnbsp;the objeds is admitted; and fecondly, the inflection of light is avoided by the enlargement of thenbsp;hole.

It will appear from the following defcription, that the eye is a moft excellent camera obfcura, havingnbsp;�il the neceflary properties of it to a moft accuratenbsp;�^^ree of nicety. It is a dark room, with onenbsp;aperture for the admilTion of light, with lenfes fit tonbsp;form a pidure of external objeds on the hind partnbsp;of its cavity, and is capable of all the neceflary ad-juftrnents within certain limits.

-f^ig- 8, Plate XXL exhibits the fedion of a human eye, larger than its real or moft ufual fize. Its figure is nearly globular. What is feen of the

VOL. III. nbsp;nbsp;nbsp;eye

cye in 2 living fubje�t, is part of a convex white protuberance, with a circular tranfparent fpot in thenbsp;middle, which refts upon a radiated bafis, differentlynbsp;coloured in different perfons, amp;c. This vifible partnbsp;of the eye is reprefented by the front view, fig. 9�nbsp;and by theportion ABECD, in the fe�lionfig. �,BECnbsp;is the tranfparent part thereof. The hind pjrt AFDnbsp;is furrounded by fix mufcles, the extremities ofnbsp;which are firmly faftened to the external coat of thenbsp;eye, and are deftined to move it in the different ne-ceffary direftions. The particular defeription ofnbsp;thofe mufcles does not bfelong to this work.

The bulb of the eye confifts of the external coats or tunics ABE CDF A, and of internal hU'nbsp;mours that fill the whole cavity, and keep thenbsp;tunics inflated in a form nearly globular.

The membranes, coats, or integuments of the eye are as follows: the external firm one is callednbsp;the Jclerotica, the anterior portion of which, vi2*nbsp;BEC, is tranfparent and more convex, and is callednbsp;the cornea, (or tranfparent horny fubftance.) Thenbsp;portion adjacent to the cornea, viz. DC, is white;nbsp;hence it is commonly called the white of the eye*nbsp;The next membrane within the fclerotica, is callednbsp;the choroides. This is extended at ab under thenbsp;cornea, and forms the coloured part of the eye,

Ic, called the iris, (fee alfo fig. 9, which repre-quot; fents a front view of the eye.) This iris is form^*^nbsp;of mufcular fibres, difpofed in two directions, vi2*nbsp;fome are like radii, tending towards the centre, andnbsp;,nbsp;nbsp;nbsp;nbsp;�nbsp;nbsp;nbsp;nbsp;othet^

others are circular. The iris is perforated near its middle, and that perforation is called the pupil. ,nbsp;This pupil is not always of the fame fize ; for bynbsp;the contraftion or relaxation of the fibres of the iris,nbsp;the pupil becomes larger or fnaller, viz. when thenbsp;radial fibres contraft and the circular arc relaxed,nbsp;the pupil becomes enlarged �, but when the latternbsp;are contraTed and the former relaxed, then thenbsp;pupil becomes diminifiied*. The colour of thenbsp;iris is different in different perfons ; but it does notnbsp;feem that its colour is connedted with any peculiarity of conftitution, or of configuration of the human body.

Under the iris there is a prolongation df o( the choroideSj which forms a circular fibrous band, tonbsp;which the cryftalline humour d of 1% attached.nbsp;This circular band is called the ligamentum ciliare.

At the hind part F of the eye, but not exaftlyop-pofite to the pupil, a prolongation of the coats of the eye is to be obferved. This prolongation envelopes

* This is an admirable ftructure, which, whether in an enlarged or contradled hate, does always preferve the circularnbsp;figure of the pupil.nbsp;nbsp;nbsp;nbsp;, ,

The pupil is feldotn quite concentric with the iris. Its aperture varies conliderably, and differently with differentnbsp;individuals. In Corns perfons its diameter, in its contraffednbsp;ffate, is about one-tenth of an inch, and in the moll: en^^nbsp;larged ftate it exceeds a quarter of aa i.nch ; in others itnbsp;varies lefs.

R 2 nbsp;nbsp;nbsp;a nerve

2^14 nbsp;nbsp;nbsp;J^ejcripticn �f the Eye, i�c.

a nerve that comes from the brain, and is called the optic nerve, the inner or medullar part of whichnbsp;fpreads itfelf over the choroides, as far as the ciliarynbsp;procefs df, and forms the innermoft coat of the eyegt;nbsp;called the retina. This is a thin and whitilh membrane, looking like the fineft fort of net-work, or ofnbsp;linen¹.

The above-mentioned membranes contain three tranfparent humours, which are called the aqueous,nbsp;the cryjialline, and the vitreous.

The cryjialline humour dosf \s a confiftent cellular tranfparent fubftance in the fliape of a double convex lens, whofe hind lurface d o � is more convexnbsp;than the other furface' dsf\. The edge of this lens is

The infertion of the optic nerve is not exaftly in the axis of the eye (viz. in the ftraight line E I, which is fup-pofed to pafs through the eye in a dire�tion perjjendicularnbsp;to the cornea, and to the cryftalline lens;) but in each eye itnbsp;deviates from the axis towards the nofe, by about the 14thnbsp;part of the whole circumference of the eye : but this diftancenbsp;is not the fame in all eyes.

For a more particular account of the membranes of the eye, as alfo of the veflels, glands, amp;c. that belong to thenbsp;fame organ (which would be unneceflary for our prefentnbsp;purpofe,) fee the anatomical writers.

f It is pretty much the opinion of anatomills, that the cryftalline lens confifts of mufcular fibres. Its fpecificnbsp;gravity is about 1,1, viz. very little above that of water. Itsnbsp;r.onfiilence, and of courfe its refra�live power, is not uniform

Dejcription of the Eye, ^c, nbsp;nbsp;nbsp;245

attached all round to the ligamentum ciliare. All the cavity between the cornea and the cryftallirie humour,nbsp;IS filled with a fluid called the aqueous fluid *. Thenbsp;remaining, which forms the greateft, part of thenbsp;Cavity of the eye, viz. of m I k do, is filled with another fluid called the vitreousf.

The figures 8 and 9, reprefent the eye, for the fake of perfpicuity, larger than its natural fize, andnbsp;^ot in exaft proportion ; but the proper dimenfions,nbsp;as deduced from a great number of actual mea-forements, will be found in the note

form throughout. � On the whole. Dr. Thomas Young � fays, it is probable that the refraftive power of the centrenbsp;� of the human cryftalline, in its living Hate, is to that ofnbsp;� Water nearly as 18 to 17; that the water imbibed afternbsp;death reduces it to the ratio of 21 to 20; but that, onnbsp;^ account of the unequable denfity of the lens, its effedhnbsp;' in the eye is equivalent to a refraflion of 14 to 13 for itsnbsp;whole fize.� Phil. Tranf. vol. for i8oi, p. 42.

* This is a very lijnpid water, and like water in refpecl ^ confiftency, fpecific gravity, and refradlive power.

This, which is by far the moft abundant humour of eye, confifts of fmall cells diftended with a limpidnbsp;'''atery fluid, ft but a little exceeds water in refpedt tonbsp;fpecifle gravity and refractive power.

246 'E^Jcription of the Eye, ^c.

When the eye is open, and illuminated objedls are before it, inverted pidtures of thofe objedts arenbsp;formed upon the retina, by the refradtive powers ofnbsp;the above-mentioned three humours. If the thickernbsp;coats on the back of a frefh eye be removed, andnbsp;the eye thus prepared be turned towards objects thatnbsp;are well illuminated, their pidtures may be clearlynbsp;perceived through the remaining thin coat.

Whoever will trace the progrefs of parallel rays (viz. fuch as come frorft a very diftant luminousnbsp;point) which may be eafily done from the meafure-

Thicknefs of the fclerotica, about . nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;_ o,o25

The fine of the angle of incidence to the fine of refraction at E, viz. from the air into the cornea and aqueous humour, is (as from air into water) or as 4 to 3 nearly*nbsp;At j, viz. from the aqueous into the cryftalline, the rationbsp;of refradfion is nearly as 13 to 12. At 0, viz. from thenbsp;cryftalline into the vitreous humour, the ratio of re�ad�oanbsp;is nearly as 12 to 13.

�merits that are mentioned in the note, and from �what has been faid in page 190, will find, that bynbsp;*^he refra�tions of all the humours through whichnbsp;they mull pafs, they will be colleded to a focus onnbsp;the retina, which therefore is the true place of thenbsp;image. But at the fame time it is evident that ifnbsp;that be the focal diftance for parallel rays, it cannot be the focal diftance for diverging rays; or, innbsp;other words, when the objefts are fituated at a fewnbsp;feet diftance from the eye, their true images muftnbsp;be formed farther back ; confequently their imagesnbsp;hpon the retina muft be imperfe6|:, unlefs the retinanbsp;be fituated farther back by an elongation of the axisnbsp;of the eye, or the focal diftance be (hortened by thenbsp;alteration of fome other part. But fince we maynbsp;perceive either diftant or near objeds diftindly, itnbsp;is evident that fome fuch alteration does aduallynbsp;nnd neceflarily take place. This is called the ad-jnftment or accommodation of the eye for diftindnbsp;'I'ifion; but the difficulty is to determine how thisnbsp;^djuftment is effeded.

By fome perfons it has been attributed to a change in the length of the eye, and by others to anbsp;change of curvature-in the cornea ; but fome verynbsp;��ccent experiments render thofe alterations unlikely,nbsp;^t leaft to the full amount of what may be required.nbsp;Other ingenious perfons have'attributed the alteration to a change either of the fhape of the cryftal-hne lens, or of its fttuatLon, or of bqth; and this

That the eye cannot fee both near and remote cbjefts diftindtly at the fame time, may be eafil/nbsp;proved. Let a tree, a houfe, or fome other ob-jeft be upwards of 50 feet from you; fhut one eyegt;nbsp;and whilft you are looking with a fingle eye at thenbsp;tree, amp;c. hold a pin, a pencil, or fome other objeftnbsp;in the fame direction at about a foot diftance fromnbsp;the eye; and it will be found that whilft you feenbsp;the pin diftinftly, the tree will appear indiftinftjnbsp;but if you adjuft your eye fo as to fee the tree dif-tinftly, then the pin will appear indiftind. .

The eyes of fome perfons are more capable of adjuftment than thofe of others. In old perfons thenbsp;humours grow thicker, and the parts lefs pliable;nbsp;hence their eyes are lefs capable of adjuftment thannbsp;in young perfons.

The eyes of fome perfons can be adjufted for dif-taut objedts. better than for near objedts, and vice ver/a. When the eye is defedlive, and by its fizenbsp;or other conformation, parallel rays form their focinbsp;before they arrive at the retina, then the perfon cannbsp;fee very near objefts only. Such perfons are faid

For farther information on this fubjedf, the reader may perufe Prieftley�s Hiftory of Vifiqn, Light, and Colours; OVoamp;t^'�s de oculi mutatimibus internis-, and Young�¹nbsp;Paper in the Philofophical Tranfadtions, vol. for i8oigt;nbsp;Art. II.

to be near-fighted, or they are called myopes. Vv hea the eye is flatter than ordinary, then the foci of raysnbsp;htoin pretty near objedls are formed beyond the retina. Perfons with fuch eyes are callednbsp;nbsp;nbsp;nbsp;; �

they can adjuft their eyes for objefts beyond a certain diftance only. The latter is generally the cafe 'vith old perfons j but the eyes of old perfons fome-tirnes are incapable of adjuftment both for very nearnbsp;�nd for very diftant objefts. This comes from anbsp;tigidity or want of pliability in the parts ¹.

Thofe imperfeftions may in great meafure be temedied by the ufe of proper glalTes or 1'pedacles;nbsp;for fince in near-fighted perfons the rays of lightnbsp;converge to a focus too loon, viz. before theynbsp;come to the retina, concave lenfes, which diminiflinbsp;the convergency, mufl: remove the imperfeftion.nbsp;And for thofe who can fee diftant obje�ts only withnbsp;tolerable diftinfbion, viz. in whofe eyes the rays donbsp;converge foon enough, convex lenfes, whichnbsp;tocreafe the convergency, muft remove the imper-

When the defe�l comes from rigidity, as in fomc �^id perfons, then thofe perfons require concavenbsp;Slaftbs for viewing diftant objefts, and convex glalTes

Thofe defers are frequently brought on or increafed y habit, as by the conftan^ cuftom of viewing objedtsnbsp;either from too near or from too great a diftancej as alfa

The

* The eflential and extenfive ufe of fpeflades, which affords comfort to fo great a number of individuals, whonbsp;would otherwife be a burden to themfelves and to focietygt;nbsp;is an inftance of the great ufefulnefs of the fcience of optics.

No pains have been fpared to render fpecfacles as perfect as poffible, and a variety of contrivances .have beef* from time to time offered to the. public. Spedtacles havenbsp;been made with two lenfes for each eye; alfo th� lenfesnbsp;have been made plano-convex or plano-concave, or of othernbsp;fhapes; but upon the whole, fingle lenfes, either doublenbsp;concave, or double convex, of clear glafs, well polifhed andnbsp;regularly formed, are the beff.

When the eyes of perfons firft begin to be affeiSed by age, the opticians furnifh them with fpectacle lenfes, ofnbsp;about 40 inches focus, which glafles are therefore callednbsp;number iff, or glafies of the firft fight; viz. for the fighfnbsp;when it firft begins to be impaired by age. But I find con-fiderable difference between the focal diftances of fpedtacles�nbsp;N� I. made by different opticians. When the focal length*nbsp;is about 16 inches, the lenfes are called Nquot; 2. About twelvenbsp;inches is the focal length of N� 3. Ten inches is what thefnbsp;call N� 4. Nine inches is that of N� 5. Eight inchesnbsp;the focal length of N� 6. Seven inches is the focal length ofnbsp;number 7- Six inches is the focal length of N� 8.;nbsp;fometimes they jnahe fpetlacles of a focus fliorter ftd^'nbsp;Concave fpedfacles are alfo named by fimilar numbers.

The capability-of adjuftment is greater or kfs in different eyes, and it is - frequently different in thenbsp;two eyes of the very fame perfon �, but in all eyesnbsp;tHere is a limit, within which vifion is not diftindt.nbsp;This is called the limt of dtftinSi vifion; and withnbsp;^orne perfons it is as fhott as one inch, whilft innbsp;others it exceeds 20 inches ; but in common it willnbsp;found to lie between fix and 10 inches.

All the retina, as far as it is extended, is capable receiving the moft perfcdt image of objedls.nbsp;There is, however, a fingle fpot where no vifionnbsp;takes place j and this fpot, which is about a 40thnbsp;of an inch in diameter, lies exadly'upon the infer-lion of the optic nerve; fo that we cannot perceivenbsp;the image of any cbjedl that falls upon this fpot atnbsp;the hind part of the eye, provided the other eye benbsp;^ot. The exiftence of this (which we may callnbsp;thanis j,j4 fufneient either for reading, or for other neceffarynbsp;P�^^tpofes.

^Vhen a variety of fpedfades cannot aftually be tried, the '^^fedl of the fight may be expreifed by mentioning the dlftanccnbsp;otn which the perfon can read, or other peculiarities, fromnbsp;'''hich the neceffary glaffcs may be determined pretty nearly.

�nftrument for meafuring the exact limits of difiinCl ''dioii Was feme years ago contrived by Dr. potterfield,nbsp;''ho named it an Ofiemeter (fee his Work on the Eye,nbsp;h) and an improved one for the fame purpoie wasnbsp;^tely contrived by Dr. Thomas Young. See his Paper onnbsp;^ e Mechanifm of the Eye, in the Philofophical Tra;ifac-

Let three pieces of paper of different fhapes, Agt; Bj C, fig. ic, Plate XXL be faftened on a wallgt;nbsp;at the diftance of about two feet from one another,nbsp;and let a perfon, keeping one of his eyes fhut, placenbsp;himfelf nearly oppofite to the middle paper B, andnbsp;beginning pretty near to it, let him retire graduallynbsp;backwards, whilft the open eye is turned obliquelynbsp;towards the outfide paper, viz. that paper whichnbsp;is next to the eye that is fhut-, he will find a fituatioonbsp;(which generally is at the diftance of about lo feetnbsp;from the papers) where the middle paper will en-tirely difappear, while the outermoft papers continue vifible. In that fituation the image of thenbsp;middle paper falls exaftly upon the infertion of thenbsp;optic nerve.

This obfervation has been often adduced as the foundation of an argument to prove, that the featnbsp;of vifion is not exaftly at the retina, and that eithernbsp;the choroides or fome other part of the eye receivesnbsp;the impreffion of light, amp;c.: but as nothing pofitivenbsp;is known with refpeft to this fubjedt, viz. of thenbsp;manner in which the perception of objedls is conveyed to the fenforium; and having fhewn that anbsp;pidture of the objedls, amp;c. is adtually painted on thenbsp;retina, which is going as far as we can in tracingnbsp;the adtion of light; I fhail not detain my readernbsp;with long and unprofitable difquifuions relativenbsp;to it.

There

There is another remarkable adjuftment of the that requires to be explained; and this is thenbsp;^ontraftion and enlargement of the pupil.

It has been fliewn above, that of the innumerable ^ays which proceed from every Angle point of aiinbsp;caleris paribus, a greater or lefs quantitynbsp;lalls upon a lens in proportion as the lens is larger ornbsp;Analler, and in the fame proportion is the refradfednbsp;focus or image, more or lefs bright. Now, by in-fpcdling fig. 11, Plate XXL it will appear, thatnbsp;^fter the fame manner more or lefs rays from everynbsp;fingle point of the objedt A C B, will enter the eyenbsp;proportion as the pupil is open more or lefs, andnbsp;the correfponding points a, b, c, of the image willnbsp;proportionately more or lefs bright. But as thenbsp;^�ght from certain objefts, fuch as the fun, a brightnbsp;amp;c. would be hurtful to the eye, and in othernbsp;t^afes the infufficient quantity of light would ren-the perception of objedts too faint; thereforenbsp;Ptovident nature has furniflied the eye with a me-of enlarging or contradling its aperture, whichnbsp;^ffedted by the adlion of the iris, which, as hasnbsp;fliewn above, is a prolongation of the cho-*'Qides j and fo eafy and involuntary is the contrac-of that membrane, that without the lead confi-Station we readily adapt it to receive a propernbsp;quantity of light in mod cafes. Let a perfon turnnbsp;^ eyes towards a pretty dark place, and in thatnbsp;ation by looking at his eyes, you will find thenbsp;Pupils much dilated; then place a lighted candle

before bis eyes at about three or four inches diftanc^' and you will perceive the pupils to become remarkably narrow.

In fome perfons the pupil is in ail cafes larger thaf^ in others, nor can they contraft it fufficiently. Sud'nbsp;perfons fee befl; with little light. Other perfoo*nbsp;have their pupils naturally narrower than ordinary�nbsp;and of courfe thofe perfons fee beft in a brigh*-light. Sometimes the pupil lofes its contraftio*�nbsp;entirely.

Though a more open pupil will admit more lig^�' than one which is lefs open, and of courfe objefts tha*-are lefs luminous may be perceived by the fortfl^*^nbsp;eye than by the latter; yet the total want of lig^^nbsp;renders object's invifible to any eye. Fair and ik'nbsp;tisfacftory experiments prove that, in a room p^^'nbsp;feftly dark, no objedt can be perceived even by tk�nbsp;eyes of a cat.

Having defcribed the ftruflure of the human eVC? and the progrefs of light through it; our next oO'nbsp;je�t is to explain feveral phenomena of vifio*^'nbsp;which' othervvife might be confidered irreconcileabi^nbsp;to the common theory of light; and in thenbsp;place, it may be naturally inquired how is it that'''�nbsp;perceive objedts fingle, if they are llngle, or of th^knbsp;real number, though we look at them with twonbsp;and though a pidture of each objedt is formednbsp;each eye.

Of the various opinions, which have been vanced in explanation of this difficulty, the. tnoft

tisfadory

tisfaftory is, that in the two eyes there are cotref-ponding parts of the retinas which are probably fufeeptibie of the fame imprefiion in equal degree,nbsp;3nd convey it to the fenforium in that equal degree:nbsp;hence as long as fimilar points of the iftiages fallnbsp;��'pon the correfponding points of the retinas, thenbsp;perception of the fame object is fingle, otherwifenbsp;is double.

Fig. 12, Plate XXI. exhibits two eyes directed to the fame objeft A B; and it is likely that at oppo-fite diftances from the infections of the optic nervesnbsp;the retinas have correfponding tenhons, irritabilities,nbsp;orfufeeptibilities; for ihftance, a may correfpond tonbsp;^ to ^; and as long as the like parts of the imagesnbsp;fall upon thofe correfponding parts, the obje�t appears fingle. It is evident that for this purpofe thenbsp;^xes of the eyes, that is the eyes themfelves, mullnbsp;he turned more or lefs towards each other, accord-as the object to which they are dlredted isnbsp;Clearer or farther, and this is aftuaily the cafe ; fonbsp;that according to the diftance of the objedt, we notnbsp;t^tily adjuft each eye for dilliud: vifion at that dif-tance, but alfo adjuft the diredlion of both eyes innbsp;^tder to produce fingle vifion of fingle objects. Innbsp;^Confirmation of this theory, hold up a finger beforenbsp;your eyes at the diftance of 8 or 10 inches, vvhilftnbsp;^ ftian, a window, c.r other object, is before you atnbsp;^ much greater diftance, and you will find that ifnbsp;you endeavour to look fteadily at the finger, viz. bynbsp;^iireding the axis of both eyes towards it, the man,

^cc. will appear not only indiftindt, but alfo dbiibte* If ygu endeavour to fee the man diftindtly an^nbsp;fingle, the finger will at the fame time appear double and indifiinft.

It is from this adjuftment that we are in great meafure enabled to judge of the diftances of ob-jeAs, when thofe diftances are not very great. Itnbsp;is from this adjuftment, or from the direAion ofnbsp;the two eyes, that we judge whether a perfon isnbsp;looking at us or not.

When from particular configuration, or from bad habit, the axes of the two eyes do not t ppear to benbsp;direded to the objeA which they aAually have innbsp;view, then the perfon is faid to Jquint. But it doesnbsp;not follow that the fquinting perfon fees every ob'nbsp;jeA double ; for the apparent improper direAioUnbsp;of the eyes may be owing to the unufual lituatioUnbsp;of the parts of the eye j yet the like parts of thenbsp;two images may fall upon correfponding parts ofnbsp;the retinas.

In the next place it may be inquired how do perceive objeAs ereA or in their proper fituationS)nbsp;confidering that the image is inverted upon the rO'nbsp;tina. Various opinions have been advanced in explanation of this difficulty; but the moft plaufibl^nbsp;is, that the mind contemplates the objeA and not i^snbsp;image, and that by experience we are accuftornednbsp;to confider the lower part of the piAure as indicating the upper part of the objeA, and vice verja.nbsp;by referring the fituation of objeAs to other fut'

Snc^whether he turns his head one way or the othergt; and even upfide down, the houfe does always appear ereft.

The perceptions of our fenfes are fo difficultly inveftigated, and fo influenced by affuefaftion, amp;c.nbsp;that we can hardly comprehend any of them withnbsp;full certainty and fadsfadlion.

Our judgment of the diftances as alfo of the fize of ohjeifls which we perceive by our fight, is influenced bynbsp;the concurreiice of leveral circumftances; viz. w'e arenbsp;flirefled to form ourjudgment, i ft, from the apparentnbsp;tnagnitude of the objects ; 2dly, from the ftrength ofnbsp;the colouring and diftindtnefs of their minute parts;nbsp;�3^1ygt; from the direftion of the two eyes; and 4th]y,nbsp;from their ficuation in relation to other objedts. Andnbsp;Ourjudgment is more, or lefs liable to be wrong, according as one or more of thofe circumftances arenbsp;tvanting. Thus a perfon with one eye is lefs capable to judge of the diftance of an objedt than anbsp;with two eyes, as in that cafe the third cir-oomftance is wanting. Thus if a man fix feet high

fituated at 40 feet diftance from us, and a boy 3 feet high be fituated at 20 feet diftance, they willnbsp;fr'btend equal angles at our eyes, and thereforenbsp;^bey Ought to appear equally high ; yet from thenbsp;frfmation of their limbs, and their fituation relativelynbsp;^o other tabjedls, we do by no means think themnbsp;equally high,

^ fmall objedt near us, and a layge one at a pro-poitionate diftance, fubtend tire fame vilual angle ;

but the diftanc objeft appears indiftinft. Hence if 3 fiTiali and near objeft is by any means rendered in-diftindl, we are apt to take it for a large diftant object. Thus a fly or other infeft often paffes by ournbsp;eyes, when the eyes are diredted to fome other ob-jedl; in which cafe the fly appears indiftindl, andnbsp;we frequently take it for a crow at a diftance.

When the moon is near the horizon, the thick-nefs of the atmofphere renders it lefs bright and lefs diftindb than when it is higher up : hence we imaginenbsp;it to be farther off in the former cafe than in thenbsp;latter; and'becaufe we imagine it to be farther offgt;nbsp;we take it to be a much larger objedl than when itnbsp;is higher up; in which fituation we imagine it to benbsp;nearer to us, from its appearing much brighter.nbsp;For it appears from actual meafurement, that thenbsp;, fize of the moon is fmaller near the horizon thannbsp;when it ftands higher up. So that this v/ell knownnbsp;phenomenon of the horizontal moon is merely an iknbsp;lufion.

The very remarkable exhibition made in London for fome yeats paft, under the name of Panoramtifnbsp;produces a furprizing effedl from the fame above*nbsp;mentioned caufes. A circular pidlure, in a circulatnbsp;building whofe diameter is about 40 feet, is exhibited to the fpedator, who ftands near the centranbsp;of the circle, and every other object with whic^nbsp;the painted objedls might be compared, arenbsp;moved from his fight; in confequence of which, an�1nbsp;on account of the indiftinftnefs of the painted ob-9

lie is led to imagine that they are real ob-jefts of the natural fize at much greater diftances. With relpe�t to apparent motion, our judgmentnbsp;likewife apt to be miftaken j for when our eyesnbsp;diredled to any particular objeft, and follow itnbsp;^nfenfibly, every other objed; which deviates fromnbsp;�^hat diredion, is frequently taken for a movingnbsp;objed. Thus when the clouds are paffing fwiftlynbsp;by the moon, if we look fteadily at the clouds, thenbsp;fttoon appears to run fwiftly by. If we look fteadilynbsp;at the moon, then the clouds appear to move onnbsp;fapidly. Thus alfo a perfon in a boat, keeping hisnbsp;^yes either immoveable, or looking at fome part ofnbsp;boat, will frequently imagine that the coaft isnbsp;*^ving away.

A queftion is frequently aflced with refped to Our perception of black objeds, viz. that lincenbsp;blacknefs is a privation or abforption of all colours,nbsp;quot;'hat do we fee when we perceive a black objed ?nbsp;*�'he anfwer to this queftion is, that we fee not thenbsp;black objed itfelf, but we fee the objeds that fur-^uund it, the boundaries of which on that fide arenbsp;*-he fame as the boundaries of the black objed. Anbsp;hole from which no light is refleded, and anbsp;black fpot of the fame fize appear alike to our eyes.nbsp;When we look at a black hat, or other like objed,nbsp;perceive the bendings, edges, and other prominent parts of it, becaufe thofe parts are not perfedlynbsp;�lark; but they refied fome light to our eyes, fuffi-�^icnt to diftinguifli one part from another,

The above-mentioned deceptions, to which our eyes are liable, inftruct us not to believe the fup-pofed infallible evidence of the fight, when reafon isnbsp;againft it.

There is one phenomenon more of fimple fight, which deferves to be explained before we pafs on tonbsp;the exarrination of vifion through ienfes.

When the eye-lids are pretty clofe, or almoft Ihut, and efpecially when they are moift, on lookingnbsp;at a candle, two long irradiations NM are feen tonbsp;dart from the candle upwards and downwards, as innbsp;fig. 13, Plate XXL the caufe of which is, thatnbsp;the rays of light which fall upon the edge of thenbsp;lower eye-lid, as at I, are by it reflected into thenbsp;eye at L D, where it forms a long fpeCtrum, onnbsp;account of the curvature of the edge of the eyelid; and for the fame reafon the rays w'htch fallnbsp;upon the edge of the upper eye-lid at H, are reflected by it, and form the long fpeCtrum O X, onnbsp;the oppofue fide of the eye. In faCt, if by ih^nbsp;jnterpofition of an opaque body P, the upper raysnbsp;be intercepted, then the lower fpeCtrum or irradiation will vanifli; and, if the lower rays be in*nbsp;tercepted, then the upper irradiation will vanilh.

The eyes of different perfons, in all probabiHtygt; do not receive the fame imprcffions from the fam^nbsp;colours} and this' is fometimes the cafe withnbsp;fame perfon at'difFerent times, efpecially whennbsp;body is not in a found ftate. To fuch perfons all

objects fometimes nppear tinged either yellow,

green, or red, amp;c. Several cafes are recorded, in the Philofophical and other Tranfadlions of learnednbsp;focieties, of perfons who laboured under thcfenbsp;imperfeftions of fight, as alfo of fome who couldnbsp;not diflin^iifli certain colours from one another,nbsp;fhofe imperfedlions may arife from a mixture o!nbsp;particular juices with the humours of the eye, ornbsp;firom particular configurations, with which howevernbsp;�We are not acquainted.

With refpeft to the phenomena of vifion througla glafs lenfes, perhaps what has been faid in the preceding pages might be deemed fufficient, viz. thatnbsp;convex lenfes, by inclining the rays more towardsnbsp;each other, before they come to a focus, in-creafe the vifual anglCj and enlarge or magnifynbsp;the appearance of the objeft ; and that, on thenbsp;Contrary, the concave lenfes dirainifh the vifuainbsp;^ngle, amp;c. But notwithflanding the univerfalitynbsp;this principle, moft beginners find ft difficultnbsp;comprehend the real adtion of convex lenfes,nbsp;�tgt;d to account for ajl their effedls. It will therefore be neceflfary to .give a more particular expla-t^ation of the effefts of the laft mentioned lenfes,nbsp;^fpccially as the fame is of confiderab'.e affiftance innbsp;^^plaining the properties of moft ol the opticalnbsp;hiftruments, which will be deferibed in the nextnbsp;chapter.

^^'hea an objeft, fituated in the focus of a. con-'''sx lens, is viewed by an eye fituated on the other fide of the lens, that objeift will always appearnbsp;^^rger than it would if tlie lens were not inter-

pofed : but if, when the lens is removed, the objedfc be brought nearer to the eye, then ic will appear asnbsp;large as it did in its former fituation, viz. whennbsp;it was viewed through the lensj for by bringing thenbsp;objeft nearer to the naked eye, the vifual angle isnbsp;enlarged, as it was enlarged in the former cafe bynbsp;the refra�lion of the lens that was interpofed.

Could we bring objefts unlimitedly near to our eyes, and could we adjuft our eyes for viewing tholenbsp;objedts diftindlly at any diftance, then convex lenfesnbsp;would be ufelefs. But our eyes are capable of ad-jullment within certain limits, viz. for rays thatnbsp;come from any fingle radiant point, either parallelnbsp;or nearly fo. When the objedt is nearer than fix,nbsp;or eight, or ten inches, the rays are too divergentnbsp;for the eyes of moft perfons. This diftance therefore,nbsp;viz. eight inches, may be reckoned as the ordinarynbsp;limit of d^ftindl vifion.

It has been alfo obferved, that few men can diftin-guifh an objedt which fubtends at the eye an angle fmaller than half a minute: therefore an objedl,nbsp;whofe diameter is fmaller than the chord of half ^nbsp;minute to a radius of eight inches, is the leaft objectnbsp;which the naked eyes of moft men can diftinguilh-The diameter of fuch an objedl is o,ooii6 of annbsp;inch ¹.

Now the great ufe of lenfes is, to enable us to diftinguilh objedls that are otherwife invifible to

us, viz. when the whole obje�t, or any part of it that uiay be defired to be diftinguifhed, fubtends annbsp;angle fmaller than half a minute, or is- finallernbsp;than 0,00116 of an inch. The lens, or the combination of lenfes, that perform this office are callednbsp;^nicrojcopesand in the former cafe it is called anbsp;fngle microjcope, whereas in the latter it is called anbsp;compound microjcope.

A fingle lens (as has been already obferved in the preceding pages) when an objedt is placed beforenbsp;h, converges the rays of each radiant point to anbsp;focus on the other fide, where, if a fcreen be fitu-^ted, an image of that objeft will be formed; other-wife the rays will proceed divergingly beyond thatnbsp;focus. It muft likewife be recolledted, in confequencenbsp;tgt;f what has been faid above of the conjugate focinbsp;�f a lens, that if the conjugate foci are equidiftantnbsp;footn the lens, then the image will be equal to thenbsp;otherwife the fize of the one will exceed thatnbsp;the other, in proportion as the former is farthernbsp;fooni the lens than the other. Or in other words,nbsp;foe lengths of the objedt and of the image are asnbsp;foeir refpedlive diftances from the lens. Now withnbsp;^^fpedl to the eye, it muft not be imagined that thenbsp;fons forms, by its refradlion, an image of the objedlnbsp;the retina, in the fame manner as it forms thenbsp;^nnage upon the fcreen ; becaufe from the lens towards the fcieen the rays proceed conyergingly tnbsp;whereas, if proceeded convergingly to the eye,nbsp;foe humours of that organ would converge them a

great deal more than is neceflary to form a focus of image upon the retina. But the real office of a lensnbsp;that is adapted to the eye, is to render the rays ofnbsp;every fingle radiant point parallel j for the eye receives thofe parallel rays, and in virtue of its ownnbsp;refradtive power, converges them to a focus, ofnbsp;forms an image of the objefl upon the retina.

Fig. 14, Plate XXL reprefents an objeft AB fituated at the principal focal dilhance of the lensnbsp;�D: therefore, from what has been faid above,nbsp;(page 218,) the rays which proceed from everynbsp;fingle radiant point, as A, or I, or B, or any other,nbsp;fall divergingly upon the lens, and after havingnbsp;palfed through it, proceed in a parallel direftion,nbsp;viz. the conical rays A D E proceed cylindrically,nbsp;or become the parallel rays EP, CO, D W ; thenbsp;rays IDE become the parallel rays EF, CG,nbsp;DH i and fo on. Now if an eye be fituated nearnbsp;the lens, and QJR.� be confidered as the diameternbsp;of the pupil, it is evident in the firft place, that thenbsp;rays from every fingle point of the objeft, enternbsp;the pupil in a parallel direftion ; fecondly, the extreme rays, AEQ^and BDR, enter the eye andnbsp;limit the field of view j for the other rays, as ACO,nbsp;A D W, amp;c. which come from the fame extremenbsp;point A, do not enter the pupil Qji. Hence itnbsp;that if part of the lenfe�s furface be covered, a pot'nbsp;tion of the objeft will likewife be rendered invifibL�nbsp;or, which is the fame thing, the field of view is ptO'nbsp;portionate to the aperture of the lens. Thirdly, tt

appears that the objeft will be feen diftinftly, whether the eye be placed nearer to or farther from the lens 5 but a fmaller part of it will be feen when thenbsp;pupil is farther off, as at S T, than when nearer, asnbsp;at QJ^ ; becaufe, when 'S T is the aperture of thenbsp;pupil, the extreme rays AEB, BDM, which entered it at QjR, will not now enter it. Fourthly,nbsp;�with refpeft to the magnifying power, it inufl: benbsp;obferved that the axes of any two pencils, as for in-ftance, the axes AGO, B C L, form an angle AC Bnbsp;at the centre of the lens, which is equal to the anglenbsp;E^/D, formed at the eye by the extreme raysnbsp;A E B D which arifes from the parallelifm ofnbsp;the rays AGO, Ed, and BCL, D d. Thereforenbsp;the diftance of thofe tw'o points, or the length ofnbsp;the objefl A B, will be feen under the fame anglenbsp;of vifion as if the naked eye were fituated at C ;nbsp;but the naked eye cannot fee an objedl diftinftly atnbsp;^ diftance lefs than 8 inches j therefore the eye atnbsp;or at ST, will be enabled, by the adtion ofnbsp;the lens, to fee the objedt AB enlarged ; as if thenbsp;Oaked eye itfelf, fituated at C, faw the objedt at thenbsp;biftance GY of 8 inches, and as large as ZX. Sonbsp;that the fize of the image is to the fize of the objedlnbsp;8 inches is to the focal diftance of the lens. Thusnbsp;^ lens, whofe principal focal diftance (or focus ofnbsp;parallel rays) is one inch, will magnify 8 times. Ifnbsp;^he focal length be half an inch, the lens will magnify 16 times, and fo forth. In Ihort, to find thenbsp;rnagnifyiog power of a lens, divide 8 inches (or

the fhorteft diftance of diftinfl vifion for any parti' cular eye) by the focal length of the lens, and thenbsp;quotient fhews how many times the length or diameter of the image will exceed that of the objeft*nbsp;The magnifying powers of lenfes are generally ex-prefled by the length or diameter of the image,nbsp;otherwile called the lineal dimenfons. Thus when anbsp;kns is faid to magnify the objeft four times, thenbsp;meaning is, that the image, or the appearance ofnbsp;the objefh through the lens, is five times as long ornbsp;as broad as the objed: itfclf. Some writers fome-times reckon the magnifying power by the furface,nbsp;and others even by the fblidity. Thus fpeaking ofnbsp;the fame above-mentioned lens, it may be faid thatnbsp;k magnifies five times in length, or 25 times in fur-face, or 125 times in folidity; fince the furfaces ofnbsp;firnilar bodies are as the fquares of their lengths, ornbsp;of other like drmenfions, and their folidities are asnbsp;the cubes of their lengths, or of other like dimen-fions.

It has been mentioned above that the fpherical curvature of lenfes does not converge the rays ofnbsp;the fame radiant point exactly to one refraifled focus,nbsp;and that the aberration or indiftinftnefs which arifesnbsp;therefrom, increafes w;ith the thicknefs of the lensnbsp;and with the increafe of curvature. But we muftnbsp;here farther obferve, that from the properties ofnbsp;fpherical furfaces, from the refradion of glafs, amp;c.nbsp;it has been demonftrated by the writers on Optics,nbsp;that the aberrations of lenfes, which have the fame

curvature but different apertures (viz. areas) are as the cubes of the apertures refpeftively; and thatnbsp;�when the lenfes have equal apertures, then the aberrations are inverfely as the fquares of the radii ofnbsp;curvature Whence it follows, that when largenbsp;Menfes which do not magnify much, are ufed fingly,nbsp;the aberration is tolerable in mofl cafes; but whennbsp;the lenfes have a confiderable magnifying power, andnbsp;^re to be ufed for nice purpofes, then the aberrationnbsp;or indiftinftnefs is very detrimental.

In order to obviate this inconvenience, various contrivances have been offered; but the only onenbsp;which anfwers the purpofe to a confiderable degree,nbsp;IS a combination of two fiiallow lenfes, fet at anbsp;little diftance from each other, which are ufed as anbsp;fingle lens.

Fig. 15, Plate XXL reprefents fuch a com-hination of two lenfes; and they are plano-convex, hnce that figure admits of lefs aberration than anynbsp;other. Let F be the focus of the fingle lens N M;nbsp;lo that an object placed at F may be feen magnifiednbsp;through that lens. Now when the other lens GHnbsp;ts placed between the firft lens and its focus, thenbsp;f^ys which proceed from the obje�l, by paffingnbsp;through both lenfes, are bent more than by a finglenbsp;lens; hence the focus is fliorcened, viz. the focusnbsp;of both conjointly will be at �; fo that thofe two

lenfes aft as a fingle lens of a much greater curvature. But the advantage which the former have over the latter is, that the curvatures of the twonbsp;lenfes conjointly are lefs than the curvature of thenbsp;fingle lens, that has an equal magnifying power; ionbsp;confequence of which lefs aberration and a largernbsp;field of view is obtained by the combination of thenbsp;two lenfes.

The advantages of the two lenfes is derived from the fpace which is left between them. In faft, bynbsp;altering that diftance the magnifying power is alfonbsp;altered. I Ihall not detain my reader with a longnbsp;Inveftigation of the. precife degree of aberration,nbsp;field of view, and other particulars relative to thenbsp;above-mentioned combination of lenfes. Whoevernbsp;wilhes to be ir)formed particularly thereon, may bynbsp;himfcif trace the rays of light through thpfe lenfes,nbsp;after the manner which has been fufficiently de-feribed in the preceding pages; or he may perufenbsp;any of the principal works which have bee/i writtennbsp;cxprefsly upon Optics within thefc 50 or 60 years.nbsp;I Ihall only add a rule neceffary for determining thenbsp;compound focus of fuch lenfes; viz. the focal lengthsnbsp;of two lenfes, and the diftance between them, being given,nbsp;to find the focal length of a fingle lens that has the fam^nbsp;magnifying power as the tzvo combined lenfes.

Rule. Subtraft the diftance from the fum of the two focal lengths, and note the remainder;,nbsp;multiply the two focal lengths together. Dividenbsp;this produft by the above remainder, and the

Exampk. Let the focal length of one lens be S inches, that of the other be 6 inches, and let thenbsp;lt;3ifl:ance between the two lenfes be 4 inches; thennbsp;*^he fum of the two focal lengths is 14 inches, fromnbsp;'''^hich fubtra�l 4, and there remains 10. The pro-du6t of 8 by 6 is 48, which, being divided by 10,nbsp;quotes 4,8 inches; and this quotient is the focalnbsp;length of a fipgle lens, which will have the famenbsp;magnifying power as the combination of the twonbsp;given lenfes ; but not the fame diftinftnefs, nor fonbsp;large a field of view.

I quot;^HE life of the fimpleft optical inftruments, fuch as lenfes, fpeftacles, refle�lors, amp;c.nbsp;has been fufficiently explained in the precedingnbsp;chapters; fo that the inftruments which remain tonbsp;be defcribed in the prefent chapter, arc thofe of anbsp;more complicated nature; viz. where the effectnbsp;arifes from the combination of two or more of thenbsp;fimple inftruments.

One of the fimpleft and pleafanteft of thofe inftruments ia the camera cbjcura, the principle of which has been already defcribed in elucidationnbsp;of the conftru�lion of the eye: but as in that cameranbsp;obfcura the pidure of the objeft is inverted, wenbsp;muft point out in what manner the image is rendered eredt, and at the fame time we ftiall defcribenbsp;the moft ufual conftru�lion of a portable cameranbsp;obfcura.

Fig. 16, Plate XXI. reprefents a box confiftin^ of two parts. The external ACBDEFG has a

dutter or cover L. N, which moves round an hinge and when open, as in the figure, it carrksnbsp;lateral boards, which ferve to exclude thenbsp;*gt;ght as much as poffible from the rough glafs O,nbsp;''^hich is difcovered on opening the Ihutter L N P Q�nbsp;3nd upon which the obferver is to look. Thenbsp;^te fide or part of the box is wanting, and in thatnbsp;aperture another narrower box EHIKG Aides,nbsp;f his box w'ants the inner fide, and has a convex glafsnbsp;^ns fixed at I. If this machine be turned with thenbsp;�^ns I towards any objedls that are well illuminated,nbsp;it is evident that an inverted pidiire of thefe objedlsnbsp;''��ill be formed within the box on the fide ABCD;nbsp;and that picTcure may be rendered diftin�l by movingnbsp;the Aiding box E H G K in or out, in order to adjufl;nbsp;the focus according to the diAance of the externalnbsp;t*bje(f|;s_ Now at the back part of the box a fiatnbsp;piece of looking-glafs is fituated at an inclination ofnbsp;half a right-angle, as is fliewn by the dotted linesnbsp;BR; in confequence of which the rays of light failnbsp;^pon the looking-glafs, and are refieded upwards tonbsp;the rough glafs O, which forms that part of thenbsp;iide of the box, which lies under the cover LNP Q__.nbsp;'^he pifture then is formed upon that rough ornbsp;^^rnitranfparent glafs, and will appear ereft to anbsp;spectator fituated behind the box, and looking downnbsp;^Pon the glals O; becaufe that part of the pifture,nbsp;^hich fallsnbsp;nbsp;nbsp;nbsp;lower part of the looking-glafs,

to the part next to the hinge P C, and vice verjdt as may be eafily conceived by the lead refledtion.

The fhape of the camera obfcura has been aP tered in a great variety of ways; fometimes thenbsp;looking-glafs is placed before the lens, and the boXnbsp;is placed flraight up. In this conftruftion the raysnbsp;are bent before they pafs through the lens, and thenbsp;image or pifture is formed \vithin at the bottom ofnbsp;the box: hence in order to view it, one lateral fidenbsp;of the box is cut off, and the obferver looks at thenbsp;pifture through that opening, or introduces his handnbsp;through it for the purpofe of drawing an outline ofnbsp;the pi'fture.�N. B. A curtain of fome dark fluffnbsp;mtift be laid over the obferver, in order to preventnbsp;the introduftion of any extraneous light.

Fig. I, Plate XXII. reprefents the machine� with the effedi it produces. By means of this in-ftrument fmall coloured images painted upon glafsnbsp;are confiderably magnified, and thrown upon thenbsp;wall of a dark room, in their natural and' vividnbsp;colours, to the great entertainment of the by-ftanders�nbsp;efpecially of children.

Fig. 2, Iliews the internal parts of the machine fig �, placed at their proportionate diftances.nbsp;lantern contains a candle A, or fometimes two�nbsp;or three, or mort burners placed clofc to

other ; a refleftor M N, which is fo fituated to have the light A in its focus. On thenbsp;fore part of the lantern there is a thick doublenbsp;convex lens CD, or a plano-convex (ufuallynbsp;oalled a bull�s eye) of Ihort focus. The lanternnbsp;clofed on every fide, fo that no light can comenbsp;otit of it, but v/hat paffes through the lens C D.nbsp;Jo the direftion of this lens there is a tube*nbsp;ot apparatus, fixed to the lantern, which has anbsp;lateral aperture from fide to fide, through which thenbsp;glafs Aider with the painted fmall images, is movednbsp;Iti an inverted pofition. G H reprefents one ofnbsp;fhefe images. The fore part of the tube containsnbsp;another Aiding tube, which carries the double con-'''cx lens EF. The effedl of thofe parts is asnbsp;follows;

The thick lens CD throws a great deal of light from the candle A upon the image G H. And tonbsp;Iricreafe'that light ftill more, the refiedtor M N isnbsp;often, but not always, placed in fuch lanterns j fornbsp;the flame is in the focus of the refleftor, the. lightnbsp;proceeds in parallel lines from the refleftor to thenbsp;lens c D. The image G H being thus well illu-ttt'nated, fends forth rays from every point, which,nbsp;paffing through the lens E F, are converged tonbsp;^ focus upon the wail, and form the large imageinbsp;is (hewn in fig. \.

^hat has been faid above of the conjugate foci of a lens will (hew the neceffity of moving thenbsp;lens EF nearer to, or farther from, the painted imagenbsp;VOL. III.nbsp;nbsp;nbsp;nbsp;Xnbsp;nbsp;nbsp;nbsp;GH,

G Hj according to the diftance of the wall and does likewife fhew why is the rcprefentationnbsp;upon the wall fo much larger than the paintednbsp;image G H.

in fome magic lanterns, inftead of the fingle lens E F, two lenfes are ufed of lefs curvature, and fet atnbsp;a little diftance from each other j which aft rathernbsp;better than a fingle lens. See fig. 3, where bb is ^nbsp;diaphragm.

The magnifying powers of lenfes have been Ihewn to be inverfely as their principal focal lengths jnbsp;from which it follows, that very diftant objedls ar^nbsp;not fenfibly magnified by the interpofition of ^nbsp;fingle lens j but that efi��t may be produced bynbsp;a combination of two or more lenfes, as alfo by ^nbsp;combination of refleftors and lenfes. The formernbsp;are called dioptric telefcopes, and the latter are callednbsp;catadioplric, or refiebling telefcopes.

The dioptric telefcope, from the various com' bination of its lenfes, as alfo from its principal ufeSjnbsp;derives different appellations; viz.

The aftronomical telefcope, (which confifts of tw^ convex lenfes, AB, KM, fig. 4, Plate XXII.) fixednbsp;the two extremities of a tube, which confifts at leal^nbsp;of two parts that Aide one within the other, for ad'nbsp;jufting the focus in proportion to the diftance of th�nbsp;objefts that are to be feen through the telefcope ¹�

P Q^reprefents the femidiameter of a very diftant ohjedt, from every point of which rays come fo verynbsp;kittle diverging to the objedb lens K M of the te-^efcope, as to be nearly parallel, �p q is the picturenbsp;of the obje�l P Q__, which would be formed upon anbsp;Screen fituated at that place. Beyond that placenbsp;rays of every Angle radiant point proceed di-vergingly upon another lens A B, called the eyenbsp;glafs, which is more conyex than the former, andnbsp;^re by this caufed to proceed parallel to one another,nbsp;Jfi which direftion they enter the eye of the obferv-or at O.

The two lenfes of this telefc'ope have a common 9xis O L Qj Ly is the focal diftance of the objedtnbsp;lens, and E y is the focal diftance of the eye lens.nbsp;P L is the fum of both focal diftances. An objedtnbsp;''icwed through this telcfcope, by an eye fituated atnbsp;will appear diftindt, inverted, and magnified jnbsp;'quot;'2- the objedl feen without the telefcope will be tonbsp;Its appearance through the telefcope, as 7 E to 7 L;nbsp;that is, as the focal diftance of the eye lens to thenbsp;local diftance of the objedl lens.

For the rays which, after their crofting at the place rqp, proceed divergingly, fall upon the lens

S in the fame manner as if a real objedl were htuated a,t rqp (viz. at the focus of that lens) jnbsp;2nd of courfe on the other fide of that lens the raysnbsp;of each pencil will proceed parallel (lee what hasnbsp;heen Paid of a Angle lens in p. 228,231.) Now to thenbsp;at O, the apparent magnitude of the objedl, ornbsp;T 2nbsp;nbsp;nbsp;nbsp;of

of the part P Q^gt; is meafured by the angle EOAgt; or by its equal qV^p �, but to the naked eye at L�nbsp;when the glafs is removed, the apparent magnitude of the objed; is meafured by the angle QLPgt;nbsp;or by its equal q l.p j therefore the apparent magnitude to the naked eye is to the apparent magnitude through the telefcope, as the angle q 'L.p is tonbsp;the angle qEp-, or as the diftance yE is to thenbsp;difbance q L.

This telefcope is moftly ufed for aftronomical obfervations; for as it inverts the objed, the repre-fentation of terreftrial objeds through it would notnbsp;be pleafant.

It is evident from the above explanation, that h the two lenfes of this telefcope have equal focal dif-tances, the telefcope will not magnify. It alfo appears thar, with a given objed lens, the fhorter thenbsp;focus of the eye lens is the greater will the magnifying power be. But when the difproportion of thenbsp;two focal lengths is very great, then the aberrationnbsp;arifing from the figure of the lenfes and from thenbsp;difperfive power of glafs, becomes fo very greatnbsp;to do more damage than can be cOmpenfated by thenbsp;increafed magnifying power. Hence, in order to ob'nbsp;tain a very great magnifying power, thofe telefcope�nbsp;have fometimes been mad every long, as for inftance ofnbsp;I CO feet, or upwards: and as they were ufed foraftrO'nbsp;nomical purpofes, or moftly in the night time, the/nbsp;were frequently ufed without a tube, viz. the objelt;^

of motion in any required dire�tion, and the eye lens was fixed in a '(hort tube that was held in thenbsp;hand of the' obferver. The diftance as well as thenbsp;direftion of the two lenfes was adjufted by a ftrongnbsp;cord ftretched between the frame of the obje�l lensnbsp;and the tube of the eye lens.

In this conftruftion the inftrument has been called an aerial telefcope. Its ufe is evidently incommodious ; bu: it was with fuch a telefcope that five fa-tellites of faturn, and other remarkable objedls werenbsp;difcovered.

The imperfeftions of fuch a telefcope arife principally from the difperfive power ofglafs, which, efpe-cially at the edge of the field of view, frequently introduces circles of prifmatic colours; but fince the invention of achromatic lenfes, tlje telel'copes havenbsp;been made much fhorter, by fubftituting either anbsp;double or a triple achromatic lens in lieu of a Amplenbsp;objelt;5f lens K M j for with an achromatic lens thenbsp;bad efFeft of the difperfion is in great meafure, ifnbsp;not entirely, removed.

In a telefcope of a given length, the quantity of objedt which is taken in at once, or the field ofnbsp;view, depends upon the breadth of the eye lens ;nbsp;for as A E is larger or Imaller, fo the angle ALE ornbsp;Its equal PLQ^, is larger or fmaller (fee page 264);nbsp;and this angle takes in all the objeft, or the part ofnbsp;an objedt that can be feen at one view on one fidenbsp;of the axis of the telefcope. But in order to increafenbsp;the field of view as much as polTible, and in great

meafure the magnifying power alfo, the eye lens, or what performs the office of a fingle lens, as is noWnbsp;ufed, confifts of two plano-convex lenfes, fet at anbsp;little diftance from each other. The advantage ofnbsp;fuCh a combination has been explained in page 267-The objedt, which appears inverted through thenbsp;above-defcribed telefcope, will �appear upright andnbsp;diftindt, if two more convex eye glaffes be fubjoinednbsp;to it, as in fig. 5, Plate XXII. which reprefentsnbsp;the fame telefcope as that of fig. 4, but with thenbsp;addition of two other convex lenfes B, C, fituatednbsp;at a diftance from each other, which is equal to thenbsp;fum of their focal diftances; and when their focalnbsp;diftances are equal, the bbjedl will' be m.agnified asnbsp;much as without thofe additional glaffes j but throughnbsp;them it will appear ftraight up or redtified, and notnbsp;inverted. Hence this telefcope has been moftly ufednbsp;for viewing terreftrial objefts, and is therefore callednbsp;the terreftrial telefcope or perfpeSive glafs.

For the pencils of rays EOF, AOB, amp;c. that are continued to the lens F B, will be formed bynbsp;into a fecond image S T j and the focus S of an)?nbsp;oblique pencil O B, 'will be determined by the inter-feftion of tiie line S T, perpendicular to the common axis of the lenfes, and of the oblique axis F S)nbsp;drawn parallel to the incident rays O B. Thi^nbsp;point S being the focus of incident rays on the la^nbsp;lens G C, the emergent rays C D will be parallelnbsp;to the oblique axis S G, becaufe the rays whichnbsp;proceed from T are fuppofed to emerge parallel to

the dire�; -axis; therefore to the eye at D the objed will appear diftinft and upright.

When -the lenfes B, C, are quite equals then the ' sngle CDG, which now meafures the apparentnbsp;�Tiagnitude of the objeft, is equal to the angle AOE;nbsp;hence, amp;c.

The laft lens, or the one neareft to the eye in this telefcope, is now moftly made double ; viz. infteadnbsp;of one, two lenfes are combined together, for thenbsp;purpofe of enlarging the field of view (fee pagenbsp;267): hence moft of the terreftrial telefcopes nownbsp;contain four lenfes in the tube next to the eye.

The Galilean telejcope confifts of a convex object lens, and a concave eye lens and derives its namenbsp;from the great Galileus, who is generally reckonednbsp;the inventor of it. See fig. 6, Plate XXII. whichnbsp;htews that the diftance between the two lenfes isnbsp;lefs than the focal diftance of the objedt lens; viz.nbsp;tnftead of the convex lens fituated behind the placenbsp;�f the image, to make the rays of each pencil pro-'^eed in a parallel direftion to the eye, here a con-cave eye lens is placed as much before that image jnbsp;^t^d this lens opens the rays of each pencil that con-''^^tged to q and 7gt;, and makes them emerge parallelnbsp;towards the eyes as is evident by conceiving the rays

The eye muft be placed clofe to the concave lens, tn order to receive as many pencils as poffible j andnbsp;then fuppofing an emerging ray of an oblique pencil

to be produced backwards along AO, the apparent magnitude of the objeft is meafured by the anglenbsp;AOE, or its equal which is to the 'angle yLpnbsp;(or CLLP, the meafure of the magnitude) asnbsp;to 5'E, viz, as in the aftronomical telcfcope. It isnbsp;evident that in this telefcope the objefts appearnbsp;eredt, for the rays of light do not crofs eachnbsp;other.

The field of view or quantity of objedls that are taken in at once in this telefcope, does not dependnbsp;upon the breadth of the eye lens, as in the aftrono-mical telefcope, but upon the breadth of the pupilnbsp;of the eye; becaufe the pupil is lefs than the eyenbsp;lens ABj and the lateral pencils do not now converge to, but diverge from, the axis of the lenfes.nbsp;Upon this account the view is narrower in thisnbsp;than in the preceding telefcope �, yet the objedlsnbsp;through it appear remarkably clear and diftinft.

An achromatic objedl lens, inftead of the fimpl^ lens K L, improves this as well the preceding te-lefcopcs.

The night telefcope is a Ihort telefcope, viz. about two feet long, which reprefents the objedis inverted, much enlightened, but not much magnified. Itsnbsp;field of view is alfo very extenfive.

This telefcope, in confequence of thofe propef' ties, is ufed at night moftly by navigators, for th�nbsp;purpofe of difeovering objedls that are not very

diftaut;

diftant, but which cannot otherwife.be feen for want of fufficient light; fuch as velfels, coafts,nbsp;focksj amp;c. On account of its extenfive field andnbsp;great light, this telefcope has alfo been advan-tageoufly ufed by aftronomers for difcovering fomenbsp;coeleftical objefls, whofe fituation was not exaftlynbsp;known, or for viewing at once the relative fituationnbsp;of feveral ftars and other objefis.

This telefcope has a pretty large and fimple ob-jedl lens, whence it derives its great light j for as the rays which proceed from every fingle point-ofnbsp;the objed', fall upon the whole lens of a telefcope,nbsp;and are thence refradled to a focus, it is evident thatnbsp;the larger that lens is, the greater numbej- of raysnbsp;will be thrown upon that focus and of courfe thenbsp;brighter will the image be. In this telefcope anbsp;pretty large lens may be ufed, becaufe the telefcopenbsp;is not intended to magnify more than about four ornbsp;Cx times in lineal extenfion.

WithirT this telefcope a fecond lens is often ufed for fhortening the focal length of the objed lens.nbsp;The eye lens is fometimes fingle, but moftiy double,nbsp;('quot;iz- a combination of two plano-convex lenfesnbsp;placed at a little diftance from each other) andnbsp;pretty large ; hence is derived the extenfive field ofnbsp;view, which in fome of thofe telefcopes exceeds fixnbsp;Or feven degrees.

may obferve once for all, that in every te-fofcope the diftance between the objed lens and the Other lens or lenfes muft be alterable, in order that

the focus may be adjufted according to the diftance of the objeds. Hence, every telefcope confifts atnbsp;leaft of two tubes, one of which, viz. that with thenbsp;eye lenfes, Aides within the other. To the famenbsp;telefcope feveral eye tubes, with a Aialiower ornbsp;deeper lens, or with a different number of lenfes, maynbsp;be adapted fuccefllvely, in order to give them different m.agnifying powers, fuitably to the clearnefsnbsp;of the air, of the objeds, amp;c. as alfo for converting them into aftronomical or terfeffrial te-lefcopes.

I'his is likewife called the Newtonian telefcope, or reflecting telefcope ; for if not the original proiedor.nbsp;Sir Ifaac Newton is, at leaft, the firft perfon whonbsp;executed a telefcope of this fort, which, as itsnbsp;name imports, confifts of rcAeding and refradingnbsp;parts.

The general principle of this telefcope is the fame as that of the dioptric or refrading telefcope. Innbsp;the latter the rays which come from a diftant objed:nbsp;are, by the adion of the convex objed lens, col-leded to a focus, and beyond that focus the raysnbsp;of every Angle radiant point are rendered -againnbsp;parallel by the adion of the eye lens or eye lenfes*nbsp;This is otherwife expreffed, by faying that the objed lens forms an image of the objed, whichnbsp;image is viewed by the eye lens. In the form.er,

viz. in the reflecting telefcope, the rays which come from a diftant objeCt, are, by the action of a concave reflector, fent back convergingly to a focus,nbsp;where they form an image, which is viewed throughnbsp;the eye lens.

Fig. 7, Plate XXII. reprefents the principle,of the original contlruCtion. ACDB is the feCtion of a tubenbsp;open at AB. EF is a concave refleCtor fixed at thenbsp;bottom of the tube ; mn is an arm projecting fromnbsp;one fide of the tube, as far as its middle or axis, wherenbsp;it fupports a fmall flat fpeculum G, fet aflant ¹;nbsp;fo that the rays which come from every Angle pointnbsp;of a diftant object IK, and fall upon the concavenbsp;fpeculuru EF, are reflected by it in a convergingnbsp;manner to the fmall flat fpeculum G, which bendsnbsp;their courfe fideway, and fends them with the famenbsp;convergency to an hole at H in the fide of the cube,nbsp;t^'hore the image of the objeCt I K is formed j andnbsp;this image is viewed by the eye through an eyenbsp;frns L, or through a tube with more than one eyenbsp;fros, for the purpofe of reprefenting the objeClnbsp;^teCl, as in the above-deferibed refraCling te-frfeopes.

Inftead of the flat fpeculum, a glafs prifm has been often applied, which, in a certain fituation, aCls like a reflector. See page 102.

is equal to E G plus G H 5 for the flat refledlor G does only bend the rays fideway. Without thatnbsp;fmall refleftor, the rays reficfted from EF, wouldnbsp;form an image at O (OG being equal to GH)nbsp;where indeed an eye lens might be placed, and thenbsp;obferver looking through it, with his face towardsnbsp;the refledlor EF, would fee the magnified imagenbsp;of the objedt I K ; but in that cafe the head of thenbsp;obferver would intercept a great part of the rays;nbsp;yet, by fetting the refledlor EF a little aflant, itsnbsp;focus may be thrown to P, where the eye lens beingnbsp;applied, the head of the pbferver would obflrudtnbsp;little or none of the light, efpecially when the reflector F E is of a con fide rable fize. This forms the,nbsp;fecond variety.

The magnifying power of this telefcope is determined after the fame manner as in the refradling te-lelcope, viz. the focal length of the refledlor EF (which is analogous to the object lens of the re-fradling telefcope) is divided by the focal length ofnbsp;the eye lens, and the quotient Ihews how manynbsp;times the objedl feen through the telefcope appearsnbsp;larger (meaning in lineal extenfion) than withoutnbsp;the telefcope.

In this telefcope there is an adjuftment at FI, viz* the fliort tube with the eye lens, or lenfes, may benbsp;Aid a little way in or out; for the focal diftance ofnbsp;the refleftor E F increafes or decreafes according.nbsp;the diftance of the objedt.

Jyefcripion of Optical Injiruments. 285 and is reprefented in fig. 8, Plate XXII. Thenbsp;large concave fpeculum BE of this telefcope isnbsp;perforated with a hole quite through its middle.nbsp;Within the tube of the telefcope a fmall concavenbsp;fpeculum xy, is fupported by the arm H, diredllynbsp;facing the large fpeculum B E. Two 'lenfes, WXnbsp;and n 0, are contained in the eye tube, and thenbsp;obferver applies his eye to a fmall hole at P, innbsp;�Order to view the magnified diftant objedt G.

The large refiedlor BE receives the rays a c, b d, from the diftant objeti:, and reflefts them to itsnbsp;focus Cy where they form the inverted image, ornbsp;where they crofs each other, and then fall diverg-ingly upon the fmall refie�or xy, whofe focus isnbsp;at/; viz. a little farther than the focus e of thenbsp;large refiedor; hence the rays are refieded backnbsp;upon the lens W X, not in a parallel, but in a con-verging m.anner; and that convergency is increafednbsp;f�y the adion of that lens, fo as to come to a focus,nbsp;O'quot; �O form a fccond image RS much larger than thenbsp;former, and ered like theojojed. Laftly, this imagenbsp;IS viewed through the eye lens no-, or, in othernbsp;^ords, the rays from every fingle point of the ob-jed, after this fecond crofting, fall diverginglynbsp;opon the eye len.s which fends them nearly pa-rallel to the eye at P, through a very fmall hole.nbsp;Sometimes the eye lens no, is double, viz. itnbsp;oonfifts qP lenfes, which perform the officenbsp;of a fingle lens, as has been explained abovenbsp;(P^ge 267).

If the firft lens WX were removed, the imag^ would be formed fomewhat larger at z; but thenbsp;area or field of view woyld be fmaller and lefs plea-fant. At the place of the image R S, there isnbsp;fituated a circular piece of brafs, called a diaphragm,nbsp;with a hole of a proper fize to circumfcribe thenbsp;imao-e, and to cut off all fuperfluous or extraneousnbsp;light, in order that the objedt may appear as diftindtnbsp;as poffible.

If this telefcope confifted of the two refledtors only, and thefe were fituated fo that e were thenbsp;focus of each refledlcr; then the rays which camenbsp;parallel from the diftant obje�dt to the large re-fledtor, and divergingly from that to the fmall re-fiedlor, would, after the fecond refledtion, go parallelnbsp;to the eye at P, and of courfe the objedt would appear magnified in the proportion of the focal dif-tance of the large refledlor to the focal diftance ofnbsp;the fmall refledlor; fo that if the focal diftance ofnbsp;the former be to that of the latter as 6 to i, thennbsp;the objedl would be magnified 6 times in diameter.nbsp;But fince the firft image is magnified into a fecondnbsp;image much larger, which is viewed through thenbsp;eye lens �, therefore the whole magnifying power isnbsp;in a proportion compounded of de to ex, and ofnbsp;xz to zo. If the former proportion be as 6 tonbsp;and the latter as S to i i then the objedl will ap'nbsp;�lt;5^nbsp;nbsp;nbsp;nbsp;peil**

pear 48 (viz. 6 by 8) times larger in diameter through the telefcope than to the naked eye.

The fourth fpecies of refledling telefcope goes under the name of Cajfegrainian telefcope. It difiersnbsp;from the preceding, in having the fmall refleftornbsp;convex, inftead of concave; in confequence ofnbsp;'vhich the fmall refledlor muft be placed nearernbsp;to the large refledtor than the focus of the latter;nbsp;then the rays from the large refledtor fall convergenbsp;ingly upon the convex fmall refledtor, and are by itnbsp;fent back convergingly upon the lens W X, amp;c.

The only difference that is worth remarking between this and the preceding telefcope is, that in this the objedb appears inverted, becaufe in it therenbsp;is no image formed, or the rays do not crofs eachnbsp;other, between the two .refledtors. Aifo with thenbsp;lame magnifying power, amp;c. this telefcope is fliorternbsp;than the Gregorian, by twice the focal length of thenbsp;fmall fpeculum.

To both thofe telefcopes a long wire is fixed all along the outfide of the tube, at the end bf whichnbsp;there is a fcrew which works into an external pro-jedtion g of the internal arm H, and ferves to movenbsp;that arm with the fmali fpeculum nearer to or farther from the large fpeculum, in order to adjuft thenbsp;focus of the inftrument, according to the diftance ofnbsp;the objedt. The adtion of this wire is eafily under-ftood; for it paffes through a hole at F, wherenbsp;It IS prevented going forwards or backwards by twonbsp;Ihoulders, which are indicated by the figure : hence,

when the obferver looks throusfh the hole P, he turns with his hand the wire by the nut Qjnbsp;which fcrews the projeftion g of the arm nearernbsp;or farther, amp;c. until the objett appears very dii-tinft.

Upon the whole, the reflefting telefcopes may be rendered rriore powerful than the refradlingtelefcopesnbsp;of the fame length j which arifes principally from thenbsp;rays of light not being difperfed by reflection as theynbsp;are byrefraflion, andlikew'ife from the prafticabilitynbsp;of giving the' large refledtors a' form either parabolical, or at leafl: fuch as anfwers better than the fphe-rical figure*. But the refiedling telefcopes are.nbsp;larger and heavier than the refrattors; hence, w�he.nnbsp;(hort and portable telefcopes are wanted, the achromatic telefcopes may be preferred; but for aftrono-micaf obfervatories, where large and very powerfulnbsp;telefcopes are wanted, the refledors Ihoukl be preferred.

The largefl; refleding tclefcope now exifting? was . conftrtided by that excellent aftronomer, Dt*nbsp;Herfchel. -It is a telefcope ,of the fecond fpecies,

�* If the reader with to learn the method of forming� poliftiing, amp;c. the reflectors of thofe telefcopes, which at�,nbsp;univerfally made of metal, he may confult Dr. Smith snbsp;Optics, Book III. Chapter II.; Mudge�s Paper m thenbsp;67th Volume of the Philolophical Tranfa�tions; thenbsp;John Edwards�s Diredions for making thebeft Compofluounbsp;for the Metals of refleding Telefcopes, amp;c.

Viz. where the obferver looks through an eye lens down upon the large refleftorj whofe polilhed fur-face is 48 inches in diameter. Its focal length isnbsp;about 40 feet¹; and I do not know that a refradlingnbsp;felefcope was ever made, whofe power equallednbsp;�^hat of this gigantic telefcope, or of another of 20nbsp;f^et, which was conftrufted and ufed by the famenbsp;perfon, or even of one of his feven feet refie�lors,nbsp;to which Dr. Herfchel can give a magnifying powernbsp;of fome thoufands j-.

The above-mentioned methods of compyting the magnifying powers of tolefcopes, are no? innbsp;general very pradticable, as the lenfes and fpeculumsnbsp;cannot eafily be removed from the telefcopes, innbsp;order to have their particular focal diftances afcer-tained ; therefore it will be proper to Ihew how thisnbsp;tgt;bjeft may be accomplilhed experimentally.

There are feveral experimental mctliods of afcer-taining the magnifying powers of telefcopes j;; but I lhall fubjoin one of the eafieft, which is defcribednbsp;the Rev. John Edwards in the following words;

quot; At the dlftance of 100 or 200 yards from the telefcope, put up a fmall circle of paper, of anynbsp;determined diameter, an inch for inftance. Upon ,

fhi). Tranf. yol. for 1795' Art. XVIII. f Phil. Traaf. vol. 72, Art. XT.nbsp;t See Dr. Smith�s Optics, Notes to Art. 109 and 485.nbsp;Alfo, my Defcription and Ufe of tjie mother-of-pearlnbsp;Micrometer, London 1793.

� a card, or any piece of ftrong paper, through � which tire light cannot be eafily tranfmitted, drawnbsp;two black parallel lines, whofe diftance fromnbsp;� each other is exa��y equal to the diameter of thenbsp;fmall circle. Adjuft the telefcope to diftinlt;3:nbsp;� vifion, and through it view the aforefaid fmallnbsp;� circle with one eye, and with the other eye, opennbsp;� alfo, view at the fame time the two parallel lines.nbsp;� Let the parallel lines be then moved nearer to, ornbsp;� farther from your eye, till you fee them appearnbsp;� exa�tly to .cover the fmall circle viewed in thenbsp;� telefcope. Meafure now the diftance of thenbsp;� fm.all circle, and alfo of the parallel lines, fromnbsp;� your eye. Divide then the diftance of thenbsp;former by that of the latter, and you willnbsp;have the magnifying pow'er of the telefcope re-quired.�

In the preceding pages we have taken no notice of the tubes, ftands, and movements that arenbsp;ufually given to telefcopes j firft, becaufe thofenbsp;particulars are not neceflary for illuftrating thenbsp;principles of optical inftruments; and fecondly, becaufe the particular defeription of thofe externalnbsp;parts, in all their variety, would require a greatnbsp;many more pages than can poffibly be allotted to itnbsp;in thefe elements. There are however two ufefnlnbsp;appendages to telefcopes, which deferve to benbsp;briefly deferibed.

pofe^

pofe of finding out an objeft expeditioufly. This finder does not magnify the objeft more than 4, 6,nbsp;8 times ; but it has a great field of view, fo thatnbsp;through it a great part of the heavens may be feennbsp;once. In the inflde of its tube, and cxadlly atnbsp;^he focus of the eye glafs, there are two flendernbsp;quot;'ires, which crofs each other in the axis of the te-i^fcope. Now the finder is adjufted by means ofnbsp;Icrews upon the tube of the great telefcope, in fuchnbsp;^ rnanner as that when an objedt, feen through thenbsp;finder, appears to be near the croffing of thenbsp;above-mentioned wires, it is at the fame time vifi-ble through the great telefcope : hence, when thenbsp;obferver wilhes to view a Imall diftant objeft, as anbsp;ftar, a planet, amp;c. he moves the inftrun1.ent to one fidenbsp;nr the other, until, by looking through the finder,nbsp;brings the ob]eft nearly to coincide w'ith thenbsp;^tofling of the wires, and when that takes place,nbsp;immediately looks through the large te-iefcope, amp;c.

A. micrometer is an inftrument, which is ufed �'''ith a telefcope, for the purpofe of meafui ing fmallnbsp;ingles. great variety of micrometers have beennbsp;Contrived by various ingenious perfons; and theynbsp;more or lefs complicated, more or lefs expen-as alfo more or lefs accurate. If the readernbsp;to examine the conftrudlion of any of the va-micrometers, he may perufe the works thatnbsp;XT 2nbsp;nbsp;nbsp;nbsp;are

are mentioned in the note ¹. I fhall only fubjoigt;^ the defcription of a very fimple, and, at the famenbsp;time, accurate micrometer, which I contrived fomenbsp;years ago ; but we may previoufly obferve, that mnbsp;general the micrometers meafure the fize of thenbsp;image, which is formed in the focus of the eye lensnbsp;or of the eye lenfes within the telefcope ; for knoW'nbsp;ing the magnifying power of the telefcope, one maynbsp;eafily calculate to what angle fuch meafurementnbsp;correfponds. For inflance, if the telefcope magnifynbsp;30 times, and the length of die image of the objectnbsp;is fl|^ewn by the micrometer to fubtend an angle ofnbsp;two minutes; then we may conclude that the realnbsp;objeft fubtenu'j an angle of the 30th part of twonbsp;minutes, viz. an angle of four feconds; and fonbsp;on.

My micrometer confifts of a fmall femitranfparent fcale or flip of m-other-of-pearl, about the 20th partnbsp;of an inch broad, and cf the thicknefs of commonnbsp;writing paper. It Is divided into a number of equal

Dr. Smith�s Optics, Book III. Chapter VIII. for th� carlieft micrometers. Dollond�s Micrometer, Phil. Traofnbsp;vol. 61, p. 536¹ Bofcovich�s Adicrometer, Phil. Tranfnbsp;vol. 67, p. 7II95 799� Rochon�s Micrometers ;nbsp;de 'Mem. fur la Mecaniqus et la Phyfque. RamfdeH�nbsp;Micrometers, Phil. Tranf. vol. 69, p. 419. Herfchel �nbsp;Lamp-Micrometer, Phil, Tranf. vol. 72. Art. XHl'nbsp;Smeaton�s Equatorial Micrometer, Phil. Tranf. vol. 7?'nbsp;Art. XXXill.

parts by means of parallel lines, every fifth and tenth of which divifions is a little longer than thenbsp;reft.

This micrometer, or divided fcale, is fituated gt;!vithin the tube at the focus of the eye lens of th�nbsp;telefcope, where the image of the obje�l is formed,nbsp;and with its divided edge pafllng through the Centrenbsp;of the field, of view though this is not abfolutelynbsp;neceflary. It is immaterial whether the telefcopenbsp;be a refradtor or a refledtor, provided the eye lensnbsp;be convex, and not concavcj as in the Galilean telefcope.

The fimpleft way of fixing it, is to ftick it upon the diaphragm, which generally hands within thenbsp;tube, at the focal diftance of the eye lens.

By looking through the telefcope, the image of the objedl and the micrometer will appear to coih-t^ide ; hence the obferver may eafily fee how manynbsp;^iv^ifions of the latter meafure the length or breadthnbsp;�f the former j and knowing the value of the divi-Bons of the micrometer, he may eafily determinenbsp;the angle which is fubtended by the objedl.

There are feveral methqds of afcertaining the Value of the divifions of a micrometer in a givennbsp;telefcope. The following is one of the eafieft.

I^iredl the telefcope to the fun, and obferve how ttiany divifions of the micrometer meafure its dia-�vieter exadtly; then take out of the Nautical Almanack the diameter of the fun for the day in whichnbsp;the obfervation is made j divide it by the above*-

mentioned number of divifions, and the quotient is the value of one divifion of the micrometer�nbsp;Thus, fuppofe that 26 j divifions of the micrometer meafure the diameter of the fun, and the Nautical A-lmanack gives for the meafure of the angle?nbsp;which is fubtended by the fame diameter, 31', 22 gt;nbsp;or (by reducing it all into feconds) 1882�. Dividenbsp;1882'' by 26,5, and the quotient, neglefting a fmallnbsp;remainder, is 7or 1', 11quot; j which is the value ofnbsp;one divifion of the micrometer; the double ofnbsp;which is the value of two divifions; the treble isnbsp;the value of three divifions j and fo forth *.

The Microjcope is another moll ufeful, inftruc-tive, and pleafant optical inftrument. As the te-lefcope enables us to diftinguifh objedls, or the parts of ebjefts that are otherwile invifible on account of their being too remote from us; fo thenbsp;microfcope enables us to perceive fuch fmall ob-jefts and their parts, as are otherwife abfolutely invifible to us.

It has already been obferved, that there are two forts of microfcopes, viz. the fimple, which confift*nbsp;of one lens, and the compound, which confifts of

* For farther particulars relative to this mother-of-pead micrometer,fee thePhilofophical Tranfadions, vol.LXXXfnbsp;Art. XIX. or its feparate defcription.publiQied in London rhnbsp;the year 1793, wherein other methods are deferibed of af'nbsp;certaming the value of its divifions, when the tclefcope doe*nbsp;not take in the whole dife of the fun.

nriore than one lens. The Jolar and the liicernal microfcopes are fometimes confidered as two othernbsp;fpecies of microfcope �, but in truth they are onlynbsp;firnple microfcopes, wherein the objeds are illu-niinated either by the fun�s light or by a lamp,

Of the properties of the fimple microfcope, viz. of the magnifying power, amp;c. of a fingle fmall lens,nbsp;fufHcient mention has been made in the precedingnbsp;pages. We fhall only obferve with refpedt to itsnbsp;limits, that fmall lenfes have been made, whofenbsp;focal diftance was fliorter than one 200th part ofnbsp;an inch, and which of cotirfe magnify the objeft upwards of 1600 times in diameter; but very feldomnbsp;any fuch lens turns out fufficiently well lhaped andnbsp;well poliflied. Globules of glafs have alfo beennbsp;conftrudted by means of a lamp and blow-pipe,nbsp;whofe diameter was about the locoth part of annbsp;^�^ch, and of courfe their magnifying powers werenbsp;prodigioufly great. But fuch fmall lenfes or globulesnbsp;^te managed with very great difficulty ; their field

view is extremely fmall, and, as the objeft muft be brought exceedingly near their furface, they arenbsp;thereby eafily dirtied or fcratched, and confequentlynbsp;tendered ufelefs. It muft be acknowledged, how-ever, that through a fingle lens, when it is w^ellnbsp;^^Ped and well polifhed, an objeft appears muchnbsp;clearer and more diftinft, though a little more dif-tlt;3rted, than through a compound microfcope of

Fig. 9, Plate XXII. r�prefents the two lenfes of a compound microfcope. acbhz fmall obje�l, placednbsp;not precifely at, but near the focus of the fnnallobjeflnbsp;lens def, and the rays of light which proceed fromnbsp;any fingle point of the objeft, are by the adtion ofnbsp;that lens converged to a focus fomewhere aboutnbsp;ABC, where an image is formed, which is largernbsp;than the objeft, in proportion as the diftancenbsp;exceeds the diftance e d Another larger lens D Fnbsp;is fituated fo that its focus may be at B ; then thenbsp;eye of the obferver at I will view the Image A C,nbsp;magnified by that large lens DF j or in other words,nbsp;the rays which proceed from each fingle point, fornbsp;inftance c of the objeeft, by pafling through thenbsp;iens^/, are converged to a focus B, where theynbsp;crofs each other, and then proceed diverginglynbsp;through the eye lens DF, which caufes them tonbsp;proceed nearly parallel to the eye.

The magnifying power of this microfcope is eafily computed ; for firft of all, the image A C isnbsp;to the objedt as the diftance B ^ is to the diftancenbsp;Cl?; and fecondly, the image _AC will beften bynbsp;the eye at I, under the angle DIF, which is equalnbsp;to the angle AEC; and therefore that image widnbsp;appear as much longer than to the naked eye, asnbsp;the diftance BE is fliorter than 8 inches (or thenbsp;limit of diftindt vifion with the naked eye) ; fo thatnbsp;if the diftance e c be one inch, ^B fix inches, andnbsp;EB two inches, then the image A C is fix timesnbsp;longer than the objedf a h, and that image is mag'nbsp;nified four times by the lens liF j fo that upon the

whole, to the eye at I the objedl a h will appear magnified 4 times 6, or 24 times.

This microfcope has a larger field of view than a fimpie microfcope of the fame power �, and its fieldnbsp;of view is rendered larger ftill by the addition ofnbsp;one or two more ler.fes inftead of the fingle lensnbsp;DF. The magnifying power of the inftriiment,nbsp;with more than two lentes, mnft be computed fromnbsp;the efFeft of all the lenfes; or it may be afeertainednbsp;experimentally in the following manner. Placenbsp;part of a divided ruler before the microfcope, fonbsp;that, looking through the inftrument, you may feenbsp;one of its divifions magnified ; then open the othernbsp;eye alfo, and looking with it at the ruler out of dienbsp;microfcope, you will perceive the image of thenbsp;magnified divifion as it were projefted upon thenbsp;ruler; and you may eafily fee how many divifions ofnbsp;the unmagnified ruler meafure, or are equal to, thenbsp;^lugle magnified divifion, and that number is thenbsp;magnifying power of that microfcope. Thus, if thenbsp;ruler be divided after the common way into inchesnbsp;^ud tenths, and if you find that one magnifiednbsp;tenth is equal to three inches, you may concludenbsp;that the microfcope magnifies 30 times.

Different fliapes, and likewife different ufes, of fimple or compound microfcope, have givennbsp;thofe inftruments a variety of names, which, innbsp;^ruth, are dependent not upon the principle, butnbsp;upon the apparatus, which is necellary either tonbsp;tender it portable, or fteady, or applicable to any

particular purpofe. Thus, we hear of the aquatic mcrojcope, opaque microjcope, (viz. for viewing opaquenbsp;objedts) Wiljon�s micro/cope, See.

Microfeopes have been alfo made by means of refleftors ; hjence they are called reflecting mi�nbsp;crojeopes. The principle of their conftrudlion maynbsp;be eafily derived from what has been faid abovenbsp;with refpeft to the refiefting telefcope. But, uponnbsp;the whole, refiecling microfeopes are neither fonbsp;ufeful nor fo manageable as thofe with lenfes. '

Micrometers have been applied to compound microfeopes, as well as to telefcopes, and generallynbsp;in the fame manner, viz. at the place where thenbsp;image is formed within the body of the microfcof)e.nbsp;There are however fome micrometers applied to thenbsp;objedt itfelf; viz. the objec5t is laid upon a dividednbsp;flip of glafs, of ivory, or of metal, amp;c. and are bothnbsp;magnified at the fame time. But thefe micrometersnbsp;are by no means fo eafy of application, nor fo generally ufeful, as thofe which meafure the imagenbsp;within the microfeope; amongfl which the mother-of-pearl micrometer (fuch as has been deferibednbsp;above for the,telefcope) is by far the moft accurate, as well as the fimpleft. It is only neceflTary tonbsp;obferve, that with the microfeope this micrometernbsp;meafures the lineal dlmenfions of the objedl; andnbsp;the value of its divifions are afeertained by placingnbsp;an objeft of a known dimenfion before the mi-crofeope, and by obferving how many divifions ofnbsp;the micrometer meafure its magnified image; for

�tiftance, place a piece of paper, which is exaftljr one-tenth of an inch long, before the microfeope,nbsp;anti if you find that 50 divifions of the micrometernbsp;naeafure its magnified image, you may concludenbsp;that each divifion is equal to, or rather denotes annbsp;extenfion of the 500* part of an inch in the ob-jedl; for if 50 divifions meafure one-tenth, 500nbsp;divifions mull meafure the whole inch; and fonbsp;forth.

The laft inftrument which I fiiail mention in this chapter, is called photometer, or meafurer of light;nbsp;its office being to indicate the different quantities ofnbsp;light; for inflance, in a cloudy or bright day, ornbsp;between different luminous bodies. But as a commodious inftrument of this fort is rather a defdera-tum in philofophy ; I fhall only mention in generalnbsp;terms, that the ratio of the intenfides of two luminous objedfs have been attempted to be meafurednbsp;by, placing them at different diftances from a givennbsp;objeiff, until that objecl call two fhadows of equalnbsp;darknefs; or by x)brcrving when two equal objedlsnbsp;appeared to be equally illuminated, each by one ofnbsp;the luminous objeds; for then the proportion ofnbsp;the intenfities of their lights was reckoned to be asnbsp;that of the fquares of the diftances. For inflance, ifnbsp;two equal objeds appear to be equally illuminated,nbsp;'''hen one of them is three feet from a tallow candle, and when the other is nine feet from a waxnbsp;candle j' then it is concluded that the intenfity of the

The intenfity of light has alfo been meafured by means of an extremely fenfible thermometer, andnbsp;the contrivance is a very curious one -f; but thisnbsp;proceeds upon the fuppofition that heat and lightnbsp;are the fame thing, or that they are always accompanied in equal degree; or that the fame quantitynbsp;of light does always excite the fame quantity ofnbsp;heat i which is not the cafe.

See Count Rumford�s Paper in the Philofophical Tranf-aftions, Volume for the year 1794. Art. IX. as alfo Prieftley�s Hiftory of Light, Colours, and Vifion; P. VLnbsp;Sea. VII.

t Nicholfon�s Journal of Natural Philofophy, Chemillfy� amp;c. vol. III. pages 461, and 518.

[ nbsp;nbsp;nbsp;301nbsp;nbsp;nbsp;nbsp;1

WE have referved for this chapter the account of fuch natural phenomena refpe�i;ing light,nbsp;as could not be inferred in the preceding chapters,nbsp;without interrupting the general theory of optics.nbsp;The rainbow is undoubtedly the moft frequent,nbsp;the moft remarkable, and the m.ore generallynbsp;known, of thofe phenomena. We lhall, in thenbsp;firft place, ftate the particular circumftances thatnbsp;attend its appearance, and lhall then fubjoin thenbsp;'Jfoal explanation, which is derived from the abovenbsp;defcribed theory of optics.

When the fun is on one fide of the fpedator, and rain falls on the other fide, a beautiful coloured archnbsp;is frequently feen in the fky on the fide of the rain.nbsp;This coloured arch is called the rainbow and oftennbsp;two fuch arches are feen one within the other, as innbsp;%� �O, Plate XXII,

The colours of the inner bow EABF, are much more vivid than thofe of the outer bow GCDH-Each bow exhibits all the prifmatic colours, ar-

ranged in the fame order as in the prifmatic fpecr-trum, viz. red, orange, yellow, green, blue, indigo, and violet; but the order of thofe colours in thenbsp;upper bow is contrary to that of the lower; thenbsp;latter having the violet at A, and the red at B; butnbsp;the former having the red at C, and the violet atnbsp;D. Thofe colours are blended into each other, fonbsp;that no eye can diftinguilh their boundaries; andnbsp;indeed for moft eyes it is difficult to diftinguilhnbsp;more than the three or four more predominant colours.

Sir Ifaac Newton calculated the breadth of each bow, as alfo the diftance between them ; but ortnbsp;the fuppofition that the light which comes from thenbsp;fun and forms the bows amongft the drops of rain,nbsp;came from a fingle point, viz. from the centre ofnbsp;the fun. The refult of that calculation is, that atnbsp;the eye of the obferver, the breadth of the internalnbsp;or lower bow ffiould fubtend.an angle of i�, 45-,nbsp;the breadth of the external, which is much broader,nbsp;ffiould fubtend an angle of 3quot;, 10'; and that thenbsp;diftance between the two bows ftaould fubtend annbsp;angle of 8% 55'. But as the fun is not a point,nbsp;and as the light proceeds from every part of itsnbsp;furface, the diameter of which fubtends an anglenbsp;of about half a degree; therefore the breadths of thamp;nbsp;bows are larger; and the diftance between them isnbsp;lefs than the above-mentioned refults. Aftuafnbsp;meafurement with a quadrant, when the coloursnbsp;gre vivid, conftanfly Ihews that the breadth of the

lower

lower bow fubtends an angle of 2�, 15'} the breadth the upper bow fubtends an angle of 3�, 40', andnbsp;the diftance between both bows fubtends an anglenbsp;8�, 2 5'. Alfo the femidiameter of the circle, ofnbsp;'vhich the external part of the lower bow is an arch,nbsp;fubtends an angle of 42�, 17'; and the femidiameter of the circle, of which die internal part ofnbsp;the upper bow is an arch, fubtends an angle ofnbsp;50% 42'.

The fituation of the rainbows changes according as the eye of the fpectator changes fituation ; fornbsp;otherwife their breadths, amp;c. could not lubtendnbsp;conftantly the fame angles j hence no two perfonsnbsp;can fee the fame bow precifely, or the fame colour,nbsp;in the very fame place.

When the fpeftator is upon a plain, and the fun clofe to the horizon, the rainbow is a femicircle;nbsp;^ut, according as the fun is higher above the hori-fo the rainbow is a fmaller part of a circle,nbsp;inner or lower bow cannot appear when thenbsp;elevation of the fun exceeds 42�; and even thenbsp;^Pper bow difappears, when the elevation of the funnbsp;^^ceeds 54%

^hen the fpedlator is upon an eminence, and fun is near the horizon, then the rainbow' maynbsp;exceed a femicircle j and if the elevation of thenbsp;Ipeflator be very great, and the rain, near him, thennbsp;rainbow may form a complete circle: for in allnbsp;^afes the Centre of the bow, the fpe�ator, and the

Therainbowfometimes is complete from one part of the ground to the other} and at other times it isnbsp;interrupted, either in the middle or in fom'e othernbsp;part. This happens when the rain is partial ; fornbsp;it is in the drops of rain that the bows are formed,nbsp;or that the light is difperfed into its coloured rays*nbsp;The interruption, however, may alfo be producednbsp;by the interpofition of clouds, amp;c.

It follows likewife, from the various diftances of the rain, and from the wind, which impels the rainnbsp;obliquely, that fometimes the rainbow appears inclined, or even of an oval form.

The ufual way of accounting for the formation of the rainbow, or for the difperfion of whitenbsp;light into colours, amongft the drops of rain, is asnbsp;follows :

Let j/D, fig. II, Plate XXII. reprefent a drop of water in the llcy. Sj is a beam of thenbsp;fun�s light that falls upon it. This ray, on accountnbsp;of the refraftive power of water, will not proceednbsp;ftraight towards F, but will be bent towards thenbsp;perpendicular ^C, fo as to impinge upon thefurfacenbsp;of the drop at t. At that place part of the ligl^^nbsp;pafies through the drop into the air; but anothetnbsp;part of it is-refle�led, making the angle of refleftio*^nbsp;equal to that of incidence, and in coming out of th^nbsp;v/ater into the air at e, is refrafted, viz. bent fr^nn

the

the ftraight direfiion .ef, fo-as to make the angle psO, with the perpendicular Cp, larger than thenbsp;angle Cet (fee page 169.) In fliort, the beam ofnbsp;light S s, by going in and out of the drop, fuffersnbsp;two refraftions, viz. at s and r, and one refledionnbsp;at t. By calculating the diredions it muft take atnbsp;thofe places, (according to the method cjiefcribednbsp;in page 190,) it will be found that the angle SFOnbsp;is 42�. 2'.

By thele refradions the light is difperfed into the prifmatic colours O^B; the red light, as thenbsp;leaft refrangible, being next to e and the violetnbsp;next to therefore an eye fituated at O willnbsp;perceive a red light at, e. If the eye be raifednbsp;gradually higher, it will perceive the orange next,nbsp;then the yellow, then the green, amp;c. and laft of allnbsp;will perceive the violet.

Now this would be the cafe if there were a (ingle ^��Opof rain in the fky, and that drop remained im-ttioveable : but it is eafy to conceive that if the eyenbsp;the fpedator remain immoveable, and the dropnbsp;^efeend gradually from C to E, then the eye willnbsp;likewife perceive all the colours fuccefTively, fromnbsp;the red to the violet j and fince, in a (hower of rainnbsp;^ vaft number of drops are to be found at the famenbsp;titne between C and Ei therefore the eye will atnbsp;the fame time receive the red light from the dropsnbsp;C, or near it, the orange from drops that are anbsp;liule lower, the yellow from ihofe that are lowernbsp;ftih, amp;c. .nbsp;nbsp;nbsp;nbsp;laftly, the violet from the lowed;

306 Natural Phenomena relative to Light. at E. Hence the violet, which is feen in the direction OE, is the loweft colour of the firft rainbow}nbsp;and the red, which is feen in the direction O e, is thenbsp;higheft.

Since the incident and the refradled ray mufi; make a given angle, as S F O, in order to fhew anbsp;certain colour it follows, that the rainbow muft benbsp;the arch of a circle, or rather the bafe of a cone,nbsp;the axis of which (viz. the line of afpeEl) paffesnbsp;through the eye of the fpeftator, and through thenbsp;fun,',which forms the vertex of the cone ; for in thatnbsp;cafe only ftraight lines drawn from any point of thenbsp;rainbow to the fun, and to the eye of the obferver,nbsp;form the fame requifite angle. Hence we fee why,nbsp;when the line of afped is upon the horizon, the bownbsp;muft be a femicircle; alfo, why it muft be lefs thannbsp;a femicircle, when the line of afped is inclined fromnbsp;the fun downwards, amp;c.

Having fpoken above of the incident ray, or beam of light S Sy it may perhaps be neceffary tonbsp;obferve, for the fake of perfpicuity, that this is notnbsp;the only light that falls from the fun upon the dropnbsp;stD; for there are numberlefs rays that fall uponnbsp;its whole furfacej but as they fall with differentnbsp;inclinations, fo all their emergent parts cannot comenbsp;to the fame eye: hence we have taken notice ofnbsp;that light only, which impinging upon the drop ihnbsp;the diredion S s, can (after the two refradionsnbsp;s and e, and a refledion at t,) come to the eyenbsp;at O.

There

There is, however, another part of the light inci-lt;3ent upon a drop of rain, which, after two refractions, and two refleftions, can come to the fame eye when placed at a proper diftance; and this is thenbsp;light which forms the fecond or external rainbow.

Let dGs (fig. the fame) be a drop of rain higher than the drop j/D. Y j is a ray of light, which enters it at s, and inftead of proceeding ftraight towards a, is refradted towards the perpendicular sC ;nbsp;it is then partly reflefted from d to e, and againnbsp;from e to gi making both at and at e, the anglesnbsp;of refle�tion refpeftively equal to the angles of incidence. Laftly, on going out of the drop at thisnbsp;ray is refrafted from the perpendicular q C, and isnbsp;difperfcd into the coloured felt;5tor B^O, having thenbsp;violet colour, which fuffers the greatefl refradlion,nbsp;t^ext to and the red, which is the lead: refran-gible next to ;g;0 ; fo that the colours of the uppernbsp;rainbow are in an order contrary to that of thenbsp;lower rainbow. By calculating the changes of thenbsp;^iredtion which take place at the two places of re-fraftion j,^, (fee page 190,) and at the two placesnbsp;�f reflection d, e, it will be found that the emergentnbsp;red ray ^-o, makes with the incident ray Y h, annbsp;^^gle O^Y of 50�, 57'.

account of the light fuffering one refle�lioa more, and continuing longer in the drop Gds thannbsp;m the drop r/D, the angle of difperfion 'Qgo isnbsp;larger than the angle of difperfion OeB: hencenbsp;the upper rainbow is broader than the lower j but

I need not repeat what has been faid above in explanation of the particulars relative to the form,nbsp;extent, amp;c. of the lowxr rainbow j for the fame explanation, with few obvious changes, is applicablenbsp;to the upper rainbow.

A rainbow is alfo produced, and for the fame reafons, by the light of the moon but (as it maynbsp;naturally be expedled) the colours of the lunar rainbow are not nearly fo vivid as thofe of the folarnbsp;rainbow *.

Such a coloured bow is not unfrequently feen at fea in the fpray or drops of water, w'hich the windnbsp;difperfes or carries away from the tops of the waves.nbsp;The colours of, this bow are not fo lively as thofenbsp;of the common rainbow; the moll vivid are anbsp;yellow next to the fun, and a green next to thenbsp;fea. Thofe bows, of which a great many are oftennbsp;to be feen at the fame time, have a pofition contrary to that of the common rainbow; viz. thenbsp;curve part is towards the fea, and the legs upwards.

A coloured bow is always to be feen amongft th^ fcattered water of a jet, a^^broken cafeade, and thenbsp;like, when the fun and the Ipedlator are in propernbsp;fituations.

* See the account of a remarkable lunar bow in Philofophical Tranfadlions, N� 331.

Sometimes

Sometimes a coloured bow is caufed by the re-fi'aiftion of the fun�s rays in the drops of dew upon the grafs. The convex part of fuch bow is turnednbsp;towards the fpedtator.

In fhort, a coloured bow, larger or fmalkr, ftronger or weaker, according to circumftances, isnbsp;always to be feen when drops of water, the fun, andnbsp;the fpeftator, are properly fituated. A perfon maynbsp;fee it if he turns his back to the fun, and forcesnbsp;fome water violently, and in broken ftreams, fromnbsp;his mouth. But the bell way of imitating a rainbow is to fallen a number of fmall (olid glafs balls,nbsp;or a number of fnail glafs bubbles full of water,nbsp;upon a dark board, and to prefent the board thusnbsp;furnilhed to the fun at a proper inclination, whichnbsp;experience eafily finds, whilfl: you turn your back tonbsp;the fun and look at the board.

Another fort of luminous appearances under the tiarne of halos or coronas, may be frequently ob-^^rved in the flty. Thefe are circular zones of palenbsp;hght, moftly white, but fomecimes varioufly coloured, which are feen round the fun, the moon,nbsp;^nd even round fome very bright ftar or planet,nbsp;quot;f he halo is fometimes quite clofc to the luminousnbsp;body, cc Yhofe which have been feen about Siriusnbsp;Atid Jupiter were never more than 3, 4, or 5 de-gf'^es in diameter j thofe which furround thenbsp;�^oon are, alfo, fometimes no more than 3 or 5nbsp;^ degrees. But thefe, as well as thofe which fur-round the fun, are of very different magnitudes.

viz. from 12% to ^0�^ or even larger than this. Their diameters alfo fometimcs vary during thenbsp;time of obfervation; and the breadths both of thenbsp;� coloured and white circles are very different, viz.nbsp;� of 2, 4, or 7 degrees.

� The colours of thefe coronas are more dilute � than thofe of the rainbow ; and they are in a dif-ferent order, according to their fize¹.�

Coronas may be produced by placing a lighted candle in the midft of fteam in cold weather.

Various opinions have been entertained by different philofophers concerning the real caufes of fuch halos or coronas. But whether they are owingnbsp;to the refraftion, or the refledion, or the infledion,nbsp;of light, or to all thofe caufes, and in what propor-

� In thofe which Newton obferved in 1692, they were � in the following order, reckoning from the infide. Iquot;nbsp;� the innermoft were blue, white, and red; in the middlenbsp;� were purple, blue, green, yellow, and pale red j in thenbsp;' � outermoft, pale blue, and pale red. M. Pluygens obfervednbsp;� red next the fun, and a pale blue outwards. Sometimesnbsp;� they are red on the inlide, and white on the outfide. iVlr.nbsp;� Weidler obfervod one that was yellow on the infide, andnbsp;white on the outfide. In France one was obferved innbsp;� 1683, the middle of which was white ; after which fol'nbsp;� lowed a border of red, next to it was blue, then green,nbsp;� and the outermoft circle was a bright red. In 1728 onenbsp;was feen of a pale red outwardly, then followed yellovi^jnbsp;and then green, termin.ited by white.� Prieftley�snbsp;�f Vifion, Light, and Colours, P, VI, Sedt, XI,

is not yet fatisfiidlorily determined ¹ ². It appears, however, that they are formed in fuch aggregations of vapours as are not heavy enough to fall in the form of drops f.

A more remarkable, but much lefs frequent, fpecies of phenomena are fometimes feen in thenbsp;heavens; they are called parhelia and parafeleneSinbsp;vulgarly called mock-funs and mock-moons. Theynbsp;leem to be refledlions of the fun and of the moonnbsp;from zones of denfe vapours that happen to benbsp;Collected in the fity.

� The apparent fize of parhelia is the fame as � that of the true fun; but they are not alwaysnbsp;round, and alfo, they are not always, thoughnbsp;quot; they are fometimes faid to be, as bright asnbsp;� the true fun. When there are numbers of

* The various opinions are collected by Dr. Prieftley in Ihe above-mentioned Sedlion of his Hiftory, amp;c. See allbnbsp;anonymous publication, entitled, An Account of hides ornbsp;^oroHis, London 1799.

t Ariftotle, Pliny, Gaffendi, De la Hire, CalKni, Def-�^artes, Newton, Mr. Grey, Dr. Halley, amp;c. but a concife of all their obfervations, as alfo of the opinions

0 have been entertained concerning the formation of y.- 1^^^�io'uena, may be feen in Dr. Prieftley�s Hillory ofnbsp;1 ion, Light, and Colours, P. VI. Sed. XI. from whichnbsp;he above account is taken.

qf them, fome are not fo bright as others. Ex-ternally they are tinged with colours, like the � rainbow, and many have a long fiery tail oppo-fite to the fun, but paler towards the extremity.nbsp;� Dr. Halley obferved one which had tails extend-ing both ways, and fuch a one alfo M. Mufchen-� broeck obferved in 1753, the tails being in anbsp;right line drawn through both the funs. Bothnbsp;� of them, alfo, v/ere in coloured circles. M*nbsp;Weilder faw a parhelion with one tall pointingnbsp;upwards and another downwards, a little crook-ed; the external limb, with refpedt to the fun,nbsp;� being' of a purple colour, and on the other fide itnbsp;� was tinged with the colours of the rainbow. Thenbsp;tails of thefc parhelia, for the mofi part, appearnbsp;� in a white horizontal circle.

� Coronas generttlly accompany parhelia, fome tinged with the colours of the rainbow, andnbsp;others wliite. They differ in number and fize,nbsp;� but they are all of the fame breadth, which isnbsp;� that of th� apparent diameter of the fun.

A very large white circle, parallel to the ho- � � rizon, generally paffes through all the parhelia ';

and if it were entire, it would go through the � centre of the fun. Sometimes there are arcs ofnbsp;�� leffer circles concentric to this, touching thofenbsp;coloured circles which furround the fun. Theynbsp;� are alfo tinged with colours, and contain othernbsp;parhelia.� .nbsp;nbsp;nbsp;nbsp;,nbsp;nbsp;nbsp;nbsp;'

� nbsp;nbsp;nbsp;make

� make mention in the next fedtion, under the title of Electricity: but we fhall juft obferve in thisnbsp;place, that fometimes, though by no means frequently, a pale white light more or lefs extended,nbsp;is feen in the Iky, the caufe of which is not known.nbsp;It diiTcrs from the northern light principally by Itsnbsp;being fteady and uniform, whereas the northern lightnbsp;is lambent and changeable. The former is like-wife more denfe than the latter j for it generallynbsp;eclipfes the ftars over which it paftes. A remarkable appearance of this fort was obferved in London on the night of March the 27th, 1781 *.

' The zodiacal light is a fort of pyramidal white-nefs, which is fometimes feen above the horizon after the fetting of the fun, or before its rifing. Itsnbsp;whitenefsis not much unlike that of the via lahlea.nbsp;Or milky way. Its bafe is towards the fun, and itsnbsp;oxtenfion is in the plain of the zodiac. Caffininbsp;feetns to have firft taken notice of it in 1683. Innbsp;the torrid zone the zodiacal light is frequently, ornbsp;almoft conftantly feen. At or near our latitude itnbsp;ttiay be feen about the time of the equinoxes. Thenbsp;breadth of this whitenefs is various j at the horizonnbsp;Varies from 8 to 30 degrees; its extenfion,nbsp;reckoning from the fun to the apex of the light, generally exceeds 45�. Mr. Pingre, being in thenbsp;torrid zone, faw it of 120 degrees.

At prefenr, fays de la Lande^ it feems to be � generally believed, that the zodiacal light is thenbsp;quot; atmofphere of the fun; for it always accompaniesnbsp;� that luminary, and the equator of the fun is innbsp;the diredlion of the zodiacal light. Therefore innbsp;all probability the zodiacal light is an atmofpherenbsp;� fituated round the fun in the diredtion of its equa-� tor, and flattened by its rotatory motion*.�

Various accounts of peculiar luminous appearancesquot; that are feen in particular places, and which are evidently ov/ing to certain peculiar difpofitionsnbsp;of mountains, houfes, rivers, and other objects, arenbsp;to be met with in different books ; but nonenbsp;of thefe feems to be more remarkable, and lefs un-derftood with refpeft to its caufe, than the famousnbsp;Lata Morgana, or apparition fo called, which isnbsp;frequently feen near the city of Reggio, fituatednbsp;towards the extremity of the kingdom of Naples,nbsp;and facing the ifland of Sicily.

� When the rifing fun fliines from that point, � whence its incident ray forms an angle of aboutnbsp;45 degrees on the fea of Reggio, and the brightnbsp;� furface of the water in the bay is not difturbednbsp;� either by the wind or the current, the fpeclatornbsp;being placed on an eminence of the city, with hisnbsp;� back to the fun and his face to the fea;�on anbsp;quot; fudden there appear in the water, as in a catop-� trie theatre, various multiplied objefts, viz. num-

berlefs feries of pilafters, arches, caftles well delineated, regular columns, lofty towers, fuperb palaces, with balconies and windows, extendednbsp;alleys of trees, delightful plains with herds andnbsp;flocks, armies of men on foot and horfeback, andnbsp;many other ftrange images, in their natural colours and proper aflions, paffing rapidly in fuc-� cefliori along the furface of the fea during thenbsp;whole of the Ihort period of time while the above-� mentioned caufes remain,

� But if, in addition to the circumflances before defcribed, the atrnofphere be highly impregnatednbsp;� with vapour, and denfe exhalations not previ-� oufly dilperfcd by the action of the wind ornbsp;waves, or rarefied by the fun, it then happensnbsp;that in this vapour, as in a curtain extended alongnbsp;� the channel to the height of about 30 palms, andnbsp;� nearly down to the fea, the obferver will beholdnbsp;� the fcene of the fame objcdts not only refle�lednbsp;� from the furface of the fea, butlikewife in the air,nbsp;� though not fo diftinfl or well defined as the for-mer objedts from the fea.

� Laftly, if the air be flightly haxy and opake, ^rtd at the fame time dewy and adapted to formnbsp;the iris, then the above-mentioned objedts willnbsp;appear only at the furface of the fea, as in thenbsp;firft cafe, but all vividly coloured or fringed withnbsp;ted, green, blue, and other prifmatic colours*.�

This phenomenon is related with fome variety of circumftances, and has been differently explained,nbsp;by various writers. Upon the whole, it feems thatnbsp;the appearances of hoiries, trees, amp;c. are only thenbsp;reflexions of the objeXs of the city of Reggio, andnbsp;pf the coaft. They feem to be refleXed from thenbsp;furface of the fea, and from the furface of denfe vapours 6r clouds in the air clofe to the fea; and ac-cordine to the various forms and number of the re-fleXing furfaces, thofe objeXs are multiplied, magnified, inverted, elongated, or otherwife diftorted.nbsp;But the exaX explanation of the phenomenonnbsp;muft be left for the ingenuity of future ob-fervers.

1 flrall clofe this chapter with a concife account of phofphorefcent bodies, among which I fhatlnbsp;reckon the ignis fatuus, ov jack-a-lantern.

The name of phofphorus has of late been given to a particular primitive fubftance, of which fuffi-dent mention has been made in the fecond volumenbsp;of thefe Elements; but in its more extenfive appli'nbsp;cation, that nam.e means every fubftance thatnbsp;fhines' in the dark, without the produXion of fen-fible heat.

The phofphorefcent bodies may be divided into five fpecies, viz. I. The living animals which havenbsp;the property of (hining �m the dark, fuch as glowworms, lanter.t flies, of which there feems to be fe'nbsp;veral fpecies, but of ihe mechanifm which produces

fome of them, in their beft ftate, afford light barely enough to read the hour on a watch that has a clearnbsp;dial. In warmer climates their light is much morenbsp;powerful. The light of thofe infefbs generallynbsp;ceafes after death; but whilft living they may eithernbsp;ftiow it or not at pleafure.

A vaft number of fubftances have the property of fhining for a certain time in the dark, after having been previoufly expofed to light; but they havenbsp;it in different degrees of intenfity as well as of duration. Several precious ftones, and calcareousnbsp;bodies, efpecially after calcination, have this property,'-as alfo paper, and almoft all vegetable andnbsp;animal fubftances when very dry, or after folutionnbsp;in nitrous acid. Metallic fubftances and water havenbsp;not this property ; yet congealed water, viz. ice,nbsp;^nd efpecially fnow, have it in a confiderable degree ¹.nbsp;nbsp;nbsp;nbsp;'

There is a mineral, called the Bolognian ftone, ^hich, after due preparation, has this property in anbsp;remarkable degree f. Thofe ftones are moftly

t The proper or effeiftual method of preparing this ftone feems to be kept fecret. Several trials made in this countrynbsp;have fucceeded but partially. Kircher diredls to reduce the

found in the neighbourhood of Bologna. This is an heavy grey fpar of the barytic genus, properly callednbsp;larofelenite, and, from its weight, manner metal'nbsp;ticim. .

If this ftone, after due preparation, be expofed tfgt; the day light, and then be brought into a darknbsp;room, it will be found to Ihine with a darkifli rednbsp;light, or to appear like ignited coals. This fhiningnbsp;continues a few minutes, gradually decaying, andnbsp;laftly vanifhing. By expofing it again to the daynbsp;light for a few feconds, its fhining property is renewed as often as one pleafes. It will become luminous even by expofing it to candle light.

The refiduum of the diftillation of chalk and nitrous acid has the fhining property, fimilar to that of the bolognian ftone, though not in fo great anbsp;degree. This is called (from its inventor) Bald-zvin's phojphorus.

Several other preparations have the property of abforbing light, and then of yielding it in the dark ;nbsp;but none has it in fo eminent a degree as that

ftoae into a fine powder, together with white of egg, water or linfeed oil. T he pafte thus formed muft be put in n,nbsp;furnace, and muft be calcined to a certain degree. Othersnbsp;dire�t to place the bolognian ftone amongft lighted charcoal,nbsp;and to leave it undifturDed therein until the coals arc con-fuined. Other methods have alfo been deferibed ; but, n^tnbsp;knowing which of them is the beft, I fhall not trouble thenbsp;reader with any more of them.

which was difcovered by tlie late Mr. Canton, and which is prepared in the following manner :

� Calcine fome common oyfter-fliells,� (if they be old, and half calcined by time, fuch as are foundnbsp;tipon the fea-fhore, they are fo much the better)nbsp;� by keeping them in a good coal-fire for half annbsp;hour; let the pureft part of the calx be pulve-� rized and lifted; mix with three parts of thisnbsp;powder one part of the flowers of fulphur ; letnbsp;quot; this mixture be rammed into a crucible of about

quot; and let it be placed in the middle of the fire, � where it mull be kept red hot for one hour atnbsp;leaftj and then fet it by to cool; when cold, turnnbsp;� it out of the crucible, and cutting or breaking itnbsp;quot; to pieces, fcrape off, upon trial, the brighteft part,nbsp;� which, if good phofphorus, will be a white pow-quot; der, ^nd may be preferved by keeping it in a drynbsp;� phial with a ground Hopple*.�

If this phofphorus, whether in the phial or not, be ^ept in the dark, it will give no light, but if it benbsp;^xpofed to the light, either, of the day, or of anynbsp;other body fufficiently luminous, and afterwards benbsp;brought into a dark place, it will appear luminousnbsp;for a confiderable time, viz. a few minutes. Its lightnbsp;IS white with a lhade of blue or green.

A little of this phofphorus, when firft brought Into a dark room, after having been expofed for a

few feconds on the outfide of a window to the common day light, will give light enough to difcovef the hour on a watch; provided the eyes of the ob-ferver have been Ihut or in the dark two or threenbsp;minutes before.

It has been long queftioned whether thofe phof-phori fhine by yielding the light which they have firft imbibed, or by yielding their own light, kindlednbsp;as it were by the adlion of foreign light �, and thoughnbsp;the former opinion be by far the moft probable,nbsp;yet the queftion is not quite fatisfaftorily determined.

In order to elucidate this point, various ingenious perfons have attempted to illumine thofe phofphorinbsp;by coloured light, as for inftance, by red, or green,nbsp;or blue, or yellow light; but their refults do notnbsp;agree. Algarotti having illuminated the bologniannbsp;phofphorus, by difTerently coloured light producednbsp;by a prifm, found that the phofphorus was faintlfnbsp;illuminated by this means, but he could not diftin-guilh any difference of colour in it ¹.

Beccaria of Turin obfervcd, that pieces of artificial phofphorus, much fuperior to the bologniaOj inclofed in tubes into which the light was admittednbsp;through pieces of colpured glafs, exhibit that paf'nbsp;ticular colour only j yet this effedt has been deniednbsp;by fubfequent obfervers.

The determination of this queftion would go a great way towards proving that light is real matternbsp;emanated by the luminous body, rather than a modification of a fluid univcrfally difperfed. But independent of this queftion, what principally feemsnbsp;to prove the materiality of light, is the change whichnbsp;light alone produces on various bodies, viz. on vegetables, on folutions of filver, amp;c. ¹

III. The bodies which produce light when heated, form the third fpecies of phofphory. Thenbsp;beft method of heating bodies in a dark roomnbsp;for this purpofe, � is to reduce the body to anbsp;� moderately fine powdetj and to fprinkle it, bynbsp;� fmall portions at a time, on a thick plate of iron,nbsp;� or mafs of burnt luting made of fand and clay,nbsp;� heated juft below vifible rednefs, and removednbsp;quot; into a perfedlly dark place.� f

A great variety of lubftances fhine when they arc fo treated, viz. fluoric ftones, feveral marbles, dia-n^ond and other precious ftones, calcareous earth,nbsp;i^etallic fubftances, fea coal, oils, wax, butter, paper,nbsp;and feveral other animal and vegetable fubftances.

See Count Rumford�sPaper in the Philofophical Tranf-a�ions, Volume for the year 1798, Art. XX.

t See Mr. T. Wedgwood�s Paper in the Philofophical T ranfadtions, Volume for the year 1792, Art. III.nbsp;t Ibid.

unequal �, in fome the light is almofl: momentary� � in others it lafts for fome minutes, and may benbsp;� prolonged by ftirring the powder on the heater.nbsp;�It foot! attains its greateft brightnefs, and diesnbsp;� away gradually from that point, never appearing

in a fudden flalh, like the light of quartz pebbles � rubbed together. If blown upon, it is fuddenlynbsp;� extinguilhed, but immediately reappears on dif*nbsp;� continuing the blaft.�

The light of the preceding fpecies of phofphori is alfo expelled more effectually by heat; but whennbsp;the quantity of light previoufly imbibed has beennbsp;once yielded, then they will ceafe to ftiine unlefsnbsp;they be expofed again to the light; whereas thenbsp;phofphori of this third fpecies give out light and heat,nbsp;without the neceffity of having been previouflynbsp;expofed to external light. It muft be obferved,nbsp;however, that feveral bodies are phofphori of bothnbsp;Ipecies.

IV. Several fubftances yield a light either quite white, or with different lhades of red, or blue, bynbsp;attrition, viz. when they are rubbed or knocked onenbsp;againft the other. The light is generally Ipreadnbsp;beyond the touching parts, and fometimes it fpreadsnbsp;all over the bodies.

Almoft all the ftones of the filiceous genus, ft\ch as quartz, flints, agates, amp;c. have this property,nbsp;as alfo glafs, porcelain, hard baked earthenware, amp;c.

peculiar fmell. Some of thofe bodies during attrition,

Natural Phenomena relative to Light. 323

tiorij emit now and then reddifh fparks of a vivid light, which retain their brightnefs in a paflage ofnbsp;2,. and even 3 inches through the air.

V. The phofphori of this laft fpecies are rhofe '^hich emit light whilft they are in an evident ftatenbsp;of decompofition. Of this fort are moft animalnbsp;matters, and fome vegetable fubftances, efpeciallynbsp;rotten wood. In fome of them the light feems tonbsp;belong to the extrication of phofphorus properly fonbsp;called ; whilft in others a pure light feems to benbsp;produced. Upon the whole it appears, that lightnbsp;enters into combination with various bodies, andnbsp;forms one of their conftituent principles, efpeciallynbsp;with animal and vegetable fubftances; and that whennbsp;thofe fubftances are in a ftate of decompofition, thenbsp;light being one of the ingredients, is feparated fromnbsp;the reft, amp;c. It alfo. feems that the light is feparated either very flowly,,in which cafe it is not per-c^cived; or quickly, when it becomes vifible. Innbsp;fome bodies it is the firft produce of the decompo-htion, viz. before any putrid effluvium is perceived;nbsp;m others, it is yielded at different periods.

Of all the animals, filh feems to afford the greateft quantity of light, and they yield it in thenbsp;greateft quantity before the putridity takes place.nbsp;Almoft every body is acquainted with the Ihiningnbsp;property of fifti; but the moft recent and entertaining experiments upon this fpecies of animals, andnbsp;particularly die herring and the mackerel, were

made by Dr. Hulme, and are deferibed in th� Philofophical Tranfa�tions for the year 1800, Ar-^nbsp;tide IX. from which the following particulars havenbsp;been extradted.

Herrings and mackerels (and probably moft other fifties) begin to appear luminous about thenbsp;fecond day after their having been taken out of thenbsp;�water. The light increafes whilft they are perfedlynbsp;good and fweet j but it begins to decreafe -when thenbsp;filh begins to putrefy, and it decreafcs according asnbsp;the putrefcence increafeslt;

It is not the external furface only of thofe animals that is capable of fliining; but the light feems to benbsp;incorporated with their whole fubftance, and tonbsp;make a part thereof, in the fame manner as anynbsp;other conftituent principle ; for if the filh be cut irfnbsp;various pieces, the whole furface of every piece becomes luminous, if kept in a dark and rather coolnbsp;place 5 efpecially the foft roe both of the herringnbsp;and of the mackerel, which look like a completenbsp;body of light at about the third or fourth night�nbsp;�which generally is the period of their greateftnbsp;brightnefs.

Hence it feems that the decompofition of the fift^ begins very foon, but the light is the firft principlenbsp;that efcapes, and which takes place long beforenbsp;any foetid or putrid effluviurii can be perceived.

This light is not accompanied with any degre� pf heat that may be difeovered by the thermo*nbsp;meter.

The

The luminous matter of fifhes, or the thickifli liquor with which this light is incorporated, may benbsp;fcrapcd off by means of a knife from over their fur-faces, and may be prcferved in a phial, where it willnbsp;Continue to (bine for a day or two, or longer, according to circumftances. But there are fome fubftaneesnbsp;which, being mixed with this luminous matter in anbsp;Certain proportion, will extinguilh its light; and itnbsp;is very remarkable that fome of thofe very fub-ftances, but mixed in another proportion, will in-creafe or preferve it for fome time longer.

� Thofe which extinguifh it are, water alone ; water impregnated with quicklime j water impregnated with carbonic acid gas j water impregnatednbsp;with hepatic gas; fermented liquorsj ardent fpirits;nbsp;tnineral acids, both in a concentrated and in a dilutednbsp;ftate J vegetable acids j fixed and volatile alkalies,nbsp;when diflblved in water; neutral falts, \\z. faiuratednbsp;folutions of Epfom fait, common fait, and of falnbsp;Ammoniac ; infufions of chamomile flowers, of longnbsp;pepper, and of camphor, made with boiljng-hotnbsp;'^ater, but not ufed till quite cold j pure honey, ifnbsp;'Jfed alone.�

On the other hand, a very moderate folution of fome of the above-mentioned fubftanees, as for in-ftance, about a dram of Epfom fait, or of Glauber�snbsp;fait, or of Rochelle fait, or of phofphorated foda, ornbsp;of nitre, or of common fait, or of honey, or of fugar,nbsp;diflblved in an ounce of water, and then mixed with

the luminous matter of fifhes, will render their light ftronger and more durable.

illurninatlon. The fea water preferved its lumi-� noufnefs for feveral days.�

Any of the laft-mentioned folutions, being impregnated with the luminous matter, and left feme time at reft, are rendered more lucid by a moderatenbsp;degree of heat j but an higher degree of heat, fuchnbsp;as that of about boiling water, extinguifties themnbsp;totally and permanently.

Cold extinguifties this light in a temporary manner ; for the light is revived in its full fplendour as foon as it is expofed to a moderate degree ofnbsp;heat.

The light of thofe mixtures is rendered more vivid by motion, viz. by agitating the phial whichnbsp;contains the liquor, or by drawing fome hard bodynbsp;through it. This feems fully to explain the caufenbsp;of that phofphorefcent light which at, night is feennbsp;on the furfacc of the fea, when the water i^

Natural Phenomena relative to Light. latter fort; for inftance, if the light be extinguifliednbsp;by the admixture of a faturated.folution of fait, addnbsp;more water to the mixture, fo as to diminilh thenbsp;proportion of fait, and the light is thereby re-''ived j and on the contrary, if to the latter morenbsp;fait be added, the light will be extinguilhed, andnbsp;lb on.

� Some fhining matter, fays Dr. Huline, was � taken from a mackerel, and mixed with a folu-quot; tion of feven drams of Epfom fait in one ouncenbsp;*' of water j and its light was immediately extin-� guilhed. The fame effe�t enfued, but in a lefsnbsp;� degree, with a folution of fix and of five drams.

In a folution of two drams, in the fame quantity � of water, the liquid was luminous; but muchnbsp;� more fo when only one dram of fait was ufed.nbsp;� Obferving the extinftion of light to take place,nbsp;as above, in the more faturated folutions, whilenbsp;rhe diluted folutions were luminous, it occurednbsp;to me to endeavour to dlfcover what became ofnbsp;quot; the extinguilhed light, in the former cafe, andnbsp;quot; whether it might not be revived by dilution.

� For this purpofe I took the folution of feven � drams of fait in one ounce of water, in whichnbsp;the lucid matter from a mackerel had been ex-tinguifhed, and diluted it with fix ounces of coldnbsp;pump water j when, to my great aftonifhment,nbsp;light in a moment burll out of darknefs,, and thenbsp;'''^bole liquid became beautifully luminous ! Thisnbsp;revived light remained above 48 hours, that is.

quot; as long as other light in general does, which has � never been extinguilhed. Hence, it had loftnbsp;� nothing of its vivid luminous powers by its ex-

The fleiTi of quadrupeds fometimes has alfo been obferved to emit light¹. Light has alfo fometimesnbsp;been feen on burying grounds, which is attributednbsp;to the fame caufe, viz. to the decompofition of animal matter.

Vegetable fubftances in aftate of decompofition, and efpecially rotten wood, are fometimes feen tonbsp;Ihine in the dark j but amongft the various luminous appearances which feern to owe their origin tonbsp;a decompofition of animal and vegetable matter,nbsp;none is fo famous, and yet fo irnperfeftly known asnbsp;the ignis fatuus, or jack-a-lantern, which has beennbsp;v^rioufly related by ignorance, apprehenfion, andnbsp;exaggeration,

It has been the opinion of certain philofbphers that the ignis fatuus is produced by Ihining infe�ls.nbsp;Sir Ifaac Newton called it a vapour jhinbig withoutnbsp;heat; and this feems to be the moft probable opi'nbsp;nion, efpecially if it be allowed to owe its originnbsp;to the decompofition of animal and vegetable fub-ftances.

Waving however any farther cohjefture, I lhall juft add two of the moft authentic accounts of fuchnbsp;appearances that I find recorded.

See T. Bartholin de luce animalium, p. 183. Boyle�¹ Works, vol. III. p. 304. Phil. Tranf. vol. XI. p. 599-

It Is related by Dr. Derham, that, having ob^ ferved an ignis fatuus in fome boggy ground, between two rocky hills, in a dark and calm night,nbsp;lie got by degrees within two or three yards of it,nbsp;and thereby had an opportunity of viewing it to thenbsp;greateft advantage. It kept fkipping about a deadnbsp;thiftle, till a flight motion of the air, occafioped, ,asnbsp;he fuppofed, by his near approach to it, made itnbsp;jump to another place ; and as he advanced, it keptnbsp;flying before him. He was fo near to it, that, hadnbsp;it been the flrining of glow-worms, he was fatisfiednbsp;that he could not but have diftinguiftied the fepa-rate lights of which it muft have confifted ; whereas it was one uniform body of light. He thereforenbsp;thought that it muft be an ignited vapour *.

Mr. Beecari made particular inquiry concerning the ignis fatuus. He found that two, which appeared on the plains, one to the north and the other tonbsp;the eaft of Bologna, were to be feen almoft everynbsp;tJark night, efpecially the latter ; and the light theynbsp;Save was equal to that of an ordinary figgot. Thatnbsp;to the call of Bologna once appeared to a gentlemannbsp;tgt;f his acquaintance, as he was travelling, and keptnbsp;h'ti Company above a mile, conftantly moving before him, and calling a ftronger light upon the roadnbsp;than the torch, which was carried along with him.nbsp;All thefe luminous appearances, he fays, gave lightnbsp;enough to make all neighbouring objedls vifible,

and they were always obfervecl to be in motion, but this motion was various aud uncertain. Sometimesnbsp;they would rife up, and at other times finkj butnbsp;they commonly kept hovering about fix feet fromnbsp;the ground. They would alfo difappear of a fud-den, and inftantly appear again in fome other place.nbsp;They differed both in fize and figure, fometimesnbsp;fpreading pretty wide, and then again contraftingnbsp;themfelves j fometimes breaking into two, and thennbsp;joining again; fometimes floating like waves, andnbsp;dropping., as it were, fparks of fire. He wasnbsp;aflured that there was not a dark night all the yearnbsp;rouhd in which they did not appear, and that theynbsp;were obferved more frequently when the groundnbsp;was covered with fnow than in the hotteft fummcr;nbsp;nor did rain or fnow in the leafl hinder their appearance ; but, on the contrary, they were obfervednbsp;more frequently, and call: a ftronger light in rainynbsp;and wet weather j nor were they much afleded bynbsp;the vvind ¹.

Prieftley�s Hiftory of Vifion, amp;c. p. 581. Phil. Tranf. vol. 3amp;, p. 204.

The mod enlightened and inquificive perfons of the third or fourth century before thenbsp;Chriftian aera, were acquainted with a remarkablenbsp;property of, at lead, two mineral bodies, one ofnbsp;which was amber, and the other was a hard done,nbsp;called lyncuriutn' by Theophradus (probably thenbsp;fame as is at prefent known under the name of ,nbsp;tourmalin.) They knew that either of thofe bodies,nbsp;3i^d in particular the foumer, after a flight fri�tion,nbsp;'vould attraft any'kind of fmall bodies, fuch as bitsnbsp;of draw, alhes, amp;c. that might be prefented to itnbsp;''^ithin a certain didance.

They knew likewife that another mineral, which called magnet, would attraft iron-, and all fuchnbsp;^dies as contained a fufficient quantity of thatnbsp;o^etal. But a wide difference obvioufly exided be-*^ween the power of the magnet, and that of the-other above-mentioned bodies. The magnet at-trafted iron only, and its attractive property requirednbsp;no previous friCtion j the other bodies could notnbsp;aCl without previous friflion, but then they would

attract

attradt bodies of every kind indifcriminately, provided diey were fufficiendy light.

In procefs of time it was found that feveral other bodies, fuch as precious ftones, fulphur, glafs, amp;c.nbsp;poflefled precifely the fame attradive property, notnbsp;as the magnet, but as the amber; therefore theynbsp;were faid to have the property of the amber, which,nbsp;jn the Greek language, was callednbsp;nbsp;nbsp;nbsp;whence

the word eleSlruily has been derived, and hence thofe bodies were faid to be pofltfled of (;l0ricityynbsp;or of the eleSlrk property. After the laple of fomenbsp;centuries it was found that larger bodies, moderatelynbsp;warm, dry, and propedy rubbed, would attraft fromnbsp;a greater diftance, and with greater force, thannbsp;fmaller bodies of the ftame fort j and as it was eafynbsp;to procure large pieces of glafs and fulphur, attempts to increafe die attraftive property of thofenbsp;fubftances were foon made, and thofe attempts grarnbsp;dually difclofed 1�everal other properties of the famenbsp;cleftric power.

It was difeovered that the fame body would not only attraft, but alfo repel, the light bodies j thenbsp;attradlion and repulfion fucceeding each other repeatedly,

It was difeovered that this attractive property might be communicated from the glafs or fulphur,nbsp;or amber, amp;c. to other bodies, which could not ofnbsp;themfelves acquire it by rubbing.

It w'as obferyed that on touching the body which Vnbsp;nbsp;nbsp;nbsp;had

Had been rubbed (otherwife faid to be excited, or to be eledrlfied), luminous fparks were feen to pro*nbsp;cfed from it, and thofe fparks were accompaniednbsp;�'^ith fnappings, viz. an audible noife; and when anynbsp;part of the furface of the animal body was prcfentednbsp;to the eleftrified fubftance, a ftnfation was perceived,nbsp;as if fomething (truck the part at the time that thenbsp;fpark was manifefted.

Thofe difcoveries were moftly made in the 17th century, and being incomparably more furprizingnbsp;than the mere attradlion of a bit of amber, they pro*nbsp;duced an aflonilbing degree of curiofity, of induf-try, and of emulation amongft the philofophers ofnbsp;Europe. The multiplicity of labourers, the varietynbsp;of machines that were contrived, and of experimentsnbsp;that were inftituted, produced farther difcoveries killnbsp;more furprifing than the preceding, and renderednbsp;the fubjeft of eledlricity highly interefting in the eyenbsp;of the philofopher.

It was difeovered that the eledtrlc power could be accumulated in what is called the Leyden phialsnbsp;Ib that inftead of a Angle fpark fronh a piece of ex- -t^'ted fulphur or of excited glafs, the force of feveralnbsp;fuch fparks could be colledted in that phial, andnbsp;Could afterwards be difeharged all at once' upon anynbsp;given body, upon which it would produce very ex-maordinary effedts. In (hort, fuch difeharge willnbsp;inftantly melt even the moft refradlory metallic bo-bies, it kills animals, fires combuftible bodies,

Indeed; not long after the difcovery of the Leyden phial, the identity of eledlricicy and thenbsp;power which produces the thunder and lightning,nbsp;was fully and fatisfaftorily proved.

Subfequent to this it was difcovered that electricity is excited by a variety of other means befides friftion, fuch as by heating, cooling, evaporation,nbsp;condenfation, efFervefcence, amp;c.; or rather it appears that there is hardly an operation of nature innbsp;which the electric power i;; not concerned. Butnbsp;whilft we admire the univerfality of its influence,nbsp;whilft we applaud the ingenuity of philofoph^rs fornbsp;having acquired the knowledge of fo many wonderful fabfs, we muft confefs our utter ignorance of thenbsp;caufe which produces them; and thus we are atnbsp;once forced to acknowledge the ftrength and thenbsp;weaknefs of the human underftanding.

The very extenfive influence of eletSlricity throughout the operations of nature, its greatnbsp;power, and its conftant adtion, feem to indicatenbsp;that it muft be eflentially concerned in variousnbsp;grand and neceffary procefles. Yet in the invefti'nbsp;gation of its nature, of its influence, and of its ufe,nbsp;we have had only fuppofitions for guide, and vvenbsp;have nothing but hypothefes, viz. fuppofitions, eonbsp;offer.

' nbsp;nbsp;nbsp;The

The ftatement of thofe fads, the moft ufefui application of the fame, and the beft hypothefesnbsp;which have been offered for their explanation, willnbsp;form the materials of the prefent fedion

[ 336 ]

F a perfon, holding with one of his hands a clean and dry glafs tube, rubs it with his other hand,nbsp;which muft alio be clean and dry, by ftroking itnbsp;alternately upwards and downwards j and after �inbsp;few ftrokes prefents to it fmall light bits of paper,nbsp;thread, metal, or of any other fubftance, the rubbednbsp;tube will immediately attraft them, and after a fhoftnbsp;time will repel them. It will prefently attradl: themnbsp;again, then repel them, and fo on j continuing thisnbsp;alternate attradion and repulfion for a confiderab'enbsp;time.

If the glafs tube be rubbed in the dark, and after having been ftroked a few times, a finger be pre-fented to it at the diftance of about half an inch,nbsp;lucid Ipark will be feen between the finger and thenbsp;tube, and this fpark is accompanied with a Ihappingnbsp;noife ; the finger at the fame time receiving a pufhgt;nbsp;as if it were from air iffuing with violence out of ^nbsp;fmall pipe *.

In this experiment, the attraftion, repulfionj Sparkling, amp;c. are the efPedls of that unknownnbsp;caufe, which is called EleSridty; and hence theynbsp;ate called eledirual appearances. The glafs tube it-felf is called the ekdric, and all thofe bodies whichnbsp;are capable of producing fuch effedts after friftion,nbsp;are called eledrics j and as the rubbing awakes, asnbsp;it were, in them the power of producing fuchnbsp;efFefts, they are therefore faid to be excited by thenbsp;rubbing. The hand, or any other body that rubsnbsp;an eledlric, is called the rubber j and if, inftead ofnbsp;the perfon rubbing the glafs tube, a machine be contrived capable of excitinganeledlric, that mechanifmnbsp;is called an eleSlrical machine.

Let an oblong piece of metal, fuch as a poker, a long metallic fpoon, amp;c. be fufpended in the air bynbsp;gt;Taeans of a dry filk firing, upwards of a foot long,nbsp;any convenient fupport, as in fig. t, Platenbsp;and let fmall light bodies, fuch as havenbsp;i^^en mentioned in the preceding experiment, benbsp;P'^efented to its lower extremity, within about annbsp;^'ich of it; then having rubbed the dry glafs tub�nbsp;as before, place it near the upper etid A of the fuf-Pfnded metallic body, and you will find that thenbsp;lower end B of that body will attradf and repel thenbsp;^'ght bodies, alfo will give fparks, amp;c. exadtly likenbsp;the excited tube itfelf; which fliews that tire eleftricnbsp;Virtue paffes through the metal from one end A tonbsp;the other end B.

If, inftead of the metallic body A B, yo� fufpend III.nbsp;nbsp;nbsp;nbsp;znbsp;nbsp;nbsp;nbsp;in

fealing wax, and repeat the lafl defcribed experiment, you will find that the lower part of the fuf-pended flick of glafs or of fealing wax, will not at-tradl the light bodies, nor will it give any fparks; which Ihews that the eleftric virtue will not pafsnbsp;through glafs or through fealing-wax.

Now the above-mentioned metallic body, and ail thofe bodies through which the electric virtue cannbsp;pafs, are called conduliors of electricity. But thenbsp;glals flick, the fealing-wax, and all thofe bodiesnbsp;through which the cledric virtue cannot pafs, arenbsp;called non-condu�lors. A body refling entirely upon�nbsp;or fufpended by, non-condudtors, is faid to be in-Julated.

All the bodies we are acquainted with, may be divided into condudtors and non-condudtors ofnbsp;eledtricityj and as it has been found that the non-conduiflors may be excited by fridion, whereas thenbsp;condudtors cannot be excited by fridtion j thereforenbsp;eleSirics. and mn-condultors mean the fame bodies¹�nbsp;and condudtors have alfo been called non-eleElrics.

Such is the outline or the general idea of thofe principles ofeledtricity; but thofe diftindtions arefai^

Eledtrics have alfo been called etedlrics per Je. Itmuft obferved, however, that certain fubftances, fuch as oils, certain powders, amp;c. which are non-condudlors, are calk^^nbsp;eledlrics from analogy; for they cannot be fubmittetlnbsp;friction.

fi'om being accurately fettled and determinate. For mftanccj we are not acquainted with any bodynbsp;which, ftridtly fpeaking, may be faid to be a per-fedt eledtric or a perfedt condudtor j the eledtricnbsp;virtue finding fome refiftance in going throughnbsp;the beft condudlors, and being partly tranfmittednbsp;through, or over the furface of, moft^ if not all thenbsp;eledfrics. The lefs perfedl condudlor any fubftancenbsp;is, the nearer it comes to the nature of ah eledtric jnbsp;and, on the other hand, the lefs perfedl eledtricsnbsp;come neareft to the nature af condudtors. In fadt,nbsp;there are certain fubftances which may adlually .benbsp;excited by means of fridlion, and at the fame timenbsp;are pretty good condudtors.

The following lifts contain, in general, all the eledtrics and the condudtors, difpofed, as much asnbsp;it is pradticable, in the order of their perfedtion,nbsp;beginning with the moft perfedt of each clafs.

Wax.

Almoft all the above-mentioned fubftances, when heated beyond a certain degree, become conductors. Thus red-hot glafs, melted rofin, amp;c. arenbsp;condudors of eledricity ¹. The focus of a burningnbsp;lens, or concave refledor, is not a condudor. Sometimes glafs of a hard quality is fo bad an eledricnbsp;as to be almoft a good condudor. It is remarkablenbsp;that often the nature of the fame pieces of glafs is-changed by time, and byufe, fo as to become goodnbsp;elecftrics, though at firft they were almoft conductors, and vice verja.

A glafs vefTel is excited beft when the air in it is a little rarefied j but a glafs velfel entirely ornbsp;almoft ent rely exhaufted of air, on being rubbed,nbsp;fhews no figns of eledrieity on its external fur-

Hot air has been reckoned a condudor ; but this is denied by Mr. Read. See his Summary View of Spontaneous Eledricity, p. 8,

face, but the eleiStric power appears within the vefTel. A glafs veflel with condenfed air in its' cavity, or full of fome conducing fubftance, cannot benbsp;excited; yet a folid ftick or lump of glafs maynbsp;be excited.

Congealed water, viz. ice or fnow. But when cooled down to �13� of Fahrenheit�s thermometer, Mr. Achard of Berlin found thatnbsp;ice loft its conducing property, and becamenbsp;eledtric.

Charcoal is very equivocal in its condudtiiig power; for fome pieces of it will hardly condudfat all,' whilft others

Almoft all the above-mentioned fubftances, when heated beyond a certain degree, become conductors. Thus red-hot glafs, melted rofin, amp;c. are-condufliors of electricity ¹. The focus of a burningnbsp;lens, or concave reflector, is not a conductor. Sometimes glafs of a hard quality is fo bad an electricnbsp;as to be almoft a good conductor. It is remarkablenbsp;that often the nature of the fame pieces of glafs is-changed by time, and by ufe, fo as to become goodnbsp;electrics, though at firft they were almoft conductors, and mce verf a,

A glafs velTel is excited beft when the air in it is a little rarefied; but a glafs veflel entirely ornbsp;almoft ent rely exhaufted of air, on being rubbed,nbsp;fhews no figns of electricity on its external fur-

Hot air has been reckoned a condutor ; but this is denied by Mr. Read. See his Summary View of Spontaneout¹ Electricity, p. 8.

GtMral Idea of Ek^ricity. nbsp;nbsp;nbsp;341

face, but the eleftric power appears within the veflel. A glafs veflel with condenfed air in its' cavity, or full of fome conducing fubftance, cannot benbsp;excited; yet a folid ftick or lump of glafs maynbsp;be excited.

quot;Water (efpecially fait water), and all fluids, excepting the aerial, and oils.

Congealed water, viz. ice or fnow. But when cooled down to �13� of Fahrenheit�s ther-niometer, Mr. Achard of Berlin found thatnbsp;ice loft its conducing property, and becamenbsp;sn eledlric.

z 3

Elecftricity pervades alfo fuch a vacuum, or ab-fence of air as is caufed by the beft air-pump j but not the perfeft abfence of air, or the tor-ricellian vacuum, formed by boiling thequick-filver in a barometer tube ¹-

It needs hardly be obferved, that compound bodies partake more of the nature of conduftors or of eleftrics, according as a greater quantity of the

In rarefied air the attraftion of elcdlricity is weakened, and the elefilric light becomes more diffuled, but lefs denfe, innbsp;proportion to the rarefadlion; but, though in a very fmall degree, they are, however, vifible even in the beft vacuumnbsp;that can be produced by the moft efficacious air-pump, viz.nbsp;when the air which remains in the receiver is about th�enbsp;thoufandth part of the original quantity. AH this feems natural j for, fince the air is an eleiftric, the more accuratelynbsp;this eledtric is removed from a given fpace, the more effectually can the eledlric power pafs through it; and hence itnbsp;might be expedfed, that the eledfric power would pafs freelynbsp;through the perfedt torricellian vacuum. But it feetnsnbsp;to have been fully afeertained by Mr. Walfh and Mr. Morgan, that fuch a vacuum is not a conductor of cledlricity-See Mr. Morgan�s Paper in the Philofophical Tranfadtions,nbsp;vol. 75th, and my Treatife on Electricity, fgurth edition,

former or of the latter enters into their compofi-tion. Thus green vegetables, frelh wood, amp;c. are conductors on account of the water which they contain. Hence it follows, that all eledrics, previouflynbsp;to their being ufed as eledrics, muft be properlynbsp;cleaned and dried.

Baked wood is a very good eledric, but it foon lofes that property by imbibing moifture from thenbsp;air: hence, in order to preferve it in a non-con-duding ftate, it thould be varnifhed as foon as itnbsp;comes out of the oven ; and then again thoroughlynbsp;dried in a warm .^lace, or in the oven itfelf.

7.4

[ 344 ]

�F the perfon who rubs the glafs tube, as irien-tioned in the preceding chapter, be infulated, viz. be fufpended by means of filk firings, or {landsnbsp;upon a cake of rofin, amp;c. and in that fituation rubsnbsp;the tube with his hand; after a few ftrokes it willnbsp;be found that the perfon and the glafs tube are bothnbsp;eledtrified ; for if any light bodies be prefented tonbsp;any part of the perfon�s body, they will be attradednbsp;and repelled in the fame manner as they are by the ^nbsp;tube. The infulated perfon will alfo give out fparksnbsp;to another conductor that may be prefented to anynbsp;part of his body ; but the eledricity of the infulatednbsp;perfon is different from the eledricity of the tube,nbsp;and the difference principally confifts in the following three charaderifiic properties.

I. Whenever an infulated light body, as for in-ftance, a fmall piece of cork fufpended by a filk thread, has been attraded by ihe tube, and afterwards repelled ; that cork will not be attradednbsp;again by the excited tube, but will be repelled by itgt;

provided the cork in this ftate of repulfion is not touched by any conducing body. T he fame thingnbsp;takes place if an infulated light body, like the cork,nbsp;amp;c. be attracted and repelled by the perfon�s body,nbsp;viz. it will continue to be repelled by it. But if thenbsp;infulated cork, which is a�tually repelled by the tube,nbsp;be brought near the perfon, a ftrong attraftion willnbsp;take place between the cork and the perfon; andnbsp;in the fame manner, if the other cork, which is repelled by the perfon, be brought within a certainnbsp;diftance of the tube, the former will be ftronglynbsp;attracted by the latter. Or if the two infulatednbsp;corks, which are repelled, viz. one by the tube, andnbsp;the other by the perlon�s body, be brought within anbsp;certain diftance of each other, they will attraft, andnbsp;will rufii towards, each other.

The fame thing may be obferved in a more convincing manner, by prefenting more than one light body to each of the elehlrified bodies. Thus letnbsp;B, fig. 2, Plate XXIII. be two cork ballsnbsp;biftened by a linen thread ACB, and let the partnbsp;CD be a� filk thread fattened to a proper fup-at fome diftance from the wall or othernbsp;Jjj fituation, if you bring the ex-cited glafs tube near the balls A, B, the tube willnbsp;^�ttra�b them, and will foon after repel them. Nownbsp;let the tube be removed, and the cork balls will benbsp;found to repel each other, and to remain for a

quire anj eleflricity, then the other body will certainly acquire the other eledricity.

It muft likewile be remarked, that almoft all the eleftrics may be made to acquire, at pleafure,nbsp;the one or the other of the two eleftricities j viz. bynbsp;ufuig particular rubbers. Thus, if a glafs tube benbsp;drawn acrofs the back of a cat, it will acquire thenbsp;refinous eledtricity j but if rubbed with any othernbsp;fubflance, it will then acquire the vitreous eledricity.nbsp;Thus alfo a ftick of fealing-wax will acquire the vitreous eledricity, when rubbed with any inetallicnbsp;fubftance ; but it will acquire the refinous cledriritynbsp;when rubbed with leather, or paper, or the humannbsp;hand - amp;c,

A llight alteration, either of temperature, or of furface, or of prelTure, will difpofe a body to acquire one eledricity rather than the other; thenbsp;rubber always acquiring the oppofite eledricity.

When the difference between the two eledricities was firft obferved, it was imagined that the twonbsp;t powers were both owing to emanations of two particular elaftic fluids, which, when mixed in due proportion, would counterad each other, or v/ouldnbsp;form a fort of neutral compound. But a fuppofi'nbsp;tion much fimpler, which goes under the name ofnbsp;the Franklinian theory, and which is peculiarly corroborated by the above-mentioned third differencenbsp;between the tv.�o eledricities, viz. that of the currentnbsp;from the vitreous- to the refinous eledricity, isnbsp;follows:nbsp;nbsp;nbsp;nbsp;'

All

' All the phenomena, called eleftrical, are fup-pofed to be produced by an invifible and fubtile fluid exifting in all the bodied of the terraqueousnbsp;globe. It is alfo fuppofed that this fluid is verynbsp;lt;-da(l;ic, viz. repulfive of its own particles, but attractive of the particles of other matter.

When a body does not fliew any electrical appearances, it is then fuppofed to contain its natural quantity of this eleClric fluid ; (but whether thatnbsp;Quantity bears any proportion to the quantity ofnbsp;matter, or not, is utterly unknown) therefore, thatnbsp;body is faid to be in its natural, or non-eknrifiednbsp;fate: but if a body fhews any eleCfrical appearances, it is then laid to be eleSirified, and it is fuppofed that it has either acquired an additionalnbsp;lt;iuantity of eleftric fluid, or that it has lolt lome ofnbsp;Its natural fliare. And from the above-mentionednbsp;^�tcumftance of the current, amp;c. (page 347,) wenbsp;led to fuppofe that the vi;reous electricity arifesnbsp;ftotn an over-charge of that fluid, and that the refi-nous electricity arifes from ah under-charge, or di~nbsp;mmutlon of the natural quantity of that fluid. Hencenbsp;the vitreous eieCtricity has alfo been called the plus,nbsp;pofitive eleBricity �, and the refinous has been-'^^lled the minus, or the negative electricity *.

350 nbsp;nbsp;nbsp;Of th� 1'wo EleBrldties.

As this hypothefis is fufficient to account for aH the electrical appearances, at leaft much more lonbsp;than any other, we are authorized to adopt it,nbsp;until forae other hypothefis may feem to be betternbsp;entitled to our afl�nt.

In the Brft place, this theoty fhews that when an eleClric and a conducting fubftance are rubbednbsp;againft each other, the ekftric fluid is not generated j but, by the aCtion of rubbing, one bodynbsp;pumps, as it were, the electric fluid from the othernbsp;body. Hence, if one body becomes overchargednbsp;with it, or eleCtrified pofitively, the other muftnbsp;become undercharged, or eleClrified negatively,nbsp;unlefs its deficiency is fupplied by other bodies thatnbsp;communicate with it¹. Hence alfo we fee thenbsp;reafon why, when an eleCtric is rubbed with anothernbsp;eledtric, or with an infulated rubber, it can acquirenbsp;but little electricity, viz. becaufe in that cafe thenbsp;rubber cannot be fupplied with eledtric fluid fromnbsp;other bodies.

Electric attraction is cafily ejtplained; for this does not exift, except between bodies that are differently electrified, where the fuperfluous eleCtrie¹

By what mechanifm one body extraCis the eleclric from another body during the rubbing, is by no nieafgt;snbsp;known. The increafed capacity of the electric for the electric fluid in certain fituations, feems to afford a plaufibknbsp;explanation. The nature of thofe capacities will be explained hereafter.

fluid of the bodies that are eleftrificd pofitively attracts, according to the theory, the underchargednbsp;matter of thofe which are eleftrified negatively.

We might now proceed to apply this theory to the other phenomena of eleftricity ; but it will benbsp;more fatisfaftory to fubjoin this application to thenbsp;defcription of the experiments which will be givennbsp;in the courfe of this Section.

WHENEVER any eleftricity is communicated to a body, be it pofitive or negative, it is confined upon it only by ele�trics, and will remain with that body a longer or a Ihorter time,nbsp;Recording as the eleftrics which confine it are morenbsp;or lefs perfeil:. Thus the eleftrreity which is fuper-induced upon a glafs tube by rubbing ir, remainsnbsp;upon the tube, infomuch as it is furrounded by thenbsp;air, which is an ele�tric; and as the air is in a morenbsp;or lefs perfefl; eleftric flate on account of itsnbsp;moifture, drynefs, amp;c. fo the eledric virtue is retained upon the glafs for a longer or a fhorter period;nbsp;and fotnedmes an excited glafs tube will remainnbsp;fenfibly eleftrified for upwards of 20 hours.

If a finger, or any other condudor, be prefented to an excited eledric, it will receive a fpark, and'innbsp;that fpark a certain portion only of the eledricity ofquot;nbsp;the excited eledric, becaufe that eledric cannotnbsp;convey the eledricity of all its furface to that part to

if a conduftor be prefented fucceflively to different parts of the excited eleftric, it will receive a fparknbsp;at every approach, until all the power of that electric is exhaufted, and then a new excitation is ne-ceflary in order to revive it.

Whenever a conduftor, which communicates with the earth, (viz, not infulated) is prefented atnbsp;a convenient diftancc to an excited eleflric, itnbsp;acquires, on that prefented fide, an ele�lricity contrary to that which is poffefied by the eleftric.nbsp;This eleftricity increafes as the body is approached, and at laft, there being an eager attradtionnbsp;between pofitive and negative eleftricities, thenbsp;conduftor receives a fpark from the cleftric, bynbsp;which means the balance is reftored.

If the conductor do not communicate with the earth, but be infulated, then on being prefented, asnbsp;before, to the excited eleftric, not only that fide ofnbsp;it which is towards the eledlric, but the oppofitenbsp;fide alfo will appear eledlrified ; with this difference,nbsp;however, that the fide, which is expofed to the influence of the eleftric, has acquired an eleflricitynbsp;contrary to that of the excited eleftric, and the op-pofite fide has acquired the fame eledlricity as thatnbsp;of the eleftric. Thofc two different eleftridcies ofnbsp;the condudtor increafe as the conductor comesnbsp;uearer to the eledric, and at laft it receives a fparknbsp;rom the eleftric, and becomes throughout poffclfednbsp;of the fame eleftricity with the eledric.

All thofe effefls will take place in the fame manner, if a thin plate of glafs, or of rofin, or of other eleftric fubftance, be interpofed between the con-dudlor and the excited eleflric; but then a fparknbsp;cannot come from the eleclric to the conduftor,nbsp;unlefs it opens its way by burfting the interpofednbsp;eledtric, as it always does in paffing through thenbsp;air. This difplacing and fubfequent collapfing ofnbsp;the air is what caufes the noife that attends anbsp;Ipark.

An infulated conduftor that has received the cledfricity from an excited eledlric (in which ftatcnbsp;it is faid to be eleSlrified hy communication) will adlnbsp;in every refpe�: like the excited eledlric itlelf, excepting that when it is touched by another con-dudtor which is not infulated, the former will givenbsp;one fpark to the latter, difcharging at once all itsnbsp;eleftricity, becaufe the eledlricity which belongs tonbsp;every part of its furface is eafily conducfed throughnbsp;its fubftance to that fide to which the other con-dudlor is prefented¹. Hence it follow's that thenbsp;eledfricity, which is difcharged by an eleclrified andnbsp;infulated condudfor, is in general ftronger than thatnbsp;which is difcharged by an excited eledlric.

It muft be obferved, however, that when the electrified Condudlor is large, and much extended, a very trifli'^S refiduum of eledlricity generally remains upon it, vvhich vr'U

which only is eleftrifiedj and if this conduftor be touched by the other, then the eledlricity will benbsp;divided amongft thofe cpnduflrorsj but it will benbsp;divided neither equally nor in proportion to theirnbsp;quantities of matter. But if the condudlors be quitenbsp;alike, and be fimilarly fituated with refped to thenbsp;furrounding bodies j then the eleftricity will be divided equally among them. If their furfaces benbsp;equal but diffimilar, as for inftance, a fquare foot ofnbsp;tin foil in one piece, and another fquare foot of thenbsp;fame cut into a long flip 5 then the latter, viz. thenbsp;body whofe furface has a greater extenfionj will acquire more eledlricity than the other. If, when thenbsp;two condudors are equal and fimilar, one of themnbsp;lies contiguous to an imperfeft conductor, and thenbsp;other is contiguous to the air only then the formernbsp;will acquire a greater quantity of eledlricity thannbsp;the latter.

The eledric fpark (viz. a feparate quantity of eledlricity) will go a greater or lefs diftance throughnbsp;the air, in order to reach a condudlor, according asnbsp;its quantity is greater or lefsj as the parts fromnbsp;^hich it proceeds, and on which it flrikes, arenbsp;harper or more blunt, and as the condudlor is morenbsp;tgt;r lefs perfedl.

The noife and the light, which accompany the fpark, are greater or lefs, according to the quantity of eledlricity, alfo as the parts from which itnbsp;proceeds, and on which it flrikes, are blunter ornbsp;fliarper, and as the condudlor is more gr lefs perfedl.

feci. Thus a fharp-pointed body will throw off cle�lricity to, and receive it from a greater diftancenbsp;than a body of any other fhape; but that paf-fage occafions no remarkable noife, and is attended with little light; for in this cafe the electricity comes not in a feparate large body, but bynbsp;little and little, or rather in a continuate ftream.

If a pointed wire be concealed in an open glafs tube that projefts a Ihort way beyond the point, ornbsp;if it be covered with tallow, or bees-wax, or ful-phur, amp;c. then it will take a ftrong Ipark from annbsp;cleflrified condudlor.

It is remarkable, that when points are throwing off, or are receiving eleftricity, a current of air always appears to proceed from the point, and thatnbsp;is the-cafe whether the eleftricity is pofitive or negative.

A pretty large quantity of electricity pervades the fubftance of a conductor of confiderable lengthnbsp;with furprifing and inappreciable velocity; but anbsp;fmall quantity of it has been found to take a littlenbsp;time in pafling through a long and lefs perfect con-dudor.

The eledric fpark taken upon any part of a living animal body, caufes a difagreeable fenfation, whichnbsp;is more or lefs fo, according as the fpark is ftronge*'nbsp;or weaker, and as the part is more or lefs delicate,nbsp;or the perfon more or lefs fenfible.

It has been repeatedly afferted and denied, that eiedricity communifateci to infulated anicnalnbsp;6nbsp;nbsp;nbsp;nbsp;bodies^

bodies, quickens their pulfe, and increafes their perfpiration; adly, that if it be communicated tonbsp;infulated fruits, fluids, and other bodies which arenbsp;aftually in a ftate of evaporation, it increafes thatnbsp;evaporation ; and, 3dly, that it promotes vegetation.

With refped to the firft circumflance, the moft accurate experiments (hew that eleftrization, whether by pofitive or negative eledricity, does notnbsp;accelerate nor retard the ordinary number of pulfa-tions in a found perfon; but that the quickening ofnbsp;the pulfation, which is often obferved in fuch cafes,nbsp;arifes from fear or apprehenfion ¹.

The perfpiration of animal bodies, fruit, and other fubftances that are a(5lually in a ftate of evaporation, is increafed but little by eleftrization jnbsp;provided thofe fubftances are expofed to the ambient air with a free furface.

With refpedl to vegetation, the moft impartial, diverfified, and conclufive experiments have Ihewn,nbsp;^hat eleftrization does neither promote nor retardnbsp;'vegetable life f.

^f the face, or any part of the body, be prefented to an excited ele�tric, or to a condudtor ftrongly

See Van Marum�s Account of the Teylerian Eleflrt ^ach. of Harlem, and my Treatife on Eleift. 4th Edition,nbsp;''ob III. p. 277.

t See Dr. Ingen-Houfz�s two Letters in Journal de Phyfique^ for February 1786 and May 1788.

eleftrified, a fenfation will be felt as if a wind were blowing, or rather as if a fpider�s web were drawnnbsp;over it.

. If the noftrils be prefented to an excited elcftric, a fmell will be perceived which much refemblesnbsp;that of phofphorus; but communicated eledtricitynbsp;does not occafion any fuch fenfation, except whennbsp;a large quantity of it palTes fuddenly from one bodynbsp;to another.

If the ftream of eledtricity which ifliies from an cledtrified point be diredted on the tongue, a peculiarnbsp;tafte is perceived ; and bodies that have been anbsp;certain time expofed to that ftream, or to ftrongnbsp;eledlric effluvia in general, retain a certain fmell,nbsp;fuch as has been mentioned above, for a confidera-ble time after.

If elediricity be communicated to an infulated vefl�l containing water, and the water be adtuallynbsp;running out of it through a hole or pipe; the ftream,nbsp;if lefs than a tenth of an inch in diameter, will be afc^nbsp;celerated, and more fo in proportion as its diameternbsp;is fmaller; it will even drive the water in a conci -nuate ftream out of a very finall capillary tube, cutnbsp;of which, without the aid of eledtricity, the waternbsp;�will not even be able to drop. When above anbsp;tenth of an inch in diameter, the ftream, thoughnbsp;it divides and carries the fluid farther, is, hov/-ever, neither fenfibly accelerated nor retarded bynbsp;ckdtricity.

Towards the beginning of this chapter it has

been

been faid^ chat when a conductor is prefented to an eieftrified body, it acquired, on the prefented fide,nbsp;an eledtricicy contrary to that of the eledlrifiednbsp;body. We muft now add a very remarkable law,nbsp;viz. that no eleftricity can be obferved upon thenbsp;furface of any electrified body, unlefs that furfacenbsp;is contiguous to an eledric, which can in fomenbsp;manner or other acquire the contrary eledricity atnbsp;a little diftance; or, in other words, no eledricitynbsp;can appear upon the furface of any eledrified body,nbsp;Unlefs that furface is oppofite to another body whichnbsp;has aduaily acquired the contrary eledricity} andnbsp;thofe contrarily eledrified bodies muft be feparatednbsp;by an eledric. Thus, when an infulated body,nbsp;which ftands at a diftance from other condudors,nbsp;IS eledrified, the air which furrounds it performsnbsp;at once the office of an eledric and of a condudor 5nbsp;fur it acquires the contrary eledricity at a littlenbsp;biftance from the eledrified body, whilft the inter-'^^ning ftratum of air feparates thofe two elec-^'�icities.

With refped to the pafiiige of eledricity from �ue body to another, we may in general remark,nbsp;that if repulfion exifting between bodies thatnbsp;pofiefled of the fame kind of eledricity be ex-t^pted, all the other eledrical phenomena are produced by the pafifage �f eledricity from one body tonbsp;another.

With refped to attradlon and repulfion, this general law muft be remembered; namely, that thofe A A 4nbsp;nbsp;nbsp;nbsp;bodies.

bodies, which are poffeffed of the fame fort of electricity, repel, or tend to repel, each other; but bodies, which are poirefTed of different eleftricities, attradl, or tend to attraft, each other; and there isnbsp;po eledric attradion but between bodies which arenbsp;pofTeffed of different eledricities

This laft affertion may at firft fight appear to be contradided by, the effed which takes place whennbsp;fmall bodies' are prefented to an excited tube, or tonbsp;any other eledri filed body j for they are attraded bynbsp;it, though they have not been previoufly expofed tonbsp;any eledrization; but the difficulty will vaniffi, ifnbsp;what has been faid above be remembered, namely,nbsp;that the fmall bodies naturally acquire the contrarynbsp;cledricity merely by their being brought within thenbsp;fphere of adion of an eledrified bqdyj fo thatnbsp;when they are attraded, they are adually poffeffednbsp;pf the contrary eledricity.

^�^HE eleiftric virtue may alfo be communicated to eleftrics; but this communication to electrics is attended with feveral circumftances, differentnbsp;from thofe which attend the communication ofnbsp;eleflricity to conduftors; for when one fide of any'nbsp;of the latter receives fome eleftricity, that electricity inftantly pervades its whole fubftance;nbsp;whereas when an eledric is prefented to an eleftri-fied body, a fpark from the latter will eleftrify thenbsp;former in a fmall fpot only 5 for, on account of itsnbsp;non-condu(5ling quality, the eledlricity cannot expand itfelf through it. In fhort, v/hen an eledricnbsp;is prefented to an eledrified body, the former willnbsp;acquire different eledricities on different fides, (asnbsp;has been faid of condudors in the preceding chap-*or) j thefe eiedricities increafe accoi ding as thenbsp;diftance between the two bodies diminifhes, viz. asnbsp;they are brought nearer ; but if at laft a fmallnbsp;quantity of eledricity be communicated to one partnbsp;of the cledricjthat eledric will not become throughout

362 Of EleBricity comnunicated to EleB/ ics. out pofTeffed of one eledricity, but will, in fomenbsp;cafes, ftill fliew different eledricities on differentnbsp;fides; and in certain circumftances many repeatednbsp;changes from pofitive to negative eiedricity may benbsp;' cblerved upon the very fame eledric.

If to one fide of an eledric fufficiently thin, fuch 2S a pane of common window glafs, a plate of feal-ing v/ax, or of taic, amp;c. you communicate one electricity, and to the oppolite fide you communicatenbsp;the contrary eiedricity, that plate in that flare isnbsp;feid to be charged, and the two eledricities cannotnbsp;come together, and annihilate each other, unlefs anbsp;communication by means of conduding lubftancesnbsp;be made between both fides, or the eledric platenbsp;be broken by the force of eledric atiradion.

When the two eledricities of a charged eledric are by any means united, and of courfe their powersnbsp;deftroyed, then that eleclric is faid to be difeharged;nbsp;and the ad of union of thofe two oppofite powers isnbsp;generally called the eleBric Jhock; becaufe when anbsp;living animal body forms the circle of communication between the two fides of the charged plate, thenbsp;difeharge which muft pafs through it, occafions anbsp;fudden motion, by contrading the mufeies throughnbsp;which it pali'es, and gives a peculiar fenfation, whichnbsp;proves more or lefs difagreeable according to thenbsp;different conftitutions of perfons.

In order to avoid the difficulty of communicating eiedricity to an eledric plate, it is cuftomary tonbsp;coat the fides of it with fome conduding fubflance,

fijch as tin-foil, gold-leaf, fheet-lead, amp;c. by which means the charging and difcharging becomes verynbsp;eafy; for when the cleftricity is communicated tonbsp;one part of the coating, it immediately fpreads it-felf through all the parts of the eleftric that are innbsp;contaft v/ith that coating; and when the eledric isnbsp;to be difcharged, it will be fufficient to make anbsp;conducing communication between the coatings ofnbsp;both fides.

Thofe coatings muft not come very near to each other towards the edge of the plate, for in that cafenbsp;a communication between thofe coatings is ready atnbsp;hand; and though the coatings are not abfolutely innbsp;contadl, yet when they are eledlrified, the electricity will eafily force a paffage through the air, and,nbsp;by paffing over the furface of the eledlric plate fromnbsp;one coating to the other, renders it incapable of receiving any confiderable charge *.

The curious properties of charged eleftrics, and the farprifing effeds of the difeharges, entitle it tonbsp;the following more accurate enumeration of particulars.

If a glafs plate (and the fame thing mull; be underftood of other eledric fubftances), whethernbsp;fmooth or rough, be coated with fomc condudingnbsp;litbltance, fo that the coatings do not come very

tie property of conduding the elcdricity over their furluce is fo great in fome kinds of glafs, as to render themnbsp;lt;^uite ludtfor thepurpofe of charging

Titzx the edge of the plate; and if fome cledlriclty be communicated to one of thofe coatings, whilft thenbsp;other coating communicates with the earth, or withnbsp;a fufficient quantity of conducting bodies; then thenbsp;lafb mentioned coating will of itfeif acquire aboutnbsp;an equal quantity of the contrary electricity: but if,nbsp;whilft one fide is acquiring eleftricity, the oppofitenbsp;fide does not communicate with the earth, or with anbsp;fufficient quantity of conducling fubftances; then thenbsp;glafs plate cannot be charged, except in a very trifling degree.

Now the reafon why, when one fide of the glafs is receiving one eledricity, the oppofite fide acquiresnbsp;the other ele�lricity, is the fame as was mentionednbsp;above, viz. the property which bodies have ofnbsp;acquiring an electricity contrary to that which isnbsp;poflefled by a contiguous eleiftrified body j and thenbsp;interpofition of the glafs plate keeps thofe electricities feparate: but if the charge be too high, andnbsp;the glals plate be too thin, then the great attraction between the two different eledtricicies forces anbsp;paflage through the glafs, difcharges it, and, by breaking it, renders it unfit to receive another charge.

Thofe effeds take place in the fame manner if� the glafs be not in the form of a plate, but in anynbsp;other fhape whatfocver, provided it be fufficientlynbsp;thin ; it being not the form but the thicknefs of thenbsp;glafs that renders it capable of receiving an high^*',nbsp;or a lower charge. The thinner glafs receivesnbsp;the higheff charge, but it is more liable to be brokef^nbsp;bv ir.

This remarkable property was difcovered by Von Kleift in 1745*, but it was firft fatisfa�torily noticednbsp;at Leyden, where the experiment was performednbsp;with a phial; hence a phial or bottle coated on thenbsp;infide and outfide for the purpofe of charging, amp;c.nbsp;has been called the Leyden Phial., otherwife an electric jar'-, and the charging and difcharging of anbsp;coated eledtric, in general, has been called thenbsp;Leyden Experiment.

A coated glafs is capable of holding a greater charo-e in condenfed than in rarefied air, providednbsp;the air be dry.

If a coated glafs plate or jar, after having been charged, be infulated, and only one of its coatings,nbsp;or fides, be touched with fome condudtor; that fidenbsp;will not part with its eleSlricity, becaufe the electricity of one fide exifts in confequence of the contrary eledfricity on the oppofite fide, and they, bynbsp;their mutual attraftion, confine each other oii thenbsp;furface of the glafs. Therefore, in order to dif-charge that glafs, both coatings muft be connefted

means of a conducing body, and then the dif-charge is made through that conducler. The difi-charge may alfo be made by connecting each coating 'vuh a large quantity of conducting bodies.

When, in order to difcharge a jar, one of its coatings is touched firft wdth a conduftor, as for in-ftance, with one end of a brafs chain, no particular-

phenomenon will take place; but as foon as the other end of the chain comes within a fufficientnbsp;diftance of the other coating, a fpark will be feennbsp;between this end of the chain and that coating, accompanied with a report, and the jar is inflantlynbsp;difcharged.

The fpark thus produced by the difcharge of a charged ele�lric or Leyden phial, is much brighter,nbsp;much louder, but at the fame time much Ihorternbsp;than that which is taken from an infulated condudtornbsp;that contains an equal quantity of eleftricity.

If the communication between the two coatings of a charged jar be made by means of imperfedtnbsp;conduftors, as a flender piece of wood, or wet packthread, amp;c. the difcharge will be made filently, butnbsp;not fo fuddenly, and of courie its efFedls will not benbsp;fo great, as when it is difcharged fuddenly.

The force of the fhock, which is produced by coated glafs of a given thicknefs, is proportionatenbsp;to the quantity of coated furface, fuppofing that thenbsp;charge has been carried up to the utmoft degree.nbsp;Hence by increafing the quantity of coated furface,nbsp;the charge, and the etfefts of the difcharge or Ihock,nbsp;may be increal�d almoft to any degree. A numbernbsp;of coated jars, connedted together in fuch a mannernbsp;as to unite their forces and act like one jar, confti-tutes what is called an ekSlrical battery.

In making the difcharge, the eledricity, which goes from one fide of the jar to compenfate thenbsp;contrary eledricity of the oppofite fide, thro^gh

The force and the noife of an eledlric difcharge is not affefted by the inflexions of the conduftornbsp;through which it paflesj but is fenfibly weakenednbsp;by its length.

It evidently appears that the eleftricity finds fome obflrruXion in going through even the beft conductors ; for in fome cafes it will prefer a Ihort paffagenbsp;through the air, to a long one through the beftnbsp;conduXors. The obftrinflion is greater where thenbsp;conduXors, which form the circuit, are not in per-feX contaX, and efpecially where the eleXricitynbsp;mufl pafs from a more perfeX to a lefs perfeXnbsp;conduXor.

A ftrong fhock fent through an animal or a plant, puts an end to animal as well as to vegetable life f.nbsp;If a fmall interruption of the circuit be made innbsp;'vater,, on making the difcharge (notwithftandingnbsp;that the water is a conduXor) a fpark will be feennbsp;in it, which never fails to agitate the water, andnbsp;often breaks the velTel that contains it. If, bynbsp;tnaking a fmall interruption of the circuit between

Pricftley�s Hiftory of EleXricity, Period VIII. ScX. II. t The common Bafam [Impatiens] i.s the plant wnich,nbsp;as far as I know, is killed eafieft by eleXricity. The (liock,nbsp;of a fmall jar, fuch as a coated 4 ounce phial, is fufficlent tonbsp;deftroy the life of a full groWn ballam. The plant begins tonbsp;droop immediately after the fhock.

^68 nbsp;nbsp;nbsp;Of the heyden Phial,

the two Tides of a Leyden phial, in water, the fhock is pafied through it, fo as to produce afpark in thenbsp;water, that difcharge will be found to produce annbsp;exceedingly fmall bubble of elaftic fluid ; and, bynbsp;repeating the difcharge a vaft number of times, anbsp;certain quantity of that elaftic fluid may be accumulated, which is inflammable, and appears to be anbsp;mixture of hydrogen and common air or oxygennbsp;air, viz. the components of water. By inflammation this elaftic fluid explodes, and is convertednbsp;again into water ¹.

If the circuit be interrupted by one or more electrics, or imperfeft conduftors, of a moderate thick-nefs, the eledtric Ihock will break them, and in fome circumftances will difperfe them in every di-redtion, and in fuch a manner as if the force proceeded from the centre of every one of the inter-pofed bodies. In feveral inftances the effect of th�nbsp;(hock upon an interpofed body is evidently greaternbsp;on that fide of it which communicates with the po-fitive fide of the jar or battery.

A ftrong (hock fent through a (lender wire, or a fmall piece of metal, makes it inftantly red-hot,nbsp;melts it, and, when the fufion is perfedl, reduces it

See a letter on the fubjedl from Meflrs. Padls, Vai� Trooftwyk, and Deiman, to Mr. De la Metherie, ornbsp;Treatife on Eledtriclty, 4th edition, vol. HI. page ih8¹nbsp;Alfo Dr, Pearfon�s Paper in the Philofophical Tranfadh¹^^�¹nbsp;Volume for the Year 1797, page 142.

into globules of different fizes, or even into a fcoria. If the metal be placed between pieces of glafs, thenbsp;Ibock, by melting it, will force it into the very fub-ftance of the giafs. The glafles themfelves arenbsp;generally fbattered to pieces.

If thole glaffes which inclofe the metal be preffed by heavy weights, then a remarkably fmall Ibocknbsp;is often capable, not only of lhaking off the weights,nbsp;but alfo of breaking fuch thick glaffes as otherwifenbsp;Would require the force of a large battery. A thicknbsp;piece of glafs may likewife be broken into innumerable fragments, by only fending a Ibock over anbsp;fmall part of its furface, when that part is preffednbsp;by weights, without the interpofition of any metal.nbsp;When fuch pieces of glafs are not broken by thenbsp;explofion, they then will frequently be found marked with the moft lively prifmatic colours, whichnbsp;be fometimes confufed, and at other times in theirnbsp;prifmatic order. The coloured fpot is evidentlynbsp;owing to thin plates or fcales, partly feparatednbsp;from the glafs; and it generally occupies a 1'pacenbsp;of about one inch in length, and half an inch in

The force which is required to melt wires of the fame metal, muft be greater or lefs, according to thenbsp;length and thicknefs of the wire; but it.is far fromnbsp;bearing any direfl proportion to the quantity of metal , for if jj qP ^nbsp;nbsp;nbsp;nbsp;length and diameter be

When a moderate Ihock (meaning a Hiock that is not fufficient to melt the metallic � circuit) is fentnbsp;through an imperfect metal, efpecially when thenbsp;circuit confifts of feveral pieces, as a chain ; a blacknbsp;dull, in the form of fmoke, will proceed from thatnbsp;metal, which is a metallic oxide. If fuch circuit benbsp;laid upon paper, glafs, or other non-condu6tor, this,nbsp;after the explofion, will be found ftained with indelible marks, and often Ihews evident figns of havingnbsp;been burnt. A long and permanent track may benbsp;marked upon glafs, and upon feveral other bodies,nbsp;efpecially upon certain painted furfaces, by paffingnbsp;an eleftric Ihock over their furfaces ¹.

A Ihock fent through feveral metallic oxides, when thefe form part of the circuit, frequently reduces them into the metallic ftate.

A fufficiently ftrong Ihock fent through a magnet has fometimes deftroyed its virtue, and at other times has invigorated it, or even reverfcd its poles.nbsp;The following particulars will fltew the circum-ftances that are likely to produce fuch effedls. Whennbsp;the charge of eight feet of coated glafs- furface, ornbsp;even lefs, is fent through a fine fcwing-iieedle, thenbsp;needle will thereby often acquire a magnetic polarity�

fo as to traverfe when laid gently upon water. If the needle be ftruck, laying eaft and weft, then thatnbsp;end of it which is entered by the ftiock, viz. thatnbsp;which communicates with the pofitive fide of thenbsp;battery, or jar, will afterwards point north j but ifnbsp;the needle be ftruck laying north and fouth, thennbsp;that end of it which ftands towards the north will,nbsp;in any cafe, point north, and the needle will acquirenbsp;a ftronger virtue in this than in the former cafe ;nbsp;and laftly, if the needle be fet ftraight up, and thenbsp;ekftric ftiock enters it at either point; then thenbsp;lower extremity of the needle will acquire the property of pointing north ¹. This however cannotnbsp;take place in all parts of the world, for a reafonnbsp;which will appear in the next feclion.

A fmall fhock is fufficient to inflame feveral in-flamnnable fubftances; and inflammable fpirits may be fired even by a fpark proceeding from an electrified condufftor.

If the moderate charge of a large battery be dif-charged between two fmooth furfaces of metallic bodies, laying at a fmall diftance from each other;nbsp;Of if the explofion of a battery, ilTuing from a pointed body, as the point of a needle, be repeatedlynbsp;talken upon the fmooth and plain furface of a metallic body, fituated at a little diftance from thenbsp;point; in either cafe the metallic furface or furfaces

372 nbsp;nbsp;nbsp;Of the Leyden Phial.

will be found marked with circles of partly fcaled or fufed metal round a central fpot, and, efpecially innbsp;the latter cafe, they will frequently exhibit all thenbsp;prifimatic colours

When thedifchargeof abattery is made by bringing the conduftors which proceed from the coatings of anbsp;battery, in contadwith, or at a little diftance from,nbsp;the furface of certain conduding fubftances, as water,nbsp;raw meat, moift wood, amp;c. the eledricity, infteadnbsp;of going through thofe fubftances, will go over theirnbsp;furfape in a luminous track; fometimes preferring anbsp;much longer pafiage over the furface to a Ihort onenbsp;through the fubftance. In this cafe the explofionnbsp;never fails to give a concuflion to the body overnbsp;which it paffes.

The eledric explofions taken upon the leaves of delicate flowers frequently cliange their colours-f.

The colour of the ele�lric fpark, when taken in hydrogen or in ammoniac gas, is purple j in carbonicnbsp;acid gas it appears wliite. �

The eledric fpark taken repeatedly in common air, diminifhes a little its purity. In other permanently elaftic fluids fometimes it increafes, and innbsp;others it diminifties, their bulk, and alters their quality in a certain degree J.

* For farther particulars concerning thofe circles, fee the Phil. Tranf. vol. 58-

J See Dr. Prieftley�s fecond vol. of Obfervations on different kinds of air; and Dr. van Marum�s Account uf Experiments with the Teylerian Elec. Machine at Harlem-

By naakins; the eleftric difcharge a great many times in a mixture of oxygen and common air, ornbsp;of oxygen air and azotic gas, the nitrous acid is produced ¹.

According to the theory, the eieftric fluid which is communicated to one fide of the glafs drivesnbsp;away the electric fluid from the other fide, or thenbsp;eledricity of one fide induces a contrary electricitynbsp;on the oppofite fide ; but it is impoffible to fay hownbsp;this virtue or this repulfion can operate through thenbsp;glafs, which is impervious to the eledric fluid jnbsp;much lefs do we know where the fuperinducednbsp;eledric fluid refides.�Is it lodged in the fur-face of the glafs, or in the air contiguous to thenbsp;glafs ? In the firft cafe, if the additional eledricnbsp;fluid penetrates a certain way into the fubftance ofnbsp;the glafs, it follows, that a plate may be given fonbsp;thin as to be permeable to the eledric fluid, and ofnbsp;t^ourfe incapable of a charge ; yet glafs balls blownnbsp;exceedingly thin, viz. about the �ooth part of a,nnbsp;inch thick, when coated, amp;c. were .found capable ornbsp;holding a charge f.

Mr. Canton charged fome thin glafs balls about ^ i inch in diameter, having necks or tubes of about

See Mr. Cavendifli�s Experiments, which produced this remarkable difcovery, in the 75th and 78th volumes ofnbsp;the Philofopkical Tranfadions. See alio the Phil. Xranf.nbsp;for 1800, p. igo, and 202.

t The chargmg of a jar does by no means difplace the air from its in�dei neither does the charge heat or cool it.

374 nbsp;nbsp;nbsp;Of the Leyden Phial.

nine inches in length, and afterwards fealed the ends of the tubes hermetically. If thofe balls were pre-fented to an electrometer, they.fhewed no fign ofnbsp;eledlricity ; but if they were warmed, by being keptnbsp;a fhort time before the Hre, then they appeared tonbsp;be ftrongly ele�lrical, and appeared poflefled of thatnbsp;eledricity which had been communicated to theirnbsp;infide ; which Ihews that heat renders the glafs permeable to the eleftric fluid. This dedricicy is notnbsp;that which properly conftitutes the charge, but is thenbsp;fuperfluous eledricity of their infide j for an electric jar may always retain a little more eledricitynbsp;on one fide, than what is jufl; fufficient to counteradnbsp;the eledricity of the oppofite fide. If a chargednbsp;jar be infulated, and then be difcharged by con-neding its coatings with an infulated difchargingnbsp;rod, after the difcharge, both thefides of the glafsnbsp;together with the difcharging rod, will be foundnbsp;nightly poflTelled of the eledricity contrary to thatnbsp;of that fide of the jar which was touched laft.

Some very remarkable phenomena, the caufe of which is far from being clearly underftood, are exhibited by flat glafs plates, jointly charged like anbsp;Angle plate. If two flat glafs plates be placed onenbsp;upon the other, and their outward furfaces be coatednbsp;with tin-foil, in the ufual manner of coating a finglenbsp;plate for the Leyden experiment; and if thefe benbsp;charged by prefenting one coating to an eledrifiednbsp;body, and communicating the other with the earth jnbsp;. the plates (which we lliall call A and B) after the

charge wi*]] adhere firmly to each other j but if fepa-rated. A, whofe coating was charged pofitively, will appear pofitive on both fides, and B negative onnbsp;both fides. If thefe plates be laid one upon thenbsp;other as before, and be difcharged, by making anbsp;communication between the two coated fides; theynbsp;will afterwards be found ftill to adhere to each other,nbsp;and if feparated, they will ftill appear to be eleftri-fied, but with this remarkable difference, viz. that Anbsp;is negative on both fides, and B pofitive on bothnbsp;fides. U', after the difcharge, the feparation be madenbsp;in the dark, flafties of light will be perceived between their internal furfaces. By laying the platesnbsp;together, touching their coatings, and feparatingnbsp;them fucceffively, the flafhes may be obferved for anbsp;confiderable number of times, diminifliing by degrees until they vanifh.

But thofe effefts are not conftantly the fame with sll forts of glafs. Crown glafs and common platenbsp;glafs exhibit the above-mentioned phenomena; but

was obferved by Mr. Henly, that Dutch glafs plates, when treated in the fame manner, have eachnbsp;^ pofitive and a negative fide. He alfo obfervednbsp;ftgt;me other irregularities. Beccaria endeavourednbsp;to account for thofe and fimilar phenomena by fup-pofing that when two bodies, cither a conduftornbsp;�od an eleftric, or two contrarily and equally electrified eledtrics, are put one upon the other, they adhere to each other, and their eledlricities difappear,nbsp;becaufe the two oppofite powers counteradl each

other ; but as Toon as they are feparated, the electrics ihew a power or a tendency to recover their eleftricities. This is what he called vindicatingnbsp;eleBricity ¹.

We (hall laftly obferve, with refpecl to, communicated eledlricity, that the application of it either as fimple eledlrization, or in the form of fparks andnbsp;Ihocks to the human body, has been found un-queftionably ferviceable in various diforders, fomenbsp;of which had refifted every other medical application. But it muft at the fame time be confelTednbsp;that this application is not.frequently fuccefsful tonbsp;any remarkable degree.

Without entering into any particular difeuffion refpefling its power, or the particular effedls whichnbsp;are attributed to it in particular diforders, I thall innbsp;general obferve, that the application of eledlricitynbsp;has moftly proved beneficial in recent cafes of ob-ftrudlion, whether of motion, of circulation, or ofnbsp;fecretion; and that a gentle application has, uponnbsp;the whole, proved more advantageous than ftrongnbsp;fhocks.

The moft general pradlice is to infulate the patient, to place him in contadt with the eledlrified condudlor, in the manner which will be fhewnnbsp;hereafter, and then either to prefent a pointed body

For farther particulars relative to this vindicating electricity fee Beccaria�s Art. Elec. Part II. Sec. VI. or my Trcatife on Elec. 4th edition, vol. II. Appendix N� I,

towards the part affeded, (which produces rather an agreeable lenfac�ton, and is called giving the electrical aura) j or to draw fparks from the part, or atnbsp;moft to pafs very flight Ihocks through it.

The novice in this branch of natural philofophy can hardly underftand the meaning of feveral fadsnbsp;that are mentioned in this and the preceding chaptersnbsp;of this fedion. They have been put together fornbsp;the fake of reference, and in order that the leadingnbsp;principles of the theory might be feen under onenbsp;point of view j but the experiments which will benbsp;defcribed ia the fequel will probably rem.ove everynbsp;dilRculty.

J78 3

^T^HE eleftrical apparatus confifts ofinftruments .J- neceflary either for producing ele�tricitVj ornbsp;for accumulating, retaining, and employing it;, ornbsp;laftly, for meafuring its quantity and afcertaining itsnbsp;quality.

The principal inflrument for the produdtion of eleftricity is a machine capable, by any means, ofnbsp;exciting an eledlric, fo as to produce eledtrical appearances, Th� moft eflential parts of this machinenbsp;are the eledtric, the moving engine, the rubber,nbsp;and the prime condudtor, viz. an infulated conductor, which immediately receives the eledlricity fromnbsp;the excited eledlric.

The eledlric was formerly ufed of various fub-ftances and various (hapes. At prefent glafs globes, or glafs cylinders, or circular glafs plates, are almoftnbsp;all the variety that is ufed, and which indeed are thenbsp;moft advantageous. The moft ufual fize for thenbsp;globes is from g to 12 inches in diameter; and theynbsp;are moftly made with one neck. The cylinders arenbsp;1nbsp;nbsp;nbsp;nbsp;made

made with two necks, and they are of all fizes, e ven as far as 24 inches in diameter. The glafs plates arenbsp;alfo of various fizes. The glafs generally ufed innbsp;this country for fuch purpofes is the beft flint glafs,nbsp;and the articles fhould be well annailed.

With refpeft to the engine, which is to give motion to the eleftric, multiplying wheels have been generally ufed, which might move the eledlric withnbsp;confiderable velocity, whilft they are commodiouflynbsp;turned by a winch. A wheel and an endkfs ferewnbsp;has alfo been ufed, but this is apt to make a rattlingnbsp;noife, and loon wears away. But either a cylindernbsp;or a circular plate may be moved quite quick enoughnbsp;by means of a fimple winch, to which the hand isnbsp;immediately applied.

The rubber is the next article which muft be de-feribed. After a variety of trials it appears that the beft rubbers for a globe or a cylinder are made ofnbsp;leather ftuffed with hair, and a pretty long piece ofnbsp;fine filk is faftened to one fide of the rubber, andnbsp;after having paffed over the rubber, viz. betweennbsp;the culhion and the globe or cylinder, fpreads overnbsp;�^ore than one third part of the circumference of thenbsp;latter. For a plate the rubbers moftly confift of anbsp;piece of leather with a piece of filk at its extremity, or of cufhions, amp;c.

quot;The proper conftrueftion of the rubber requires, that the ftde of it which the furfaefe of the glafs enters in whirling, may be as perfedt a conduftor asnbsp;poflible, in order to fupply the glafs with eledtric

fluid,

fluid, and that its other fide be as much a non-con-dudor as poflible, in order thar none .of the fluid �which is accumulated upon the glafs may return tonbsp;the rubber.

The rubber fliould be fupported by a fpring, by which means it may eafily fuit the inequality of thenbsp;glafs, and the fpring fhould be fixed fad upon anbsp;glafs pillar or other infulating ftand gt;, it being ufefulnbsp;to have the rubber infulated in feveral experiments �,nbsp;but when its infulation is not required, a chain ornbsp;wire is eafily fufpended to it, and thus it may benbsp;made to communicate with the earth, or with anynbsp;ether body at pleafure.

The prime conduSior is nothing more than an in-fulatcd condudor which is fituaced with one of its extremities contiguous, but not quite in contad,nbsp;with the eledric, and nearly oppofite to the rubber.nbsp;This condudor may be made of hollow brafs, or ofnbsp;tin plates, or of pafteboard covered with tin-foil, ornbsp;of wood covered with tin-foil, amp;c. Its fhape is generally cylindrical with feipi-globular terminations.nbsp;But be the fhape what it may, care fliould be had tonbsp;make it as free as poflible from points, fharp corners, fharp edges, amp;c. for thefe throw off and diffi-pate the eledric fluid; but on the end which is contiguous to the eledric, it muff have a fhort pointednbsp;wire, or two, or more, which are called the collet'nbsp;tor, and will readily receive the eledricity.

The fize of the condudor fliould be propot-tionate to the fize and power of the eledric. The

larger the prime condu6lor is, the denfer and longer fparks'may be drawn from it, provided the electric be fufficiently powerful. But beyond a certainnbsp;fize, the diffipation from the furface may be greaternbsp;than what the eledric can fupply, and in that cafenbsp;the large condudor is diladvantageous.

Upon thofe principles eledrical machines of a vaft variety of lhapes and fizes have been conftrud-ed in this as well as in other countries- But amongflnbsp;all that variety, we (hall deferibe two only, which,nbsp;upon the whole, are the mod commodious, and arenbsp;more generally ufefui.

Fig. 4- Plate XXIII. reprefents an eledrical machine of the fimpleft fort. GEF is a flrong board, which fupports all the parts of this machine, andnbsp;which may be fattened to a ftrong table by means ofnbsp;one or more iron or brafs clamps, as at Thenbsp;glafs cylinder AB, quite clean and dry in its infide,nbsp;is about lo inches in diameter, and is furniflred withnbsp;two caps, either of wood or brafs, into which its twonbsp;Fiort necks are firmly cemented *. Each of thofenbsp;Caps has a pin, or projedion, or pivot, which turnsnbsp;io a hole through a wooden piece, that is cementednbsp;oo the top of a glafs pillar, as at A and B on thenbsp;glafs pillars B E, A G, which are firmly fixed tonbsp;the bottom board GEF. One of the above-men-

Ehe beft cement for this purpofe is male by melting and incorporating together 5 parts of rofin, 4 of bees-wa.v,nbsp;and 2 parts of powdered red ochre..

tioned projeftions pafles quite through the wooden piece, as at A, and has a fquare termination, to whichnbsp;the winder A D is applied and fecured on by meansnbsp;of a fcr,ew nut. Then by applying the hand at D, thenbsp;operator may turn the cylinder, amp;c. Sometimesnbsp;the part AC of the winder is made of glafs, in ordernbsp;the more effefliually to prevent the efcape of thenbsp;eleftric fluid from the cylinder. IR is the rubber,nbsp;and IRK is the filken flap¹ ². This culhion ornbsp;rubber is faftened to a fpring which proceeds fromnbsp;a focket cemented on the top of the glafs pillar S.nbsp;The lower part of this pillar is fixed into a fmallnbsp;board which Aides upon the bottom board of thenbsp;machine, and by means of a fcrew nut and a flit atnbsp;H, may be fixed more or lefs forward, in order thatnbsp;the rubber may prefs more or lefs upon the cylinder, NF is a glafs pillar which is fixed in the bottom board, and fupports the prime conduftor ML,nbsp;of hollow brafs or tin plates, which has the colledlornbsp;or pointed wires at L, and a knobbed wire at M.nbsp;From this brafs knob O, a longer fpark may benbsp;drawn than from any other part of the conduftor.nbsp;But this knobbed wire is only ferewed into the con-duclor, and may be eafily removed from it.

As glafs is apt to attract moifture from the air, in which cafe it conducts the eleCtricity over its fur-face; therefore it is proper to cover with fealing-

The fillc generally ufed for this purpofe is what is corn monly called black mode.

Of the EkSlrical Apparatus, nbsp;nbsp;nbsp;383

wax, or to varnilh over, the glafs pillars of this machine, as alfo all thofe glafs articles which ferve for infulating ; for when varnilhed, and efpecially whennbsp;covered over with fealing-wax in the dry way, theynbsp;attraft the moifture, either not all, or in an incomparably fmaller degree, and of courfe they infulatenbsp;vaftly better ¹.

The fimple rubber, fuch as has been defcribed, �will produce a very flight excitation of the cylinder jnbsp;but its power is vaftly increafed by laying upon it anbsp;little amalgam of tin, and efpecially an amalgamnbsp;of zinc f. The beft way of ufing this emalgam is

In Order to cover glafs with fealing-wax in the dry way, warm the glafs gradually near the fire, and when fuffi-ciently warm, rub a fticlc of fealing-wax gently over its fur-face; for by this means the fealing-wax is melted, and adheres to the glafs. In the humid way, the fealing-wax multnbsp;he diffolved in very good fpirit of wine, for which purpofenbsp;you need only break the fealing-wax into fmall bits, andnbsp;leave it in the fpirit of wine for a day or two, fhaking it nownbsp;and then.�This folution muft be laid upon the dry andnbsp;clean glafs, by means of an hair pentil, and when the firftnbsp;coat of it is quite dry, then a fecond, a third, and even anbsp;fourth Coat fhould be laid on.

The heft varnifh for this purpofe is the amber varnifb, which indeed anfwers as well as the fealing-wax in the drynbsp;way, but it mufl; be made with great care and caution.�Seenbsp;the particular defcription of the procefs in my Treatife onnbsp;Eledlricity, ^th edition, vol. III. p. 2.96.nbsp;t The amalgam of tin is niade with two parts of quicks

as follows ; Firft nnake the rubber with the filk flap very clean and dry, and put it in its place, as at R I jnbsp;then fpread a little of the amalgam upon a piece ofnbsp;leather, and apply it to the under part of the cylinder,nbsp;while this is revolving in the direftion of the lettersnbsp;a, B, for by this means particles of the amalgamnbsp;will be carried by the glafs itfelf to the lower part ofnbsp;the rubber, and will increafe the excitation prodigi-oufly. The leather with the amalgam needs not benbsp;kept againft the cylinder longer than it may be required to produce the defired effeft �, for when thenbsp;excitation decreafes, the leather may be appliednbsp;again.

The fimpleft conftruftion of the plate machine is reprefented by fig. 5, Plate XXIII. which requiresnbsp;very little explanation. ABC DM is a woodennbsp;frame, to which the four rubbers are affixed, which

filver, and one of tin-foil, with a fmall quantity of powdered chalk, mixed together until it becomes a mafs like pafte.

To make the amalgam of zinc, let four or five parts of quickfilver be heated higher than the degree of boiling water�nbsp;and let one part of zir.-e be melted in a crucible of in an ironnbsp;ladle. Pour the heated quickfilver into a wooden box, andnbsp;immediately after pour the melted zinc into it. Then ftiutnbsp;up the box, and fhake it for about half a minute. After thisnbsp;you muft wait until the amalgam is quite cold, or nearly fo,nbsp;and then you may mix it, by trituration, with a fmall quantity of greafe, fuch as tallow or mutton-fuet, a very fmallnbsp;portion of finely pow'dered whitening, and about a fourthnbsp;of the above amalgam of tin.

by means of the fcrews g,gig,gi may be made to bear with proper preflure upon the circular glafsnbsp;plate HK*. This plate has a hole through itsnbsp;middle, to which an axis M L is firmly fixed, innbsp;the manner indicated by the magnified fide view,nbsp;fig. 6, and is turned by means of the winch L G.nbsp;The prime conduftor has a branched terminationnbsp;quot;with points at the extremities, which collefl thenbsp;eleftric fluid from the fore part of the glafs plate.

Some plate machines have been made with two glafs plates and eight rubbers, and when properlynbsp;conftrudted, efpecially as they are made by Mr.nbsp;Cuthbertfon, their power is very great. Indeed thenbsp;moft powerful, eledtrical machine now extant is, asnbsp;far as I know, one of this conflru�lion made by thenbsp;above-mentioned philofophical inftrument maker,nbsp;for the mufeum of Teyler, at Harlem; a particularnbsp;defcription of which was given to the public by Dr,nbsp;Van Marum

This machine confifts of two circular plates, each inches in diameter, fixed on a common axis,nbsp;parallel to each other, and 7 f inches afunder. Eachnbsp;plate is excited by 4 rubbers the prime conductor

The rubbers generally confift of oblong cufliions that frequently affixed to fprings; butfometimes they are onlynbsp;pieces of leather fpread upon wood, to which filken flaps arenbsp;affixed, amp;c.

t See a compendious defcription of its effedls in ni/ Treatife on Elei^ricity, 4th edition, vol. II- p- 273'

is divided into two branches, which enter between the plates, and, by means of points, colledt the electric fluid from their inner furfaces only.

The plate machines may in general be made more compa6l and more powerful than other eledtricalnbsp;machines, but they are liable to a confiderable degree of fri�ion, and of courfe they are not eafilynbsp;�worked.

in the plate machines the, rubbers are not eafily infulated, yet this has been accomplilhed by various,nbsp;rather complicated, means *.

Befides the eleftrical machine, the operator ought to have fome glafs tubes, and one or two prettynbsp;large fticks of fealing wax, which are of great ufe innbsp;a variety of experiments.�The bell rubber for thenbsp;excitation of a glafs tube is the rough fide of blacknbsp;oiled filk, efpecially when a little amalgam ha^nbsp;been rubbed over it; but foft new flannel is the beftnbsp;rubber for fealing-wax, fulphur, rough glafs, ornbsp;baked wood ; every one of which fubftances, whennbsp;rubbed with flannel, will acquire the negative elec-�tricity.

The inftruments necelTary for the accumulation of eleflricity, are coated eledtrics, amongft whichnbsp;glafs has juftly obtained the principal place. The

form is immaterial; but the thicknefs and the quality of the glafs Ihould be noticed. Thin glafs can receive a greater charge; but it is at the fame timenbsp;more liable to be broken by the difchargei Anbsp;fingle jar may be pretty thin, but fuch jars as arenbsp;to form a large battery mull be ,a little thicker.nbsp;When their openings are narrow, thofe jars maynbsp;be coated on the infide with brafs filings, which arenbsp;ftuck by means of gum-water, or pafte; or meltednbsp;wax ; but when their openings are fufficiently large,nbsp;they may be coated on their infide as well as on thenbsp;oiitfide with tin-foil, or fheet-lead, or gilt paper,nbsp;either of which may be ftuck with pafte, or varnifti,nbsp;or gum-w'ater, amp;c.

Fig. 7. Plate XXIII. reprefents an eledrlc jar, coated with tin-foil on the infide and outfide, within about three inches of the top of its cylindricalnbsp;part; and having a wire wfith a round brafs knob,nbsp;or ball A, at its extremity. This wire pafles throughnbsp;^he cork or wooden ftopple D, and its lower extre-quot;^ity touches the infide coating.

Fig. 8, Plate XXII1. reprefents a battery con-fifting of r6 jars, coated with tin-foil, and difpofed in a proper box. The wires, which proceed fromnbsp;the infide of every four of thofe jars, are fcrewed, ornbsp;foldered or faftencd to a common horizontal wire E,nbsp;which is knobbed at each extremity, and by meansnbsp;of the wire5 F. F, F, the infide coatings of 8, or ij,nbsp;or all the i6 jars may be connefled together.

The infide of the box which contains thofe jarsj is likewife lined with tin-foil or tin-plates, for thenbsp;purpofe of connefling more efFedtually the outfidenbsp;coatings of all the jars. On one fide of this boxnbsp;there is a hole, through which a ftrong wire or hooknbsp;paffes, which communicates with the lining of thenbsp;box, and of courfe with the outfide coatings of thenbsp;jars. To this hook a wire is occafionally fattened,nbsp;which conneds it with one branch of the difcharg-ing rod BBC A.

The difcharging rod confitts of the glafs handle A, cemented into the brafs focket C, and thenbsp;curved wires B,B, which may be opened and Ihut,nbsp;like a pair of compaffes, by a joint at C. The extremities of thofe wires�are pointed, and the pointsnbsp;enter the brafs knobs D,'D, to which they arenbsp;fcrewed, and from which they may be unfcrewednbsp;at pleafure. With this conftrndion we may ufenbsp;either the points or the balls, and the inttrumentnbsp;may be ufed fbr difcharging jars of various fizes.

Fig. 9, Plate XXIII. reprefcnts Henley�s Uni-verfal Difcharger, which is a very ufeful inttrument in a great variety of experiments.. A is a flat boardnbsp;or pedeftal about 15 inches long, 4 broad, and inbsp;thick. B, B, are two glafs pillars, fixed fatt intonbsp;the board A, and furnilhcd at top with brafs caps?nbsp;each of which has a vertical joint, and fupports 3nbsp;fpring-tube, through which the wire DC Aides-Each of thofe caps confitts of three pieces fo con-

nefted as that the wire D C, befides its Aiding through the fpring-focket, has two other motions,nbsp;viz. an horizontal and a vertical one. Each of thenbsp;wires DC, DC, is turned into a ring at one end,nbsp;and at the other end has a brafs ball D, which, bynbsp;nieans of a Ihort fpring focket, is llipt upon itsnbsp;pointed extremity, and may be removed from it atnbsp;pleafure. E is a ftrpng piece of wood, or tablet,nbsp;about 5 inches in diameter, having on its furface anbsp;dip of ivory inlaid, and is furnifhed with a ftrongnbsp;cylindrical foot that fits 'the cavity of the focket F,nbsp;which is faftened into the bottom board A, and hasnbsp;a fcrcw G, which ferves to detain the foot of thenbsp;circular tablet E at any required height. H is anbsp;fmall prefs which belongs to this inftrument. Itnbsp;confifts of two oblong pieces of board, 'which maynbsp;be preffed againft each other, or againft any thingnbsp;that may be interpofed, by means of the, fcrews andnbsp;nuts 0, a. The lower of thofe boards has a cylindrical foot equal to that of the board E. Whennbsp;this prefs is to be ufed, it is fixed into the focket F,nbsp;in the place of the circular board E, which muft, innbsp;that cafe, be removed.

The infiruments which either manifefl; the prefence, or manifefl: the prefence and the quality, or meafure the quantity of eleflricity, are called elecr-trometers or eleE�roJcopes ; and they have been made ofnbsp;a great variety of fnapes, from which, as alfo fromnbsp;their ufes, thgy derived peculiar appellations.

A Ample thread, or a feather, or other light body, c c 3nbsp;nbsp;nbsp;nbsp;fimply

{imply fufpcnded by a fine thread, may be ufed for exploring whether a body be eleftrified or not; fornbsp;if the body be eledfrified, and be brought near it,nbsp;the thread, or other light body, will be attradtednbsp;by it.

The fimpleft eledfrometer for afcertaining the quality as well as the preience of eledtricity, hasnbsp;been already defcribed j it is reprefented at fig. 2,nbsp;and 3, Plate XXIII. and is called, from its inventor, Canton's EkSlrometer.

Fig. lo, Plate XXill. reprefents drant EleSlrometer, fixed upon a final! circular Hand,nbsp;from which it may be occafionally feparaced, andnbsp;may be fixed upon the prime condudtor, or elfe-where. This eledfrometer indicates the quantity,nbsp;or rather the condenfation, of eledtricity. It con^nbsp;fifts of a perpendicular ftem of box wood, with anbsp;globular termination at top, and having a brafs ferrule at its lower extremity, by which it may benbsp;fixed upon the prime condudtor, or upon the electrical battery, amp;c. To the upper part of the ftetnnbsp;a graduated ivory femicircle is fixed, about thenbsp;middle of which is a brafs arm, which contains anbsp;pin or axis of the index. The index confifts of anbsp;very flender ftick of box wood, which reaches fromnbsp;the centre of the graduated femicircle to the brafsnbsp;ferrule, and has a fmall cork ball faftened to itsnbsp;lower extremity. When this eledlrometer is notnbsp;eledtrified, the index hangs parallel to the pillar,nbsp;and its cork bail touches the brafs ferrule, as m

6 nbsp;nbsp;nbsp;fig

fig. 10; but when eleftrified, the index is repelled by, or recedes from, the ftem more or lefs, according to the intenfity of the cleflricity; and the graduation on the ivory femicircle Ihews the force ornbsp;the elevation of the index, as at P in fig. 5.

A vaft number of alterations have been made to this eledlrometer, viz. the index has been enclofednbsp;between two ivory femicircles; the whole has beennbsp;made of brafs, with multiplying wheels, and a coun-terpoife has been put to the index, in order to rendernbsp;a fmall force of eleftricity more perceptible, amp;c.nbsp;but, after all, the fimple original conftruftion, asnbsp;defcribed above, feems preferable.

The principle of Lane�s Difcharging Electrometer, as is now commonly ufed, efpecially by the practitioners of medical electricity, is fhewn innbsp;fig. 13, Plate XXIII. It confifts of a glafs arm D,nbsp;vvhich proceeds from a focket on the wire of thenbsp;cledtrical jar F, and to the top of which a bra'snbsp;fpring-focket E is cemented ; through this focketnbsp;a brafs wire, with the ball B at one end and thenbsp;fing C at the other, may be Aid backwards and forwards. The wire B C is generally marked withnbsp;divifions of inches and tenths. When the jar F is

ict in Contact with the prime (;onduCtor, as repre-fented in the figure, and the ball B is fet at the diftance, for inftance, of one-tenth of an inch fromnbsp;the ball A, let a wire CK be fixed between thenbsp;nng C of the eleCtrometer, and the outfide coatingnbsp;of the jar; then, when the eleCtrical machine is in

39� Of the EleBrical Apparatus. action, the jar F cannot be charged beyond a certain point; for when the charge is ftrong enoughnbsp;to leap from the ball A to the ball B, the difchargenbsp;will take place, and the fhock will pafs through thenbsp;wire C K, or through a human body, or throughnbsp;any other conducting body that is placed, inftead ofnbsp;the wire C K, to form the communication. Tliusnbsp;by fituating the ball B farther from the ball A,nbsp;ftronger (hocks may be given, as far as the fame jarnbsp;is capable of.

This eleClrcmeter has likewife undergone a great many alterations. An improvement of it, and anbsp;combination of this and other eleClrometers wasnbsp;made by Mr, Cuthbertfon ¹.

In performing feveral atmorpherico-electrical experiments about the year 1776, I found that the ufe of Canton�s cork-ball electrometer was much ob-ftrucled by the wind, in confequence of which Inbsp;attempted,to enclofe it in a bottle, and after a variety of trials and alterations the inftrument was innbsp;the year 1777 brought to the ftate which is repre-fented in fig. n, Plate XXIIl. which is about thenbsp;half of the original fize; but the lhape as well as thenbsp;fjze of it has been frequently altered by the philofo-phical inftrument makers. The three parts of thenbsp;figure reprefent the inftrument in its cafe, the fame

See a defcription of it in Nrcholfon�s Journal of Nat. Phil. ore. vol. II. p. 528.

CDMN is an open glafs veflel narrower at top than at bottom, and cemented into the woodennbsp;piece AB, by which part the inftrument is heldnbsp;when it is to be prefented to the atmofphere, ornbsp;it may be refted upon a table for other experiments.nbsp;This wooden piece allb ferves to fcrew the inftrument into its wooden cafe O. The upper part ofnbsp;CDMN is tapering like the neck of a phial, andnbsp;a Ihort glafs tube is cemented into it, fo as to p'ro-je�l a little above and a little within the neck of thenbsp;former. Then the upper part of the inftrument,nbsp;from C D to L, is covered with fealing-wax, bynbsp;means of heat, which gives it the appearance of onenbsp;continuate body. The inner part G of the fmallnbsp;glafs tube is alfo covered with fealing-wax. Intonbsp;this tube a brafs wire is cemented, the lower partnbsp;H of which is flattened, and is perfor^ited with twonbsp;holes; the upper part L is formed into a fcrew,nbsp;upon which the brafs cap E F is fcrewed. Thenbsp;office of this cap is to defend the upper part of thenbsp;inftrument from the rain. The conical, or oval, ornbsp;globular, corks P of this eleflrometer, are as fmailnbsp;as can be made, and are fufpended by exceedinglynbsp;fine fiiver wires, die upper parts of which are formednbsp;in rings, which pafs through the holes at H, and arenbsp;thereby pj loofdy fufpended, that they are caufed tonbsp;diverge when the brafs cap E is expofed to a verynbsp;fli^htiy ele�frjfjejj atniofphere. 1 M and K N are

two narrow flips of tin-foil ftuck to the infide of the glafs, and communicating with the wooden bottom A B; �they ferve to carry off that ele(51:ricity,nbsp;which, when the corks touch the glafs, is communicated to it, and if accumulated would difturb thenbsp;free motion of the corks.

An ufeful alteration of this eleftrometerwas made by Mr. Bennet. It confifts of two flips of gold-leaf, or filver-leaf, fufpended from the cover of, andnbsp;hanging within, a cylindrical glafs veffel, inftead ofnbsp;the corks fufpended by wires or threads. The flipsnbsp;of gold are about 2 \ inches long, and fometimesnbsp;they are narrower at their lower extremities. Thisnbsp;eledlrometer is the molt fenfible inftrument of thenbsp;kind, and very ufeful in nice experiments; the goldnbsp;flips being caufed to diverge in a ready and unequivocal manner by very fmall quantities of eleftri-city; but the inftrument, thus furniflied, is by nonbsp;means portahje¹. If very.fine threads ftiffened withnbsp;glue, be ufed without any balls, they will be foundnbsp;nearly as fenfible as the flips of gold leaf.

Such are the rnoft efl�ntial parts of the eledrical apparatus. But there is a great variety of particular inftruments, which are to be ufed for the performance of peculiar experiments ; but the deferip-ition of thefe, as well as the necelTary inftrudtions fornbsp;the mananement of the fame, and for the generalnbsp;performance of experiments, will be found in thenbsp;lequel.

[ 39S 1

ThE principal objecSt of this chapter is, to de-fcribe fuch experiments as are more eflen-tially neceflary for proving the laws which have been ftated in the preceding chapters of thisnbsp;feftion.

A few very trifling articles, fuch as a glafs tube, a ftick of fealing-wax, or a piece of amber, andnbsp;two or three eleftrometers, will be fulEcient tonbsp;prove the leading propofitions of eledlricity; butnbsp;the eledtrical machine being the principal article ofnbsp;a pretty large eledtrical apparatus, we fliall beginnbsp;by explaining the proper management of thenbsp;fame.

When the weather is clear and dry, efpecially in fetene and frofty weather, the eledlrical machinenbsp;always works well. In very hot or damp weather,nbsp;machine does not work well; therefore morenbsp;attention is required in the latter circumftance thannbsp;in the former j yet, with proper care, the eledlricalnbsp;machine may at all times be made to work with fu-f-ficient power, by attending to the following in-flrudfbns.

Before the machine be ufed, the cylinder fhould be wiped very clean and dry, and in cold weather itnbsp;fhould be gently warmed by keeping it a little whilenbsp;at a moderate diftance from a common fire. Thisnbsp;done, if the winch be turned, when all other thingsnbsp;are removed, and the knuckle be held at a littlenbsp;diftance from the furface of the cylinder, about thenbsp;middle of it, and oppofite to th� rubber, the electric fluid will come from the cylinder to the knuckle,nbsp;and the fpark.s, accompanied with a crackling noife,nbsp;will Toon be perceived. But fhould this not takenbsp;place after about 20 or 30 turns of the cylinder,nbsp;take off the rubber from its glafs pillar, clean itnbsp;well, and place it near the fire, in order to dry atnbsp;leaft the fiik flap wipe the cylinder well with anbsp;warm flannel or warm filk handkerchief, and replace the rubber, ib that it may bear upon the cylinder with fufficient force; then hold the piece ofnbsp;leather with the amalgam againft the cylinder at itsnbsp;under part while you turn the winch, and the machine vvill foon acquire its power. When this hasnbsp;taken place, remove the leather with the amalgam ;nbsp;place the prime condudor before the cylinder, as innbsp;hi. 4, Plate XXIII. wipe its (land NF quite cleannbsp;and dry, and make a good communication, by meansnbsp;�of a wire or otherwife, between the rubber and thenbsp;ground ; then turn the winch, and the eledric fluid,nbsp;lie form of fparks, may be drawn from the

by prefenting a blunt uninfulated condudor to its furface. The longeft fpark may

drawn from the knob O. If the point of a pin be prcfented to the prime conduftor whilft the cylindernbsp;is revolving, a luminous globule of light will be feennbsp;upon the point, which is not attended with anynbsp;noife. If the communication between the earthnbsp;and the rubber be removed, and it be made between the earth and the prime condudlor 5 then, onnbsp;prefenting a pointed pin to the rubber, a brufh ornbsp;pencil of light will be feen iffuing from the point,nbsp;and tending towards the rubber.

If, when the communication is made between the earth and the prime conduftor, a fimple eleftrome-ter, viz. two cork balls fattened at the ends of twonbsp;threads, be fufpended to the knobbed wire MOjnbsp;thefe will hang down touching each other, as longnbsp;as the machine is not in aftion j but the leatt turning of the cylinder will make them diverge, or flynbsp;from each other. If, in this ttate of repulfion, younbsp;touch the prime conduftor with an eleftric, as withnbsp;a piece of glafs, or fealing-wax, or amber, or ful-phur, amp;c. the cork balls will continue to diverge .nbsp;but if you touch it with any uninfulated conduftor,nbsp;fuch as your finger, or a wire, or a piece of char-amp;c. the threads with the balls will immediately collapfe. And this is a ready way of tryingnbsp;whether a given body be a conduftor or not.

Now, according to the theory, the cylinder is enabled, by the friftion, to draw the eleftric fluid,nbsp;which naturally exitted in the rubber, and throws itnbsp;upon the prime conduftor, from which, on account

of the infuktion, it cannot fly away, except what is communicated to the air, or what flies off in thenbsp;form of fparks to any conductor that may be pre-fented to the prime conduftor.

If the rubber be infulated, the eleftrical machine will lofe almoft all its power, becaufe the rubber,nbsp;after having fupplied the cylinder with its ownnbsp;fluid, cannot receive any more, except a very littlenbsp;quantity of it from the furrounding air, which isnbsp;feldom, if ever, a perfeft eleflric.�The influx ofnbsp;eleftric fluid to the rubber, and the efflux from thenbsp;prime conduftor, is (hewn by the luminous pencilnbsp;or ftar, which is leen on the pin or pointed conduftornbsp;that is prefented to them.

If, when the cork balls are diverging at the end of the prime condu�lor, as mentioned above, younbsp;prefent to them an excited glafs tube, or any othernbsp;body pofitively eleftrified, the balls will fly from it jnbsp;but they will run towards an excited piece of fealing-wax, or towards any other body negatively eledlri-fied ; and this Is a ready way of trying whether annbsp;eleftrified body be pofitive or negative.

Sometimes another prime conduftor is placed In contadl with the rubber R I; then the communication being made between the prime condudtor MLnbsp;and the earth, the above-mentioned experimentsnbsp;may be made with the other prime conduftor, butnbsp;with this difference, that in the latter cafe they arenbsp;affedled by negative eleiffricity, and fhew figns of

Take an excited glafs tube in one of your hands, and let a fmall light feather be left in the air, at thenbsp;diftance of about 8 or lo inches from the tube. Thisnbsp;feather will be immediately attradled by the tube,nbsp;and will adhere very clofely to Its furface during anbsp;few feconds, and fometimes longer j then, havingnbsp;acquired the fame fort of eleftricity, it will be repelled, and by keeping the tube under it, the feather will continue to float in the air at a confiderablenbsp;diftance from the tube, without coming near itnbsp;again, except it firft touches fome conducting fub-ftance, upon which it can depofit the acquirednbsp;electricity. By managing the tube dexteroufly younbsp;may drive the feather to any part of the room atnbsp;plcafure.

A remarkable circumftance attends this experiment, which is, that while you keep the feather from the tube, and move the latter about the former, the feather always prefents the fame part towards the tube; the reafon of which is, that whe.nnbsp;the equilibrium of the eleCtric fluid amongft thenbsp;parts of the feather is once difturbed, it is not ealiJynbsp;reftored, on account of the feather being a very bad

conductor.

Place upon an infulatihg ftand, (viz. a ftool with glafs legs) a metal pint or quart mug� or fome

other conducting body nearly of the fame fliape ; then fallen a Ihort cork-ball eledlrometer, like thatnbsp;of fig. 2, at the end of a filk thread, proceeding fromnbsp;the ceiling of the room, or from any other propernbsp;fupport, fo that the eleftrometer may be fufpendednbsp;entirely within the mug. This done, eleflrify thenbsp;mug, by giving it a fpark with an excited eledlric,nbsp;or otherwile, and you will find that the electrometer, whilft it remains in that infulated fituation, andnbsp;even ,if it be caufed to touch the infide furface ofnbsp;the mug, will not be attrafted by it, nor will itnbsp;acquire any eledtricity j but if a condudtor, partlynbsp;Handing out of the mug, be made to communicatenbsp;�with the eledtrometer, then the latter will be immediately attradted by the mug.

In this experiment the eledtrometer is adted upon from all fides by the eledtricity of the mug, andnbsp;having no body upon which it can depofit its eledirricnbsp;fluid, or acquire any from, cannot acquire the contrary eledtricity, and of courfe cannot be attradted ;nbsp;but when another condudtor is prefented to it, thennbsp;the attradlion takes place, becaufe the eledtrometernbsp;in that cafe acquires fome eledtric fluid from, ornbsp;can depofit its fluid upon, that condudtor.

Let a body be eledtrified, for inftance, pofitively� and if at fome diflance from it you hold an eledtrometer of cork balls, this eledtrometer will be foun'^

tl) diverge, but vv'uh negative ele6lricity; which niay be eafily proved; for if you prefent to it annbsp;excited piece of glafs, the cork balls will run to-�wards it; but they will fly away from excited feal-ing wax, fuppofmg this-to be excited always negatively, and the glafs always pofitively.

Infulate in an horizontal pofition a metallic rod with blunt terminations, as A B, fig. 14, Platenbsp;XXIIL about two feet long, and having a cork-ball electrometer at its extremity A; then bringnbsp;within 8 or 10 inches of its ofner end B an excitednbsp;glafs tube ; and the balls C will immediately divergenbsp;with the fame, viz. with pofitive eleftricity. Ifnbsp;the tube be removed, the balls will immediatelynbsp;come together, and no eleftrichy will remain innbsp;them or in the rod. But if, while the tube is nearnbsp;one end B of the rod, and the cork balls diverge with pofitive eleftricity, the other end A benbsp;touched with a finger, or with any unlnfulated con-duiftor, the cork balls will immediately collapfe,nbsp;remaining as if the rod were perfedlly unelecltifi'ed;nbsp;bt)t if, in this ftace of things, the excited tube benbsp;removed, the balls will immediately diverge withnbsp;negative eledlricity, fhewing that the rod A B is

This experiment is eafily explained; for-when the rod is in a natural date wich refpea to eleftricity,nbsp;then the eleftric fluid naturally belonging to it, isnbsp;equably diffufed throughout the rod; but when thenbsp;excited tube is brought within 3. certain diftance of

one of Its ends,, as B, then the fluid belonging to that end will be driven lowards the extremity A jnbsp;which extremity therefore becomes overcharged,nbsp;and the other extremity B undercharged, yet thenbsp;rod has no more eledtric fluid now than it had before j and when the tube is removed beyond thenbsp;Iphere of its action, the fuperfluoiis fluid of the extremity A returns to its former place B, and thenbsp;equilibrium is reftored. But if, whilft the extremitynbsp;A is overcharged, this fame extremity be touched,nbsp;then its fuperfluous fluid will be conduced away bynbsp;the touching body, leaving the extremity A in anbsp;. natural ftate ; but at the fame time the extremity Bnbsp;is undercharged ; therefore, when afterwards thenbsp;tube is removed, part of the fluid naturally belonging to the extremity A, goes towards B, and ofnbsp;courfe the whole rod will remain undercharged, ornbsp;eleftrified negatively.

This experiment, which may be endlefsly diver-ftfied, and fo fimplified as to be perform^ed with a fimple cork-ball eledlrometer, llrews how an electrometer or other body may be eledlrified negatively by means of a body eleflrified pofitively, ornbsp;mce verja.

EkSirical E!gt;cp�rm�n!s. nbsp;nbsp;nbsp;403

gold-leaf, bran, amp;c, and. whilft the machine is in aftion, hold the faid plate^direftly under the primenbsp;condudor at about 3 or 4 Inches dillance from itsnbsp;furface,; and the light bodies will foon move between the plate and the condudor, leaping alternately from the one to the other. In this experiment the finajl bodies and the plate, by being withinnbsp;the fphere of adion of the elcdrifiLd prime con-dudor, become adually pofTeffed of the contrarynbsp;eledric.ity, leaving their eledric fluid upon the handnbsp;of the operator, or other body that communicatesnbsp;v/ith the plate: hence the light bodies (on accountnbsp;of the attradion between bodies differently ekdri-fied) are arttraded by the prime condudor. Nownbsp;as foon as thefe bodies touch the prime condudor,nbsp;they become inftantly poffefled of the fame electricity with it ; therefore they are repelled ^on accountnbsp;of the repulfion between bodies pofi�fTcd of thenbsp;fame fort of eledricity), but they are attradecf bynbsp;the plate, which is in a contrary ftate, amp;c.

If the condudor be fuppofed to be eledrified negatively, the explanation requhes a very triflingnbsp;and very obvious alteration of e.xpreffions.

That the fmall light bodies cannot be attraded the Condudor, unlefs they beconie firft pofTeffednbsp;of the contrary eledricity, may be proved in thenbsp;following manner: � Place the faid light bodiesnbsp;�upon a clean and dry pane of gUfs, inftea^l of thenbsp;metaliic plate, and holding the glafs by one corner,nbsp;place it under the eledrified prime condudor. It

^04- nbsp;nbsp;nbsp;EleElrical Exferiments.

will be found that the ftnall bodies are not attracted, becaufe in this cafe they have no opportunity ofnbsp;parting widi their natural eleftric fluid, and confe-quently cannot acquire the contrary ele�lricity. Butnbsp;if a finger or anv other conductor be prefented tonbsp;the under fide of the pane of glafs, then the lightnbsp;bodies will be inftantly attrafted, repelled, amp;c. fornbsp;thefe bodies can now depofit their ele�lric fluidnbsp;upon the upper fur face of the glafs plate, whilft thenbsp;under furface of the gl^fs depofits its fluid uponnbsp;the finger, or other condudtor. If this experiment be continued, the pane of glafs will foon be

Experiments

The preceding experiments fhew the following fails, or laws, which we fhall alTume as axioms, to prove that thenbsp;repulfion of bodies poffeffed of the fame fort of eleilricity,nbsp;be it pofitive or negative, feems to be clearly explicable onnbsp;the theory of a fingle eleclric fluid.

1. nbsp;nbsp;nbsp;A body pofleffed of either fort of eleclrlcity will induce, or tend to induce, the contrary eleilricity on anynbsp;other body that comes within its fphere of ailion, viz. within a certain diftance of its furface.

2, nbsp;nbsp;nbsp;A body cannot appear electrified on any part of itsnbsp;furface (meaning that the eleilrical power cannot manifeftnbsp;itfelf, or, according to the theory, the eleiiric fluid cannot benbsp;equably diffufed through it,) unlcfs that furface is oppofitenbsp;to fome other body which is adtually poflefled of the contrarynbsp;-eleiftricity. And thofe two contrarily eledirified bodies attract or tend to attract each other.

Place a coated jar, fiich as that of fig. 7, upon the table where the electrical machine ftands, and

with

3. According to the Franklinian hypothefis, the eledric fluid is elaftic, that is, repulfive of its own particles, butnbsp;attradlive of the particles of other matter.

Now let A and B, fig. 15, Plate XXIII. be two fpheres of conducing matter fufpended in the open air, contiguousnbsp;to each other, and capable of being eafily moved. Let fomenbsp;eledfricity be communicated to them, and it is evident thatnbsp;this eleflricity cannot be difFufed equably over their furfaces,nbsp;but it muft be thicker or more condenfed on the parts thatnbsp;are remote from the point of contact, becaufe there the airnbsp;is at liberty to acquire the contrary electricity ; whereasnbsp;near the point of contadt, the electricity cannot be manifeft-cd, becaufe in that place there is no air or other body whichnbsp;can acquire the contrary eledtricity. Therefore the at-mofpheres of contrary eledtricities cannot be concentric withnbsp;the fpheres A and B, but muft be fituated fomewhat likenbsp;^ the dotted reprefentation of fig. 15 ; then the fpherical bodiesnbsp;being attradted towards the centres of thofe fpheres, appearnbsp;to,repel each other, as ftiewn in fig. 16; fo that when thenbsp;bodies are eledfrified pofitively, negative atmofpheres will benbsp;formed round them, and the additional eledlric fluid of thenbsp;bodies will attraa, and be attraded by, thofe negative at-niofpheres. When the bodies are eledtrified negatively,nbsp;pofitive atmofpheres will be formed round them, which at-tradt the undercharged bodies.

40^ 'EleEincal ExperimentSt

with its knob A, in contadt with the prime con-^ duflo�, alfo place Henley�s quadrant ekdirorneternbsp;upon the prime condudor; then work the machine,nbsp;and the index of the eledrcmeter will rife graduallynbsp;as far as a certain height, which depends upon thenbsp;force of the machine, fize of the jar, See. beyondnbsp;V. h.ich It w�ll not rife. You may then xoncludenbsp;that the ja- has received its full charge *. Take anbsp;difeharginy rod, and, holditig it by its glafs handle,nbsp;apply gt;me t.f its knobs to the outfide coating of thenbsp;jar, ti'en bring its'other branch towards the knob A

This expianauoii may be eafily applied to bodies of any pt'ner i'hape ; proper allowance being made for their more ornbsp;lefs perfect conducting or nonconducting nature.

* Some lore of glafs is rnore apt to difeharge itfelf over its furrace than, ottiers, A battery cannot in gene�ral benbsp;chargeil fo high as a linglc jar. The dampnefs or drynefsofnbsp;the air does alfo influence the charge. Yet Mr. Cuthbertfonnbsp;found, that by breathing into a jar through a glafs tube,nbsp;previous to the charge, the jar will be enabled to hold a muchnbsp;greater charge. He judges of the force of a battery or jar by 'nbsp;the lengtn of wire which its difeharge is able to fufe. 1'husnbsp;fpeaking of his experiments with a certain battery, he fays,nbsp;� This battery contained 17 fquare feet of coated glafs, andnbsp;� was compofed of 15 j^rs; it was found iri the then ftate ofnbsp;, � the atmofphere to be incapable of fufing a greater lengthnbsp;� of wire than 18 inches. But after breathing into eachnbsp;�� jar through a g'afs tube, it took a charge which fufed 60nbsp;V inches.�- Nicholfon�s Journal of Nat. Phil. amp;c. vol. fl*

of the jar, and you will hear a report, and will fee vivid Iparks between the difcharging rod and thenbsp;condufling fubftances that communicate 'with thenbsp;fides of the g�afs. This operation difcharges thenbsp;jar. If, inftead of ufing the difcharging rod, younbsp;touch the outfi le of the jar with one hand, and itsnbsp;knob vvith the other kand; then, behdes the report,nbsp;amp;c. you will feel a p-culiar Ihock, which, accordingnbsp;to the height of the charge, fize of the jar, amp;c, willnbsp;affed either your wrifis, or elbows, or bread. See. �nbsp;If a number of perfons join hands, .and the fiirft ofnbsp;them touches the outfide of the jar, and the laftnbsp;touches the knob, they will all feel the Ihock, andnbsp;preciiely at the fame perceivable inftant. But thofenbsp;who are nearer to the coatings of the jar, or who arenbsp;at the extremities of the circuit of communication,nbsp;will feel the Ihock ftro.nger than the reft ; for thenbsp;eleftricity of either fide becomes lefs condenfed,nbsp;and of courfe lefs aeftive in proportion as it expands itfelt through a greater quantity of conducing matter.

The force of the dilcharge may be manifefted by ^ great variety of experiments.�Take a card ornbsp;quire of paper, or two cards kept a little afunder

^^he interpofition of little bits of wax here and there; place either of thofe articles flat againft thenbsp;outfide coating of a charged jar, and put one of thenbsp;knobs of the difeharmna: rod over it, fo that the

card qr quire of paper, or the two cards, may be jnterpofed between that knob and the coating of the

D D 4 nbsp;nbsp;nbsp;jar j

jar j then, by bringing the other knob of the dlf-oharging rod near, the wire of the jar, make the diicharge ; end the eleftric matter, rufliing throoghnbsp;the circuit from the pofuive to the negative fide ofnbsp;the jar, will pierce a hole, and frequently more thannbsp;one hole, quite through the card or cards, or quirenbsp;of paper, amp;c.; and each hole wdl be found to havenbsp;a bur raifed on each fide, unlefs the card be preflTednbsp;too hard againft the fide of the jar *. If the noftrilsnbsp;be immediately prefented to fuch perforation, anbsp;fmell, fomewhat like that of pihofphorus, will benbsp;perceived. If, inftead of paper, a very thin platenbsp;. of glafi, or of rofin, or of fealing-wax, be inter polednbsp;between th� difcharg�ng rod and the cutfide coating of the jar, on making the difcharge, this will benbsp;broken in feveral pieces.

If a piece of white fugar be interpofed, and the Ihock be fu.fiiciently ftrong, the iiigar will benbsp;broken, and in the dark it will appear beautifullynbsp;illuminated, remaining fo for nearly a minutenbsp;after.

Put the extremities of two wires upon the furface of a card, or, which is the fame, place the cardnbsp;flat upon the tablet E of the univerfal difcharger,nbsp;'fig. 9, and having removed the knobs D, D, incline the wires, fo that their extremities rnay reftnbsp;upon the card, and at about an inch diftancenbsp;* This fliews, that the bur and the perforation are made by

� nbsp;nbsp;nbsp;from

from eachorhcr; then, by conneding one of the rings, or wires C, with the outfide of a chargednbsp;jar, and the other wire C with the knob of the jar,nbsp;the fhock will be can fed to pais over the card ; andnbsp;after the fame manner it may be caui'�d to pafs overnbsp;the furface of any other body.

If the card be very dry, the difeharge v/ill leave upon the card between the extremities of the twonbsp;wires a lucid track, which will remain upon it during fome fcconds. If the fhock be palled over anbsp;piece of writing paper, this will be torn into verynbsp;fmali bits. If the fhock be fent over a piece ofnbsp;glafs plate, the furface of the glafs will thereby benbsp;marked with an indelible track. In this experimentnbsp;the glafs pi vte is feldom broken but Mr Henlynbsp;found that it may be eafily broken if weights havenbsp;been previoufly laid upon it. He ufed to place anbsp;thick piece of ivory upon that part of the glafsnbsp;which ftood between the extremities of the wires,nbsp;and upon that ivory he placed any v/eight from anbsp;quarter of an ounce to fix pounds. On making thenbsp;difeharge, ,the glafs would generally be broken intonbsp;innumerable pieces, fome of it being abfolutely re-duced into an impalpable powder. If the glafs benbsp;too thick to be broken by the force of the explo-fion, it Will be found marked with the moft livelynbsp;ptifinatic Colours, which are occafioned by verynbsp;thin laminre of the glafs, partly feparated by thenbsp;fhock. The weight is always fliook by the ex-

If the card, over which the iliock is fent, be pafnctd with any particular colour, a permanentnbsp;black Tnark is generally left upon it, efpecially if itnbsp;be painted with vermillion ¹.

In order to fire gun-powder by means of the Leyden phial, make a frnali cartridge of paper, andnbsp;fill it with gun-powder, or elfe fill the tube of a quillnbsp;with it, and infert the pointed extremities of twonbsp;vfires in it, fo tlrat their extremities w^ithin the powder may be about one-fifth part of an inch fromnbsp;each other. This done, fend the charge of a I.eydennbsp;phial through thofe wires, and the gun-powder willnbsp;be fired. It the pov.'der be mixed with fteel filings,nbsp;thc' experiment wilb fucceed even with a fmallnbsp;Jlibck.

If the gun-powder be placed loofely upon any Hand, and the interruption of the wire circuit benbsp;made in it j on making the difcharge of the jar, thenbsp;fpark which takes place at that interruption, willnbsp;fcatter the gun-powder without firing it. -But thenbsp;loofe gun-powder, may be fired, if the fliock benbsp;ti anfmitted through lefs perfect conductors j innbsp;which cafe the difcharge being lefs fudden, or rather proceeding in aftream, the powder will be fired.

hquot; is the ^un powder, placed upon the fame tabic up ,n w .ich the jar A B vs dtuated ; C D is a glafsnbsp;tub.- about one root long an.i a quarter of an inchnbsp;in diarticter, full of water, and having two corks atnbsp;its cxcre �iitieo. Into thefe corks two wires arenbsp;thrult, the inner txcremitics of which juft touch thenbsp;Water, viz. rhe ihort w ire at D, and the long wirenbsp;�C A, which ir.ak.,.'s the condmunicadon between �nbsp;the w ater of t!te tube and the knob of the jar. Onnbsp;making die diicharge, which mull: pafs through thenbsp;flTiait quant'ty of water in C D, and through thenbsp;tabic F B, both imperfe 't conductors, the eledlricnbsp;flui d com.es cur at D, in the form of a cienfe ftream,nbsp;which generally li.'-es the gun-powder at F.

If a fpoon, containing fpirit of wine, be con-nefted with the outfide of a Leyden phial, and the knob of a wi e, communicating with the infide ofnbsp;the phial, be brought juft over the fuiface of thenbsp;fpwitj at a final! diftance from it, the di.ftharge ofnbsp;the phial will fet fire to the ijvirit of wine, providednbsp;this has been previoufly warmed. But the famenbsp;thing inay be done by paffing a fimpie fpark fromnbsp;the prime conduiftor i f the machine through thenbsp;'�warmed (pint of wine.

A very fine flender wire may be fufed by the flifeharge of a fingie j.ir. For this purpofe younbsp;need only make that wire part of the circuit; for

inftance,

inllance, place it between the extremities of the vrires of the iiai\^erfal difcharger. The fine turnings or fiiavmgs of fteel, which may be had at thenbsp;phllofophical inftruraent milkers, are very eafilynbsp;fufed, even by a faaall fhock. But a wire of thenbsp;50th part of an inch or upwards, requires a confi-derable battery to melt it ¹,

Take two flips of common window-glafs, about three inches long and half an inch broad i put anbsp;fmall flip of gold, or filvcr, or brafs-leaf betweennbsp;them, leaving a little of the metallic leaf out of thenbsp;glafles at the two ends, and place thofc glafs flipsnbsp;between the boards of the prefs H of the univerfalnbsp;difcharger, fig. 9, which prefs mull: be put in thenbsp;pdace of the tablet E then by connfcfting the wiresnbsp;D, D, with the projecting extremities o'! the metallic leaf, amp;c. fend the charge of a pretty large jarnbsp;through itj the confequence will be that the glalTes

inch in diameter; another battery belonging to the fame perfon, and containing 225 fquare feet of coated furface,nbsp;could melt, with its higheft charge, 300 inches of the firft-inentioned wire, or 10 inches of the laft ; alfo the higheftnbsp;charge of a third battery, which contained 550 fquare fes¹-of coated furface, could tufe 25 inches of the latter wir^'nbsp;Nicholfon�s Journal ot Natural Philofophy, amp;c. vol�nbsp;page 527.

It appears that the highcll charge of a battery, belonging to Dr. Van Marum, and containing 135 fquare feet of coated furface, could juft fufe 180 inches of iron wire,

are generally lliattered by it; but whether they are broken or not, they will be found indelibly markednbsp;by the metal, which is forced fo far into the pbresnbsp;of the glafs, as not to be affected even by the men-ffrua which otherwifc are wont to diffolvc it.

Take a wire bf the fi'ze of a common knitting-needle, or larger, and by means of any eafily flexible �wire or chain, let one end of it communicate with thenbsp;nutfide coating of ajar, that contains at leaft ten fquarenbsp;inches of coated furfacc. Round the ether end ofnbsp;the firft-mentioned wire, fome cotton muff benbsp;loofely twifted, fo as to form a head round it, andnbsp;thus corceal the end of the wire. Roll this headnbsp;of cotton in powder of lycopodium, or in powdernbsp;of rofin; this done, charge the jar, and bring thenbsp;Cotton head rather quickly towards its knob; bynbsp;�which means the difeharge will be caufed tb palsnbsp;through the faid cotton, which will thereby be in-ftantly fet on fire.

If ajar be difeharged with a difeharging rod that has not an eleftric handle, the hand which holds thenbsp;tod, on making the difeharge, feels a partial {hock.nbsp;In other words, a perfon, or any conduding fub-flance that is conneded with one fide of a Leydennbsp;phial, but that forms no part of the circuit, will feelnbsp;a kind of fhock, or fome effed of the difeharge.nbsp;Thus, if conned a piece of a chain with thenbsp;outfide of ajar, or,place it very near the jar; thennbsp;difeharge the jar through another circuit, as for in-ftance, by means of a common difeharging rod ;

on making the difcharge in the- dark, Tparks ^il! be feen between the links of the chain, alio between thenbsp;chain and the.jar; which (hews that the eledtric fluidnbsp;of the chain is affeded by the proximity at the jar.nbsp;If this chain be infulated, it will be found, after thenbsp;difcharge of the jai-j not to be eleflrifled ; hencenbsp;Dr. Prieftley (who firft deferibed this effedl out ofnbsp;the circuit, and to which he gave the name of lateralnbsp;explofion) thinks that this lateral fpark flies from thenbsp;coating of the jar to the chain, and inftantly returnsnbsp;to the former.

Thus far I have deferibed fuch experiments as fhew the effefts or the power of charged eledlricsjnbsp;and which may be moflly pet formed with a Anglenbsp;jar. That power may be fhewn in a much morenbsp;furprifing manner by the ufe of a large battery jnbsp;but the management of fuch battery being fimilarnbsp;to that of a Angle jar, it is needlefs to give any par-*nbsp;ticular diredtions refpeding the ufe of the fame.nbsp;We may only obferve, by way of precaution, thatnbsp;more care and attention is requ'red in the n.anage-ment of a large battery, lelf the fliock, which mightnbsp;be very hurtful, fhould unexpectedly pafs throughnbsp;the operator, or any of the by-ftanders.

After having difeharged a large battery, the operator Ihould once more apply the difeharging rod to the outfide and infide coatings of the battery; fot*nbsp;arefiduum of the charge generally remains in it afternbsp;the firft difcharge, which might afterwards give an

I fhall now add fuch experiments as may illuftrate the theory of the Leyden phial, and the hypothefisnbsp;of a fmgle eledric fluid.

Place a coated jar on an infulating ftool, and �with its knob, not in contaft, but within an inch ofnbsp;the prime condudor ; then work the machine, andnbsp;after a certain time you v/ill find, upon trial, thatnbsp;the jar is not charged, becaiife its outflde, beingnbsp;infulated, could not part with its eleflric fluid, andnbsp;�of courfe its imide could not receive any additionalnbsp;quantity of it. But if you hold the knob of a wirenbsp;at fuch a dillance from the outfide coating of thenbsp;jar, as the knob of the jar is from the prime con-duftor j then, on working the machine, you willnbsp;find, that whenever a Ipark goes from the primenbsp;condudtor to the wire ot the jar, anodrer fpark paffesnbsp;fiom the outfide coating of the jar to the knob ofnbsp;the wire that is pi'efented to it; which fnevvs thatnbsp;according as a,quantity of eleftric fluid enters thenbsp;jar, about an equal quantity of the ele�ric fluidnbsp;^�hich belongs to the outfide of the jar, leaves thatnbsp;outfide. In this manner the jar becomes charged.nbsp;If in this experiment the fame fluid which goes fromnbsp;*^he prime conduftor to the knob of the jar, camenbsp;through it, and paffed to the oppofed knob, the jarnbsp;could not poffibly become charged.

hen the jar is charged, if you prefent the pointed extremities of the difeharging rod at a

certain diftance from the outfide coating and from the knob of the jar, as fiiewn in fig. 7, you willnbsp;perceive (if the experiment be performed in thenbsp;dark) both points illuminated, viz. the upper pointnbsp;with a little ftar, and the lower, B, with a brufh ofnbsp;light, provided the jar has been charged pofitivclynbsp;in the infide; but if the jar be charged negatively innbsp;the infide, (viz. byprefenting its knob to the negative conduftor) then the ftar and the brufh will benbsp;reverfed, viz. the brufh will iftue from the upper,nbsp;and the ftar will appear on the lower, point. Bynbsp;this means the jar is filently difcharged.

Difpofe the apparatus as in the above-mentioned experiment, (p. 408} with the card j viz. lay a cardnbsp;upon the tablet E of the univerfal difchargerj fig. 9,nbsp;but with this difference, that inftead of laying thenbsp;extremities of both wires upon the fame fide ofnbsp;the card, one of them be placed under the card;nbsp;then fend a ftiock through the faid wires, as in thenbsp;above-moutioned experiment, and it will b�e foundnbsp;that the eledlric fluid will run over that furface ofnbsp;the card, upon which ftands the wire that communicates with the pofitive fide of the jar; and in ordernbsp;to pafs to the other wire, it will break a hole throughnbsp;the card juft over the extremity of that other wire.nbsp;Thus let A B, fig. 17, Plate XXIII. reprefent anbsp;fedlion of the card; C and D the extremities of thenbsp;wires laid upon the oppofite furfaces of the card;nbsp;then, if the wire D be connedled with the pofitivenbsp;fide of the jar, on making the difcharge the elebbric

fluid will run over the card from D to E, and at E it will break a hole and pafs to the wire C, whichnbsp;communicates with the negative fide of the jar; burnbsp;if the wire C be connefted with the pofitive fide ofnbsp;the jar, then, on making the difcharge, the eiedricnbsp;fluid will run along the furface of the card from. Cnbsp;to F, and at F it will break a hole and pafs to thenbsp;wire D.nbsp;nbsp;nbsp;nbsp;^

The courfe of the eleftrie fluid in this experiment may be feen either by the luminous track, if the experiment be performed in the dark, or bynbsp;previoufly painting the card on both fides withnbsp;Vermillion and gum-water j for the patTage of thenbsp;eleftric fluid will leave a permanent dark tracknbsp;upon it.

Take a fmall coated phial, and by breathing upon its external uncoated part, render that part flightlynbsp;damp; then holding it by its outfide, prefent itsnbsp;knob to the prime conductor, while the machine isnbsp;in aftion, and you will find that, after the phial hasnbsp;received a fmall charge, a beautiful brufh of raysnbsp;quot;'ill proceed from the cork, which, after going anbsp;little way into the air, bends its courfe towards thenbsp;Outfide coating of the phial. If the phial be charged negatively in the infide (viz.- if its knob benbsp;prefented to the infulated rubber), then the luminous brufh will ifiue from the outfide coating, andnbsp;will proceed towards the cork or wire of the phial.nbsp;In this experiment the outfide of the phial muft be

Remove the circular board E from the univerfal diicharger, fig. 9; fix the wires DC, D C, fbnbsp;that their knobs D, D, may be about two inchesnbsp;afunder, and upon the focket F fix a piece of wax-taper lighted, fo that its flame may be midway between the two knobs D, D. This done, if younbsp;conned, by means of a chain or otherwife, the out-fide of a charged Leyden phial with one of thenbsp;wires C, and bring the knob of the phial to thenbsp;other wire C, you will obferve that on making thenbsp;difeharge, which mufl: pafs from one of the knobsnbsp;D to the other, the flame of the wax-'taper is alwaysnbsp;driven in the diredion of the eledric fluid j that is,nbsp;it will be blown upon the knob of that wire whichnbsp;communicates v/ith the negative fide of the phial.

In this experiment the phiaL muft have a fmalL charge, which experience will prefently determine.nbsp;quot;With high charges the experiment does notfucceed,nbsp;becaufe the charge paffes too fuddenly, and like-wife becaufe on approaching the phial to the wire,nbsp;a confiderable eledrical atmofphere is formednbsp;round the knob of that wire, which difturbs thenbsp;flame, amp;c.

If a Leyden phial be clofcly flopped, and a narrow and open tube, containing a drop of water, be �paffed through and cemented into its cork, it isnbsp;evident that if the air within the jar be at all rarefied

or condenfed, the drop of water within the tube muft be moved from its place. Now on chargingnbsp;this phial either pofitively or negatively in the in-fide, the water within the narrow tube will not benbsp;moved from its place; which llrew's that the chargenbsp;does by no means dilplace the air. Nor will thenbsp;water be moved'on making the difcharge, unlefs anbsp;Ipark happens between the infide coating and thenbsp;wire, or between the various parts of the infidenbsp;coating; for a fpark always rarefies a little and dif-places the air.

Take a naked phial, and for a coating on the outfide ftick a piece of tin-foil with a little wax, fonbsp;that it may juft adhere to the glafs; and for an infidenbsp;coating ufe fmall leaden fliot, or quickfilver; laftly,nbsp;infcrt a wire into the phial. This done, hold thenbsp;phial, thus coated, by its outfide, and charge it in thenbsp;ufual manner. When charged, turn it upfide down,nbsp;and pour its contents into an infulated cup for examination ; alfo remove the outfide coating. Bynbsp;this operation the phial does not lofe its charge, andnbsp;if the quickfilver or the fhot which formed the infide coating be examined by means of an ele�lro-meter, it will be found flightly eleftrified, viz. asnbsp;much as any other like infulated ,condu6tor that hasnbsp;been in contad with the prime condudor. Pournbsp;the fame lliot or quickfilver, or elfe fome othernbsp;quickfilver again into the phial, and replace thenbsp;outfide coating; then touch th� outfide coating-withnbsp;one hand, and the infide with the other hand, by

mearis of a wire, amp;c. and you will feel a Ihock, which will convince you that the phial had not loftnbsp;its charge, and will at the fame time prove that the.nbsp;charge does not refide in the coating.

The illuftration which the preceding experiments afford to the theory of a Angle ele�lric fluid is fonbsp;obvious as to require no farther explanation. Anbsp;vaft number of other experiments with the Leydennbsp;phial might now be added, which, however, are innbsp;general only variations of thole which we have already deferibed. The inquifltive reader may findnbsp;abundance of fuch experiments deferibed by the numerous writers on eledricity.

Hitherto we have taken notice of one mode of producing eleflricity, namely, bynbsp;means of friftion j and have ftated its properties,nbsp;together with its moft rational theory. But electricity is alfo produced by other means, whichnbsp;remain to be deferibed, and which indeed are intimately concerned in feveral grand natural pro-ceffes.

There is hardly an operation of nature which does not produce fome ele�lricity, or with which eleftri-oity does not Teem to be in foine meafure concerned. Probably all the different produftions of electricity follow one general law; however, for thenbsp;fake of perfpicuityitwillbe neceffary to fpecify thofenbsp;various fources, befides friftion, and to reduce themnbsp;to the following fpecies.

3. nbsp;nbsp;nbsp;It is produced by evaporation and by the con-denfation of vapour.

4. nbsp;nbsp;nbsp;It is to be found in the atmofphere at all timesnbsp;more or lefs,

6. nbsp;nbsp;nbsp;It is produced by the mere contact, or by thenbsp;natural aftion of certain conducing bodies uponnbsp;each other,

We fhall defcribe thofe different fources of electricity in the following chapters; comprehending the firft three under the title of eleSiricity producednbsp;ly meltings heating, cooling, and evaporation ; the 4thnbsp;under the title of atmofpherical electricity-, the 5thnbsp;under the name of animal eleSlricity; and the laftnbsp;under the appellation of Galvanifm.

But previous to this it will be neceffary to defcribe, in the prefent chapter, the principal methods thatnbsp;have been contrived for difcovering the prefence,nbsp;and for afcertaining the quality, of very fmall quantities of eledtricity; for fometimes the eleftricity,nbsp;�which is produced by the above-mentioned fources,nbsp;is fo very fmall as to require the utmoft attentionnbsp;and mechanical contrivance on the part of the phi-Ipfopher,

The adlion of eledtric atmofpheres is the principle which has furnilhed the methods of manifeft-ing the prefence of fmall quantities of eledlricityj viz. of fuch quantities as of themfelves could notnbsp;affecl: an eledlrometer fenfibly.

Let an eledlrometer be affixed to an inful^t^*^

metallic plate. Communicate fome electricity to this plate, and the eleftrometer will diverge. Innbsp;this ftate bring the plate near a conduflor not infu-lated, and you will find that the eledlrometer col-lapfes in proportion as you approach the plate tonbsp;the uninfulated conduftor. Remove the electrifiednbsp;plate, and the electrometer will again diverge to itsnbsp;former degree very nearly; which fliews that by thenbsp;vicinity of the uninfulated conducting body, whichnbsp;could eafily acquire the contrary eleCtricity, the in-tenfity of the eleCtricity in the eleclrified plate wasnbsp;diminiflied; or, which is the fame thing, that thenbsp;capacity of that plate for containing eleCtricity wasnbsp;increafed, becaufe in that fituation a greater quantity of eleCtricity mull be communicated to the plate,nbsp;in order to raife the electrometer to the fame heightnbsp;as when the plate is not oppofed to an uninfulatednbsp;conductor.

It eafily follows, that according as the conductor which is oppofed is larger or fmaller, and alfo as itnbsp;is nearer or farther, fo the capacity of the plate maynbsp;be increafed more or lefs.

Now if there be a fource of eleCtricity which, when communicated to an electrometer, is too weaknbsp;to affeCt it; let an ample infulated plate be fituatednbsp;very near another plate not infulated, and in thatnbsp;ftate let the former plate communicate with thenbsp;body which furnifhes the weak eieClricity ; and thenbsp;plate fo fituated will acquire a confiderable quantitynbsp;of that eieClricity, which, whilfl; this plate is oppofed

to the other, will not afFefl: the eledlrometer; but if afterwards the receiving plate be removed fromnbsp;the vicinity of the other plate, its capacity for containing eleflricity will be diminifhed, and of courfenbsp;the abforbed eledVricity will appear much ftrongernbsp;upon its furface, amp;c.�Such a receiving plate wasnbsp;Called a conde^fer by Mr. Volta.

Farther, it muft be remarked that when a body is eledlrified, if an infulated plate be brought near it,nbsp;and in that (late be touched, for inllance, with anbsp;finger, the plate will thereby acquire the contrarynbsp;eledlricity. Now remove the finger, alfo removenbsp;the plate, and give its eledlricity to an infulatednbsp;body, as to an eledlrometer, by touching it withnbsp;that plate j then repeat the operation, viz. bring thenbsp;lame plate near the original ele�trified body, andnbsp;touch it, by which means you can communicate tonbsp;it as much eleflricity as before, which may alfo benbsp;communicated to the fame eledlrome'ter ; and thusnbsp;by degrees the eledlrometer will be caufed to diverge fufficiently ; whereas the mere contadt of thenbsp;original eledlrified body might not be nearly fuffi-cient to affedi: it fenfibly. In this cafe the eledlri �nbsp;city which Is communicated to the eledtrometer isnbsp;evidently contrary to that of the original eledlrifiednbsp;body; viz. it will be pofifive if that was negative,nbsp;and vice verja^

See my Treatife on Ele�ricity, 4tb edition, vol. If p. 49, and following ; alfo p. 244, and following,

and upon this principle feveral machines have been contrived for rendering manifeft: a fmall quantity ofnbsp;eledlricity ¹.

Before the year 1795� I contrived a machine for this purpofe, to which, by way of diftindlion, I gavenbsp;the name of Multiplier of EleSlricityy and which,nbsp;after long ufe, feems (if the partiality for my ownnbsp;contrivance do not deceive me) to anfwer the pur-pofe in a manner more commodious and much lefsnbsp;equivocal than any other inftrument of the kind.nbsp;This machine is delineated in Plate XX'II. fig. 19,nbsp;which is about one-third of the real fize.

Q^R S is the bottom board, upon which are fteadily fixed on the glafs flicks H, G, two flatnbsp;brafs plates, A and C. B'is a fimilar brafs platenbsp;fupported by a glafs flick I, which is cemented intonbsp;a hole made in the wooden lever KL. This levernbsp;moves round a - fteady pin or axis K, which isnbsp;fcrewed tight in the bottom board. By moving this

Mr. Bennet�s Doubler is an ingenious contrivance for the purpofe of manifeftlng very fmall quantities of eleiSlri-which afls upon the above-mentioned principle. Itnbsp;v/as afterwards improved by Mr. Nicholfon. But in all itsnbsp;ftates it ij 2pt to contract a certain permanent eledlricity,nbsp;which renders its effed: equivocal in moft cafes. See thenbsp;Philofophical Tranfadions, vol. 77 and 78; alfo tny Treatifenbsp;on Eledricity, 4th edition, vol. III. p. 76, and following.

See Mr. Volta�s Method in the Phil. Tranf. vol. 72, or in my Treatife as above, vol. II. page 244, amp;c. See alfonbsp;vol. III. page 91, amp;c.

lever alternately from L to X, and back again, the plate B, with the lever, may be placed in the twonbsp;fituations, viz. the fituation LIBK, and that whichnbsp;is Ihewn by the dotted reprefentation of the fame.nbsp;N is a thick brafs wire fixed tight into the bottomnbsp;board. Om is a crooked wire that proceeds fromnbsp;�the brafs focket on the back of the plate B.

There is likewife a fourth brafs plate D, fimilar to the others, which is fupported, not by giafs, butnbsp;by a wire j and this wire is ferewed faft to an oblongnbsp;piece of brafs F P, which Hides in a groove madenbsp;for the purpofe in the bottom board Q_^RS ; fo thatnbsp;by applying a finger�s nail to the notch at the endnbsp;F, the fiiding piece F P may be drawn out eithernbsp;entirely or to a certain length, and of courfe thenbsp;plate D will be removed to any required diftancenbsp;from the plate C. When F P is puQied quitenbsp;home, the plate D Hands parallel to C, and at

The parts of this inftrument are fo adjufted, as that when the lever is in the fituation of the ftiadednbsp;part of the figure, viz. is pufhed as far as it can gonbsp;towards Q, then the plate B comes parallel to thenbsp;plate A, and at about ^^th of an inch diftance fromnbsp;it. At the fame time the extremity of the wire O mnbsp;juft touches the fixed wire N, and of courfe rendersnbsp;the plate B uninfulated. But as foon as the levernbsp;begins to move towards S, the communication ofnbsp;the plate B with the wire N, or with the ground,nbsp;is interrupted, and B remains infulated. When thenbsp;3nbsp;nbsp;nbsp;nbsp;lever

lever has been moved as far as it can go towards S, the wire�? comes in contadt with the plate C, as is iliewnnbsp;by the dotted part of the figure. Then the twonbsp;plates B and C communicate with each other, butnbsp;they are other wife in'lilated.

When this inftrument is fituated in the manner which is indiened by the (haded part of the figure,nbsp;the plate A has its capacity for eledlricity increafednbsp;by ti'.e proximity of the uninfulated plate B : hencenbsp;A, if it be cauled to touch a body weakly eleflrified,nbsp;will acquTe a greater quantity of eledlricity from itnbsp;than it would otherwife do. Now fuppofe that Anbsp;has acquired a fmall quantity of eleclricity, for in-dance, pofitive (fince by changing the words pofi-tive for negative, and vice verfa, the following explanation is applicable to the cafe in which A- is electrified negatively) ; then B will acquire the negative eledlricity. On moving the lever L, the communication between B and the ground, or the wire N,nbsp;is difeontinued, and B remains infulated and eledlri-fied negatively. With this eleftricity B is carriednbsp;towards C, until the wire ni touches the plate C,nbsp;and then the negative eleftricity ot B will pafs al-moft entirely to C, becaufe the capacity of C fornbsp;holding ele�lricity is confiderably increafed by thenbsp;proximity of the uninfulated plate D. If after thisnbsp;the lever be moved back to its fird fituation, B willnbsp;be made negative a fecond time as before; and bynbsp;pufhing the lever again towards S, that fecondnbsp;charge of negative electricity will be communicated

fronq

frcm B to C. And thus by repeating the operation, which confifts in merely moving the lever alternatelynbsp;from L to X, and fromX toL, a confiderable quantitynbsp;of electricity will be accumulated upon C. Then if thenbsp;Aiding piece F P be drawn out about one inch, thenbsp;plate D will, of courfe, be removed as much fromnbsp;C : hence the capacity of C will be much diminished. Therefore, if an eleftrometer be broughtnbsp;into contact with it, the negative eleftricity, (viz.nbsp;the electricity contrary to that of the original electrified body in queftion), will be manifefted; whereasnbsp;the electricity originally communicated to the platenbsp;A could perhaps not have affeCted an electrometernbsp;in any fenfible degree.

The principal caufe which renders this infl.ru-ment certain in its efFecSts, is, that all the refiduum of electricity which can remain upon the plate Anbsp;after the performance of an experiment, and afternbsp;having touched that plate, is too inconfiderable tonbsp;induce a contrary eleCtricity in B j the electricitynbsp;which is originally communicated to A, being notnbsp;increafed upon it in the courfe of the experiment¹.

For fa ther particulars relative to this inftrument fee my Treatiie on EleCtricity, 4th edition, vol. Ill, page 98,nbsp;and following.

OF THE ELECTRICITY WHICH IS PRODUCED BY MEANS OF MELTING, HEATING, COOLING, ANDnbsp;EVAPORATION.

IF fulphur be melted in an earthen veflel, and the whole be left to cool upon condu6lors } and ifnbsp;afterwards the fulphur, when cold, be taken out ofnbsp;the velTel, it will be found ftrongly eleftrical ,� butnbsp;not at all fo if it be left to cool upon eleftrics.

If fulphur be melted in a glafs veflel, and be left to cool, both the glafs and the fulphur will acquirenbsp;a ftrong eledricity; the former pofitive and thenbsp;latter negative! and that will be the cafe whethernbsp;they be left to cool upon eledrics or upon con-dudors.

If melted fulphur be poured into a veflel of baked wood, if will acquire the negative, and thenbsp;wood the pofitive, eledricity; but jf it be pourednbsp;into fulphur, or rough glafs, it will acquire no fen-fiblc degree of eledricity.

Melted fulphur poured into a metal cup, and there left to cool, Ihews no figns of eleftricitynbsp;whilft Handing in the cup j but if they be feparated,nbsp;then they will both appear ftrongly cleiflrified, thenbsp;fulphur pofitive, and the cup negative. If the fulphur be replaced in the cup, every fign of eledtri-city ^ill vanifh ; but if, whilft feparate, the eleclrl-city either of the cup or of the fulphur be takennbsp;off; then on being replaced they both v/ill appearnbsp;pofleffed of that eleftricity which has not been takennbsp;off. .nbsp;nbsp;nbsp;nbsp;.

Melted wax, being poured into glafs or wood, acquires the negative eleftricity, and the glafs ornbsp;wood becomes pofitive. But fealing-wax, pourednbsp;into a'fulphur veffel, acquires the pofitive eledlri-city, and leaves the fulphur negative.

Chocolate frefh from the mill, as it cools in the tin pans in which it is received, becomes ftronglynbsp;eledlrical. When turned out of the pans, it retains this property during a certain time, but lofesnbsp;it prefendy by handling. By melting it again innbsp;an iron ladle, and pouring it into the tin pans as atnbsp;firft, you may renew its power Qiice or twice; butnbsp;when the mafs becomes very dry and powdery innbsp;the ladle, the electricity is no longer revived bynbsp;fimple melting; but if then a little olive oil benbsp;added, and be mixed well with the chocolate innbsp;the ladle, and be afterwards poured into the tinnbsp;pans, as at firft, it will be found to have completely

The property of becoming eleftrified merely by heating or cooling, was firft obferved in, and isnbsp;eminently pofleHed by, an hard pellucid ftone callednbsp;tourmalin, which is generally of a deep red, or purple, or brown colour; which feldom, if ever, exceedsnbsp;the fize of a fmall walnut; and which is found in fe-veral parts of the Eaft Indies, efpecially in the iflandnbsp;of Ceylon; but on farther examination it has beennbsp;found that feveral other precious ftones, and efpecially the Brafilian emerald, poflefs the like properties more or lefs : hence the following particulars,nbsp;which have been principally obferved with the tour-^nbsp;malin, muft be underftood to belong likewife tonbsp;moft other precious ftones.

I. The tourmalin, while kept in the fame temperature, ftiews no figns of eleftricity; but it will

Refinous or oleaginous eleilrics, when once excited, retain their eleclric power for a very confiderable time,nbsp;fometimes for feveral days. ' But that power gradually di-minifhes, and at laft vaniflies; nor do we know of anynbsp;eleclric which retains the eleflric virtue as permanently asnbsp;the magnet retains its magnetic povver.

When a flick of glafs, or of fulphur, and efpecially of feallng-vvax, is broken into two pieces, the extremities^nbsp;which were contiguous, will generally be found electrified,nbsp;one pofitive and the other negative. This is probably oc-cafioned by the rufhing in of the air, which may produce a

become eleftrical by increafmg or diminirhing its heat, and ftronger in the latter circiimftance than innbsp;the former. A very trifling alteration of temperature is often fuflicient to produce the effeft.

2. nbsp;nbsp;nbsp;Its eleftricity does not appear all over its fur-face, but only on two oppofite fides of it, whichnbsp;may be called its poles, and which always are in onenbsp;right line with the centre of the ftone, and in t)ienbsp;direftion of its ftrataj in which diredtion the ftonenbsp;is abfolutely opaque, though in the o:her it is femi-tranfparent.

3. nbsp;nbsp;nbsp;Whilft the tourmalin is heating, one of itsnbsp;fides (call it A) is eledtrified plus, or pofitive, andnbsp;the other, B, minus; but when cooling, A isnbsp;minus, and B is plus. Hence, if one fide of thenbsp;ftone is heating, whilft the other is cooling, thennbsp;both fides will acquire the fame eledlricity; or ifnbsp;one fide only changes its temperature, then that fidenbsp;only will appear eledtrified.

4. nbsp;nbsp;nbsp;If this ftone be heated, and fuflfered to coolnbsp;without either of its fides being touched, then A willnbsp;appear pofitive, and B negative, all the time of itsnbsp;heating and cooling.

5. nbsp;nbsp;nbsp;This ftone. may be excited by means ofnbsp;fridtion like any other eledtric, and either of its fides,nbsp;or both, may be rendered pofitive.

6. nbsp;nbsp;nbsp;If the tourmalin be heated or cooled uponnbsp;fome other infulated body, that body will be foundnbsp;cledirified as well as the ftone j but it will be found

poffefled of the eleftricity contrary to that of the contiguous fide of the ftone.

7. nbsp;nbsp;nbsp;The eledlricity of either fide, or of both, maynbsp;be reverfed by heating or cooling the tourmalin innbsp;contadt with various fubftances, fuch as the palmnbsp;of the hand, a piece of metal, amp;c.

8. nbsp;nbsp;nbsp;Thpfe properties of the tourmalin are alfonbsp;obfervable in vacuo, but not fo ftrong as in thenbsp;open air.

g. If a tourmalin be cut into feveral parts, each piece will have its pofitive and negative poles, cor-refponding to the pofitive and negative fides of thenbsp;original ftone.

10. If this ftone be covered all over with fome eledlric fubftance, fuch as fealing-wax, oil, amp;c. itnbsp;will in general fhew the fame properties as without it.

I!. A vivid light appears upon the tourmalin, whilft heating in the dark, and by a little attentionnbsp;one may be eafily enabled by this light to diftin-guifh which fide of the ftone is pofitive, and whichnbsp;negative. Sometimes, when the ftone is ftronglynbsp;excited, pretty ftrong flafties may be feen in thenbsp;dark, to go from the pofitive to the negative fidenbsp;of it. .

^2. Lafliy, it has been found that with refpedt to the eledtric properties, the tourmalin is fome-times injured by the adlion of a ftrong fire, at othernbsp;times is improved, and fometimes is not at all altered by it.

VOL. III. nbsp;nbsp;nbsp;FTnbsp;nbsp;nbsp;nbsp;The

' The evaporation of water, as alfo of fome othef fluids, produces ele�lricity, viz. thofe bodies fromnbsp;which the water has departed, will remain in a negative ftate of eledlricity, indicating that the waternbsp;by its converfion into vapour has its capacity fornbsp;the eleftric fluid iiicreafed, as it has its capacity in-creafed for containing heat*. But though the effedlnbsp;is in general fuch as has been mentioned above, yetnbsp;there are tv/o exceptions which involve the fubjedl:nbsp;in fame difliculty, and which will require farther experiments and confideration.

The exceptions are, ift, that if water be evaporated by being put in contadl with a red-hot piece or pieces of very rufty iron, It will leave the ironnbsp;electrified pofitively; whereas, if the iron be notnbsp;rufly, the evaporation of the water from its furfaeenbsp;will leave it eledtrified negatively f. adly. If waternbsp;be evaporated by throwing into it impure red-hotnbsp;glafs (fuch as the green glafs of common bottles)nbsp;the vefTel, or the remaining water, will be eledtri-ficd pofitively

* Mr. Volta, who made this remarkable difcovery, like-wife obferved, that the flmple combuftion of coals, as alfo the effervefcence of�.ron filings and diluted fulphuric acid producenbsp;the fame effedlwhich-is, in ail probability, owing to thenbsp;evaporation which attends thofe proceffes.

This

This curious prod�clion of eledtricity by evaporation, as alfo the produftion of ekdlricity by the condenfation of vapour, may be eafll'y obferved innbsp;the following manner:

Place a metallic cup, or a pewter plate, upon an infulating fcand, and connect a fenfible eleiflrometernbsp;with it. Alfo place one or two lighted coals in thenbsp;cup or plate; then pour a little water at once uponnbsp;the coal or coals, which will produce a quick evaporation, accompanied with a great hifiing noife,nbsp;and at the fame time the dedlrometer will divergenbsp;with negative eledricity.

If the ftcam, which iflues copioufly from water quickly boiling, be received under a pretty largenbsp;and infulated metallic plate, that plate, by the condenfation of the fleam upon it, will be eledrifiednbsp;pofitively, as may be afeertained merely by con-neding a fenfible eledrometer with it.

On throwing a variety of other fubftances upon adually burning, or o.nly hot and infulated, coals,nbsp;the coals, amp;c. either fliewed negative eledricity, ornbsp;no eledricity at all. Either fpirit of wine, or ethernbsp;�''vhen thus treated, left the coals negative i but iGnbsp;coals being fufEciently hot) the fpirit of winenbsp;or the ether took fire, and burned in their ufualnbsp;then no eledricity was produced.

{ 436 3

The memorable year 1752 produced the remarkable difcovery of the identity of lightning and electricity, which, previous to that year, had only been fulpeCled by philofophers.

The fimilarity of lightning to artificial eleCtricity is not to be remarked in a few appearances only,nbsp;but is obfervable throughout all their numerousnbsp;effects i and there is not a fingle phenomenon ofnbsp;the one, which may not be Imitated by the other.nbsp;Lightning deftroys edifices, animals, trees, amp;c.�nbsp;Lightning goes through the beft conductors in itsnbsp;way ; and if its paflage be obftruCted by el�Clrics,nbsp;or lefs perfect conductors, it rends and difperfesnbsp;them in every direction; � lightning burns com-buftible bodies;�it melts metals;�a ftroke ofnbsp;lightning often difturbs the virtue of a magnet, andnbsp;gives polarity to ferruginous fubftances; and allnbsp;thefe effects may be produced upon a much fmallernbsp;fcale by means of artificial electricity. But independent of the great fimilarity between the effeCls

of liglvtning and thofe of eleftricity', what fully proves their identity, is, that the matter of lightningnbsp;may be a�tually brought down from the clouds bynbsp;means of infulated metallic rods, or of eleftricainbsp;kites, and with it any known eleftricai experimentnbsp;may be performed.

Clouds, as well as the rain, fnow, and hail, which fall from them, alfo fogs, are almoft always eleftri-fied, but oftener negatively than pofitively; and thenbsp;lightning, accompanied with the thunder, is thenbsp;effeft of the eleftricity, which, darting from anbsp;cloud, or a number of clouds highly eleftrified,nbsp;ftrikes into another cloud, or elfe upon terreftrialnbsp;objefts; in which cafe it prefers the lofciefl, moftnbsp;pointed, and beft condufting objefts ; and by thisnbsp;ftroke it produces all thofe dreadful effefts, whichnbsp;are known to be produced by lightning.

The air, at fome diftance from houfes, trees, mafts of ftiips, amp;c. is generally eleftrified almoftnbsp;always pofitively, efpecially in frofty, clear, or foggynbsp;Weather; but how the air, the fogs, and the cloudsnbsp;become eleftrified, has not yet been fully and clearly afcertained. The moft probable conjefture isnbsp;grounded upon the elFefts of the evaporation of watery fluids, and the condenfation of vapour j but wenbsp;lhall in the firft place defcribe the inftruments |:hatnbsp;are moft ufeful for difcovering this eleftricity j thennbsp;fhall ftate the principal fafts whieh have been ob-ferved with refpeft to this atmofpherical eleftricity;nbsp;and lhall, laftly, fobjoin the moft plaufible explana-

F F 3 nbsp;nbsp;nbsp;tion,

tlon, together with the advantage which is derived fi-om the knowledge of the fubjeft.

My ele�l:rometer in a phial, which has been already defcribed (p. 392) is the beft portable inftru-ment for this purpofe j for if you hold this eledlro-meter by its lower part, and raife it juft above the level of your head in the open air, when the air isnbsp;ftrongly eleiftrified, or in a fog, or when eledtrifiednbsp;clouds are over head, and fometimes even whennbsp;they are a little way above the horizon j the divergency of the eleftrometer will announce the pre-ence of eledlricity, and by the approach of an excited ftick of fealing-wax, or of any other eledlrric,nbsp;you may eafily determine whether the eledtricity benbsp;pofitive or negative ; obferving that the eledtricitynbsp;of the eledtrometer in this cafe is the contrary of thatnbsp;of the clouds or fog: but if the eledlrometer be electrified by the rain, or fnovy, or hail, falling upon it;nbsp;then the eledlricity of the rain, amp;c. is the fame asnbsp;that of the eledlrometer; for in the latter cafe thenbsp;eledtrometer is eledlrified by the contadf, but in thenbsp;former cafe it is eledtrified by the adlion of eledlricnbsp;atmofphere. Seepages 353 and 359.

When the eledlricity of the air is not fo ftrong as to be difcovercd by this inftrument, then an eledtrometer muft be extended farther out into the air.nbsp;For this purpofe I have long ufed the following moftnbsp;commodious inftrument or atmofpherical eledtrometer.

fifliing-rod, wanting the laft or fmallefl: joint. From the extremity of this rod proceeds a flender glafsnbsp;tube orglaft {tick C, v/hich is covered '.vith fea�iig-Wax, and has a cork D at its extremity, to which anbsp;cork-ball eledrometer, E, is fufpended. HGI is anbsp;piece of common pack-thread, fattened to the rodnbsp;at A, and fupported at G by a fhort firing F G.nbsp;At the extremity I of the pack-thread, a pin, crnbsp;pointed wire, is fattened, which when pufned intonbsp;the cork D, renders the eledrometer E uninfu-Jatcd.

When I wifh to obferve the eleflricity of the at-mofphere with this inftrument, I thruft the pin I into the cork D, and holding the rod by its lower end A,

I projecl it out from an upper window, railing the end B with the ele�trometer, fo as to make an anglenbsp;of about 50� or 60�, with the horizon. In thisnbsp;fituation I keep the inftrument for a few feconds;nbsp;then pulling the pack-thread at H, I difengage thenbsp;pin from the cork D ; which operation caufes thenbsp;firing to drop in the dotted fituation HK, andnbsp;leaves the electrometer poflefled of the eledlricitynbsp;contrary to that of the atmofphere.�This done, Inbsp;draw the inftrument within the room, and�examinenbsp;the quality of the eleflricity, without any obftrudiortnbsp;cither from wind or darknefs.

If any perfon wifh to obferve the eleftricity of the rain, he may either occafionally ufe, or have al-v/ays fixed, a rod or an afTemblage of wires roundnbsp;4 rod covered with fcaling-wax, cerx^nted into a

glafs tube, by which it may be either held in the hand occafionally, or may be permanently fixednbsp;within a room, and projecting about two or threenbsp;feet out of a window j for which purpofe either thenbsp;window muft be opened occafionally, or the rodnbsp;muft pafs through a hole fufficiently large. Tonbsp;that end of this rod which is within the room, annbsp;electrometer muft be attached, and it will frequently happen, that when it rains, and the rainnbsp;falls upon the projecting part of the rod, the electrometer at its internal extremity is eleClrified.

But an infulated wooden rod, with a wire round if, and projecting about 15 or 16 feet above the houfe,nbsp;will anfwer every purpofe; for a wire proceedingnbsp;from this rod may bfe made to communicate withnbsp;an electrometer within the room, where the intenfitynbsp;as well as the quality of the eleCtricity may be ob-ferved. Such a rod however is very dangerous innbsp;time of a thunder ftorm. In order to avoid thenbsp;danger, a conducting communication, viz. a ballnbsp;of brafs lliould be placed at about two inchesnbsp;diftance from the rod, and a thick wire Ibould benbsp;carried from this ball to the ground or to the pump,nbsp;amp;c. in order that if a large quantity of cleCtricicynbsp;from a cloud ftrike the rod, that eleCtricity may benbsp;conveyed by the wire to'the ground, without hurt-�nbsp;ing the by-ftanders¹.nbsp;nbsp;nbsp;nbsp;When

For the cpnftruCtion of fuch a rod, fee Mr. T. Read�s Summary Fievj of the fpontaneous Ele^ricity of the Earthnbsp;gnd Atmofphere. London 1793.

When the eledricity of the air, or rain, amp;c. is too weak to be difcovered by thofe inftrurnents,nbsp;then my multiplier may. be ufed in conjunction withnbsp;any of them j or the eledtricity of the atmofpherenbsp;may be difcovered by means of an eledtrical kite jnbsp;which is nothing more than a common paper kite,nbsp;fuch as is ufed by children, only having a firingnbsp;which is rendered a better condudlor by having anbsp;flender wire through it. The paper of the kitenbsp;ihould likewife be covered with drying linfeed oil,nbsp;in order to defend it from the rain.

A kite of about four feet in height is the moft commodious for this purpofe. The firing is thenbsp;mofl material part of this apparatus; for accordingnbsp;as the firing is longer or fhorter, abetter or a worfcnbsp;condudlor, fo is the eledlricity which is broughtnbsp;down by it flronger or weaker. The kite onlynbsp;ferves to keep the firing up into the atmofphere.nbsp;After a variety of trials the bell firing proved to benbsp;one which I made by twilling a copper thread, (viz,nbsp;fuch as is ufed for trimmings, amp;c. in imitation ofnbsp;gold thread, which is nothing more than filk or linennbsp;thread covered over with a thin lamina of popper)nbsp;with two very thin threads of twine.

When the kite is flying, the lower part of the firing mufl be infulated by means of a filk firing ofnbsp;about two or three feet in length, or by means of a

The famous Fr. Beccaria ufed a long chord extended in atnpfphere between two houfes. Sec his �lelt;ftricity.

' � nbsp;nbsp;nbsp;glafs

glafs ftick, amp;c.; then at the icwer extremity of the firing you may not only ele�lrify an eledlrometer,nbsp;but you may alfo draw fparks, or charge a Leydennbsp;phial, amp;c. and that at every hour of the day ornbsp;night, and at all times of the year, and will feldomnbsp;fail. The reader is rcquefted to obferve and tonbsp;remember, that this kite is dangerous (firing anbsp;ftorm.

It appears, i. That there is in the atmofphere, at all times, a quantity of eledlricity i for whenever Inbsp;life the abovc-defcribed fifliing-rod-eledtromcter innbsp;an open fituation, it always acquires fome eledlri-city, and that eleftricity is always of the fame kind,nbsp;viz. negative j which fliews that the eleftricity ofnbsp;the air or of fogs, is almoft always pofitive, except when the inftrument is influenced by cloudsnbsp;near the zenith. �

2. nbsp;nbsp;nbsp;That the ftrongefl: eledlricity is obfervable iqnbsp;thick fogs, and likewile in frofty weather i but thenbsp;weaken, when the weather is cloudy, warm, andnbsp;very near raining ; but it does not feem to he lefsnbsp;at night than in the day time.

3. nbsp;nbsp;nbsp;That in a more elevated place the eledricitynbsp;is generally ftronger than in a lower one. Thus Inbsp;have often obferved the ele�trometer to divergenbsp;more in the iron than in the ftone gallery on thenbsp;outfide of the cupola of St. Paul�s cathedral.

4. nbsp;nbsp;nbsp;That the rain, fnow, and hail, are more ornbsp;left, but almoft always eledlfified, much more

After a vaft number of experiments with eleftri-cal kites during upwards of two years, I was enabled to form the following conclufions:

I. The air appears to be eleftrihed at all times; its eleflricity is ccndantly pofitive, whether by daynbsp;or night, and much ftronger in frofty than in warmnbsp;weather. My experiments have been made in everynbsp;degree of temperatui-e between 15� and 80�.

a. The prefence of clouds generally leffens the tleftricity of the kite ; fometimes it has no effedtnbsp;upon it, and feidom increafes it.

3. nbsp;nbsp;nbsp;During rain the eledlricity of the kite is generally negative, and feidom pofitive.

4. nbsp;nbsp;nbsp;The aurora borealis, or northern light, doesnbsp;not appear to zffoQi the eledtricity of the kite.

5. nbsp;nbsp;nbsp;The fpark taken from the ftring of the kite,nbsp;or from any infulated condudtor which is connedtednbsp;with it, cfpecially when it does not rain, is very fel-dum longer than a quarter of an inch; but it is remarkably pungent j fo that the operator will frequently feel the elTedl of it even in his legs; itnbsp;appearing more like the difeharge of an eledtric jarnbsp;than like the fpark which is taken from the primenbsp;condudlor of an eleftrical miachine.

6* The eledlricity which is brought down by the ftring of the kite is, upon the whole, ftronger ornbsp;weaker, according as the ftring is longer or Ihorter;nbsp;jout it does not keep any exadt proportion to it; for

inftance, the ele�lricity from a ftring of an loo yards may raiie the index of a qu?¹irant ele(5i:rometer 20%nbsp;whereas with double that length of ftring the indexnbsp;will not rife higher than about 25�.

7. When the weather is dampj and the cleflricity is pretty ftrong, the index of the cledtrometer, afternbsp;taking a fpark from the ftring, or prefenting thenbsp;knob of a coated phial to it, rifes with furprifingnbsp;quicknefs to its ufuaJ degree j but in dry and warmnbsp;weather, it rifes remarkably flowly.

After the difcovery of the identity of ele�lricity and the matter of lightning, as alfo of the conftantnbsp;exiftenc� of eleftricity in the atmofphere, philofo-phcrs endeavoured to attribute fame other atmof-pherical and even terreftrial phenomena to the agency of eledlricity. Thus the accenlions, commonlynbsp;called fatting ftars or Jhooting ftars j meteors, water-fpouts, hurricanes, whirlwinds, amp;c. have been con-fidered by leveral perfons as being eledlrical phenomena; but of this we have no pofitive proofs.

The aurora borealis^ or northern light, feems moft likely to be an eledtrical phenomenon; and this onnbsp;two accounts, viz. firft becaufe a magnetic needlenbsp;appears a little difturbed at the time of a ftrong au-yora borealis; and fecondly, becaufe the aurora bo-yealis may be partly imitated by means of artificialnbsp;cleflricity ¹.

The aurora' bcrealis is a fheuomenon pretty weH known to the prefent generation throughout Europe at leak¹

Take a glafs phial nearly of the Ikape and fize of a Florence flalk ; fix a ftop-cock, or a valve to itsnbsp;neck, and exhau:-; it as much as you can by rneansnbsp;of a good air-pump. If then this glafs be rubbednbsp;after the manner commonly ufed for exciting electrics, it will appear luminous within, being full of anbsp;flafhing light, which plainly refembles the aurora borealis. This phial may alfo be rendered luminous,nbsp;if, holding it by either end, you bring its other endnbsp;to the prime condudlor; in this cafe all the cavitynbsp;of the glafs will inftandy appear full of light, whichnbsp;linay be feen flafhing in it for a confiderable timenbsp;after it has been removed from the prime conductor, efpecially if it be touched with the hand. Thisnbsp;efledl is eafily deduced from tlte conducing naturenbsp;of the vacuum, and from the charging and difeharg-ing of the glafs.

The moft plaufible mode of accounting for the eleftricity which is conftantly to be obferved in thenbsp;atmofphere, and which accompanies the clouds, thenbsp;fogs, the rain, or that of thunder ftorms, is to derivenbsp;it from the evaporation of water, and from the con-

denfation of vapours. For though the eledricity which is thus produced, may at firft fight appearnbsp;too fmall 5 yet if we confider that thofe procefiesnbsp;are continually carried on, both upon the furface ofnbsp;the earth and in the atmofphere, we may eafily acknowledge the fufficiency of it.

When the vapours depart from the earth, they carry away a much greater quantity of the eledricnbsp;fluid, than they had when in the form of water, andnbsp;which they have derived from the earth. Now ifnbsp;thofe vapours, as they afcend in the atmofphere,nbsp;become more rarefied, then, as they have no bodiesnbsp;at hand from which they can derive the eledricnbsp;fluid, which is required for their increafed capacity,nbsp;they mufl appear eledrified negatively. On the .nbsp;contrary, if thofe vapours are condenfed, then theirnbsp;capacity for the eledric fluid being diminished, theynbsp;muft appear eledrified pofitively. Befides, a cloudnbsp;highly electrified may eafily induce the contrarynbsp;cledricity in another contiguous cloud. From thofenbsp;caufes a variety of particular accumulations of pofi-tive or negative eledricity, or of changes from thenbsp;one to the other may be eafily conceived, apparentlynbsp;fufficient to account for the phenomena of atmofphe-rical eledricity.

One of the greateft advantages which mankind has derived from the knowledge of this branch ofnbsp;phllofophy, is a defence for houfes, fliips, amp;c. againltnbsp;the fatal efFeds of the lightning. It was propofednbsp;by Dr. Franklin to ered an iron rod, or a wire of

any metal on the top of a houfe, and to carry the communication by means of good condudlors ofnbsp;eledlricity, from that rod down to the ground ; fornbsp;fince the lightning generally ftrikes the mofl: elevated condudorsy through which it paffes to thenbsp;earth, it was natural to fuppofe that the houfe thusnbsp;furniffed with a condudtor, would be defended fronttnbsp;the pernicious effedls of lightning. This wife pro-pofal was generally adopted, and its ufefulnefs hasnbsp;been confirmed by innumerable cafes, elpecialiy innbsp;warm climates, which are much more fubjeft �0nbsp;thunder ftorms.

The ufefulnefs of condu�lors to defend buildings from the effects of the lightning, has been univer-fally acknowledged; but the proper form of thofenbsp;Gondudors, efpecially with relped to their terminations, has been the caufe of much controverfy. Itnbsp;was objeded to their having a pointed termination,nbsp;that a pointed body can attraft the ekdric fluidnbsp;from a greater diftance than a blunt termination,nbsp;and therefore it would invite the lightning wherenbsp;otherwife the lightning would not go. To this itnbsp;Was replied, that though the point will attrad thenbsp;dedric fluid from a greater diftance, yet it will attrad it in a ftream, viz. by degrees, and not in alt;nbsp;full body as a knob would do; by which meansnbsp;the force of the lightning will be diminlfhed, andnbsp;in certain cafes a full ftroke may thereby be entirely averted. In fliort, after a great variety of

arguments and experiments, the beft conftrudlloil of fuch conduftors feems to be as follows ¹.

It Ihould confift of a rod of iron, or of other metal, about three quarters of an inch thick, faften-cd'to the wall of the building, not by iron clrmps,nbsp;but by wooden ones. The rod fhould be uninterrupted from the top of the building to the ground;nbsp;of if it confift of various pieces, care muft be hadnbsp;to join the pieces as perfedtly as poftible. If thisnbsp;conduftor ftood quite detached from the building,nbsp;and fupported by pieces of wood at the diftance ofnbsp;one or two feet from the wall, it would be betternbsp;for common edifices; but it is particularly advife-able for gun-powder magazines, gun-powder millsjnbsp;and all fuch buildings as contain combuftibles readynbsp;to take fire. The upper end of the condudtornbsp;fhould terminate in one or more lharp points;nbsp;�which,_ if the conduftor be of iron, ought to be gift,nbsp;in order to prevent the ruft or the oxigenation.�nbsp;This fharp end fhould be elevated above the higheftnbsp;part of the building (as above a flack of chimnies,nbsp;to which it may be faftened) at leaft five or fixnbsp;feet. The lower end of the condudor fhould be

See what relates to the conduflors of lightning in the Philofophical Tranfadions for the year 1777, and ten ornbsp;twelve following years; alfo fee Earl Stanhope�s Principlesnbsp;ofEle�lricity,London 1779, and my Treatife on Eledlricity,nbsp;4th edition, vol. II. p. 207, and following.

driven five or fix feet into the ground, and in a diretlion leading from the foundation; or it wouldnbsp;be better to connedt it with the heareft piece ofnbsp;water.

For an edifice of a moderate fize, one of thole condudhors is perhaps fufficicnt; but a large building ought to have two, or three, or more condudtorsnbsp;at its mofl; diftant parts.

On board of fhips a chain has oftep been ufed on account of its pliablenefs; but in feveral cafes thenbsp;chain has been adlually broken by the lightning, innbsp;confequence of the obftrudtion which the eledlricnbsp;fluid meets with in going through the variousnbsp;links; hence, inltead of a chain, a copper wirenbsp;about one-third part of an inch thick, is now morenbsp;commonly ufed. One of thofe wires fliould be elevated two or three feet above the highefl: maft innbsp;the veflel; this fliould be continued down along thenbsp;maft as far as the deck, where, by bending, it Ihouldnbsp;be adapted to the furface of fuch parts as may benbsp;more convenient; and by continuing it down the fidenbsp;of the veffel, it fiiould always be made to communicate with the water.

With regard to perfonal fecurity in time of a thunder ftorm, if a perfon be in a houfe which isnbsp;not furniftied with a condudtor, it is advifable notnbsp;to ftand near any metallic articles, viz. near giltnbsp;frames, chimney-grates, bell-vvires, iron cafements,nbsp;and the like. In the middle-of a room, upon a drynbsp;chair, or table, or matralTes, or other infulating ar-

voL. nr. nbsp;nbsp;nbsp;G Gnbsp;nbsp;nbsp;nbsp;tides.

tides, is the fafell; fituation. Should a ftorm happed when a perfon is in the open fields, and far frontnbsp;any building, the bell thing he can do is to retirenbsp;within a fmall diftance of the higheft tree or treesnbsp;he can get at ; he muft not, however, go quitenbsp;near them, but he fliould ftop'at about fifteen ornbsp;twenty feet from their outermoft branches; for ifnbsp;the lightning happen -to ft tike about the place, itnbsp;will in all probability ftrike the trees in preferencenbsp;to any other much lower objedj and if a tree happen to be fplit, the perfon will be fafe enough atnbsp;that diftance from it.

UNDER this title we fliall take notice of that eledricity only which is produced from thenbsp;animal itfelf, in confequence of its particular orga-nizatioHj and not that which is produced by thenbsp;application of metallic fubftances to animals.

Three filhes have hitherto been difcovered to have, whilft living, the fingular property of givingnbsp;fliocks analogous to thofe of artificial eledricity inbsp;namely, the torpedo^ the gymnotns eleHricus, and thenbsp;filurus eleSlricus. Thofe animals belong to threenbsp;different orders of fifh j and the few particulars,nbsp;which they feem to have in common, are the powernbsp;of giving the fhock; an organ in their bodies, callednbsp;the eletlrk organ, which is in all probability employed by thofe animals for the exertion of thatnbsp;power j a fmooth fkin without fcales; and fomenbsp;fpots here and there on the furfaces of their bodies.

The torpedo, which belongs to the order of rays, is a flat fifh, very feldom twenty inches long, weighing not above a few pounds when full grown, and is

c G 2 nbsp;nbsp;nbsp;pretty

pretty common in various parts of the fea-coaif of Europe. The eleftric organs of this animal are two-in number, and are placed �ne on each fide of thenbsp;cranium and gills, reaching from that place as farnbsp;tis the femicircular cartilages of each great fin, and'nbsp;extending longitudinally from the anterior extremity of the animal to the tranfverfe cartilage whichnbsp;divides the thorax from the abdomen. In thofenbsp;places they fill up the whole thicknefs of the animalnbsp;from the lower to the upper furface, and are covered by the common fkin of the body, undernbsp;which, however, are two thin membranes or fajcia.nbsp;The length of each organ is fomewhat lefs thannbsp;ene-third part of the whole length of the animal.nbsp;Each organ confifts of perpendicular columns,nbsp;reaching from the under to the upper furface of thenbsp;body, and varying in length according to the variousnbsp;thicknefs of the fifh in various parts. The numbernbsp;of thofe columns is not conftant, differing in different torpedos, and likewife in different ages of thenbsp;animals. In a very large torpedo, one eledlricnbsp;organ was found to confift of 1182 columns. Thenbsp;oreateft number of thofe columns are either Irregu-lar hexagons, or irregular pentagons, but their fi-gtn-e is by no means conftant. Their diameters-are generally equal to one-fifth part of an inch¹.

For farther particulars, fee Hunter�s Anatomical Ob-fei vations on the Terpedo, Phil. Tranf, vol. 63.

The above-mentioned eleftric organs feem to be the only parts employed to produce the fhock* ;nbsp;the reft of the animal appearing to be merely thenbsp;condudtor of that Ihock, as parts adjacent to thenbsp;^le�tric organs; andj in faff, the animal has beennbsp;found to be a conduftor of artificial eleftricity.nbsp;The two great lateral fins, which bound the electricnbsp;organs laterally, are the beft conduftors.

If the Torpedo, whilft ftanding in water^ or out of the water, but not infulated, be touched with onenbsp;hand, it generally communicates a trembling motion or flight (hock to the hand but this fenfationnbsp;is felt in the fingers of that hand only. If the torpedo be touched with both hands at the fame time,nbsp;one hand being applied to its under, and the othernbsp;to its upper, furface, a fhock in that cafe will benbsp;received, which is exaftly like that which is occa-fioned by the Leyden phial. When the handnbsp;touches the fifh on its oppofite furfaces, and juftnbsp;over the eleftric organs, then the (hock is thenbsp;ftrongeft ; but if the hands be placed upon othernbsp;parts of the oppofite furfaces-, the fhocks are fome-what Weaker j and no Ihock at all is felt when thenbsp;hands are both placed upon the eleflric organs of thenbsp;fame furface; which fliews that the upper and low'er

* The manner in which the ekaric fluid is accumulated or generated by thofe organs, is by no means underftood, butnbsp;the fubjed of the next chapter may probably throw m

light upon it. nbsp;nbsp;nbsp;furfaces

furfaces of the eleftric organs are in oppofite ftates of elecStricity, anfwering to the plus and minus fidesnbsp;of a Leyden phial. When the fifh is touched bynbsp;both hands on the fame furface, and the hands arenbsp;not placed exaftly on the el.e^lric organs, a Ihock,nbsp;though weak, is ftill received; but in this cafe thenbsp;pppofite power of the other furface of the animalnbsp;feems to be condtiiled over the (kin.

The (hock which is given by the torpedo, when (landing in air, is about four times as (Irongas whennbsp;Handing in water gt; and when the animal is touchednbsp;on both furfaces by the fame hand, the thumb being applied to one furface, and the middle^fingernbsp;to the oppofite furface, the (hock is felt muchnbsp;ftronger than when the circuit is formed by the application of both hands. Sonjetimes the torpedqnbsp;gives the (hocks fo quickly one after the other,nbsp;that fcarcely two feconds elapfe between them i andnbsp;�when, inftead of a ftrong determinate (hock, it communicates only a iorpor, that fenfation is naturallynbsp;attributed to the fuccelTive and quick difcharge ofnbsp;a great many confecutive (hocks.

This power of the torpedo is condudled by the fame fubftances which conduft artificial eledricity,nbsp;and is intercepted by the fame fubftances which arenbsp;non-coridu6tors of eledricity; hence, if the animal,nbsp;inftead of being touched immediately by the hands,nbsp;be touched by non-eleftrics, as wires, wet cords,nbsp;amp;c. held in the hands of the experimenter, thenbsp;(laock will be communicated through them. The

circuit may alfo be formed by feveral perfons joining hands, and the fiiock will be felt by them all at tbe fame time. when the animal is in water,nbsp;the hands be put in the fame water, a ftiock will alfonbsp;be felt, which will be ftronger if one of the handsnbsp;touch the filh, whilft the other is kept in the waternbsp;at a diftance from it. In fliort, the fliock of thisnbsp;animal is conducted by the fame conductors as thatnbsp;of the Leyden phial �, thus it may pafs through morenbsp;than one circuit at the fame time i or the circuitnbsp;may be much extended, amp;c. but ip thofe cafes thenbsp;fhock is much weakened.

The fhock of the torpedo cannot pafs through ;he leaft interruption of continuity : thus it wiil notnbsp;be conducted by a chain, nor will it pals throughnbsp;the air from one conductor to the other, when thenbsp;diftance is even lefs than the aooch part of an inch;nbsp;confequently no fpark was ever obferyed to accomrnbsp;pany it.

No eleCtric attraction or repulfion was ever ob-ferved to be produced by the torpedo; nor indeed by any of the eleClric fifties, thougli feveral expernbsp;riments have been inftituted exprefsly for that pur-pofe.

Thefe ft^ocks of the torpedo feem to depend on the will of the anirhal; for each effort is accompanied with a depreflion of its eyes, by which evennbsp;his attempts to give it to non-conduCtors, may benbsp;obfcrved. It is not known whether both eleCtric organs muft always aCt together, or one of them only^

p lt;3 4 nbsp;nbsp;nbsp;may

Aimoft all thofe effefls of the torpedo may be imitated by means of a large eledrical battery weakly charged^.

The gymnotus eledricus has been frequently Called eleElrical eel, on account of its bearing fornenbsp;refemblance to the common eel. The gymnotus isnbsp;found pretty frequently in the great rivers of Southnbsp;America. Its ufual length is about three feet} butnbsp;fomie of them have been faid to be fo large as to benbsp;able to ftrike a man dea'd with their eleftric fhock.nbsp;A few of thofe animals, about three feet long, werenbsp;brought alive to England about thirty years ago,nbsp;and a great many experiments were made withnbsp;them.

A gymnotus of three feet in length generally is between 10 and 14 inches in circumference at thenbsp;thickeft part of its body. The ele�tric power ofnbsp;this animal being much greater than that of thenbsp;torpedo, its e}edric organs are accordingly a greatnbsp;deal larger, and indeed that part of its body whichnbsp;contains moft of the animal parts that are commonnbsp;to the fame order of hikes, is conhderably fmallernbsp;than that which is fubfervient to the eledric power,nbsp;though the latter muft naturally derive nouriflimentnbsp;and adion from the former. The head of the

� See Mr. Walfli�s Paper in the 6;jd volume of thePhi-lofophical Tranfudions.

animal is large, broad, flat, Imooth, and imprefled with various fmall holes. The mouth is rathernbsp;large, but the jaws have no teeth, fo that the animalnbsp;lives by fuftion, or by fwallowing the food entire.nbsp;The eyes are fmall, flattilh, and of a bluilh colour,nbsp;placed a little way behind the noftrils. The bodynbsp;is large, thick, and roundifh, for a confiderablenbsp;diftance from the head, and then diminilhes gradually. The whole body, from a few inches belownbsp;the head, is diftinguiibed into four longitudinal parts,nbsp;clearly divided from each other by lines. The Carina begins a few inches below the head, and widening as it proceeds, reaches as far as the tail, wherenbsp;it is thinneft. It has two pedtoral fins, and thenbsp;amis is fituated on the under part, more forward thannbsp;ihofe fins, and of courfe not far diftant from thenbsp;r of rum.

This animal has two pairs of eleftric organs, one pair being larger than the other, and occupyingnbsp;moft of the longitudinal parts of the body. Theynbsp;are divided from each other by peculiar membranes¹.

The nerves which go to the eledlric organs of the gymnotus, as well as of the torpedo, are muchnbsp;larger than thofe which fupply any other part of

See Hunter�s Account of the Gymnotus Ele�lncus, i.. the 65th Volume of the Philofophical Tranfadlions, for farther Particulars. Alfo my Treatife on Ele�lricity, 4thnbsp;Jidition, Vol, II, Appendix, N� VII�

the body. The eleftric organs of the gymnotus ar0 fupplied with nerves from the fpinal marrow, andnbsp;they come out in pairs between the vertebrae of tirenbsp;fpine.

The gyrnnotus poffefies all the eleftric properties of the torpedo, but in a fuperior degree. His fiiock is cortdufted by condudfors of eledtricity ; itnbsp;is communicated through water, amp;c. The ftrongeftnbsp;Ihock is received .when, the animal ftanding out ofnbsp;the water, you apply one hand towards the tail, andnbsp;the other towards the head of the animal. In thisnbsp;manner I often received Ihocks fi om one of tholenbsp;animals, which I felt not only in my arms, but verynbsp;forcibly even in my cheft. If the animal be touched with one hand only, then a kind of tremor is feltnbsp;i�i th-at fingle hand, which, though ftronger, is,nbsp;however, perfedtly analogous to that which is givennbsp;by the torpedo when touched in the like manner.

Tills pow.�r of the gyrnnotus is likewife depending on the will of the animal, fo that fometimes he gives ftrong fliocks, and at otlier times very weaknbsp;ones. He gives the ftrongeft lliocks when provokednbsp;by being frequently and roughly touched.

When fmall fiflies are put into the water, where the gyrnnotus is, they are frequently ftunned, andnbsp;arc either eft'edlually or apparently killed.

The ftrongeft Ihocks of the gymnoti, which were exhibited in London, would pafs through a verynbsp;ftiort interruption of continuity iq the circuit. Theynbsp;could be conveyed by a fhort chain when ftretched�

fo as to bring the links into a more perfedt contadt, When the interruption was formed by the incifionnbsp;made with a pen-knife on a flip of tin-foil that wasnbsp;palled upon glafs, the fhock in paffing throughnbsp;that interruption, (hewed a fmall but vivid fpark,nbsp;plainly vifible in a dark room.

This animal fliewed a peculiar property, namely that of knowing when he could, and when he couldnbsp;not, give the (hock; for if non-condudlors or Interrupted circuits were placed in the water, he wouldnbsp;not approach them j but as foon as the circuit wasnbsp;completed, he would approach the extremities ofnbsp;that circuit, and immediately give the (hock¹.

The third fifli which is known to have the power of giving the (hock, is found in the rivers of Africa;nbsp;but we have a very imperfedl account of its properties �}�.

This animal belongs to the order which the na-turalifls call ftlurus; hence its name is filurus elec-tricus. The length of fome of thofe fifhes have been found to exceed ao inches.

The body of the filurus eleftricus is oblong, fmooth, and without fcales j being rather large, and

t Meflrs, Adanfon and Forfkal make a fhort mention of it; and Mr. Bruffonet deferibes it under the French namenbsp;of le Trembleur^ in the Hif. de 1' Acad. Royale des Sciences,nbsp;fgr the yeai 1782,

flattenecj towards its anterior part. The eyes are of a middle fize, and are covered by the fllt;in, which envelopes the whole head. Each jaw is armed with anbsp;great number of fmall teeth. About the mouth itnbsp;has fix filamentous appendices, viz. four from thenbsp;under lip, and two from the upper ; the two external ones, or farthermoft from the mouth on thenbsp;upper lip, are the longeft. The colour of the bodynbsp;is greyifh, and towards the tail it has fome blackilhnbsp;fpots.

The eledtric organ feerhs to be towards the tail, where the fkin is thicker than on the reft of thenbsp;body, and a whitifti fibrous fubftance, which is probably the eledlric organ, has been diftinguiflied under it.

It is faid that the filurus elecftricus has the property ofquot; giving a Ihock or benumbing fenfation, like the torpedo, and that this fliock is communicated through fubftances that arc conduftors ofnbsp;cledricity. No other particular feems to be knownnbsp;concerning it.

Nature feems to have given thofe fifties this fin-gular power of giving the ftiock for the purpofe of fccuring their prey, by which they muft fubfifti andnbsp;perhaps iikewife for the purpofe of repelling largernbsp;animals, v/hich might otherwife annoy them.

1'he ancients confidered the fliocks of the torpedo as capable of curing various diforders; and a modern philofopher will hardly hefitate to credit their

Of Animal EkEricity. nbsp;nbsp;nbsp;4*^^

affertion�s, fmce ele�tricity has been found to be aJ tifeful remedy in feveral cafes.

A fourth fifli, faid to give (hocks like the above-mentioned, was found on the coaft of Johanna, one of the Comoro iflands, in lat. 12� if fouth, bynbsp;Lieutenant William Pateifon, and an imperfe(a account of it is given in the 76th volume of the Phf-lofophical Tranfactions.

� The fifla is defcribed to be 7 inches long, 1 � � inches broad, has a long prqjedli'ng mouth, and

feems of the genus Tetrodon. The back of the � fifh is a dark brown colour, the belly part of fea-� green, the fides yellow, and the fins and tail of anbsp;� fandy green. The body is interfperfed with red,nbsp;� green, and white fpots, the white ones particularlynbsp;quot; bright; the eyes large, the iris red, its outer edgenbsp;quot; tinged with yellow.�'

Whilft this fiih is living, ftrong (hocks, like electrical (hocks, are felt by a perfon who attemptsnbsp;to hold it between his hands. Three perfons onlynbsp;are mentioned in the account as having experiencednbsp;this property of one of thofe fi(hes ; but the want ofnbsp;opportunity prevented the trial of farther ^peri-ments.

IN the year 1791, a very remarhable difcoverv made by Dr. Galvani of Bologna was announcednbsp;to the Icient'ific world in a publication entitled, Jloyfiinbsp;Galvani �e Virihus EleUruitatis in mstu mujcularinbsp;Ccmmentarius. Bononice 1791.

The dlfcoveries of Dr. Galvani w�ere made principally with dead frogs. He in the firft place dif-covered that a frog dead and llcinned, is capable of having its mufcles brought into adlion by means ofnbsp;eledtricity, even in exceedingly fmall quantities.

Secondly, that independant of any apparent electricity, the fame motions may be produced in the dead animal, or even in a detached limb, merelynbsp;by making a communication between the nervesnbsp;and the mufcles, with fubftances that are condudlorsnbsp;of eleftricity. If the circuit of comm.unication con-fift of non-conduclors of eledlricity, as gUfs, fealing-xvax, and the like, no motion will take place.�Thenbsp;like experiments were alfo fuccefsfully inftituted

Be inherent in the animal parts, thofe experiments, or the power which produces the motion of fhenbsp;mufcles in thofe experiments, was denominated animal ele��ricity: But it being now fully afcertained,nbsp;that by the mere contadl of metallic and other conducing fubllances, fome eleftricity is generated ¹, itnbsp;is evident that the mufcular motions in the above-mentioned experiments are produced by that electricity j hence we have confined the name cf animalnbsp;eleSiricity to denote the power of the fiflres whichnbsp;give the fliock, amp;c. as deferibed in the preceding chapter. And, at leaft for the pi elcnt, \vcnbsp;flaall examine the eleftricity vdiich is produced Irynbsp;the contaft, or by the aCion, of mietallic and other'nbsp;conducing fubftances upon each other, under thenbsp;title oi Galvayiijm \ though in truth Galvanl�s dif-coveries go no farther than wliat relates to certain,nbsp;effeCs of the contaC of animal parts principallynbsp;with metallic fubftances.�I fliall briefly deferibenbsp;the principal faCs which relate to the above-mentioned fort of mufcular motion, and ftiall then proceed to thofe w'hich relate to the wonderful effeCsnbsp;of the mere contaC or aCion of one conducing fub-ftance upon another, amongft which the metallic arcnbsp;the moft confpicuous.

See Rennet�s Nnv E .perinients on EleSiricity^ 17^9gt; and my Treatife on EleCricity, 4th Edition, vol. III. p. 111,nbsp;and

and {kinned, (and indeed on other animals more or lell)) occafions a tremulous motion of the mufcles,nbsp;and generally an extenfion of the limbs.

Dr. Galvani ufed to Ikin the legs of a frog recently dead, and to leave them attached to a fmall part of the fpine, but feparated from the reft ofnbsp;the body.�Any other limb may be prepared in anbsp;flinilar manner; viz, the limb is deprived of its'nbsp;integuments', and the nerve, which belongs to it,nbsp;is partly laid bare.

If the limbs thus prepared, for inftance, the legs of a frog, be fituated fo that a little electricity maynbsp;pafs through them, be it by the immediate conta�tnbsp;of an eledlrified body, or by the adlion of eledlricnbsp;atrnofpheres (as when the preparation is placednbsp;within a certain drftance of an eledlrical machine,nbsp;and a fpark is taken' from the prime conduftor); thenbsp;prepared legs will be inftantly affecfted with a kindnbsp;of fpafmodic contraction, fometimes fo ftrong as tonbsp;jump a confiderable w'ay.

When the electricity is caufed to pafs through the prepared frog by the immediate contact of the electrified body, a much fmaller quantity of it is fufficientnbsp;to occafion the m,ovements, than when it is made tonbsp;pafs from one conductor to another, at a certainnbsp;diftance from the prepared animal¹.

Probably the lOOth part of that electricity which can affeit a very delicate electrometer, is fufficient to producenbsp;the movement of the prepared animal limb, and even ot ^nbsp;v/ho!e frog, or moufe, or fparrow, fic.

The movements are much flronger when the eleftricity is caufed to pafs through a nerve to thenbsp;mufcle or mufcles, than through any other part.

The fenfibility of the prepared animal is greateft at firfl, but it 'diminifhes by degrees till it vanifhesnbsp;entirely. Animals with cold blood, and efpeciallynbsp;frogs, retain that fenfibility for feveral hours, fome-times even for a day or two. With other animalsnbsp;the fenfibility does not laft long after death, andnbsp;Ibmetimes not above a few minutes.

The like movements may be produced in the prepared animal without the aid of any apparentnbsp;eleftricity. In an animal recently dead^ detach onenbsp;end of a nerve from the furrounding parts, takingnbsp;care to cut it not too near its infertion into thenbsp;mulcle; remove the integuments from over thenbsp;mufcles which depend on that nerve j take a piecenbsp;of metal, as a wire, touch the nerve with one extremity of it, and the mufcles with its other extremity;nbsp;on doing which you will find that the prepared limbsnbsp;move in the fame manner as when fome eleftricitynbsp;is paired through them. This however is not thenbsp;moft elFeflual way of forming the communication;nbsp;yet it will generally fucceed, and the experiment willnbsp;anfwer whether the preparation be laid upon con-dudtors Or upon eleftrics.

If the communication between the nerve and the mufcle be formed by the interpofition of non-con-dudors of eleftricity, fuch as glafs, fealing-wax, amp;c.nbsp;then no movements will take place.

When the application of the metal or metals is VOL. III.nbsp;nbsp;nbsp;nbsp;H Hnbsp;nbsp;nbsp;nbsp;continued

continued upon the parts, the contraaionswlll ceafe after a certain time, and on removing the metal,nbsp;feldom, if ever, any contraftion is obferved.

The conducing communication between the mufcle and the nerve may confift of one or morenbsp;pieces, and of the fame or, much better, of differentnbsp;bodies connefted together, as metals, water, a number of perfons, and even wood, the floor of a room.nbsp;See* But it muft be obferved, that the lefs perfeftnbsp;conduflors will anfwer only at firft, when the prepared animal is vigorous; but when'the power begins to diminifh, then the more perfect conduiftorsnbsp;only will anfwer, and even thefe will produce various effefts.

The moft effectual way of producing thofe movements in prepared animal parts is by thenbsp;application of two metals, of which filver and zincnbsp;feem upon the whole to be the belt, though filvernbsp;and tin, or copper and zinc, and other combinations, are not much inferior. If part of thenbsp;nerve proceeding from a prepared limb be wrappednbsp;up in a bit of tin-foil, or be only laid upon zinc, andnbsp;a piece of filv'^er be laid with one end upon the barenbsp;mufcle, and with the other upon the above-mentioned tin or zinc, the motion of the prepared limb wil^nbsp;be very vigorous. The two metals may be placednbsp;not in contaft with the preparation, but in any other

* The various bodies, which form this circuit, muft be placed in full and perfedl conta�t with each other, which isnbsp;done by prelHng them againft each other, or by the interpoft-^nbsp;don of water, amp;c.

part of the circuit, which may be compleated by means of other condu�lors, as water, Src.

Separate with a pair of fciflars the head and upper extremities of a frog from the reft of the body.nbsp;Open the integuments and mufcles of the abdomen,nbsp;and remove the entrails, by which means you willnbsp;lay bare the crural nerves. Then pafs one blade ofnbsp;the fciflars under the nerve, and cut off the Ipinenbsp;with the flefli clofe to the thighs, by which meansnbsp;the legs will remain attached to the fpine by thenbsp;nerves alone. This done, leave a fmall bit only ofnbsp;the fpine attached to the crural nerves, and cut offnbsp;all the reft. Thus you will have the lower limbsnbsp;G, H, fig. I, Plate XXIV. of the frog adhering tonbsp;the bit of fpine, A B, by means of the crural nervesnbsp;C, D. Thefe legs muft be flayed in order to laynbsp;bare the mufcles ; and a bit of tin-foil fhould benbsp;wrapped round the fpine A B. With this preparation the experiment may be performed various ways,nbsp;but the two which follow are the beft.

Hold the preparation by the extremity of one leg, the other leg hanging down, with the armed bundlenbsp;of nerves and fpine laying upon it. In this fituationnbsp;interpofe a piece of filver, as a half-crown, betweennbsp;the lower thigh and the nerves, fo that it may touchnbsp;the former with one furface, and the metallic coating of the latter with the other furface, or with itsnbsp;edge; and you will find that the hanging leg willnbsp;vibrate very powerfully, fometimes fo far as to ftrike

Otherwife, place two wine glalTes, both full of water, contiguous to each other, but not aftuallynbsp;touching. Put the thighs and legs of the preparation in the water of one glafs, and laying the nervesnbsp;over the edges of the two glaffes, let the bit of fpinenbsp;with its armour (viz, tin-foil) touch the water ofnbsp;the other glals. Things being thus prepared, if younbsp;form the communication between the waters of thenbsp;two glaffes, by means of filver, or put the fingers ofnbsp;one hand into the water of the glafs that contains thenbsp;legs, and holding a piece of filver in the other, younbsp;touch the coating of the nerves with it, you willnbsp;find that the prepared legs move fo powerfully a�nbsp;fometimes to jump fairly out of the glafs.

By the application of armours of different metallic fubftances, and forming a communication betweennbsp;them, the motions may be excited even in an entirenbsp;living frog, as alfo in fome other living animals,nbsp;particularly eels and flounders. The living frog isnbsp;placed upon a piece of zinc, with a flip of tin foilnbsp;pafted upon its back. This done, whenever thenbsp;communication is formed between that zinc and thenbsp;tin-foil, efpecially if filver be ufed, the fpafmodicnbsp;convulfions are excited, not only in the mufclesnbsp;which touch the metallic fubflances, but likewife innbsp;the neighbouring mufcles. This experiment maynbsp;be performed entirely under water.

The experiment may be performed with a flounder in a fimilar, eafy, and harmlefs manner. Take

Of Galvanifm, nbsp;nbsp;nbsp;469

a living flounder, fuch as may almofl: always be found at the fifhmonger�s; wipe it pretty dry, and lay it flatnbsp;into a pewter plate, or upon a fheet of tin-foil, andnbsp;place a piece of filver, as a fhilling, a crown piece,nbsp;amp;c. upon the fifh. Then, by means of a piece ofnbsp;metal, complete the communication between thenbsp;pewter plate or tin-foil and the filver piqce; on doingnbsp;which the animal will give evident tokens of beingnbsp;affedled. The filh may afterwards be replaced innbsp;water, and preferved for farther ufe.

It feems that fuch movements may be excited by the contaft of metallic fubftances in all the animals *nbsp;at leafl: they have fucceeded, but in different degrees,nbsp;in a great variety of animals, from the ox to the fly.

The human body, whilft undergoing certain chi-rurgical operations, or its amputated limbs, have been convulfcd by the application of metals. Butnbsp;the living animal body may be rendered fenfible ofnbsp;the aftion of metallic application in an harmleftnbsp;way, and both the fenfes of tafte and of fight maynbsp;be affedled by it, but in different degrees accordingnbsp;to the various conftitutions of individuals.

Let a man lay a piece of metal upon his tongue, and a piece of fome other metal under the tongue jnbsp;on forming the communication between thofe twonbsp;metals, either by bringing their outer edges in con-tadt, or by the interpofition of fome other piece ofnbsp;metal, he will perceive a peculiar fenfation, a kindnbsp;of irritation, accompanied with a fort of cool andnbsp;fubacid tafte, not exadlly like, and yet not muchnbsp;different from that which is produced by artificialnbsp;H H 3nbsp;nbsp;nbsp;nbsp;eledricity.

eleftricity. The metals which anfwer beft for thefe experimentSj are filver and zinc, or gold and zinc.nbsp;The fenfation feems to be more diftinft when thenbsp;metals are of the ufual temperature of the tongue.nbsp;The filver or gold may be applied to any othernbsp;part of the mouth, to the noftrils, to the ear, ornbsp;to other fenfible parts of the body, whilft the zincnbsp;is applied to the tongue j and on making thenbsp;communication between the tvyo metals, the taftenbsp;will be perceived upon the tongue. The efiefl:nbsp;is rather more remarkable when the zinc touchesnbsp;the tongue in a fmall part, and the filver innbsp;a great portion of its furface, than vice v�rfa. In-ftead of the tongue, the two metals may alfo benbsp;placed in conta�l with the roof of the mouth, as farnbsp;back as poffible ; and on compleating the communication, the tafte or irritation will be perceived.

Different perfons are varioufly affefted by this application of metals j with fome the fenfation or tafte is fo flight as to be hardly perceived, whilft withnbsp;others it is very ftrong and even difagreeable. Somenbsp;perfons feel merely a pungency, and not properly anbsp;tafte.

In order to affed the fenfe of fight by means of metals, let a man in a dark place put a flip of tin-foilnbsp;upon the bulb of one of his eyes, and let him put anbsp;piece of filver, as a fpoon or the like, in his mouth.nbsp;On completing the communication between thenbsp;fpoon and the tin-foil, a faint flafti of white light willnbsp;appear before his eyes, This experiment may be

Of Gahmijm. � nbsp;nbsp;nbsp;47*

performed in a more convenient mannfer, by placing a piece of zinc between the upper lip and the gums,^nbsp;as high up as poffible, and a filver piece of moneynbsp;upon the tongue ; or elfe by putting a piece of filvernbsp;high up in one of the noftrils, and a piece of zinc innbsp;conta�l with the upper part of the tongue; for innbsp;either cafe the flalh of light will appear whenevernbsp;the two metals are made to communicate, either bynbsp;the immediate contaft of their edges, or by the in-terpofition of other good conduftors.

By continuing the contaft of the two metals, the appearance of light is not continued, it being onlynbsp;vifible at the moment of making the conta�t, andnbsp;ibmetimes, though rarely, at the inftant of fepara-tion: it may therefore be repeated at pleafure, bynbsp;disjoining, and again connedling, the two metals.nbsp;When the eyes are in a ftate of inflammation, thennbsp;the appearance of light is much flrronger.

When the fcience of eledlricity was advanced no farther than the knowledge of the above-mentionednbsp;fafts, it was doubtful whether the convulfions ofnbsp;prepared animal limbs, and the fenfations whichnbsp;are produced by the application of metallic fub-ftances, were owing to fome eleftrical propertynbsp;peculiar to the animal parts, which might perhaps be conduced through the metals from onenbsp;part to the other; or to a fmall quantity ofnbsp;eledricity, which might be fupplied by the metalsnbsp;themfelves. The latter fuppofition however wasnbsp;foon verified by the refult of various experiments,

H H 4 nbsp;nbsp;nbsp;which

which prove in the moll convincing manner that eleftricity is produced by the mere contaft, notnbsp;only of metallic fubftances, but likevvife of othernbsp;bodies.

The elelt;3;rlcity thus produced by the mere con-taft of two bodies' is fo very fmall as not to be perceived without great care, and without tiling fome of thofe artifices for diicovering fmall quantities ofnbsp;eledricity, which have been mentioned above. Butnbsp;the late difcoveries of the ingenious Mr. Volta havenbsp;fhewn a method of increafing that eleftricity to anbsp;moll extraordinary degree *; by which means thenbsp;fubjefc of eleftricity has received a remarkable advancement, and has opened a moft promifingnbsp;field of wonders, wherein numerous and able labourers are daily making ufeful and admirable dif-coveries.� We fiiall now proceed to ftate thofe fa�lsnbsp;in as compendious a manner as the nature of thbnbsp;fubje�l will admit of, confiftently with perlpicuity. *

The adion of metallic fubftances upon the organs of living, or of recently dead animals, has been fully nianifefted by the above-mentioned difcoveries of Galvani and others j but, previous to thofenbsp;difcoveries, a variety of fafls, frequently afferted,nbsp;imperfeftly known, and often dilbelieved, indicatednbsp;a peculiar aftion arifmg from a combination of different metallic bodies in certain cafes.

? See his Paper in the Philofophical Tranfa�tions for the year 1800, Art. XVII,

It had been long aflerted, that when porter (and Ibme other liquors alfo) is drank out of a pewter ^nbsp;pot, it has a tafte different from what it has whennbsp;drank out of glafs or earthen ware.

It has been obferved, that pure mercury retains its metallic fplendor during a long time ; but itsnbsp;amalgam with any other metal is foon tarnifhed ornbsp;oxidated.

The Etrufcan inferiptions, engraved upon pure lead, are preferved to this day; whereas fome medalsnbsp;of lead and tin, of no great antiquity, are much corroded.�Works of metal, whofe parts are folderednbsp;together by the interpofition. of other metals, foonnbsp;tarnifh about the places where the different metalsnbsp;are joined.

When the copper fheeting of fhips is faftened on by^ means of iron nails, thofe nails, but particularlynbsp;the copper, are readily corroded about the place ofnbsp;contadt*.

It had been obferved, that a piece of zinc might be kept in water for a confiderable time, withoutnbsp;hardly oxidating at all; but that the oxidationnbsp;would foon take place if a piece of filver happenednbsp;to touch the zinc, whilft {landing in water.

Since Galvani�s difcoveries, the action ariling from the combination of three condudlors has been

� See Fabroni�s Paper on the Addon of different Metals upon each other, in the 40th Number of Nicholfon�snbsp;Journal.

examined

examined with great care, and with confiderablc fuccefs, efpecialiy by Mr. Volta, who lately difco-vered that the flight elfedl of fuch a combinationnbsp;may be increafed to a prodigious degree by repeating the combination j for inftance, if a combinationnbsp;of filver, zinc^ and water, produce a certain efi�ft,nbsp;a fecond combination (viz. another piece of filver,nbsp;another piece of zinc, and another quantity of water)nbsp;added to the fitft, will increafe the ef�eft; 'the addition of a'third combination will increafe the eftedtnbsp;iVill more, and fo on¹.

Previous to the defcription of the conftrudion, and of the very remarkable elFeds of thofe repeatednbsp;combinations, which are now generally called Galvanic batteries (though in juftice they muft be callednbsp;Volta's batteries-, or Voltaic batteries) it will be ne-ceflfary to ftate the principal laws, which have beennbsp;pretty well afcertained with refpeft to the Amplenbsp;combinations.

I. The conduclors of eleAricity, which, llriAly fpeaking, do almofl: all differ from each other innbsp;conduAing power, are neverthelefs divided into twonbsp;principal clafles. Thofe of the firfl clafs, otherwifcnbsp;called dry and perfeSl conduAors, are the metallicnbsp;fubftances and charcoal. Thofe of the fecond clafs,nbsp;or the imperfeSl conduSlors, are water and other oxidating fluids, as alfo the fubftances which contain

thofe fluids. But as the fubftances of the fecond clafs differ in conducting power mucli more thannbsp;thofe of the firff: clafs, fo they may be fubdividednbsp;into fpecies *.

JI. The fimpleft combinations capable of producing Galvanic efi��ls, (viz. to convulfe the prepared limbs of a frog, or of exciting the tafte upon the tongue, amp;c.) muff: confift of three different conductors ; for, two conductors only will not producenbsp;any fenfible effeCt. If the three conductors be allnbsp;of the firft clafs, or all of the fecond, then the effeCtnbsp;is feldom fenfible. In this cafe fuch conductors otnbsp;the fecond clafs as differ more from each other, arenbsp;more likely to produce a fenfible effeCt than thofe of

* Mr. Volta arranges thofe fubftances in the following order, commencing with the leaft aClive; obferving, however, that this order is fubjetft to a confiderable deviation,nbsp;efpeclally with refpeCt to the latter fpecies, and according asnbsp;they are combined with certain bodies of the firft clafs.

ing in folution common air, and efpecially oxigen air, is much more aClive than water deprived of air by boiling ornbsp;otherwife.') � 2. Water mixed with clay or chalk ; 3. A fo-� lution offugar; 4. Alcohol; 5. Milk; 6. Mucilaginousnbsp;� fluids; y. Animal gelatinous fluids; 8. Wine; 9. Vine-� gar and other vegetable juices and acids; 10. Saliva;nbsp;� II. Mucus from the nofe ; 12. Blood; 13. Brains;nbsp;� Solution of fait; 15. Soapfuds; 16. Chalk water ;nbsp;** I7� Cuticentrated mineral acids ; iS- Strong alkalinenbsp;� leys; 19, Alkaline fluids; 20. Livers of fulphur.�

the

the firft; clafs *. But a proper adive fimple combination muft confift of three different bodies j viz. of one condudor, of one clafs, and two different con-dudors of the other clafs. Thus (denoting thenbsp;bodies of the firft clafs by means of large capitalnbsp;letters, and thofeof the fecond clafs by fmall letters)nbsp;the combinations of fig. 2, and 3, Plate XXIV.nbsp;are adivej but thofe of fig, 4, 5, 6, 7, and 3, arenbsp;not adive, becaufe that of fig. 4, 5, or 6, con-fifts of two bodies only, and that of fig. 7, or 8,nbsp;confifts of three bodies, of which two are of thenbsp;fame fort, and of courfe ad as a fingle body.

When two of the three bodies are of the firft clafs, and one is of the fecond, the combination is faid tonbsp;be of the firft order ; otherwife it is faid to be of thenbsp;fecond order.

In a fingle adive Galvanic combination, or, as it is . commonly called, in a fmple Galvanic circle, the twonbsp;bodies of one clafs muft touch each other in one or

* Mr. Volta adduces as an inftance of an adlive Galvanic combination, conffting of three condudors of the fecondnbsp;clafs only, an experiment of Dr. Valli, in which the threenbsp;bodies concerned were, ift, 1 he leg of a frog, and particularly the hard tendinous part of the mufculus gaftroenemius \nbsp;aJ. The rump, or the mufcles of the back, or the ifchiaticnbsp;nerves, to which the faid tendinous parts are applied j andnbsp;3d. The blood or the vifeous fapoiiaceous or faline fluid,nbsp;applied to the point of contad. See his letter ro Gren in,nbsp;the Neiives Journal des Phyf vol. III. p. 4, and vol. IV-page I.

more points, at the fame time that they are con-nc�i;ed together at other points by the body of the ocher clafs. Thus, when a prepared frog is con-vulfed by the contaft of the fame piece of metal innbsp;two different places ; then the fluids of thofe parts,nbsp;which mufl be fomewh^t different from each other,nbsp;are the two condaftors of the fecond clafs, and thenbsp;metal is the third body, or the condudtor of the firfl;nbsp;clafs. If two rpetals be ufed, then the fluids of thenbsp;prepared animal, differing but little from eachnbsp;other, may be confidered as one body of the fecondnbsp;clafs. Thus alfo, when a perfon drinks out of anbsp;pewter mug, the faliva or moifture of his undernbsp;lip Is one fluid or one conduftor of the fecond clafs,nbsp;the liquor in the mug is the other, and the metal isnbsp;the third body, or conduftor, of the firfl; clafs.

III. It feems to be indifpenfably requifite, that in a fimple Galvanic circle, the conduftor or con-duftors of one clafs fhould have fome chemicalnbsp;aftion upon the other condudor or condudors;nbsp;without which circumftance the combination ofnbsp;three bodies will have either no Galvanic adion atnbsp;all, or a very flight one. Farther, the Galvanicnbsp;adion feems to be proportionate to the degree ofnbsp;chemical agency; which feems to fhew that fuchnbsp;chemical adion is the primary caufe of the eledricnbsp;phenomena *.

The moft a�tive Galvanic circles of the firfl: order, are when two folids of different degrees of oxidability are combined with a fluid capable ofnbsp;oxidating at leaft one of the folids. Thus gold,nbsp;filver, and water, do not form an adlive Galvanicnbsp;circle; but the circle will become adlive if a littlenbsp;nitric acid, or any fluid decompofible by filver, benbsp;mixed with the water.

A combination of zinc, filver, and water, forms an aftive Galvanic circle, and the water is found tonbsp;oxidate the zinc, provided the water holds fome at-mofpherical air, as it commonly does, and efpeciallynbsp;if it contain oxygen air. But zinc, filver, andnbsp;water containing a little nitric acid, form a morenbsp;powerful Galvanic circle, the fluid being capable ofnbsp;adting both upon the zinc and upon the filver.

The molt powerful Galvanic combinations of the fecond order, are when two condudtors of the fecondnbsp;clafs have different chemical adiions on the con-dudtors of the firfl; clafs, at the fame time that theynbsp;have an adtion upon each other. Thus copper, ornbsp;filver, or lead, with a folution of an alkaline fulphu-ret, and diluted nitrous acid, fono a very adlive Galvanic circle.

The prefent ftate of knowledge, relative to this fubjedt, does not enable us accurately to determinenbsp;the peculiar powers of all forts of Galvanic combinations ; however, the following lifts contain a ufefulnbsp;arrangement of the beft combinations, difpofed in thenbsp;2nbsp;nbsp;nbsp;nbsp;order

Galvanic Circles of the Firft Order, viz. which con-fifl of 'two ConduSiors of the Firjt Clefs, and one of the Second.

Zinc with gold, or charcoal, or filver, or copper, �or tin, or iron, or mercury �, and water contaln-taining a fmall quantity of any of the mineralnbsp;acids f.

Iron, with gold, or charcoal, or filver, or copper, or tin, and a weak folution of any of the mineralnbsp;acids, as above.

Tin, with gold, or filver, or charcoal, and a weak folution of any of the mineral acids, as above.

Any of the above metallic combinations, and common water, viz. water containing atmofpherical air, or efpecially water containing oxygen air.

Copper, with gold, or filver, and a folution of nitrate of filver and mercury; or the nitric acid; e'f the acetous acid.

Davy, profelTor of chemiftry at the Royal Inftitution. t Van Marum found a folution of fait ammoniac, viz. of

Galvanic Circles of the Second Order, viz. which confifl of one ConduSlor of the Firji Clefs, and twonbsp;of the Second.

The adion of a fimple Galvanic circle feems to be in fome meafure dependent upon the quantitynbsp;of furface of contaft between the ading bodies.nbsp;An higher temperature within certain limits, renders the adivity of the circle greater than a lowernbsp;temperature.

The adivity of a Galvanic circle is not altered by the interpofition of fuch condudors as have nonbsp;adion upon the adjoining condudors of the circle.nbsp;Thus, if a circle confift of zinc, gold, and water;nbsp;and if you interpofe a piece of iron, or of filver, ornbsp;both, between the zinc and the gold, the adivitynbsp;of the circle will not be altered thereby. Heijce itnbsp;appears that the adion of a Galvanic circle may benbsp;conveyed through extraneous condudors to a con-fiderable diftance; but it muft be obferved, that thenbsp;adivity is weakened by the great length of the condudors, efpecially if they be of an imperfed nature.

Of Gdvani/m, nbsp;nbsp;nbsp;481

�V. When the three bodies which form a Galvanic circle of the firft order are laid one upon the other, but the lower and the upper one do not touchnbsp;each other j then thefe two extremes are in oppofitenbsp;eledlric ftates, viz. the extremity which is next tonbsp;that metallic furface that touches the body of thenbsp;fecond clafs, is pofitive, and the oppolite extremitynbsp;is negative. Thus let copper, zinc, and rnoiftenednbsp;leather, be laid one upon the other; as in hg. p,nbsp;Plate XXIV. and the upper end W, viz. thenbsp;wetted leather, will be found poflefled of pofitivenbsp;elediricity ; whilft the lower end C, or the coppergt;

This is a very delicate experiment, and the ele�lricity Can only be rendered fenfible by means of Volta�s condenfer,nbsp;or of my multiplier. I placed a plate of zinc, 3 inches innbsp;diameter, upon a larger plate of copper, and a piece of leathernbsp;not quite 3 inches in diameter, foaked in common rivernbsp;Water, was laid upon the zinc. Then whilfl the copper plate C, fig. 9j was made to communicate with thenbsp;ground, a wire conneded the leather W, with the receivingnbsp;plate A of the multiplier, fig. 19, Plate XXIII. and bynbsp;working that inftrument after the manner which is defcribcdnbsp;in page 426, it appeared that the moift leather gave pofitivenbsp;eledricity. When the three bodies were reverfed, viz. thenbsp;rnoiftened leather was placed upon the table, and the coppernbsp;was made to communicate with the receiving plate of thenbsp;multiplier, the latter acquired the negative eleftricity. Thisnbsp;was tried repeatedly, and anfwered conft^ntly. Fromnbsp;VOL. in.nbsp;nbsp;nbsp;nbsp;11nbsp;nbsp;nbsp;nbsp;thefe

482 nbsp;nbsp;nbsp;Of Galvanifm.

V. The Galvanic efFefts may be increafed to al-moft any degree, by connefting feveral of the above-mentioned adlive combinations, or by a repetition of the fame fimple Galvanic combination (the moftnbsp;adtive fimple combinations forming the moft power-

thefe experiments, as alfo from the dedudtion which may be fairly made from the effedls of batteries, we may concludenbsp;that every adtive Galvanic combination has a pofitivc and anbsp;negative fide. Hence it is fuppofed, that when the circlenbsp;is completed, as in fig. 10, viz. by connedling the leathernbsp;with the copper, a circulation of eledtric fluid takes placenbsp;through it.

� If we form a metallic plate of two portions, the one of � zinc, the other of copper, by foldering their ends together,

� � and taking the zinc between our fingers, touch with the � copper the upper plate of the condenfer, which is alfonbsp;� of copper, the condenfer becomes negative. But if, on

the contrary, we hold the copper in our fingers, and touch � the upper plate of the condenfer with the zinc j upon re-� moving the metals and raifing the upper plate of the con-� denfer, it indicates no eledtricity, notwithftanding thenbsp;� lower plate is connedied with the common refervoir in 'thenbsp;� earth.

� But as foon as we interpofe between the zinc and the � plate of the condenfer a piece of paper moiftened withnbsp;� pure water, or any other moift condudfor, the condenfernbsp;� becomes charged with pofitive elediricity. It becomesnbsp;� alfo charged, but negatively, when we hold the zinc innbsp;� our fingers, and touch, with the copper, the humid con-� dudlor laid on the condenfer.� Report of the Nationalnbsp;Inftitute at Paris, on Volta�s Experiments made in thenbsp;courfe of the year 1801.

ful batteries, and vice verja) provided the fimple combinations are difpofed fo a� not to counteradnbsp;each other.

Thofe batteries are faid to be of the firft or of the fecond order, according as the fiinple combina¹nbsp;tions, of which they confift, are of the firft or ofnbsp;the fecond order. Thus, if a piece of zinc be laidnbsp;upon a piece of copper, and a piece of moiftencdnbsp;card be laid upon the zinC '; then a ftmilar arrangement of three other fuch pieces be laid upon them,nbsp;and a third arrangement be laid upon this, amp;c. allnbsp;in the lame order; the whole will form a battery ofnbsp;the firft order. But if the arrangement be made bynbsp;conneifting a piece of copper with a piece of clothnbsp;moiftened with water; the latter with a piece ofnbsp;cloth moiftened with a folution of fulphuret of pot-alh, and this again with another piece of copper,nbsp;amp;c. the whole will form a battery of the fecondnbsp;order ¹.

Mr. Davy diftlngulfhes the batteries of the fecond order into the following three claffes; nbsp;nbsp;nbsp;1

I. nbsp;nbsp;nbsp;Themoft feeble is compofed, whenever fingle metallicnbsp;plates, or arcs, are arranged in fuch a manner, that two of theirnbsp;furfaces, or ends oppofite to each other, are in contaift withnbsp;different fluids, one capable and the other incapable of oxidating the metal. And r�gular feries of fuch combinationsnbsp;are formed.

II. nbsp;nbsp;nbsp;When the fingle combinations or elements of thenbsp;feries confift each of a fingle plate or arc of a metallic fub-

The above-mentioned reftri�tion, viz. that the parts of a battery muft not counteraft each other,nbsp;will be eafily underftood by confidering that everynbsp;fimple, but interrupted. Galvanic combination hasnbsp;apofitive and a negative end; or that in every complete Galvanic circle, the eleftric fluid circulates innbsp;one way only. Thus, if two fimple combinationsnbsp;be difpofed, as in fig. il, this arrangement will notnbsp;have any Galvanic power, becaufe the adtions of thenbsp;two fimple combinations, or the two currents ofnbsp;eledbricity, are oppofed to each other; the two po-fltive ends being at p, and the two negative endsnbsp;being at n. But if thofe fix bodies be difpofed as innbsp;fig. 12; then the combination will be very a�live �,nbsp;becaufe, according to the hypothefis, the direftionnbsp;of the ele�tric fluid in each fimple arrangementnbsp;tends the fame way, and probably the one acceleratesnbsp;the other.

fiance capable of afting upon fulphurated hydrogen, or upon fulphurets diffolved in water, is accompanied with portionsnbsp;of a folution of fulphuret of potalh on one fide, and water onnbsp;the other.

III. The moll powerful clafs is formed when metallic fubftances oxidable in acids, and capable of adling on folu-tions of fulphurets, are connedled, as plates, with oxidatingnbsp;fluids and folutions of fulphuret of potalh, in fuch a mannernbsp;that the oppofite fides of every plate may be undergoingnbsp;different chemical changes, the mode of alternation being

What has been'faid above of the arrangement of two Ample Galvanic combinations, muA be likewifenbsp;underftood to hold good with refpedl to the con-peftiojn of any number of the fame j viz. that theynbsp;muft not counteraft each other j or, if a certainnbsp;number of them eountera�l each other, then thenbsp;remaining only form the aftive part of the battery.nbsp;For inftance, if a battery confift of 40 Ample combinations, and if 12 of them are placed in a diredtionnbsp;contrary to the others} then thofe i 2 will counteradb,nbsp;12 others, and of courfe the whole battery willnbsp;have no more power than if it confifted of 16 Ample combinations properly dilpofed.

This points out a method of comparing the powers of two batteries j for if thofe batteries benbsp;connedted in an inverted order, viz. the pofitivenbsp;end of one be made to touch the negative end ofnbsp;the other �, then, on connedting the two other extremities, or on applying them to proper inftru-ments, the whole power will be annihilated, if thenbsp;feparate batteries had equal power} othcrwife thenbsp;power of the whole will be the excefs, of the powernbsp;of the moft powerful battery above that of thenbsp;weakeA ; and the diredlion, viz. its being pofitivenbsp;or negative, will Ihew to which battery it belongs.nbsp;It muft be obferved, with refpedl to the inadtive arrangement of fig. 11, that if one of the feparate bodies Z be removed, then the remaining five bodiesnbsp;will form an adlive combination j for in that cafe,

� I 3 nbsp;nbsp;nbsp;W,

Ic is almoft fuperfluous to obferve, that (as has been faid with refpedt to fimple circles) in a Galvanic battery the interpofition of conductors thatnbsp;have no particular aCtion, or of the conductors ofnbsp;the fame clafs as the adjoining bodies, does not alternbsp;the effect of the battery.

� Thus far we have ftated the general laws, which have been pretty well afeertained with refpeCt tonbsp;Galvanic combinations. We lhall now proceed tonbsp;deferibe the practical conftruClion, and the effeCts ofnbsp;thofe combinations, efpecially of the compound arrangements or batteries.

The fimplicity of Angle Galvanic circles is fo great, that nothing more needs be faid with refpeCtnbsp;to their conftruCtion for when the three bodies arenbsp;feleCled, the operator needs only take care thatnbsp;their contaCl be perfeCl.

Voltaic batteries have been conftruCted of varl-' ous fliapes, and they may be endlefsly diverfified.nbsp;But the molt ufual forms are reprefented by fig. 13,nbsp;14, and 16, Plate XXIV. Thofe of fig. 13 andnbsp;�4, are more eafily conftructed j that of fig. 16, isnbsp;the moft commodious.nbsp;nbsp;nbsp;nbsp;;

The battery, fig. 13, confifts of feveral glaffes, or china c�jps full of water, or of water containingnbsp;fait, amp;c. and two plates unconnected with eachnbsp;other, viz. a plate of zinc and a plate of filver,nbsp;6nbsp;nbsp;nbsp;nbsp;are

are plunged In the fluid of each cup, excepting the nrft and laft cups j but each of thofe plates nnuft havenbsp;a fort of tail or prolongation, by which they arc fonbsp;connefted that the filver plate of one cup communicates with the zinc plate of the next, and fo on; thofenbsp;prolongations being foldered at a, amp;c.

The battery, fig. 14, confifts of pieces of filver, about as big as half crowns, pieces of zinc, aboutnbsp;equal to thofe of filver, and pieces of card, or cloth,nbsp;or leather, or other bibulous fubftance, a littlenbsp;fmaller in diameter than the metallic pieces, andnbsp;foaked in water or in other proper fluid.

Thofe pieces are difpofed in the order of filver, zinc, and wet cloth, amp;c. as indicated bynbsp;the letters S, Z, W. The pieces of card, ornbsp;cloth, amp;c. mull be well foaked in the fluid jnbsp;but before they are applied, they Ihould benbsp;gently fqueezed, in order that the fuperfluousnbsp;fluid may not run down the outfide of thenbsp;pile, or infinuate itfelf between the contiguousnbsp;pieces of filver and zinc. Thofe pieces, efpeciallynbsp;if foaked in plain water, lofe their moifture prettynbsp;fooii, fo that they can hardly ferve longer thannbsp;for a day or two; after which time the pile muftnbsp;be decompofed, the metallic pieces cleaned, thofenbsp;of cloth or card foaked again, and the whole arranged as before.

The three rods R, R, R, are of glafs or of baked wood, and the piece of wood, O, Hides freely up

When fuch battery is to be very powerful, viz. is to confift of numerous pieces, the beft way is tonbsp;form two or three or more piles, and to join themnbsp;by pieces of metal, as c c in fig. 15, where two pilesnbsp;are joined together, fo that a is the negative extremity, and b is the other or pofuive extremity of thenbsp;whole arrangement, or of the two piles confiderednbsp;as one.

The battery, fig. 16, confifts of a ftrong oblong veflel of baked wood, about three inches deep and about as much broad. In the fides of this veflelnbsp;grooves are made oppofite to each other, and aboutnbsp;one-eighth of an inch in depth. In each pair ofnbsp;oppofite grooves a double metallic plate, viz. anbsp;plate of zinc and a plate of filver foldered togethernbsp;at their edges, are cemented j by which means thenbsp;wooden veflel is divided into feveral partitions, ornbsp;cells, about half an inch broad, as is fufficiently indicated by the figure. The cementation of thenbsp;metallic pieces into the fides and the bottom of thenbsp;wooden veflel, m.uft be fo accurate as nor to permitnbsp;the paflage of any fluid from one cell into the next.nbsp;The cement proper for this purpofe is defcribed innbsp;pagejSi.

Thofe cells are afterwards filled almoft to the top with water, or any other fluid, according to the tablenbsp;in page 479 j and thus the whole will form a Voltaic

Of Galvanijm.. nbsp;nbsp;nbsp;489

battery, confifting of various repetitions of filver, zinc, and fluid. Two or more of fuch batteriesnbsp;may be joined, as has been faid of the precedingnbsp;battery.

I need hardiy obferve, that inftead of zinc copper and water, other combinations may be made according to the table in page 479. At prefent the lafl:nbsp;defcribed batteries are conftrudted with copper,nbsp;zinc, and water mixed with a fmall proportion ofnbsp;nitric or muriatic acid. For the conftruftion of luchnbsp;batteries it is immaterial whether the metals are quitenbsp;pure or flightly alloyed.

Theadion of all thofe batteries is greatefl; when they are firft completed or filled with the fluid; andnbsp;it declines in proportion as the metal is oxidated,nbsp;or the fluid lofes its power. Flence, after a certainnbsp;time, not only the fluid mull be changed, but thenbsp;metallic pieces mull be cleaned by removing thenbsp;oxidated furface, which is done either by filing or bynbsp;rubbing them with fand or fand-paper, or by im-merfing them for a fhort time in diluted muriaticnbsp;acid, and then wiping them with a coarfe cloth.nbsp;The metallic pieces of the battery, fig. 16, may benbsp;cleaned by the lafl: method, and may be wiped bynbsp;introducing a ftick with a rag into the cells.

Thus much may be fufficient with refped to the conftrudion of Ample and compound Galvanic arrangements. It' is now neceflary to ftate the ef�edsnbsp;of thofe combinations. Indeed the mode of applying Angle Galvanic circles and their principal efFedls,

have

have already been defcribed j yer, Tor the fake of affifting the memory^ it will be ufeful to colledfcnbsp;thofe effe�ts under the four following heads, in explanation of which we fhall add fuch farther experiments and obfcrvations as could not with proprietynbsp;be mentioned before.

I. nbsp;nbsp;nbsp;The aaion of a fingle Galvanic circle aff�dsnbsp;the organs of living animals, or of animals recentlynbsp;dead, efpecially when one end of the combination isnbsp;connedled with a nerve, and the other end is connected with a mufcle of the fame limb.

II. nbsp;nbsp;nbsp;That aflion may be tranfmitted throughnbsp;good condudlors of eleftricity, but not throughnbsp;eleftrics, or through lefs perfect conduftors.

III. nbsp;nbsp;nbsp;It affeds the electrometer by the intermediation of other inftruments.

IV. nbsp;nbsp;nbsp;That adion increafes, or otherwife modifies,nbsp;the chemical agency of the bodies concerned, uponnbsp;each other.

The limbs of animals, efpecially of frogs recently dead,. are the mofl: fenfible inftruments of Galvanic powers; and, in fad, the fimpleft Galvanic circles will affed them, when they will notnbsp;produce atiy other decifive eledrical �ffed.

The various powers of different fimple circles may be afeertained by applying them to fuch animal preparations as have their vitality, or irritability, more or lefs exhaufted. Thus Mr. Volta innbsp;his letter to Gren, fays, � If you take a frog, the

head of which has been cut off, and which has ** been deprived of all life by thrufting a needlenbsp;� into the fpinal marrow, and immerfe it withoutnbsp;� fkinning, taking out the bowels, or any othernbsp;preparation, into two glaffes of water; the rumpnbsp;� into one, and the legs into the other, as ufual; itnbsp;� will be ftrongly agitated and violently convulfed,

� when you connedl the water in both glaffes by a � bow formed of two very different metals, fuch asnbsp;� filver and lead, or, what is better, filver andnbsp;� zinc ; but this will .by no means be the cafe whennbsp;� the two metals are lefs different in regard to theirnbsp;� powers, fuch as gold and filver, filver and cop-� per, copper and iron, tin and lead. But what isnbsp;� more, the effedl will be fully produced on this fonbsp;� little prepared frog, when you immerfe in one ofnbsp;� the two glafles the end of a bow merely of tin drnbsp;� zinc, and into the other glafs the other end ofnbsp;� this bow which has been rubbed over with a littlenbsp;alkali. You may perform the experiment ftillnbsp;� better with an iron bow, one end of which hasnbsp;� been covered with a drop or thin coating of ni- .nbsp;� trous acid; and beyond all expeftation, when younbsp;� take a filver bow, having a little fulphuret of pot-� afh adhering to its extremity.�'

When a fingle powerful Galvanic combination of the fecond order is applied with one end to thenbsp;tongue, and with the other fluid end |:o fome othernbsp;fenfible part of the body, an acid tafte is perceivednbsp;on the tongue, which tafte, by continuing the

c�ntacSI:, becomes lefs diftinfl:, and is even changed into an alkaline tafte.

If a tin bafon be filled with foap-fuds, lime-water, or a ftrong ley, which is ftill better; and if � you then lay hold of the bafcn with both yournbsp;� hands, having firft moiftened them with purenbsp;� water, and apply the tip of your tongue to thenbsp;� fluid in the bafon, you will immediately be fen-� Able of an acid tafte upon your tongue, which isnbsp;in contaft with the alkaline liquor. This tafte isnbsp;� very perceptible, and, for the moment, prettynbsp;ftrong j but it is changed afterwards into a dif-� ferent one, lefs acid, but more faiine and pungent,nbsp;� until at laft it becomes alkaline and fliarp, innbsp;� proportion as the fluid adts more upon ' thenbsp;tongue*.�

Mr. Davy obferves, that � if zinc and fliver be � made to form a circle with diftilled water, hold-quot; ing in folution air, for many weeks, a confidera-� ble oxidation of the zinc is perceived, withoutnbsp;the perceptible evolution of gafs; and the water,

� at its point of contadt with the filver, becomes polfefled of the power of tinging green, red cab-� bage juice, and of rendering turbid, folution of �nbsp;� muriate of magnefia.�

The chemical adlion of bodies upon each other is increafcd by the Galvanic arrangement, fo much,nbsp;that fome of them are thereby enabled to adl upon

bodies that otherwife they would have no atSHon upon. Fig. 17, Plate XXIV. reprefents a glafs tube aboutnbsp;four inches long. Two corks are thruft into itsnbsp;apertures A and B. An oblong pieceof zinc, CDjnbsp;is fixed into one of the corks, and is made to pro-jedt within and without the tube. EFG is a filvernbsp;.wire, which, being fixed into the other cork, pro-jefls with the extremity E within the tube ; and itsnbsp;other extremity is bent fo as to come near the pro-jedling part of the zinc C.

Remove one of thofe corks, and fill the tube with water, in which you miift mix a drop or two ofnbsp;muriatic acid; then replace the cork, and you willnbsp;find that the zinc is adted upon by the diluted acid;

. is oxidated by it, and bubbles of gas are evolved from its but the filver wire E remains untouched,nbsp;and no gas whatever is evolved from it. Now, ifnbsp;you bend the filver wire F G, fo that its end G maynbsp;touch the zinc at C, then the Galvanic circle ofnbsp;filver, zinc, and diluted acid is completed, in con-fequence of which the diluted acid is enabled to adtnbsp;ftronger upon the zinc D, which is manifefted bynbsp;the more copious evolution of gas, and is, befidcs,nbsp;enabled to aft upon the filver wire ; for now younbsp;will obferve the evolution of gas from the filver Enbsp;alfo.�Break the contaft between G and C, and thenbsp;filver E will ceafe to yield gas.�Form it again, andnbsp;gas will again proceed from the filver.

Inftead of filver, zinc, and diluted muriatic acid, you may in the fame manner ufe gold, tin, and di-.nbsp;nbsp;nbsp;nbsp;luted

luted nitric acid ; and by completing the circle, the acid will be enabled to a�l upon the gold.

It has been obferved, that whenever an oxidating influence is exerted at one of the places of contaft ofnbsp;the perfect and imperfcdl conduftors, a deoxidatingnbsp;adion appears to be produced at the other place.nbsp;Thus when iron, which oxidates rapidly when forming a circle with filyer and common water, is arranged with zinc and common w'ater, it remainsnbsp;perfectly unaltered, whilft the zinc is rapidly adtednbsp;upon ¹.

Such are the fadts which have as yet been difco-vered with refpedt to the power of Angle Galvanic circles. They form a remarkable addition to thenbsp;fcience of eledlricity, and open a vaft field of fpecu-.nbsp;lation and experimental inveftigation; yet we arenbsp;unable to form a theory fufEcient to account for thenbsp;original caufe, or for the adtion of that very remarkable power j and we can only wait with patience for the probable elucidation, which may benbsp;afforded by farther difeoveries.

If the effedts of Angle circles are very remarka- ' ble, the colledted power of fevcral Angle circles, ornbsp;of the Voltaic battery, cannot fail of furprifing thenbsp;leaft refiedling mind.

The Voltaic battery not �nly convulfes the prepared limbs of a frog, or produces the appearance of a flafh of light before the human eye j but it

fliews all the phenomena of ele�tric'ity in a very confiderable degree. It gives the fhock; it afFeSsnbsp;the eledrometer; fliews a luminous fpark, accompanied with-an audible report j it burns metallic,nbsp;and other combuftible bodies j and continues innbsp;action for a very long time, viz. until the chemicalnbsp;aftion between the component parts of the batterynbsp;is quite exhaufted.�The following paragraphs contain a more particular, yet concife, enumeration ofnbsp;thofe wonderful effedts.

When Volta�s battery of the firft order (the adlion of thofe of the fecond order being weaker and muchnbsp;more tranfient) confifts of 20 repetitions of Amplenbsp;combinations, if you touch with one hand one extremity of the battery, as at b, in any one of the above-defcribed batteries, and apply your other hand tonbsp;the other extremity of the battery, as at a j you willnbsp;feel a very flight fliock, like that which is communicated by a Leyden phial weakly charged, and itnbsp;will be hardly felt beyond the Angers, or at moft thenbsp;wrifts. This Ihock is felt as often as you renewnbsp;the contafc. If you continue the hands in contadlnbsp;with the extremities b and a, you will perceive anbsp;flight but continuate irritation; and, when the hand ornbsp;other part of the body, which touches the extremitynbsp;of the battery, is excoriated or wounded, this fenfa-tion is difagreeable and rather painful.

The dry Ikin of the human body is feldom capa-ble of conducing this fhock ; therefore the touch-ing Angers fliould be well mpiftened with water. It

will be better to immerfe a wire, that proceeds frorrt one extremity of the battery, in a bafon of water,nbsp;wherein you may plunge one of your hands j thennbsp;grafping with your other hand well moiftened, anbsp;large piece of metal, for inftance, a large filvernbsp;Ipoon, touch the other end of the battery with it,nbsp;and the fhock will be felt more diftinftly. By thisnbsp;means the Ihock has been felt when the battery con-fifted of lefs than 20 repetitions.

Inftead of one perfon, feveral perfons may join hands, (which mull be well moiftened with water)nbsp;and on completing the circuit, they will all feel thenbsp;fhock at the fame inftant. But the ftrensith of thenbsp;flrock is much diminiflied by its paffing through thenbsp;feveral perfons, or, in general, by paffing throughnbsp;lefs perfedt conduftors.

The fhock from a battery confifting of 50 or 60 repetitions of the moft' aftive combinations of thenbsp;firft order may be felt as far as the elbows; and thenbsp;combined force of 5 or 6 fuch batteries will give anbsp;fhock perhaps much ftronger than moft men wouldnbsp;be willing to receive. The prepared limbs of anbsp;frog or other animal are violently convulfed, butnbsp;foon exhaufted of their irritability, by the adion of anbsp;Voltaic battery.

This fhock is fimilar to that of a large common eledVrical battery weakly charged, and not to that ofnbsp;a fmall Leyden phial fully charged. The differencenbsp;confifts in this, viz,' that the latter contains a fmallnbsp;quantity of eledric fluid highly condenfed j hence

its difcharge will force its way through perhaps an inch of air; whereas the former contains a vaftnbsp;quantity of eledrfcity, but little condenfed; hencenbsp;its fpark, viz. its courfe through the air, is fo verynbsp;Ihort, that the fingers muft be brought almoft intonbsp;perfeft contaft in order to receive the fliock; andnbsp;fuch is the cafe with the Voltaic battery; for thenbsp;Ihock from a very powerful battery of this fort willnbsp;hardly ever force its way through the air, when thenbsp;extremities of the circle of communication are morenbsp;than a fortieth of an inch diftant, even when thofenbsp;cxtreUnities confift of perfeft conduflors. In thisnbsp;cafe a fmall but very vivid fpark is feen at that extremity, accompanied with an audible but not'ftrongnbsp;report. There is no perceptible difference of appearance between the fpark of the pofitive and thatnbsp;of the negative end of the battery.

If a wire proceeding from one extremity of a pretty ftrong Voltaic battery be made to communicate with the infide coating, and a wire, whichnbsp;proceeds from the other extremity of the Voltaicnbsp;battery, be made to communicate with the outfidenbsp;coating of a common large jar or eleftrical battery ;nbsp;the latter will thereby become weakly, but almoftnbsp;inftantaneoujly, charged, in the fame manner as if itnbsp;had been charged by a few turns of a common electrical machine; and with that charge you may eithernbsp;give the Ihock, or affeft on eleftrometer, amp;c.

In fhort, every thing confpires to prove that a Voltaic battery produces a vaft quantity of eledtric

fluid, but which is little condenfed j and indeed it �would be impolflble to fuppole, that the eleftricnbsp;fluid could proceed in a very condenfed ftate fromnbsp;an arrangement of bodies, �which, whether more ornbsp;lefs, are, however, all good conductors of eleCtri-city} for if the fluid were much condenfed at onenbsp;extremity of the Voltaic battery, and much rarefiednbsp;at the other extremity, the compenfation would foonnbsp;be made through the pile itfelf. Indeed it is difficult to comprehend how this compenfation does notnbsp;tajee place in all cafes.

The eleftric fluid may probably be a neceflfary ingredient in the compofition of bodies j and perhapsnbsp;the chemical aftion of one body upon another dif-engages from the latter the eleftric fluid, as it difen-gagQs the caloric in feveral cafes: but the queftionnbsp;is, why the eleftric fluid, which is extricated fromnbsp;the bodies of a Voltaic battery, is forced to movenbsp;one way j and why is the other extremity of the battery in a negative ftate of eleftricity ?

Thofe doubts may perhaps be cleared by future difeoveries; but let us return to the ftatement ofnbsp;fafts.

Having mentioned above, that the charge of a Voltaic battery may be communicated to a common eleftrical battery; it is almoft fuperfluous tonbsp;obferve, that the fame may be communicated to anbsp;condenfef, or to my multiplier, and from it to thenbsp;eleftrometer. If the Voltaic battery confift of 20Onbsp;repetitions, the eleftrometer will be affefted by thenbsp;fimple contaft.

The

The fpark, or the difdferge of a Voltaic battery, ivhen fent through thin inflammable bodies that arenbsp;in contaft with common or oxygen air, fets them onnbsp;fire, andconfumes them with wonderful adtivity. Itnbsp;fires gun-powder, hydrogen gas, phofphorus, andnbsp;other combuftibles; it renders red-hot, fufes, andnbsp;confumes very flender metallic wires and metallicnbsp;leaves. The mode of applying the power of thenbsp;battery for fuch purpoles, is fhewn in fig. 18,nbsp;Plate XXIV. where AB reprefents a powerfulnbsp;Voltaic battery j ACDF is a wire which communicates with the laft plate of the battery at A;nbsp;B KI H G is another wire which communicatesnbsp;with the laft plate at B. D E, H I, are twonbsp;glafs tubes, through which thofe wires pals, and intonbsp;which they are fattened fufficiently tteady. Thofenbsp;tubes ferve to move the wires by ; for if the operatornbsp;apply his fingers to the middlemoft parts of tholenbsp;tubes, he may moVe the wires wherever he pleafes,nbsp;without the fear of receiving a fliock. If the twonbsp;extremities F, G, be brought fufficiently near to eachnbsp;other, the fpark will be fcen between them. It isnbsp;between thofe extremities that the combuttible fu'b-ftances, or metallic leaf, amp;c. is to be placed, in ordanbsp;to be fired or confumed. This figure reprefents thenbsp;fituation of the wires in the adt of inflaming gunpowder¹.

A battery conllfting of 2C0 pairs of metallic plates (viz. copper and zinc, each 5 inches fquare) melted 23

Under the exhaufted feceiver of the aif-pumpi the Voltaic battery adts lefs powerfully than in thenbsp;open air j but in oxygen air it afts with increafednbsp;power.

The flalli of light which appears before the eye of the experimenter, when the eye itfelfi or fomenbsp;other part not very remote from it, is put in thenbsp;circuit of a Galvanic combination, does not appearnbsp;much greater when a battery is employed, thannbsp;when two plates are applied in the manner whichnbsp;has been already mentioned j but when the batterynbsp;is ufed, the fenfation of a flalh may be produced innbsp;various ways. If one hand or both be placed innbsp;perfed contad with one extremity of the battery,nbsp;and almoil any part of the face be brought intonbsp;contad with the other extremity of the battery, thenbsp;flafh will appear very diftindly j the experimenternbsp;being in the dark, or keeping his eyes Ihut. Thisnbsp;flafli appears very ftrong, when a wire which proceeds from one extremity of the battery is held between the teeth, and refts upon the tongue, whilfl:nbsp;the other wire is held in the hand. In this cafe thenbsp;lips and the tongue are convulfed, the flafh appearsnbsp;before the eyes, and a very pungent tafle is perceived in the mouth.

If any part of the human body, forming part of the circuit of a Voltaic battery, be kept fometime in thatnbsp;inches of very fine iron wire. A platina wire about ttt **^�'*^nbsp;iicdiameter, was meltcddnto a globule.

fituation, the irritation or numbnefs is more or leis diftinct, and more or lefs painful, according to thenbsp;fenfibility of the parts concerned. This applicationnbsp;is likely to prove moft ufeful as a remedy in variousnbsp;diforders. It is faid that it has already proved beneficia} in deafnefles and in rheumatifniis. It highlynbsp;deferves to be tried by medical perfons.

The moft extraordinary phenomena of a Voltaic battery are the chemical effeefts, and the modifications which are produced by it upon the bodies concerned, or upon fuch as are placed in the circuit. Inbsp;fhall firft deferibe the fimpleft mode of exhibitingnbsp;the principal of thofe phenomena ; namely, the evolution of gas from water j from which the mode ofnbsp;conducing fimilar experiments is eafily derived jnbsp;then Ihall tranferibe the various particulars whichnbsp;relate to thofe chemical effedts, from the Journals ofnbsp;the Britifh Royal Inftitution, where they are con-cifely expreffed and to which I fhall add notpsnbsp;with farther illuftrations.

AB, Fig. 19, Plate XXIV. exhibits a glafs tube full of diftilled water, and having a cork atnbsp;each extremity. E F is a brafs or copper wire,nbsp;w�hich proceeds from one extremity of a Voltaicnbsp;battery, and, paffing through the cork A, projedlsnbsp;within the tube. H G is a fimilar wire, which proceeds from the other extremity of the battery, andnbsp;comes with its extremity G within the diftance ofnbsp;about an inch or two from the wire F.

of gas proceed in a conftaht ftream from the fur-face G of the wire which proceeds from the negative end of the battery; thefe bubbles of gas, afcend-ing to the upper part of the tube, accurhulate by degrees. This gas is the hydrogen, and may be inflamed. At the fame time the other wire F depofits a ftream of oxide in the form of a fteam or cloud,nbsp;which gradually accurpulates in a greenifh form innbsp;the water, or on the fides of the tube, and is a per-felt;5l oxide of the brafs. The wire F is readily dif-coloured and corroded. If you interrupt the circuit,nbsp;the produftion of gas and of oxide ceafes immediately.�Complete the circuit, and the production ofnbsp;gas reappears, amp;c.

This produfleion of gas may be obferved even where the battery confifts of not more than fix ornbsp;eight repetitions of filver, zinc, and water. Innbsp;fhort, if the power of the battery be fufficient tonbsp;oxidate one of the wires of communication, the othernbsp;wire v/ill afford hydrogen gas j both extremities of

In this experiment it feems that the hydrogen is fepa-rated from the water, and is converted into a gafeous ftate by the wire connected with the negative extremity of thenbsp;battery ; whilft the oxygen unites with and oxidates the wirenbsp;conne�ed with the pofitive end of the battery. If you connect the pofitive end of the battery with the lower wire ofnbsp;the tube, and the negative with the upper gt; then the hydrogen

In the above defcribed apparatus, a little hole muft be made in the lower cork B, for the purpof*nbsp;of giving exit to the water in proportion as the gasnbsp;is formed.

� In all batteries of the firft order, when the con-� nexion is completed, changes take place which denote the evolution of influences capable of pro-� ducing from common water, oxygene and hydro-�nbsp;gene, acid and alkali, in different parts of thenbsp;� feries.

If two wires of gold or platina be ufed, which are not oxi-dable; then the flr�am of gas ifTues from each, the water is diminiChed, and the colleftedgas is found to be a mixture ofnbsp;hydrogen and oxygen. It explodes violently.

Thofe two different elaftic fluids may be obtained feparatc from each other by the following means. Let the extremities of the two wires, which proceed from the battery, be im-merfed in water, at the diftance df about an inch from eachnbsp;other, and place over each of them a fmall glafs veflel inverted and full of water, as in fig. 20, Plate XXIV, However, Dr. Prieftley, who denies the convertibility of waternbsp;into hydrogen and oxygen air, thinks that the efaftic fluid innbsp;thefe experiments originates frotn the air which is containednbsp;in the water �, �.fince,� fays he, �� if by means of oil upon thenbsp;water, or a vacuum, accefs to the atmofphere be cut off, thenbsp;whole production of gas ceafes,� Nor is any air producednbsp;when the water has been exhaufted of it. See Nichol-fon�s Journal of Natural Philofophy for March iSo2,

504' nbsp;nbsp;nbsp;Of Galvanfm.

� Thus In the battery with feries of zinc plates, � filver wires, and common water, oxide of zinc isnbsp;� formed on all the plates of zinc, whilft hydrogennbsp;is produced from the filver wires; and if the waternbsp;in contaft with them be tinged with red cabbagenbsp;juice, it becomes green.

� And in the battery with filver, gold, and weak � nitric acid, the filver is diffolved, whilft the acidnbsp;� becomes green, and flowly evolves gas at its pointsnbsp;** of contadt with the gold.

*' The chemical agencies exerted in the com-� pound batteries of the firft order can be beft ob-ferved by the fubftitution of fingle metallic wires � for fome of the double plates; for, in this cafe,nbsp;� the changes taking place in the feries with wires,nbsp;� will be exaftly analogous to thofe produced innbsp;� the feries with plates; filver, and all the morenbsp;oxidable metals, oxydating in water, in the ufualnbsp;place � and gold and platina evolving oxygenenbsp;� gas.

� Thus, when into two finall glafs tubes, con-nedled by moift animal fubftance, and filled with �� diftilled water, two gold wires are introduced fromnbsp;� a large battery, in the proper order, oxygene isnbsp;produced in one quantity of water, and hydrogennbsp;� in the other, nearly in the proportions in whichnbsp;� they are required to form water by combuftion :nbsp;� and if the procefs be continued for fome time, thenbsp;apparatus being expofed to the atmofphere, thenbsp;water, in the oxygene-giving tube, will become

impregnated with an acid (apparently the nitrous) ; whilft that in the hydrogen-giving tube will be found to hold in folution an alkali, which,nbsp;in certain cafes, has appeared to be fixed.

quot; From fofne experiments it would appear probable that the quantities of hydrogene, produced in feries, are fmali, and the quantities of alkalinbsp;great, in proportion as the furfaccs of contact ofnbsp;the leaft oxidable metals with the water are morenbsp;extended.

� All the oxygenated folutions of bodies polTeffing lefs affinity for oxygene than nafcent hydrogene,nbsp;are decompofed when expofed to the a6tion ofnbsp;the metal occupying the place of the leaft oxidable part of the feries in the compound circle.

� Thus, fulphur may be produced from fulphu-ric acid 5 and copper and other metals precipitated in the metallic form from their folvents ¹.

� It is well known that hydrogen gas, in its nafcent ftate, reduces the oxydes of metals. Accordingly, when thenbsp;tube, fig. 19, is filled with a folution of acetite of lead innbsp;diftilled water, and a communication is made with the battery as above deferibed, no gas is perceived to iiTue from thenbsp;wire which proceeds from the negative end of the battery;nbsp;but, in a few minutes, beautiful metallic needles are perceived on the extremity of this wire; thefe foon increafe,nbsp;and alTume the form of a fern, or other vegetable. The leadnbsp;thus feparated is in its perfedl metallic ftate, and very briU^nbsp;Jiant.

^o6 nbsp;nbsp;nbsp;Of Galvanifm.

� But little knowledge has yet been obtained *' concerning the chemical changes taking place innbsp;the batteries of the fecond order. But from fe-veral experiments it would appear that they arenbsp;materially different in the laws of their pro-�� duiffion from thofe taking place in the firftnbsp;order.

� Thus, when fingle metallic wires with water � are placed as feries in powerful batteries of thenbsp;� fecond order, the influence producing oxygencnbsp;� feems to be tranfmitted by the point, in thenbsp;� place of that part of the plate, which was appa-*' rently incapable of undregoing oxidation j whilft

� When a folution of fulphat of copper is employed, the copper is precipitated in its metallic ftate; but inftead of appearing in cryftals, it forms a kind of button, which adheresnbsp;firmly to the end of the wire.

� On making the experiment with a folution of nitrate of fdver, the filver is precipitated In the form of a beautiful metallic brufh, the metal fhooting into fine needle-like cryftals.�nbsp;Garnett�s Annals of Philofophy, vol.I. p. 19.

If a piece of iron be immerfed in a folution of fulphat of copper, the latter metal will be precipitated in a metallicnbsp;form, and will adhere to the furface of the former. Uponnbsp;filver merely immerfed in the fame folution, no fuch eftedtnbsp;is produced ; but as foon as the two metals, viz. the filvernbsp;and the copper, are brought into contact, the filver receives anbsp;coating of copper. Phil. T, rarif. for the year 1801. Wol-lafton�s Paper, p. 428.

the hydrogen is evolved from that point, where the oxidating part of th� primary feries appearednbsp;to exift,

quot; The agency of the Galvanic influence, which occafions cliemical changes, and communicatesnbsp;eleftrical charges, is probably, in forne nieafure,nbsp;diftind from that agency which produces fparks,nbsp;and the combuftion of bodies.

� The one appears (all other circumftances being flmilar) to have little relation to furface in compound circles, but to be great, in fome unknown proportion, as the number of feries arenbsp;numerous. The intenfity of the other feems tonbsp;be as much connefted with the extenfion of thenbsp;furfaces of the feries, as with their number*.

� Thus, though eight feries compofed of plates of zinc and copper, about 10 inches fquare, and ofnbsp;cloths of the-fame fize, moiftened in diluted muriatic acid, give fparks fo vivid as to burn iron wire;nbsp;yet the Ihocks they produce are Irardly fenfible,nbsp;and the chemical changes indiftiad; whilft '24nbsp;feries of limilar plates and cloths, about 2 inches

* Van Marum obferved, that the intenfities of two columns containing an equal number of plates, appeared equal by the eledrometer, although their diameters were fo different as one and five inches. On taking Ihocks from bothnbsp;batteries, their powers alfo feemed to be equal. In the fu-fion of wire, however, the large diameter had an evidentnbsp;advantage.

� fquare,

Of Galvani/m.

fquare, which occafion fhocks and chemical agencies more than three times as intenfe, pro-duce no light whatever.

� A meafure of the intenfity of the power in Galvanic batteries, producing chemical changes,nbsp;� may be derived from the quantity of gas it is ca-� pable of evolving from water in a given time.�

The preceding fads can hardly leave any doubt with relped to the identity of the Galvanic power,nbsp;and the eledricity which is produced by means of ^nbsp;common eledrical machine, or that is brought downnbsp;from the clouds; but, what is ftill more, remarka-able, it reconciles to the fame principle the animalnbsp;eledricity, viz. the power of the torpedo, gymnotusnbsp;cledricus, amp;c. lince all the phenomena of the animal eledricity agree with thofe of the Voltaic battery.

The elfdrical fifhes give the Ihcck in water �, and in the fame manner, if the ends of the wires, whichnbsp;proceed from the extremities of the Voltaic battery,nbsp;be immerfed both in the fame bafon of water, atnbsp;fome diftance from each other; and if you plungenbsp;your hands in the fame water, you will receive thenbsp;fhock, the greateft part of which will pafs, not throughnbsp;the water, but through your body, which is the betternbsp;condudor of the two.

The ftrongeft Ihock of the gymnotus will hardly at all pafs through any interruption of circuit, andnbsp;fuch is alfo the cafe with the Voltaic battery.

eleftric organ of any of the above-mentioned fiflies leems to be conftru�ted exadlly like a Voltaic battery ; for it confifts of little laminae or pellicles arranged in columns, and feparated by moifture ¹. Itnbsp;feems, in fliort, to be a Voltaic battery, confiding ofnbsp;condudors of the fecond order ofily; but undoubtedly of different conduding powers.

Though the Voltaic battery exhibits all the leading properties of common eledricity, fuch as the attradion, the fpark, amp;c. yet in fome effeds, viz,nbsp;the decompofition of water, oxygenation of metals,nbsp;amp;c. the former feemed to differ confiderably fromnbsp;the latter; but thofe apparent differences have beennbsp;fufficiently reconciled by fome very ingenious experiments and obfervations of Dr. W. H. Wol-iafton f.

With refped to the decompofition of water, which was thought to require very powerful eledri-cal machines, he judly fufpeded, that by reducingnbsp;the furface of communication, the decompofition ofnbsp;water might be effeded with lefs powerful means; andnbsp;this was verified by adual experiments. � Having,nbsp;he faysy procured a fmall wire of fine gold, andnbsp;given it as fine a point as I could, I inferrednbsp;it into a capillary glafs tube ; and after heating

*' the tube fo as to make it adhere to the point, and � cover it in every part^ I gradually ground itnbsp;� down, till, with a pocket-lens, I could difcern thatnbsp;� the point of the gold was expofed.

� The fuccefs of this method exceeding my ex-peftations, I coated fcveral wires in the fame quot; manner, and found, that when fparks from thenbsp;� conductors were made to pals through water, bynbsp;� means of a point fo guarded, a fpark pafling tonbsp;� the diftance of ^ of an inch would decompolenbsp;quot; water, when the point expofed did not exceednbsp;� of an inch in diameter. With another pointnbsp;� which I ellimated at ttVts- of an inch, a fucceffionnbsp;� of fparks of an inch in length, afforded a cur-rent of fmall bubbles of air.

� I have fince found, that the fame apparatus � will decompofe water, with a wire of an inchnbsp;� diameter, coated in the manner before deferibed,nbsp;� if the fpark fropi the prime conductor paffes tonbsp;� the diftance of-~ of an inch of air.�

He alfo found, that with a gold point fimilar to, but much fmaller than any of the above-mentioned,nbsp;and fimilarly fituated in water, the mere current ofnbsp;eleftricity, without any fparks, would occafion anbsp;ftream of very fmall bubbles to rife from the extremity of the gold.

� Having coloured a card with a ftrong infuGon of quot; litmus, I paffed a current of eleftric fparks alongnbsp;� it, by means of two fine gold points, touching itnbsp;� at the diftance of an inch from each other. The

efFeft, as in Other cafes, depending on the fmall--nefs of the quantity of water, was moft difcernible � when the card was nearly dry; In this ftate, anbsp;� very few turns of the machine were fufficient tonbsp;occafion a rednefs at the pofitive wire, very nja-quot; nifeft to the naked eye. The negative wire, beingnbsp;� afterwards placed on the fame fpot, foon reftorednbsp;it to its original blue colour.�

Dr. Wollafton likewife remarks another ftrong point of analogy between the eleftricity of the Voltaic battery and that of a common eledtrical machine;nbsp;viz. that they both feem to depend upon oxidation.nbsp;In fadt, a common eledtrical machine will a�l morenbsp;or lefs powerfully, according as the amalgam whichnbsp;is applied to its rubber confifts of metals that arenbsp;more or lefs oxidable.

I lhall not proceed to conjedlure in what manner the oxidation of metallic fubftance can furnilh electricity, nor fhall I detain my reader any longer withnbsp;hypothefes concerning Galvanifm j a fubjedt of recent difcovery, of extenfive influence, and whichnbsp;feems promiflng of ample retompenfe to the in-duftry of diligent experimenters ; but which is ftillnbsp;involved in much doubt and obfcurity.

SECTION IV.

An hard mineral body, of a dark gf�y, or dark brown, and fometimes almoft black colour,nbsp;has been called a natural magnet, or load-Jlcne. Thisnbsp;mineral, which is an iron ore, has, from time immemorial, juftly attracted the attention of mankind, onnbsp;account of the very remarkable, and very ufeful,nbsp;properties, of which it is found naturally poffeffed,nbsp;and which are thence denominated magnetic properties ¹.

The magnetic properties may alfo be communicated to other ferruginous bodies by proper methods;

The word magnet is, by feme ancient writers, derived fretn the name of a fhephgrd, by whom they fuppofe thenbsp;magnet to have been firft difeovered on Mount Ida. It wasnbsp;in ancient times more commonly called ftderites, from itsnbsp;property of attracting iron, vvhich metal is callednbsp;in Greek ; or lapis herackus, by Pythagoras, Ariftotle,nbsp;Euripides, and others, from Heraclea, a city of Magnefunbsp;in ancient Lydia, where it was fuppofed to have been firftnbsp;found. It has alio in later times been called lapis nauticus.

To that thofe bodies will afterwards aft exaftly like natural magnets 5 hence the latter are called artificial magnets. But the magnetic properties do notnbsp;feem to have any decided agency upon any othernbsp;fubftance, befides iron ¹ j therefore the magnets,nbsp;whether natural or artificial, and the bodies, uponnbsp;which they aft, are either iron in its pure ftate, ornbsp;fuch compound bodies as contain iron. At leaftnbsp;the exceptions are rather equivocal.

A magnet, whether natural or artificial, is always poflefled of the following charafteriftic properties,nbsp;which are infeparabie from its nature j fo that anbsp;body cannot be called a magnet, unlefs it be pofi�fi�dnbsp;of all thofe properties at the fame time; neither wasnbsp;there a magnet ever produced which had one onlynbsp;or a few of thole properties f :

2. nbsp;nbsp;nbsp;When a magnet is placed fo as to be at libertynbsp;to move itfelf with fufficient freedom, as if it benbsp;fufpended by a thread, amp;c. it turns one, and con-ftantly the fame, part of its furface towards thenbsp;north pole of the earth, or towards a point not

The few and trifling exceptions to this general law will be noticed in the fequel,

In the firft volume of the Philofophical Magazine, page 426, it is faid that the ferpentine of Humboldt hasnbsp;fome of the magnetic properties only; but the account isnbsp;imperfeft, and, in all probability, incorreft.

much diftant from it; and of courfe it turns the oppofite part of its furface towards the fouth polenbsp;of the earth, or towards a point not much diftantnbsp;from it. Thofe parts on the furface of the magnetnbsp;are therefore called its poles; the former being denominated its north pole, and the latter its fouth pole.nbsp;This property itfelf is called the magnet's direSivinbsp;power, or the magnetic polarity; and when a magnetic body places itfelf in that diredion, it is faid tonbsp;traverfe. A plain perpendicular to the horizon, andnbsp;paffing through the poles of a magnet when Handingnbsp;in their natural diredion, is called the magnetic meridian, and the angle which the magnetic meridiannbsp;makes with the meridian of the place where thenbsp;magnet Hands, is called the declination of the magnet,nbsp;or more commonly of the magnetic needle at thatnbsp;place; becaufe the artificial magnets, moftly ufed fornbsp;obferving this property, are generally made of anbsp;fiender Hiape and fornetimes real fewing needles,nbsp;rendered magnetic, are ufed for this purpofe.

3. nbsp;nbsp;nbsp;When two magnets are placed fo that thenbsp;north pole of one of them is oppofite to the fouthnbsp;pole of the other, then they attrad each other; butnbsp;if the fouth pole of one magnet be placed oppofitenbsp;to the fouth pole of the other, or if the north pole ofnbsp;the one be brought near the north pole of the other;nbsp;in either cafe a repulfion takes place. In fiiort,nbsp;magnetic poles of the fame name repel each other ;nbsp;but thofe of different names attrad each other.

berty to move itfelf with fufficient freedom, it generally inclines one of its poles towards the horizon, and of courfe it elevates the other pole above it.nbsp;This is called the inclination, or dipping of the magnet,nbsp;or of the magnetic needle.

5. Any magnet may, by proper methods, be made to impart thofe properties to iron, or to fteel, or, innbsp;Ihort, to moft ferruginous bodies.

The particular laws which have been afcertained with refpedt to thole properties, their ufes, and thenbsp;inftruments neceffary for thofe purpofes, will be de^nbsp;fcribed in the following chapters.

Apiece of iron or fteel, or other ferruginous fubftance, fufficiently fmall, being broughtnbsp;within a certain diftance of one of the poles of anbsp;magnet, (be it artificial Or natural) is attrafted bynbsp;it, fo as to adhere to the magnet, and not fulfer tonbsp;be feparated without an evident effort.

This attra�lion is mutual j viz. the iron attrafts the magnet as much as the magnet attrads the iron;nbsp;for if the magnet and the iron be placed upon twonbsp;feparate pieces of cork or wood, to float upon waternbsp;at fome diftance from each other, it will be foundnbsp;that the iron advances towards the magnet, as wellnbsp;as the magnet advances towards the iron; or if thenbsp;iron be kept fteady, the magnet will move towards it.

The force or degree of magnetic attraftion varies according to different circumftances; viz. a magnetnbsp;attracts a piece of foft and clean iron more forciblynbsp;than any other ferruginous body of the like fhapenbsp;and weight, efpecially fuch as are of a harder nature.nbsp;Thus hard fteel or hard iron ores are attrafted lefs

forcibly than fofc fteel, and the latter lefs forcibly than iron. Oxygenated iron is attradtcd lefsnbsp;forcibly in proportion as it is combined with morenbsp;oxygen.

If the piece of iron be prefented fucce/Hvely to the various parts of the furiace of a magnet, it willnbsp;be found that the attraction is ftrongeft at the polesnbsp;of the magnet; that it diminilhes in proportion asnbsp;the part of the furface is more diftant from the poles jnbsp;and that it is hardly perceivable at thofe parts whichnbsp;are equidiftant from the poles of the magnet.

The attraction is ftrongeft near the furface of the magnet, and diminifhes as the diftance increafes jnbsp;viz. if a piece of iron be placed in contaCl with onenbsp;of the poles of a magnet fufEciently ftrong, they willnbsp;adhere to each other, and a certain degree of forcenbsp;is required to feparate them ; but if the fame piecenbsp;of iron be kept at a certain diftance from the famenbsp;pole of the magnet, there will aifo be perceived annbsp;endeavour to attraCt it but the force neceflary tonbsp;prevent that attraction, will be found much kfs thannbsp;that which, in the preceding cafe, was found necef-fary to feparate them; and by increafing the diftancenbsp;the attractive force will be found to diminilh. Nownbsp;it is very remarkable that the law of this diminutionnbsp;of the attractive force has not yet been afeertained,nbsp;notwichftanding a vaft number of experiments whichnbsp;have been made exprefsiy for the purpofe. Somenbsp;philofophers have found it to decreafe in proportionnbsp;to the fquares of the diftances, others in proportion

to the cubes of the diftances, and others again have found it to decreafe according to one ratio witliinnbsp;a certain diftance, and according to another rationbsp;beyond that diftance. This difference of refults arifesnbsp;from the various powers and lhapes both of thenbsp;magnets and of the iron for as the attraftion ofnbsp;the whole arifes from the attraction of the parts, itnbsp;naturally follows that if you gradually remove a piecenbsp;of iron from the magnet, the diftances between thenbsp;neareft parts may increafe in one ratio, whilft thenbsp;diftances between other parts will increafe in another ratio, and by changing the magnets, or thenbsp;fliapes of the iron, thofe ratios miift neceffarily benbsp;changed.�The only thing we can fay refpeftingnbsp;this decreafe is, that the attractive force decreafesnbsp;fafter than the fimple ratio of the diftances¹.

There is a limit in the lhape and weight of the iron which may be moft forcibly attracted by a givennbsp;magnet; viz. more forcibly than a fmaller or larger,nbsp;a more or lefs, extended piece of iron; but this limitnbsp;can only be determined by aSual experiments. Anbsp;fingle piece of iron is attracted more forcibly thannbsp;ifit be divided into feveral parts, and all thofe partsnbsp;be prefented to the fame magnet.

Such experiments are made by faftening a magnet to one arm of a balance, by placing the iron at different diftancesnbsp;below the magnet, and by counterpoifing the attraClion with

magnets has been found to begin from a greater diftance, but to be lefs powerful when in contadt,nbsp;than between fbft iron and a magnet.

Magnetic repulfion takes place only between fi-milar poles of different magnets. Thus, if the north pole of one magnet be oppofed to the northnbsp;pole of another magnet; or if the fouth pole benbsp;oppofed to the fouth pole, then thofe magnets willnbsp;repel each other, and that nearly with as much forcenbsp;as the poles of different names would attraft eachnbsp;other ¹. But it frequently happens, that thoughnbsp;magnets are placed with their like poles towardsnbsp;each other, yet they either attraft each other, ornbsp;fhew a perfedt indifference. Thefe phenomena feemnbsp;to contradidl the above-mentioned general lawj butnbsp;the following fadls will remove the difficulty ;

When a piece of iron, or of any other fubftance that contains iron, is brought within a certain diftance of a magnet, it becomes itfelf a magnet, havingnbsp;the poles, the attradtive and repulfive properties,nbsp;amp;c. like another magnet. That part of it whichnbsp;is neareft to the magnet, acquires a contrary polarity, and the oppofite part, the fame polarity. Thus,nbsp;if AB, fig. I, Plate XXV. be an oblong piecenbsp;of iron, and be brought near the north pole, N, of

The decreafe of this repulfive force, according to the increafe of the diftance between the two magnets, is asnbsp;irregular as the above-mentioned decreafe of the attradivenbsp;force.

the magnet N S, then this piece of iron, whilll, {landing within the magnet�s fphcre of aftion, willnbsp;have all the properties of a real magnet; and itsnbsp;end A will be found to be a fouth pole; viz. contrary to the neareft pole N of the magnet; whilftnbsp;the end B is a north pole. How this is to be madenbsp;evident will be Ihewn in the fequel.

The magnetifm which is acquired by being placed within the influence of a magnet, in foft iron, laftsnbsp;only whilft the iron continues in that fituation, andnbsp;when removed from the vicinity of the magnet, itsnbsp;magnetifm vanilhes immediately. But the cafe isnbsp;quite different- with hard iron, and efpecially withnbsp;hard heel; for the harder the iron or the Heel is,nbsp;the more permanent is the magnetifm which it acquires from the influence of a �magnet; but it willnbsp;be in the fame proportion more difficult to rendernbsp;it magnetic. If, for inftance, a foft piece of ironnbsp;and a piece of hard fteeJ, both of the fame fhape andnbsp;fize, be brought within the influence of a magnetnbsp;at the fame diftance, it will be found that the iron isnbsp;attradled more forcibly, and appears more powersnbsp;fully magnetic than the fteel; but if the magnet benbsp;removed, the foft iron will inftantly lofe its magnetifm, whereas the hard fteel will preferve it for anbsp;long time, having thereby become an artificialnbsp;magnet.

From thofe facfls three confequences are evidently deduced, viz. ift, That there is no magnetic at-traflion but between the contrary poles of magnets i 'nbsp;nbsp;nbsp;nbsp;for

for the iron or other ferruginous body, wh'ch is pre-fented to the magnet, muft itfelf become a m ignet before it is attrafled ; idly. It appears why a magnet attradls a piece of foft iron more f )rcibly thannbsp;hard iron, and much more than hard fteel; viz.nbsp;becaufe the lateer does not become fo ftrongly or fonbsp;eafily magnetic as the foft iron, when prefented to anbsp;magnet; and jdly, that no magnetic repulfion cannbsp;take place but between poles of the fame namej fornbsp;when the north pole, for inftan'ce, of one magnetnbsp;does not feem to attraft or repel, or it aftually at-trafts what was called the north pole of the othernbsp;magnet; the fadl is, either that the two north, ornbsp;the two fouth poles have deftroyed each other; ornbsp;that the fuperior force of one of the magnets hasnbsp;adually changed the poles of the weaker magnet;nbsp;as is, beyond a doubt, proved by experiments.

repulfion is in the leaft diminilhed or otherwife affected by the interpofition of any fort of bodies, except iron, or fuch bodies as contain iron.

The properties of the magnet are not afFerSted either by the prefence or by the abfence of air.

Heat weakens the power of a magnet, and the fubfequent refrigeration reftores it, but not quite tonbsp;its priftine degree. A white heat deflroys it entirelynbsp;or very nearly fo ; hence it appears, that the powersnbsp;of magnets muft be varying continually. Iron in anbsp;full red heat or white heat, (as I found by means ofnbsp;very decifive experiments) is not attrafted by the

magnet j

magnet; but the attraflion begins to aft as foon as the rednefs begins to difappear¹.

The attraftive power of a magnet may be in-ereafed confiderably by gradually adding more and more weight to it; keeping it at the fame time in anbsp;proper fituation, viz. with its north pole towards thenbsp;north, amp;c. A nd on the contrary, that power maynbsp;be diniinifhed by an improper fituation, and bynbsp;keeping too fmall a piece of iron, or no iron at all,nbsp;appended to it.

It feems that in thefe northern parts of the world, the north pole of a magnet has more power th.an itsnbsp;fouth pole, whereas the contrary efFeft has beennbsp;laid to take place in the fouthern parts of thenbsp;v/orld.

Amongft the natural magnets, tlie fmalleft generally polTefs a greater attraftive power in proportion to their fize than thofe of a larger fize. I have feen a fmall magnet that weighed about fix or fevennbsp;grains, and which could lift a weight of about 300nbsp;grains. Magnets of above two pounds weight fel-dom lift up ten times their own weight of iron.

It frequently happens, that a natural magnet, cut off from a larger load-ftone, will be able to lift anbsp;greater weight of iron than the original load-ftonenbsp;itfelf. This muft be attributed to the heterogeneous

See my Treatife on Magnetifm, 3d edition, Part IV. Chap. IV. for farther particulars relative to it.

As both magnetic poles together attraft a much greater weight than a Angle pole, and as the twonbsp;poles o^� a magnet generally are in oppofite parts ofnbsp;its furfare, in which c afe it is almoft impoffible tonbsp;adapt the fame piece of iron to them both at th�nbsp;fame time ; therefore it has been commonly prac-tifed to adapt two broad pieces of foft iron to thenbsp;poles of the ftone, and to let them projedl; on onenbsp;fide of the ftone for thofe pieces become themfelvesnbsp;magnetic while thus fituated, and to them thenbsp;piece of iron or weight may be eafily adapted.nbsp;Thofe two pieces of iron are generally faftenednbsp;upon the ftone by means of a brafs or filver box.nbsp;The magnet in this cafe is faid to be armed, and thenbsp;two pieces of iron are called the armature.

Fig. 2, Plate XXV. reprefents an armed magnet, where AB is the load-ftone, CD, CD, are the armature, or the two pieces of foft iron, to the pro-jedions of which DD the iron weight F is to benbsp;applied. The dots E CD CD reprefent the brafsnbsp;box with a ring at E, by which the armed magnetnbsp;rnav be fufpended.

Artificial magnets, when ftraight, are fometimes armed in the fame manner; but they are frequentlynbsp;made in the ftiape of a horfe-fhoe, having theirnbsp;poles at the truncated extremities, as at N and S,nbsp;fig. 3, Plate XXV. in which (hape it is evident,nbsp;that they want no armiature.

It has been faid above, that the magnet attradb iron only, or fuch bodies as contain iron; and asnbsp;iron is univerfally difperfed throughout the naturalnbsp;bodies, it is evident that a vaft number of bodiesnbsp;rnuft on that account be attradled by the magnetnbsp;more or lefs forcibly, in proportion to the quantitynbsp;and quality of the iron they contain. Indeed it isnbsp;wonderful to obferve what a fmall admixture of iroi�nbsp;will render a body fenfibly attraftible by the magnet.nbsp;Yet it mufi; be acknowledged, that though everynbsp;body which contains iron is in fome meafure at-trafted by the magnet, it does not follow that nonbsp;other body can be actrafled by it. Experiencenbsp;(hews that a vail number of fubftances are in anbsp;very flight degree attradiible by the magnet, andnbsp;thofc fubflances feem to contain either no iron at all,nbsp;or an exceedingly fmall quantity of it, extremelynbsp;diffufed and oxidated. To manifed this fmall degree of attradlion, the fubftances muft be placednbsp;upon a piece of paper or a light fliaving of cork, tonbsp;float upon water, and a ftrong magnet muft be gentlynbsp;approached, fideways, within fometimes a tenth ofnbsp;an inch diftance from the fubftance under trial. Innbsp;this manner it will be found that the following fubftances are in fome meafure affefled by the magnet;nbsp;viz. moft metallic ores, efpecially after their havingnbsp;been expofed to a fire. Zinc, bifmuth, and particularly cobalt, as well as their ores, are almoft alwaysnbsp;attradled. Of the earths, the coleareous is the leaft,nbsp;if at all, and the filiceous is the moft frequently, at_

traded. The ruby, the chryfolite and the tourmalin are attraded. The emerald, and particularly thenbsp;garnet, are not only attraded, but frequently acquire a permanent polarity. The opal is weakly attraded. Amber and other combuftible minerals arenbsp;attraded, efpecially after combuftion. Moft animalnbsp;and vegetable fubftances after combuftion are attraded. Even foot, and the duft which ufually fallsnbsp;upon whatever is left expofed to the atmofphere,nbsp;are fenfibly attraded by the magnet *.

About 15 years ago, I dilcovered feveral remarkable fads relative to magnetic attradion j the principal of which are as follows :

If rnoft fpecimens of brafs, which fhew no attradion towards the magnet, be hammered (viz. be hardened bynbsp;being beat with a hammer or with a ftone or other-wife) will in that hardened ftate be attraded. Thenbsp;fame piece of brafs will no longer be attraded afternbsp;being foftened in the fire j a fecond hammeringnbsp;will again render it attradible, and fo on repeatedly.

Moft of the native grains of platina have the fame property, viz. hammering renders them attradiblenbsp;by the magnet j and heat deprives them, as well asnbsp;brafs, of that property.

* See Brugman De Affinit. Magnet. This author's experiments were publlGied fome years ago ; and lately thenbsp;fame flight attradion has been (hewn under another fliape,nbsp;as a new difcovery. See La Decade Phikjophiqiie, N� 21;nbsp;or the Journals of the Britilh Royal Inllitution, N� 8.

The

The attraftion between iron and the magnet, is ^ncreafed by thea�lion of the nitric, and particularlynbsp;of the fulphuric acid upon the iron, durino- thenbsp;efFervefcence. For this purpofe the iron was placednbsp;jn a proper veffel near one end of a magneticnbsp;needle, (viz. a magnetic bar lightly fufpended) whichnbsp;was a little defle�ted from its natural diredlion bynbsp;the proximity of the iron ; but when diluted fulphuric acid was poured upon the iron, and the ef-fervefcence took place, the magnetic needle movednbsp;a little towards the iron, Ihcwing that the attraflionnbsp;was increafed by the aftion of the acid. The nitricnbsp;acid produced the like effed, but not fo powerfully.nbsp;When the efFervefcence was nearly finifhed, thenbsp;needle was found to ftand farther from the iron thannbsp;it did before the acid was poured upon the iron jnbsp;which was certainly owing to the iron remaining in.nbsp;an oxygenated ftatc ¹.

For farther particulars refpefling thofe difcoveries, fee the Philofophical Tranfa6tions, vol. 76 and 77, or mynbsp;Treatife on Magnetifni.

C H A P. II.

but it frequently happens that the fame magnet has more than two poles; viz. two or more north poles, and as many, or at leafl: as many, andnbsp;one more or lefs, fouth poles, on different parts ofnbsp;its furface j and this principally arifes from the irregular lhape of the magnet.

Thofe various poles are afcertained by prefenting the various parts of the furface of the magnet innbsp;queftion to a given pole (for inftance, the north) ofnbsp;a flender magnet lightly fufpended, and obfervingnbsp;which parts attrafl: it and which repel it; for thenbsp;latter muff be north poles, and the former, fouthnbsp;poles.

It fometimes happens, though not frequently, that two poles of the fame name, and equally powerful, are at the oppofite extremities of a magnet,nbsp;and a pole of the other name lies in the middle, innbsp;which cafe the magnet has no tendency to place it-fclf in the magnetic meridian. But good magnets,nbsp;of an uniform texture an.1 lhape, have two poles

�nly, which lie at oppofite parts of their furfaces; fo that a line drawn from the one to the other, paflesnbsp;through the centre of the magnet *. That line isnbsp;called the axis, and a line formed all round the fur-face of the magnet, by a plane which divides thenbsp;axis into two equal parts, and is perpendicular tonbsp;it, is called the equator of the magnet. Thus phi-lofophers have appropriated to the magnets, thenbsp;poles, the equator, as alfo the meridians of thenbsp;earth ; but, to complete the fimilariry, magnetsnbsp;have often been made of a fpherical fhape, with thenbsp;poles, the equator, the meridians, amp;c. marked uponnbsp;their furfaces. When thus lhaped, they have beennbsp;called terrellas, that is, fmall earths.

When a magnet having two poles is freely fuf-pended, or if it be placed to float upon water and no iron be near, it will place itfelf in the, magneticnbsp;meridian, viz. with its north pole towards thenbsp;northern, and of couife with its fouth pole towards^nbsp;the fouthern parts of the world, and that in everynbsp;part of the v/orld.

This wonderful property of the magnet forms the mod ufeful part of the fubje�l of magnetifm. It

* Here it muft not be underftood, that the polarity of a magnet refides only in two points of its furface ; for, innbsp;truth, it is the half, or a great part of the magnet, that isnbsp;poiTefled of one polarity, (viz. has the property of repellingnbsp;the like pole of another magnet) and the reft of the magnetnbsp;is pofeffe.l o the ' ther p lanty; the poles then are thofcnbsp;pcn.ts in which that power is the ftrongeff.

js this property that enables the mariners to conduft their veflels through vaft oceans, out of the fightnbsp;of land, in any given diredtion ; this diredtive property guides the miners in their fubterranean excavations, and the traveller through deferts otherwifenbsp;impaflable.

The general method is to keep a magnet, be it artificial or natural, freely fufpended, which in thatnbsp;cafe will place itfelf very nearly north and fouth ;nbsp;then the navigator, by looking at the diredtion ofnbsp;this magnet, may fleer his courfe in any requirednbsp;diredlion. Thus if a velTel felting off from a certainnbsp;place, mufl: go to another place which lies exadllynbsp;weftward of the former j the navigator muft diredtnbsp;it fo, that its courfe may be always at right anglesnbsp;with the diredtion of the magnet, keeping the northnbsp;end of the magnet on the right-hand fide, and ofnbsp;courfe the fouth-end on the left-hand fide of thenbsp;vefTcl; for, as the magnet or magnetic needle liesnbsp;north and fouth, the diredlion eaft and weft, whichnbsp;is the intended courfe of the veflel, is exadlly perpendicular to it. A little reflexion will eafily Ihewnbsp;how the veffel muft be fleered in any other diredlion.

An artificial fteel magnet, fitted for this purpofe in a proper Box, is called the magnetic needle, or thenbsp;mariner's compafs, or Jea compajs, or fimply thenbsp;compajs ¹.

It is not precifely known, when and by whom this wonderful property of the magnet was difcovcred. The

Though the north-^pole of the magnet in every� part of the world is dirc6ted towards the northernnbsp;parts, and the fouth pole towards the fouchernnbsp;parts; yet that direftion iekioni is exadlly in thenbsp;direftion of the poles of the earth. In other wordsnbsp;the magnetic meridian, and the real meridian in anynbsp;given place, feldom coincide. The angle whichnbsp;they make is called the angle of deelination, or the declination of the magnetic needle., and this declination hnbsp;laid to be eajl or weft, according as the north polenbsp;of the needle is eaftward or weftward of the aftrono^nbsp;mical meridian of the place.-

This declination is different in different places on land, as well as at fea; and is, befides, continuallynbsp;varying in the fame place. For inftance, the declination in London is not the fame as at Paris, or asnbsp;in India j and the declination in London, or in anynbsp;other place, at this time, is not the fame as it wasnbsp;fome years ago. The change, or the variation ofnbsp;the declination may be obferved even in one or two-hours time j or more properly fpeaking, the magnetic meridian in any one part of the world is con-mofl probable accounts feem to prove that this directive property of the magnet was known early in the I3dr century jnbsp;and that the perfon who fird made mariner�s compafles, atnbsp;leaf! in Europe, was a Neapolitan of the name of Flavio,nbsp;or John de Gioja, or Giova, or Gira, who likewife livednbsp;in the 13th century. See my Treatife on Magnetifm fornbsp;farther hiftorical particulars.

t'mually Ihifcing its fituation¹. This is not owing to the various conftrudion of the magnetic needle;nbsp;for in the fame place and time all good magneticnbsp;needles are diredled the fame way.

The declination and the variation of the declination in different parts of the world is fo uncertain as not to be foretold j an aftual trial is the only method of afcertaining �it. This therefore fornts anbsp;great impediment to the perfedtion of navigation.nbsp;It is true that navigators and other obfervers endeavour to afcertain the declination in various parts ofnbsp;the world, and fuch declinations are fet down uponnbsp;maps, chatts, in books, amp;c. but on account of thenbsp;continual variation, they can only ferve for a fewnbsp;years -!�; nor has it as yet been difcovered that this

This variation of the declination was difcovered by Columbus, in his firft voyage to America in the yearnbsp;1492.

f The bed charts of magnetic declination, are a chart by Dr. Halley) which was formed upon the obfervations madenbsp;in the beginning of the laft ceiituty, and a chart formed bynbsp;Meffieurs Mountaine and Dodfon, which contains the obfervations made in the year 1756. In thofe charts the obfervations are marked by means of dots, and a line is drawnnbsp;throujih all the dots, which indicate the fame declination;nbsp;but it is continued farther by conjedure or guefs; thus various lines are drawn indicating- the various declinations

The line of places, whereupon the magnetic needle points due north and fouth, is called the line of no declination. Itnbsp;is obfervable that thofe declination lines, though in fomenbsp;places very crooked, never crofs each other.

53^1 Of the Magnet's directive Property, amp;c.

variation or fluctuation is fubjeft to any law or period, though various hypothefes have been offered to the public.

The following Table contains the declination of the magnetic needle at different places upon thenbsp;eartli, as obferved in the annexed years. N. B. Bynbsp;eaft or weft declination it is meant that the northnbsp;end of the needle inclines eaftward or weftward ofnbsp;the aftronomieal meridian.

Latitude North.		LongitudeWeft.		Decli.naiion Eaft.
70�	17'	163quot;	24'	30�	2T
69	38	164	11	31	0
66	3^	167	55	27	50
65	43	170	34	27	58
63	58	165	48	26	25
59	39	149	8	22	54
5^	14	*39	19	24	40
55	12	135	0	23 �	29
S3	37	134	53	20 nbsp;nbsp;nbsp;32 Weft.
50	8	4	40	20	36
48	44	5	0	22	38
40	41	11	10	22	27
33	45	14	50	18	7
31	8	*5	3*^	17	43
28	30	17	0	H	0
23	54	18	20	*5	4
20	30	20	3	14	35
19	45	20	39	13	11
l�	37	22	50	10	33
15	25	23	36	9	15
13	32	�23	45	9	25
12	21 �	23	54	9	48 �

Years in which the Ob-fervations -were made.

1779.

1778.

1776.

Of the Magnet's dire5live Property, ^c. nbsp;nbsp;nbsp;535

IDeclination ohferved in London at different fimes.

1576	11�
1580	11
1612	6	10 1
1622	6'	� (
�633	4	5
�634	4	5 J
1657	0	oS
1665	I	22\|
1,666	�	35i
1672	2	30
1683	4	30
1692	6	0
1700	8	0
1717	10	42
1724	11	45 '
1725	11	56
1730	'3	00
*735	14	16
1740	*5	40 1
1745	16	53 /
1750	17	54
1760	*9	12
1765	20	0
1770	20	35
1774	21	3
*775	21	30
1780	22	10
1785	22	50
1790	23	34
*795	23	52
?8oo	1h	7

From the laft Table it appears, that when the variation was firft obferVed, the north pole of thenbsp;magnetic needle declined eaftward of the meridiannbsp;of London j but it has fince that time been advancing continually towards the weft ; fo that innbsp;the year 1657, the magnetic needle pointed duenbsp;north and fouth. At prefent it declines about 24�!nbsp;weftward, and it feems to be ftill advancing towardsnbsp;the weft.

It appears likewife, that the annual variation is by no means regular, and fuch is likewife the cafenbsp;with the daily or hourly variation; which thoughnbsp;evidently influenced by heat and cold, does notnbsp;however follow any known law.

The firft of the following Tables contains a fpe-cimen of hourly variation, as obferved by the late ingenious Mr. Canton, F. R. S. The fecond Ihewsnbsp;the mean variation for each month in the year, asnbsp;deduced from the fame Mr. Canton�s numerousnbsp;obfervations ¹,

^he Declination ohferved at different Hours of the fame Day, viz. June the i^th, 1759.

The

The following are axioms refpe�fing the declination of the magnetic needle colledted by L. Cottej to wtiich he adds others refpedting the northernnbsp;lights; as being concerned in the movements ofnbsp;the needle,

I. � The greateft declination of the needle from the north towards the weft, takes place about twonbsp;in the afternoon, and the greateft approximation ofnbsp;it towards the nx)rth, about eight in the morning jnbsp;fo that from the laft mentioned hour till about twonbsp;in the afternoon, it endeavours to remove from thenbsp;north, and between two in the afternoon and thenbsp;next morning, to approach it.�

a. � The annual progrefs of the magnetic needle is as follows;�Between January and March, it removes from the north; between March and Maynbsp;it approac�.ies it; in June it is ftationary j in July itnbsp;removes from it j in Auguft, September, and Odo-ber, it approaches it; its declination in Odlober isnbsp;the fame as in May; in November and Decembernbsp;k removes from the north; its greateft weftern declination is at the vernal equinox, and its greateftnbsp;approximation to the north, at the autumnal equinox.�

3. nbsp;nbsp;nbsp;� The declination of the magnetic needle isnbsp;different, according to the latitude: among us itnbsp;has always increafed fince 1657; before that periodnbsp;it was eafterly.�

4. nbsp;nbsp;nbsp;� Before volcanic eruptions and earthquakes,nbsp;the magnetic needle is often fubjed to very extraordinary movements.�

r. � The

5. nbsp;nbsp;nbsp;� The magnetic needle is agitated before andnbsp;after the appearance of the northern lights: its declination on thofe occafions is about noon greaternbsp;than iihial.�

6. nbsp;nbsp;nbsp;� The greater or lefs appearance of thefcnbsp;northern lights is variable : fome years this phenomenon is very frequent, in others uncommon; fornbsp;two or three years they have occurred very fel-dom.�

7. nbsp;nbsp;nbsp;� The northern lights are more frequent aboutnbsp;the time of the equinoxes than at other periods ofnbsp;the year.�

8. nbsp;nbsp;nbsp;� This phenomenon is almoft conftant during the long winter in the polar regions, and is thenbsp;more uncommon the-nearer the equator.�

9. nbsp;nbsp;nbsp;quot; Southern lights have been obferved alfo innbsp;the regions near the fouth pole,�

10. nbsp;nbsp;nbsp;� 1'he northern lights are often accompanied with lightning, and a noife like that of eledtri-city; while the lightning proceeds partly from thenbsp;middle of the northern lights, and partly from thenbsp;peighbouring clouds.�

[ 540 3

CHAP. Ill,

A K E a globular magnet, or, which Is more eafily procured, an oblong one, like S N,nbsp;fig, 4, Plate XXV. j the extremity N of whichnbsp;is the north pole, the other extremity S, is the fouthnbsp;pole, and A is its middle or equator ; place it horizontally upon a table C D ; then take another fmallnbsp;oblong magnet n �, (viz. a bit of fteel wire, or anbsp;frnall fewing needle magnetized) and fufpend it bynbsp;means of a fine thread tied to its middle, fo as tonbsp;remain in an horizontal pofition, when not difturbednbsp;by the vicinity of iron, or other magnet. Now ifnbsp;the fame fmall magnet, being held by the upper partnbsp;of the thread, be brought juft over the middle ofnbsp;the large magnet, within two or three inches of it,nbsp;the former will turil its fouth pole s, towards thenbsp;north pole, N, of the large magnet j and its northnbsp;pole K, towards the fouth pole S, of the large one.nbsp;It will be farther obferved, that the fmall magnet,nbsp;whilft kept juft over the middle A of the large one,nbsp;will remain parallel to it; for fince the poles of thenbsp;5nbsp;nbsp;nbsp;nbsp;fmall

fmall magnet'are equally diftant from the contrary poles of the large magnet, they are equally at-trafted. But if the fmall magnet be moved anbsp;little nearer to one end than to the other of thenbsp;large magnet, then one of its poles, namely, thatnbsp;which is neareft to the contrary pole of the laro-enbsp;magnet, will be inclined downwards, and of courfenbsp;the other pole will be elevated above the horizon.nbsp;It is evident that this inclination mull increafe according as the fmall magnet is placed nearer to onenbsp;of the poles of the large one, becaufe the attractionnbsp;of the neareft pole will have more power upon it.nbsp;If the fmall magnet be brought juft oppofite to onenbsp;of the poles of the large magnet, it will turn thenbsp;contrary pole towards it, and will place itfelf in thenbsp;fame ftraight line with the axis of the large magnet.nbsp;N. B. All thofe fituations are reprefented in thenbsp;figure.

After having obferved this very eafy experiment, the reader may eafily comprehend the phenomenanbsp;of the magnetic inclination, or of the dipping needlenbsp;Upon the furface of the earth for he needs onlynbsp;imagine, that the earth is the large magnet, (as innbsp;truth it is) and that any magnet or magnetic needlenbsp;commonly ufed, is the fewing needle of the preceding experiment; for admitting that the north,nbsp;pole of the earth is poftefted of a fouth magneticnbsp;polarity, and that the oppofite pole is poffeffed of anbsp;north magnetic polarity, it appears, as is confirmednbsp;by aftual experience, that when a magnet or magnetic

netic needle, properly fhaped and fnfpended, is Ilt;epE near the equator of the earth (or, more properlynbsp;fpeaking, near the magnetic equator of the earth jnbsp;fince neither the magnetic equator, nor the magnetic poles of the earth, coincide with its real equator and poles) it muft remain in an horizontal fitua-tion; if it be removed nearer to one of the magneticnbsp;poles of the earth, it muft incline one of its extremities, namely, that which is poflefled of the contrary polarity ; and the faid inclination muft increafcnbsp;in proportion as the needle recedes from the magnetic equator of the earth. Laftly, when the needlenbsp;is brought juft over one of the magnetic poles ofnbsp;the earth, it muft ftand perpendicular to the ground;

A magnetic needle, properly made and fufpended for the purpofe of fhevving this property, is callednbsp;the dipping needle, and its diredtion in any place isnbsp;called the magnetical line.

My reader muft not be furprized to hear, that a Jouth magnetifm is attributed to the north pole ofnbsp;the earth; it being only meant, that it has a magnetic polarity contrary to that end of the magneticnbsp;needle, which is direfted towards it; and, as we callnbsp;the lame end of the needle a north magnetic pole,nbsp;we muft of neceflity attribute a contrary polarity;nbsp;viz. a fouth magnetic polarity to the aftronomicalnbsp;north pole or northern parts of the earth. It followsnbsp;that the aftronomical fouth pole or fouthern partsnbsp;of the earth, muft be poftefled of north magneticnbsp;polarity.

If the afironomical poles of the earth coincided �with its magnetic poles; or even if the magneticnbsp;poles flood conftantly at a fixed diftance from thenrgt;,nbsp;the inclination of the needle, as \vell as its declination would remain unalterable; hence, from ob*nbsp;ferving the direclion of the magnetic needle in anynbsp;particular place, the latitude and longitude of thatnbsp;place might be afcertatned; but the cafe is far different ; for the magnetic poles of the earth do notnbsp;coincide with its real poles ; they neither are equi-diftant from them; and in fad they are continuallynbsp;fhifting their places; hence the magnetic needlenbsp;changes continually and irregularly, not only itsnbsp;horizontal diredion, but lilcewife its inclination,nbsp;according as it is removed from one place to another, as alfo whilft it remains in the very famenbsp;place. Hovrever, the change of dip or of inclination in the fame place is very trifling. In London-about the year 1576, the north pole of the dippingnbsp;needle ftood 71� 50quot; below the horizon, and in- thenbsp;year 1775 it flood at 72� 3- ; the whole change ofnbsp;inclination during fo many years amounting to lefsnbsp;than a quarter of a degree, allowing the accuracynbsp;of the obfervations.

The

544 nbsp;nbsp;nbsp;Of the Magnet's TnclinatioHi isc.

The following Table contains a few obfervations on the inclination of the needle in various places.

[ 545 ]

IT has been already mentioned, that when a ferruginous body comes within a certain diftance of a magnetj it becomes Itfelf a magnet alfo thatnbsp;this magnetifm is more eafily communicated, blitnbsp;at the fame time more eafily loft by foft iron thannbsp;by fteel. Hence it appears that the beft method ofnbsp;making artificial magnets, cohfifts in applying onenbsp;or more powerful magnets to pieces of the hardeftnbsp;fteel, becaufe thofe pieces will thereby acquire anbsp;confiderable rhagnetic power, and will retain it fornbsp;a very long time, Iii this operation care muft benbsp;had to apply the north pole of the magnet of magnets to that extremity of the piece Of fteel which isnbsp;required to be made the fouth pole, and to applynbsp;the fouth pole of the magnet to the oppofite extremity of the piece of fteel. In the fame manner anbsp;weak magnet may be rendered more powerful bynbsp;the application of ftronger magnets, or its powernbsp;may be reftored when loft.�A magnet, by communicating magnetifm to other bodies, has its ownnbsp;power rather increafed than diminifhed. � 'there

are feveral methods of performing this operatlort j-we fliall defcribe the beft prefently ; bnt let us pre-Vioufly take notice of what is commonly called the method of magnetizing fteel without any magnet.

Stridtly fpeaking, this method does not exift; for there is no magnetifm communicated but by thenbsp;adtion of another magnet; and in the above-mentioned method the magnetic power is originalJfnbsp;communicated from the earth, which is a real magnet, as is evidently and eafxly fhewn by the following experiment.

Take a ftraight bar of foft iron (one of two or three feet in length, and about three quarters of aft-inch in diameter, or a common iron kitchen poker,,nbsp;will anfwer perfedlly well), and, in thefe northernnbsp;parts of the world, if you keep it in a vertical pofi-tion, viz, with one end A towards the ground, and'nbsp;with the other end B upwards, you will find that the'nbsp;bar in that fituation is magnetic; the lower extre-.mity A being a north pole, capable of attracting thenbsp;fouth pole of a magnetic needle,- and of repelling,nbsp;the north pole of the needle; and the upper extremity B being a fouth pole.�If you invert the bar,nbsp;viz. place it with the extremity B downwards, itsnbsp;polarity will be inftantly reverfed, viz. the extre-� mitv B, which is now the loweft, will be found tonbsp;be a north pole, and the extremity Aa fouth pole

* An iron bar of four or five feet in length, and above an inch thick, when placed in this fituation, will be Capable ofnbsp;attraftiiig a finall bit of iron, or a finall common fewingnbsp;needle.

The

The explanation of this curious phenomenon is eafily deduced from the preceding laws; for fince innbsp;thefe northern parts the earth is poffeffed of a fouthnbsp;polarity, the lowed part of the iron bar, by beingnbsp;beared to it, mud acquire the Contrary, viz. thenbsp;north polarity j the other extremity of the bar becoming of courfe a fouth pole. It follows likewiicnbsp;(and it Is confirmed by aftual experiments) ; id.nbsp;That in the fouthern parts of the earth the lowednbsp;part of the iron bar acquires the fouth polarity jnbsp;sdly; That on the equator the iron bar rbud benbsp;kept horizontal, in order that it may acquire magnetifm from the earth; and jdl)?. That even innbsp;thefe parts of the earth the mod advantageous fitu-ation of the bar is not the perpendicular, but a littlenbsp;inclined to the horizon. In iliortj in every part ofnbsp;the world the iron bar mud be placed in the mag-netical line; viz, in the diredion of the dippingnbsp;needle.

A bar of hard iron, or of deeb will not anfwer for the above defcribed experiment, the magnetifmnbsp;of the earth not being powerful enough to magnc*nbsp;tize it.

After this experiment it will be eafily underdood that permanent magnetifm may be communicatednbsp;in a variety of ways. Thus bars of iron which havenbsp;dood long in a pretty favourable direction, viz.nbsp;either north and fouth, or perpendicular, amp;c. generally acquire a permanent magnetifrb, for the continual adion of the earth�s magnetifm daily commu-N N 2nbsp;nbsp;nbsp;nbsp;nicatss

tticates more and more power to it, at the farric time that the iron, efpecially if it be not very foft,nbsp;grows rather harder by rufting, or working, amp;c.

If an oblong piece of pretty hard iron be made red hot, and then be left to cool in the magneticalnbsp;line, it will thereby acquire a degree of permanentnbsp;magnetlfm.

If an iron bar, whilft ftanding in the magnetical line, be ftruck forcibly and repeatedly with an ham-rher on one of its ends, it frequently acquires permanent magnetifm from it. In Ihort, whatevernbsp;feems to render the iron or the fteel more fufcepti-ble of magnetifm (be it heat, or vibration, ornbsp;friflion), if adminiftered whilft the iron or fteel isnbsp;in the proper dirediion, is likely to fix the magnetifm in it; hence an electric firock, or a ftroke ofnbsp;lightning, or drilling, hammering, amp;c. frequentlynbsp;magnetizes the tools themfelves, or otirer pieces ofnbsp;iron and fteel concerned.

When a magnet is applied with one of its poles to one extremity of a pretty long fteel bar, the latternbsp;will thereby acquire a permanent degree of magnetifm ; but it will be found to have feveral poles^nbsp;viz. the end which has touched the magnet will benbsp;found pofTefled of the contrary polarity (fay for in-ftance, north); a little farther on. It will be foundnbsp;, pofleffed of the fouth polarity ; fome way beyondnbsp;that you will find another north polarity, and fonbsp;forth alternately. But if the bar be laot ve.'-y long,-then it will be found poftefied of two poles only, viz*

a north pole at one end, and a fouth pole at the other; which (hews that there is a limit in the lengthnbsp;of the bar, which renders it the moft eligible for annbsp;artificial magnet.

A magnet cannot communicate a degree of mag-netifm ftronger t'han that which itfelf pofiefies; but two or more magnets joined together may communicate a greater power to a piece of fteel, than eithernbsp;of them poflefles fingly : hence we have a method ofnbsp;confirudting very powerful magnets, by firfl: con-ftrufting feveral weak magnets, and thep joiningnbsp;them together, to form a compound magnet; andnbsp;to aft with great power upon a piece of fteel.

Since a bar of foft iion, fituatcd in the magnetical fine, is rendered magnetic by the earth; therefore,nbsp;if in that fituatlon we apply a fmall bit of fteel, ornbsp;a common fewing needle to it, this needle willnbsp;thereby acquire a permanent degree of magnetifm,nbsp;and thus feveral needles may be rendered magnetic; then, by joining thofe needles togedier in anbsp;little bundle* you may with them magnetize feveralnbsp;larger pieces of fteel, each of which will acquirenbsp;more power than any of the fingle needles. Withnbsp;thofe pieces of fteel, joined together, you may magnetize bars ftill larger, and fo forth.�The needlesnbsp;might alfo be magnetized by means of eleftricnbsp;fhocks, or by hammering; for a fmart ftroke of anbsp;hammer will frequently render a fmall needle magnetical. But without infifting any longer uponnbsp;thofe various methods, I fhall fubjoin Mr. Canton snbsp;N N 3nbsp;nbsp;nbsp;nbsp;procefs

procefs for conftrufting magnetical bars, the ratio-! nale of which will b? eafily deduced by the ingeniousnbsp;reader from the preceding particulars.

According to Mr. Canton Let fi.x bars be made of foft fteel, about 3 inches long, { inchnbsp;broad, and inch thick. Let alfo fix other fteelnbsp;bars be made quire hard, and about fix inchesnbsp;long, half an inch broad, and one eighth thick.nbsp;Each of thofe fets of bars muft have two pieces ofnbsp;foft iron called Jupprts or condu��cn, both equal tonbsp;one bar of the refpeftive fet. One end of each ofnbsp;thefe 12 bars muft be marked with a line, whichnbsp;end is to become the north pole. Have readynbsp;alfo an iron poker and tongs that have been longnbsp;in ufe.

Place the poker nearly upright, or rather in the magnetical line, with its point downv/ardsj and letnbsp;one of the foft fteel bars be tied, by means of anbsp;thread, to the middle of it, and with the markednbsp;end downwards ; then with the lower end of thenbsp;tongs held alfo in an upright pofuion, or in the magnetical line, ftroke the Iteel bar from the markednbsp;end upw'ards, abou!- to times, on both fides, whichnbsp;will give it power enough to keep fufpended a fmallnbsp;key. Thus commu.nicate the magnecifm to four ofnbsp;the fmall bars.

This done, lay the two ether fmall bars on a table parallel to each other, about a quarter of an inch

afunder, and between their iron conduiSlors A B, C Dgt; fig. 5, Plate XX V. taking care to placenbsp;the marked end of one of the bars on one fide, andnbsp;the marked end of the other bar on the oppofitenbsp;fide. Now place the four bars, already made magnetic, in the form lliewn in fig. 6, Plate XXV.nbsp;viz. two w'ith their north poles downwards, andnbsp;the other two with their fouth poles downwards.nbsp;The two of each pair muft be placed breadth tonbsp;breadth, and the two pairs being put contiguous tonbsp;each other at top, muft be kept open at a fmall angle by the interpofuion of fome hard fubftance I.nbsp;This fort of compound magnet, formed of the fournbsp;bars, muft be placed with its aperture on the middlenbsp;of one of the foft bars AC, taking care to let thenbsp;fouth poles H be towards the marked end of the barnbsp;A C, and the north poles F towards the other extremity. In this pofition, the compound magnetnbsp;muft be Hid from end -to end of the faid bar, viz.nbsp;when the poles H are arrived at C, move the compound magnet backwards the other way, till thenbsp;poles F come to A, amp;c. Thus ftroke the lyingnbsp;bar four times, ending at the middle j from whencenbsp;take up the cornpound magnet, and remove it tonbsp;the middle of the other lying bar BD, taking care,nbsp;as above, to let the fouth poles be towards thenbsp;marked end of the bar ; rub this in the like manner;nbsp;then turn the bars AC, B D, with the fides thatnbsp;food towards the table, upwards, and repeat thenbsp;operation on thofe other fides. This being done,nbsp;N N 4nbsp;nbsp;nbsp;nbsp;tafte

take up the two bars AC, B D, and let them forrq the inner two of the compound magnet ; and placenbsp;thofe which were before the two outfide ones, between the pieces of iron or condudlors, and rubnbsp;them with the compound magnet formed outnbsp;of the other four bars, in the fame manner asnbsp;before. This operation muft be repeated till eachnbsp;of the fix bars has been rubbed four or five times,nbsp;by which means they will acquire a confiderablenbsp;degree of magnetic power.

Wlien the fmall bars have been thus rendered magnetic, in order to com.m.unicate the magnetifmnbsp;to tlie large bars, lay two of them upon the table,nbsp;between their two condu�ors, or pieces of iron, innbsp;the fame manner, and with the fame precautions, asnbsp;w'ere ufed for the fmall bars ; then form a compound magnet with the fix fmall bars, placing threenbsp;of them with the north poles downwards, and thenbsp;three others with their fouth poles downwards.nbsp;Place thofe two parcels at an angle, as was donenbsp;with four of them, the north extremity of one parcel being put contiguous to the fouth extremity ofnbsp;the other j and with this compound magnet, ftrokenbsp;four of the large bars, one after the other, about 20nbsp;times on each fide, by which means they will acquire feme magnetic power.

When the four large bars have been fo far rendered magnetic, the fmall bars are laid afide, and fhe large ones are iflrengthened by themfelves, in thenbsp;fame manner as was done with the fmall bars.

With fome fort cf fteel, a few ftrokes are fuffi-c�lent to impart to them all the power they are capable of retaining; other forts require a longer operation ; and fometimes it is impoffible to givenbsp;them more than a juft fenfible degree of mag-netifm.

In order to expedite the operation, the bars ought to be fixed in a groove, or between brafs pins;nbsp;otherwife the attraction and fridlion between thenbsp;bars will be continually deranging them, whennbsp;-placed between the condudlors.

A fet of fuch bars are exceedingly ufeful for magnetizing other bars, or needles of compaffcs,nbsp;amp;c. their power may alfo be increafed when loftnbsp;Qr impaired by mifmanagement, amp;c. A fet of fucbnbsp;bars, viz. fix bars and the two iron conductors, qnaynbsp;be preferved in a box; taking care to place thenbsp;north pole of one contiguous to the fouth pole ofnbsp;the next, and that contiguous to the north pole ofnbsp;the third, amp;c. as Ibewn in fig. 7, Plate XXV¹,

After what has been faid above, I need not de-fcribe how a knife, or any piece of fteel, amp;c. may be rendered magnetic, or in what manner a weaknbsp;magnet may be rendered more powerful. But itnbsp;may perhaps be necelfary to fay fomething concerning the communication of magnerifm to crookednbsp;bars like ABC, fig. 8, Plate XXV.

Place the crooked bar flat upon a table, and tQ its extremities apply the magnetic bar DF, EGjnbsp;joining their extremities F, G, with the conduftornbsp;or piece of fofc iron F G ; then to its middle applynbsp;the magnetic bars placed at an angle, as ip Mr.nbsp;Canton�s method, or you may ufe two bars only,nbsp;placed as ihewn in fig. 8, and ftroke the crookednbsp;bar with them from end to end, following the di-reftion of that bent bar ; fo that on one fide of itnbsp;the magnetic bars may fland in the direftion of thenbsp;dotted reprefentation L K. Jn this manner, whennbsp;the piece of fteel ABC has been rubbed a fufficientnbsp;number of times on one fide, it muft be turned witfinbsp;the other fide upwards, amp;c.

Jn this procefs (as well as has been diredJed ip Mr. Canton�s) the magnets D F, E, G, as alfo thenbsp;magnets FI, 1, mufl: be placed fo that their fouthnbsp;poles be towards that extremity of the crookednbsp;fteel, which is required to he made the north pole^^-amp;c.

[ S55 ]

CHAP. V.

I ft, Of the principal phenomena of the earth�s magnetifrn j and 2dly, Of the fuppofed magneticnbsp;fluid.

That the earth adds as a great magnet is fo clearly indicated by a variety of fads and confiderations, that at prefent it is hardly poiTible to doubt of it.nbsp;In the firft place the diredtive property of the magnetic needle on the furface of the earth is fo analogous to that of a fmall needle upon the furface of anbsp;common magnet or terrella, as to ftrike every ob-ferver; 2dly, The magnetifm which iron acquiresnbsp;by its pofition, is another ftriking indication of thenbsp;earth�s magnetifm ; and ^dly, The vaft mafi�s ofnbsp;iron, in various ftates, which are to be found almoftnbsp;every where in the bowels .of the earth, and whichnbsp;are frequently magnetic, prove beyend a doubt thatnbsp;the earth is a vaft but irregular magnet, and that itsnbsp;magnetifm arifes from the magnetifiri of all the ferruginous bodies that are contained in it j fo that the

magnetic

rnagnetic poles of the earth miift be confidered as the centres or collefted powers of all thofe magnetic ferruginous fubftances. It follows likewifenbsp;that according as thofe malles of iron are affedednbsp;by heat, and cold, by decompofition, by mixturenbsp;with other fubftances, by volcanos, by earthquakes^nbsp;or mechanifal derangements, amp;:c. fo the magneticnbsp;poles of the earth muft Ihifc their fituation j andnbsp;this is the caufe of the variation of the magneticnbsp;needle*.

The above-mentioned caufes are fufficient to account for the daily, or hourly, or yearly variations, though it is not and perhaps it will never be in ournbsp;power to determine \yhatpart of the efFeft is due tonbsp;each of thofe caufes, or what is the precife refultnbsp;of the whole. It is therefore needlefs to fuppofe,nbsp;according to feme philofophers, that a large move-able magnet is contained within the earth, or to ad-,nbsp;mit other hypothefes ftill lefs probable.

The great defideratum in magnetics, is to know the caufe, which, in a magnet, of whatever fort itnbsp;be, produces the attradion^ repulfion, amp;c. it beingnbsp;wonderful to obferve that, by the mere contad, ornbsp;even by being brought within a certain diftance ofnbsp;a magnet, a piece of fteel, amp;c. acquires leveral remarkable properties, which it afterwards retains

� See the late Dr. Lorimer�s attempt to explain the caufe of the variation of the Magnetic Needle in my TrCut fe onnbsp;Magiietifm, 3d Edition j alfo page 254.

�wub

�with great obftinacy, and that without having its �weight, fhape, colour, or hardnefs, altered in andnbsp;fenfible degree.

Human ingenuity has contrived abundance of , hypoehefes in explanation of thofe phenomena; butnbsp;the infufficiency of mod of them renders it ufelefs tonbsp;date them in this work, excepting however onenbsp;which was propofed by Mr. Aepinus, and whichnbsp;is fimilar to the P'ranklinian Theory of t�edtri-city

The attentive reader mud undoubtedly have remarked feveral drong points of analogy between magnetifm and� cledlricicy j fuch as the analogy vnbsp;between the two poles of a magnet and the tw'onbsp;eleflricities �, the attraftion which takes place between magnetic poles of different denominationsnbsp;analogous to the attraflion between bodies differ-,nbsp;cntly eleflrified. See. Now Mr. Aepinus is led tonbsp;imagirle, that there exids a fluid produdlive of allnbsp;the magnetic phenomena, and confequently to benbsp;called the magnetic fiidd, that this fluid is fo verynbsp;dibtile as to penetrate the pores of all bodies j andnbsp;that it is of an eladic nature, viz. that its particlesnbsp;are repulfive of each other.

He farther fuppofes, that there is a mutual at-tradtion between the magnetic fluid and iron, or

other rerruginous bodies j but that no other fub-ftance has any aftion upon this fluid. He then ob-fervesj that there is a great deal of refembiance between ferruginous bodies and eledlrics, or non-condudtors of eledlricityj for the magnetic fluid paflesnbsp;�with difficulty through the pores of the former, as wellnbsp;as the eledfric fluid paffes with difficulty through th�nbsp;pores of the latter. However, there is not anbsp;body which has any adlion on the magnetic fluidsnbsp;and is, at the fame time, analogous to non-eledlrics; for inftance, there is not a body whichnbsp;attradts the magnetic fluid, and wdiich is* at th�nbsp;fame time, permeable by that fluid. In iron, indeed, a kind of gradation of this fort leems tonbsp;cxifl: j for the fofter the iron is, the more freelynbsp;does the magnetic fluid pervade its pores, and victnbsp;verfa.

According to this hypothefis, iron, and all ferruginous fubftances, contain a quantity of magnetic fluid, which is equably difperfed through their fub-ftance, when thofe bodies are not m.agnetic ; irtnbsp;which ftate they fhew no attradfion or repulfiongt;nbsp;becaufe the repulfion between the particles of thenbsp;magnetic fluid is balanced by the attradlion between the matter of thofe bodies and the faid fluid,nbsp;in which cafe thofe bodies are faid to be in a natural ftate. But when in a ferruginous body, thenbsp;quantity of magnetic fluid belonging to it, is driven

to one end, then the body becomes magnetic, one extremity of it being now overcharged with magnetic duid, and the other extremity undercharged.nbsp;Bodies thus conftituted, viz. rendered magnetic,nbsp;exert a repulfion between their overcharged extremities, in virtue of the repulfion between the particles of that excefs of magnetic fluid, which isnbsp;more than fufficient to balance, or to faturate, thenbsp;attraflion of tlieir matter. There is an attraftiojtinbsp;exerted between the overcharged extremity of onenbsp;magnetic body, and the undercharged extremity ofnbsp;the other, on account of the attraction between thenbsp;magnetic fluid and the matter of the body ; but, tonbsp;explain the repulfion which takes place betw^eertnbsp;their undercharged extremities, we mufl eithernbsp;imagine that the particles of ferruginous bodies,nbsp;when deprived of the magnetic fluid, mufl be re-pulfive of each other, or that the undercharged extremities appear to repel each other, only becaufenbsp;either of them attrafts the oppofite overcharged extremity ; both which fuppofitions are embarraflednbsp;with difficulties.

A ferruginous body, according to this hypothefis. Is rendered magnetic by having the equable diffu-lion of the magnetic fluid (Hflurbed throughout itsnbsp;fubflance; fo as to have an overplus of it in one ornbsp;more parts, and a deficiency of it in one or more othernbsp;parts: and it remains magnetic as long as its impermeability prevents the refloration of the equablenbsp;'nbsp;nbsp;nbsp;nbsp;diffufion

5-00 nbsp;nbsp;nbsp;theory of Magn�tifm.

difFufion of fluid, or of the balance betwe�n ths overcharged and the undercharged parts. Moreover, the piece of iron is rendered nnagnetic bynbsp;the aftion of a magnet, befcaufe, when the overcharged part of pole of the magnet is pfefentednbsp;tb it, the overplus of magnetic fluid in that polenbsp;repels the magnetic, fluid away from the neareftnbsp;extremity of the iron, f which therefore becornes undercharged) tb a more reriioce part of the iron whichnbsp;becomes overcharged. If the iron be magnetizednbsp;by the contact of the undercharged fide of the magnet, then the matter of the latter attradls the magnetic fluid bf the iron to that extremity of the ironnbsp;^vhich lies neareft to itlelf.

In a fimilar manner you may explain the aiftion of'two magnets upon each others .

I fhall conclude this chapter by obfervlng, that thb magnet has not been fl-und to have any aftiohnbsp;whatever upon the human body, and of courfe thenbsp;idle ftories of its bong beneficial tb perfons affliftednbsp;with tise tooth-ach, or with white fweilings, or tbnbsp;parturient Vv-omen ; as alfo of the wounds inflidlednbsp;with a magnetized knife being mortal, more than ifnbsp;the knife had not been magnetic, have not the leafbnbsp;foundation in truth or experience. The bare-fae'ednbsp;impofition which has for feveral years been praftifeefnbsp;tinder the name of animal magnetifrii is another ab-furdity.

In the Retchfanzeigeri a German periodical publication, Nquot; CCXXII. for 1797, if is faid, that s.

[ 5�� ]

TH E magnetical inftruments may be reduced to three principal heads; viz. ift, the magnets or magnetic bars, which are necelTary tonbsp;magnetize needles of compafTes, or fuch pieces ofnbsp;flee), iron, amp;c. as may be neceflfary for diverfenbsp;experiments; and which have already been fuffi-ciently explained in the preceding pages; zdly,nbsp;the compafTes, fuch as are ufed iit navigation,nbsp;and for other purpofes, which are only magneticnbsp;needles nimbly fufpended in boxes, and which,nbsp;according to the purpofes, for which they are particularly employed, have feveral appendages, ornbsp;differ in fizc, and in accuracy of divifions, amp;c.nbsp;whence they derive the different names of pocketnbsp;compafTes, ftcCring compalTes, variation CompafTes,nbsp;and azimuth compafTes; and jdly, the dippingnbsp;needle *.

* A curious contrivance, which is at once a dipping anJ a variation needle,^ was feme years ago made by the late Dr.nbsp;Lorimer. See a defeription of it in the Phil, Tranf. vol. 65;nbsp;or in my Treatile on Magnetifai.

The

The magnetic needles, which are commonly ufed lea, are between four and fix inches long; butnbsp;thofe which are ufed for obferving th� daily variation^nbsp;are made a little longer, and their extrerhities pointnbsp;the variation upon an arch or circle properly divided and affixed to the box*.

The beft ftiape of a magnetic needle is reprefent-ed in fig. 9 and 10, Plate XXVquot;.; the firft of which fliews the upper fide, and the fecond fhews a lateralnbsp;view of the needle, which is of fteel, having a prettynbsp;large hole in the middlcj to W'hich a conical piece ofnbsp;agate is adapted by means of a: brafs piece O, intonbsp;�which the agate cap (as is called) is faftened. Thennbsp;the apex of this hollow cap refts upon the point of anbsp;pin F, which is fixed in the centre of the box, andnbsp;upon which the needle, being properly balanced^nbsp;turns very nimbly j-. For common purpoles, thofenbsp;fieedles have a conical perforation made in the fteelnbsp;ilfelf, or in a piece of brafs which is faftened in thenbsp;middle of the needle jl.

� See the defcription of my new variation compafs in my Treatife oh Magnetifm, the 2d or 3d edition.

f It muft be obferved, that the needle which is balanced before it is magnetized, will lofe its balance, by being magnetized on account of the dipping, as (hewn in the thirdnbsp;chapter j therefore a fmall weight or moveable piece of brafsnbsp;is placed on one fide of the needle, as (hewn in fig. 10, bynbsp;the fhifting of which, either nearer to or farther from thenbsp;centre, the needle may always be balanced.

X The fimpleft magnetic needles are made of common fewing needles magnetized, and laid to fmm upon water.

0 0 2 nbsp;nbsp;nbsp;A mariner�s

A mariner�s compafs, or compafs generally �fed on board of fhips, is reprefented in Platenbsp;XXV. fig. II. The box, which contains th�nbsp;card or fly with the needle, is made of a circularnbsp;form, and either of wood, or brafs, or copper. Icnbsp;is fufpended within a.fquare wooden box, by meansnbsp;of two eencentric circles, called gimlalds, fo fixednbsp;by crofs axes a, a, a, to the two boxes, (fee thenbsp;plan, fig, 12, Plate XXV.) that the inner one,nbsp;or Compafs bOx, fhall retain an horizontal pofiiionnbsp;in all motions of the fhip, whilft the outer or fquarenbsp;box is fixed with refpedl to the fliip. The compalsnbsp;box is covered with a pane of gtafs, in order thatnbsp;the motion of the card may not be diidurbed by thenbsp;wind. What is called the card, is a circular piecenbsp;of paper, which is faftened upon the needle, andnbsp;moves with it. Sometimes there is a flender rimnbsp;of brafs, w�hich is faftened to the extremities of thenbsp;needle, and ferves to keep the card ftretched. Thenbsp;outer edge of this card is divided into 360 equalnbsp;parts or degrees, and within the circle of thofe di-vifions it is again divided i.nto 32 equal parts, ornbsp;arcs, which are called points of the compafs, ornbsp;thumls, each of which is often fubdividcd into quarters. The initial letters N, N E, 6?c. are annexednbsp;to thofe rhumbs, to denote the North, North Eaft,nbsp;amp;c. The middlemoft part of the card is generallynbsp;painted with a fort of ftar, whofe rays terminate innbsp;the above-mentioned divifions. To avoid confufionnbsp;on tlibfe letters, amp;c. are not'drawn in the figure.

above-

^bove-mentioned compafs, to which two fights are adapted, through which tfie fun is to be feen, innbsp;order to find its azimuth, and from thence to afcer-tain the declination of the magnetic needle at thenbsp;place of obfervadon j fee fig. 13, Plate XX V. Thenbsp;particulars in which it differs from the ufual compafs, are the fights F, G j in one of which, G, therenbsp;is an oblong aperture with a perpendicular thread ornbsp;wire ftretched through its middle; and in the othernbsp;fight F, there is a narrow perpendicular Hit. Thenbsp;thread or wire tl I is ftretched from one edge of thenbsp;box to the oppofite. The ring AB of the gimbaldsnbsp;refts with its pivots on the femicircle C D, the footnbsp;E of which turns in a focket, fo that whilft the boxnbsp;KLM is kept fteady, the compafs may be turnednbsp;round, in order to place the fights F/G, in the direction of the lun*.

There are, on the infide of the box, two lines drawn perpendicularly along the fides of the boxgt;nbsp;iuft from the points where the thread HI touchesnbsp;the edge of the box. Thefe lines ferve to fhewnbsp;how many degrees the north or fouth pole of thenbsp;needle is diftant from the azimuth of the fun; fornbsp;which purpofe, the middle of the apertures of thenbsp;lights F, G, the thread H I, and the faid lines.

� The pivots of the gimbalds of this, as well as of the common fort of compaffes, Ihould lie in the fame plane withnbsp;the point of fufpenfion of the needle, in order to avoid asnbsp;much as pofTible the irregularity of the vibrations.

003 nbsp;nbsp;nbsp;'nbsp;nbsp;nbsp;nbsp;mull

rnuft be exaftly in the fame vertical plane. The ufe of the thread HI, which is often omitted in in-ftruments of this fort, is likewife to Ihcw the degrees between the magnetic meridian and the azimuth, when the eye of the obferver Hands perpendicularly over it. Qn the fide of the box of thisnbsp;fort of compaffes, there generally is a nut or Hop,nbsp;which, when pufhed in, bears againfl the card andnbsp;Hops it, in order that the divifions of the cardnbsp;which coincide with the lines in the box, may benbsp;more commodioufly read off*.

The dipping needle, though of late much improved, is however Hill far from perfedtion. The general mode of conflrufting it is to pafs an axisnbsp;quite through the needle, to let the extremities ofnbsp;this axis, like thofe of the beam of a balance, reftnbsp;upon its fupports, fo that the needle may movenbsp;jtfelf vertically round, and when ftuated in thenbsp;magnetic meridian, it may place itfelf in the magnetic line. The degrees of inclination are fliewnnbsp;upon a divided circle, in the centre of which-thenbsp;needle is fufpended. Fig. 14, Plate XXV. repre-fents a dipping needle of the fir\ipleft conftrudlion jnbsp;A B is the needle, the axis of which FE refts uponnbsp;^he middle of two lateral b^rs C D, ^ D, which arenbsp;made fall to the frame that contains the divided

* What the azimuth of a celeftial objedt is, and how it may he afcertained, will be fliewu in the next volume of thisnbsp;y/ork,

circl^

circle AIBK. This machine is fixed on a ftand G; but, when ufed at fea, it is fufpended by a ringnbsp;H, lb as to hang perpendicularly. When the in-ftrument is furnilhed with a Itand, a fpirit level O isnbsp;generally annexed to it, and the ftand has threenbsp;fcrews, by which the inftrument is fituated fo thatnbsp;the centre of motion of the needle, and the divifiottnbsp;of 90� on the lower part of the divided circle, fnaynbsp;be exadlly in the fame line, perpendicular to thqnbsp;horizon.

The greateft Imperfeflions of this inftrument is the balancing of the needle, and the difficulty ofnbsp;afcertaining whether the needle retains its equipoife.nbsp;In making the obfervation of the dip at any particular place, the beft method to avoid the error ari-fing from the want of balance, is, firft to obfervenbsp;the dip of the needle, then to reverfe its mag-netifm, by the application of magnetic bars, fo thatnbsp;the end of the needle, which before was elevatednbsp;above the horizon, may now be below it andnbsp;laftly, to obferve its dip again; for a tnean of thenbsp;two obfervations will be pretty near the truth,nbsp;though the needle may not be perfeflly balanced.

The few experiments which follow, are principally intended to illuftrate the theory. As for entertaining magnetical experirnents, the ingenious reader may eafily derive them from the general fub-jeft which has been already explained.

I. The method of difcovering vffiether a body is attraflible by the magnet or not, and whether k has

any

any polarity or not^ or which is its fouth, and which its north pole, is fo eafily performed as not to require many words; for by approaching a magnetnbsp;to the body in queftion (which, if neceffary, may benbsp;ferto fwim upon water), or by prefenting the bodynbsp;in queftion to either extremity of a fufpended magnetic needle, the defired objeft may be obtained.

2. Tie two pieces of fofc iron wire, A B, A B, %. 15 and 16, Plate XXV. each to a feparate thread,nbsp;AC, AC, which join at top, and forming themnbsp;into a loop, fufpend them fo as to hang freely. Thennbsp;bring the marked end D, which is the north, of anbsp;magnetic bar, juft under them, and the wires willnbsp;immediately repel each other, as (hewn in fig. i6 ;nbsp;and this^divergency will increafe to a certain limit,nbsp;according as the magnet is brought nearer, and vicenbsp;�uerja. The reafon of this phenomenon is that by thenbsp;aftion of the north magnetic pole Df both the extremities B, B, of the wires, acquire the fame, viz. thenbsp;fouth polarity ; confequently they repel each ocher ;nbsp;and the extremities A, A, acquire the north polarity, in confcquence of which they ajfo repel eachnbsp;other.

Ifinftead of the north pole D, you prefent the foutli, pole of the magnetic bar, the repiilfion willnbsp;take place as before �, but now the extremities B, B,nbsp;acquire the north, and the extremities A, A, acquirenbsp;' the fouth, polarity.

On removing the magnet, the wires, if of foft iron, will foon collapfr, having loft all t.heir magnetic

power; b�t if fteel wires, or common fewing needles be ufed, they will continue to repel each other afternbsp;the removal of the magnet; the magnetic power being retained by fteel.

J- Lay a fheet of paper flat upon a table, ftrew fome iron filings upon the paper, place a fmall magnet among them; then give a few gentle knocks tonbsp;the table, fo as to lhake the filings, and you v/ill findnbsp;that they dlfpofe themfelves about the magnet NS,nbsp;as fliewn in fig. 17, Plate XXV��. the particles ofnbsp;iron clinging to one another, and forming themfelvesnbsp;into lines, which at the very poles N, S, are in thenbsp;fame diredlion with the axis of the magnet; a littlenbsp;fideway of the poles they begin to bend-, and thennbsp;they form complete arches, reaching from famenbsp;point in the northern half of the magnet, to fomenbsp;other point in the fouthern half.- The reafon ofnbsp;this phenomenon is not, as fome perfons imagine,nbsp;that a current of fluid iflues from one pole and enters at the other pole of the magnet; for if thatnbsp;were the cafe, the iron filings would be all drivennbsp;upon one of the poles. But the true reafon is, thatnbsp;each of the particles of iron is become acftuallynbsp;magnetic,- and polTeffed of the two pole.s, In confe-quence of which each particle, at the place where itnbsp;happens to ftand, difpofcs itfelf in the fame mannernbsp;as any other magnet would do; and moreover at-tradts with its extremities the contrary poles of othernbsp;particles,

fuch

iuch a piece of iron as is very little heavier tnanwhif the magnet will fupport. It is plain, that ifyou affixnbsp;this iron to one pole of the magnet, the m.omentyounbsp;remove your hand the iron will drop offj but if^nbsp;before you remove your hand, you prefent anothernbsp;larger piece of iron to the under part of the former,nbsp;and at about half an inch from it, you will then findnbsp;that the magnet will be able to fupport the firft piecenbsp;of iron which it could not fupport before, when thenbsp;fecondary piece of iron flood not below it. In ffiort,nbsp;a magnet can lift a greater weight of iron from overnbsp;another piece of iron, fuch as an anvil or the like,'nbsp;than from a table ; the reafon of which is, that innbsp;the former cafe, the iron bafis or' inferior piece ofnbsp;iron, becoming itfelf in foime meafure magnetic,nbsp;helps to increafe the magnetifm of the firft piece ofnbsp;iron, and' confequently tends to increafe the at-tradion.

5. Place a magnetic bar AB, fig. 18, Plate XXV. fo that one of its poles may proje�l a fhort way beyond the table, and apply an iron weight C to it;nbsp;then take another magnetic bar, D E, like thenbsp;former, and bring it parallel to and juft over thenbsp;other, at a little diftance, and with the contrary polesnbsp;towards each other; in eonfeq�ence of which the at-tradion of B will be diminifhed, and the iron C, ifnbsp;fuffieiently heavy, will drop off, the magnet AB being then only able to fupport a finaller piece of iron.-By bringing the magnets ftill nearer to each other^nbsp;the attradion of B will be diminiffied ftill farther;

amp;nci, 'whea the two magnets come quite into contadt, (provided they are equal in power) the attradlion between B and C will vanith entirely ; but if the experiment be repeated with this difference, viz. thatnbsp;the homologous poles of the magnets be broughtnbsp;towards each other, then the attraftion betweennbsp;B and C, inftead of being diminilhed^ will be in-Creafed.

6. Let ah iron wire �f about a quarter of an inch in diartieter, and 4 or 5 inches long, be bent fome-what like a Gothic arch, viz. v/ith a Iharp corner innbsp;the middle, as ABC, fig. 19, Plate XXVquot;. and tienbsp;it fall to any proper ftand, or let an afilftant hold it,nbsp;with the corner downwards; then apply either polenbsp;of the magnet DE to one of its extremities A, andnbsp;whilft the magnet remains in that fituation, apply anbsp;piece of iron H, of no great fize, to the corner C,nbsp;and you will find that the iron remains fufpended.nbsp;Now, if another magnet be applied to the other extremity B of the crocked iron, fo tliat the pole Gnbsp;may be contrary to the pole E, the iron H will immediately drop off; but if the pole G be analogousnbsp;to the pole E, viz. be both fouth or both north, thennbsp;the iron H not only will remain adhering to C, bufnbsp;the faid corner will be capable of fupporting anbsp;weight ftill greater than H. The reafbn of whichnbsp;is, that in the former cafe the extremities A and B,nbsp;of the bent iron, being poffeffed of different polarities, the corner C became the magnetic centre,nbsp;where there is no attraftion, nor repuliion ; whereas

in the fecond cafe, both extremities of the bent iron being poflefled of the fame polarity, the corner Cnbsp;acquired the contrary polarity. In this latter cafenbsp;the crooked iron muft have two magnetic centres,nbsp;viz. one on each fide.

7. In order to imitate in fome meafure, natural magnets, take martial tethiops, or, which is morenbsp;eafily procured, reduce into very fine powder thenbsp;fcales of iron which fall oft� from the red-hot ironnbsp;when fhammered in, blackfmiths 'fhops: mix thisnbsp;.powder with drying linfeed oil, fo as to form it intonbsp;a very ftifF pafte, and fhape it in a'proper mould,nbsp;into the form of a terrella or human head, he.nbsp;This done, place it in a warm place during fomenbsp;weeks, by which means it will become very hard ;nbsp;then render it magnetic by the application of powjnbsp;erful magriets,' and it will acquire a confiderablcnbsp;permanent power.nbsp;nbsp;nbsp;nbsp;^

Height of	Heat of boiling water,
the baro-	according to
meter.	^Mr-DeLuc.	Sir G. Sh.'
26,	205gt;I7	204,91
26,5 ,	206,07	205,82
27�	206,96	206,73
27,5	207,84	207,63
1 28,	208,69	208,25-
1 28,5	209,55	209,41
29,	210,38	210,28
29,5	210,20	211,15
30�	212,00	212,00
30,5	212,79	212,85
3igt;	2I3gt;57	1 2I3gt;^9

176 nbsp;nbsp;nbsp;Diopirics, or of Refra^sd Light.	Sine of
	inci-	pa-
.	�it'.eco.
Nitre	[,524
Camphire nbsp;nbsp;nbsp;- �nbsp;nbsp;nbsp;nbsp;-
Gum arabic	gt;477
Fluids.
Diftilled water nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	1.336	loo
Rain water	'.336
Well water between I3336 and -	1.337
Water I'aturated with common fait - nbsp;nbsp;nbsp;-	'.375	12^
Solution of common fait, water 27, fait i	1.348
Solution of fugar, water 27, fugar i -	1.346
Solution of mineral alkali, or foda -	1.352
Solution of fal ammoniac _ nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	1,382	134�
Solution of vegetable alkali, or pot-afh	1.390
Lime water ------	T.334
Sulphuric acid nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	1j426
Nitric acid _ , - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	I.4'2	154
r Euler - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;.	1.344
Diftilled vinegar lt; Rocbon - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	1.335
(, Hauklbee	1.372
Ammonia, or cauftic volatile alkali	1.349
Spirit of hartfhorn - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	1.339
French brandy ------	1.360
Ditto, a ftronger kind - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	1.365
Highly rectified fpirit of wdne, or alcohol	1.37'
Oil of olives ------	1,465
Oil of wax -	r.452
Oil of lavender nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;-	1.469
Oil of cinnamon	1.534
Oil of faffafra.1 - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;_	1.544
Oil of turpentine - nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-	1,482
Spirit of turpentine nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;_	1.562
Oil of amber ------	1.^01
The cryftalline/humour of an ox�s eye	1,463

50�	0,106.
100	1,600
150	6,7^5
160	8,740
170	11,405
180
190	18,227
200	22,703
212	29,89

January	-	-	7'	8quot;
February	-	-	8	58
March	-	-	11	17
April	m	-	12	26
May	-	-	X3	0
June	-	-	X3	21
July	-	-	13	14
Auguft		-	12	19
September	-	-	11	43
Oftober	-	-	10	36
November	-	-	8	9
December	-	-	6	58

				The North		Years in which
Latitude		Long	tude	End of the		the Obferva-
North.		Laft.		Needle	below	tions were
				thamp; Horizon.		made
53�	55'	193�	39	690	10	1778.
49	36	233	10	72	29
		Weft.
44	5	8	10	71	34	1776.
38	53	12	I	70	30
34	s1	14	8	66	12
29	18	16	7	62	17
24	24	18	11	59	0
20	47	19	36	56	15
	8	23	33	5f	0
12	I	23	35	48	26
10	0	22	52	44	12
5	2	20	10	37	25
South.
0	3	27	38	30	3
4	40	30	34	22	15
7	3	33	21	17	57
II	25	34	24	9	15
		Eaft.		South End below.
16	45	ao8	12	29	28
19	28	204	11	41	0
21	8	185	0	39	I	I777-
35	55	18	20	45	37	1774*
41	5	174	13	63	49	1777.
45	47	166.	18	70	5	1773-

stichting

n? lo Cfly ;

NATURAL OR EXPERIMENTAL

PHILOSOPHY.

TIBERIUS CAVALLO, F.R.S.amp;c,

Natural philosophy.

SECTION I.

with it.

B nbsp;nbsp;nbsp;are

B 4 nbsp;nbsp;nbsp;than

ftance

- nbsp;nbsp;nbsp;Of the Therniomeie}-, fj'c.nbsp;nbsp;nbsp;nbsp;15

pipe.

The Theory of Heat, tffc.

19

So

68^

Ds

The Theory of Heat, Csfc,

Thei uiomcters

c 3 nbsp;nbsp;nbsp;tially

Tbe T^heory of Heat, amp;c.

hiRheft

l'he Theory of Heal) Sc. , nbsp;nbsp;nbsp;27

quite proportional to the increments of heat. With �ther fluids the irregularity of expanfion is verynbsp;confiderable.

One

28 nbsp;nbsp;nbsp;The l�heory of Heat, Cfc.

One cubic inch of mer�ury, or one nneafurc whatever of it at 32� of temperature, when heated to the'

temperature

In

JO nbsp;nbsp;nbsp;^he �Theory of Heaty ifc.

1�be �Theory of Heat, iSc. nbsp;nbsp;nbsp;3 ^

34 nbsp;nbsp;nbsp;I'he 'theory of Heat, i^c.

The Theory of Heat, i^c. nbsp;nbsp;nbsp;35

212� t.

The Theory of Heat, �f?f. nbsp;nbsp;nbsp;,37

.46 nbsp;nbsp;nbsp;'The Theory 'of Heat, i�c'.

of

chanifm,

v�Uf fc Pha. T,^.

Standard

with

Of the 'Capacify of Bodies for Caloric, (fc. 6$

70 nbsp;nbsp;nbsp;Of the Capacity of Bodies for Caloric, amp;c.

itahk of the fpecific Caloric of different Bodies.

S�lutlOft

Of the Cafacity of Bodies for Caloric, Sc, 71

[ 86 ]

fun

part

Of the ProduBiofii i^c. of Heat and Cold. 91

hot

eaUed^,,.

of

vapou^quot;

The

doriBF.

of

�ream.

The

' nbsp;nbsp;nbsp;^800, Part III. p, 43S.

lens

Subltances

I 3 nbsp;nbsp;nbsp;The

' nbsp;nbsp;nbsp;� This

Th

�*796.

the

Of the TroduSliorp, of Heat and Cold. 127

128 Of the production-, of Heat and Cold.

In

SECTION ir.

ot

� dent

L � 144 ]

WheP

, ricW

be

Therefore

Catoptrics, or of Reflected Light. 157