JT may be eafily perceived by the title of this work, that it is not offered to the public as a finifhed piece on the fubjedt. To treatnbsp;of the theory and praftice of Electricity, in thenbsp;fulleft manner, would require a larger treatife,nbsp;and employ more time than I can devote to anbsp;work of this kind.

The fcience of EleClricity is now generally acknowledged to bp ufptiil and important; and there is great reafon to think, that at a future periodnbsp;it will be looked up to as the fource fromnbsp;whence the principles and properties of naturalnbsp;philofophy muft be derived; its utility to man,nbsp;whll not be inferior to its dignity as a fcience.

PREFACE.

I have not attempted to trace Eleftricity from its firft rude beginnings, or to follow the mind ofnbsp;man in its various and irregular wanderings, innbsp;fearch of the laws by which it acts, and the fourcenbsp;from whence it is derived, as this has been fo wellnbsp;executed by Dr. Prieftly. Our view of thingsnbsp;is fo eircumferibed, and the myfteries of naturenbsp;� fo profound, that it is not eafy for us to determine, whether the received theory is founded onnbsp;the bafis of truth, and conformable to nature, ornbsp;whether we lhall be confidered, by future philo-fophers, as mere children, amufed, and fatisfiednbsp;with imperfecl opinions and ill digefted theories.nbsp;When a variety of things is mixed together,nbsp;which have liitle or no connexion, they naturallynbsp;create confulion. It has been mv endeavour, innbsp;the following effay, to colleft and arrange, in anbsp;methodical and concife manner, the elTential partsnbsp;of F.leftricity, by thefe means to render its appli-cation eafy, pleafant, and obvious to the youngnbsp;praiftitioner; and by bringing together experimentsnbsp;of the fame kind, make them mutually illuftratenbsp;each other, and thus point out the ftrength, ornbsp;dilco-'er the weaknefs, of the theories that havenbsp;been deduced from thern. Though the naturenbsp;and confined limits of my plan did not admit of

much

preface. V

much variety of obfervation, or a formal enumeration of every particular, yet few things, 1 hope, ofufeand importance, have been omitted.

As I do not wifli to incur the imputation of plagiarifm, I with pleafure acknowledge the af-fiftance I have receivea from the different authorsnbsp;who have wrote on this lubjeit. I have ufed apnbsp;unreferved freedom, in feledling from their works,nbsp;W'hatever I found to anfwer my purpofe, I amnbsp;particularly obliged to Sir Jofeph Banks, for hisnbsp;politenefs in lending me Les Memoires de I�Aca-demie de Berlin for 1780, at a time when I couldnbsp;not procure them elfewhere.

The various interruptions and avocations, from which, as a tradefman, I cannot be exempt,nbsp;will, I hope, induce the reader to make fomenbsp;favourable allowances for any errors whichnbsp;he may difcover, and kindly correfl: them fornbsp;himfelf.

I beg leave to avail myfelf of this opportunity to acquaint the public, that I am now engaged innbsp;a work defcribing the mechanical parts of Mathematical and Philofophical Learning, and explain-A 3nbsp;nbsp;nbsp;nbsp;ibg

PREFACE.

ing the various ufcs of the different inftruments in their prefent ftate of improvement; which, Itruft,nbsp;will greatly tend to facilitate the attainment ofnbsp;knowledge, and accelerate its progrefs. For thisnbsp;purpofe I have been at a confiderable expence innbsp;colle�ting fuch materials as may enable me t�nbsp;offer to the public fonie effays on this fubjeft,nbsp;which I hope will not lgt;e Wnd unworthy of itsnbsp;patronage, and which I mean to publilh with allnbsp;convenient fpeed.

the-

CONTENTS.

CHAP. I.

Experiments i, 2. Excited glafs, or wax, att^adi and repel light bodiesnbsp;Eleftricity definednbsp;A definition of excitationnbsp;Eledlrics and non-ele6lrics diftlnguifhednbsp;Exp. 3, 4. To illuftrate the definition of conductors and non-condu�tors

Lift of eledrics, producing different eledricities Obfervations on thefe by Mr. Bergman, amp;c.

CHAP. ir.

Elcflrical machine defcribed Pofit�ve and negative ele�lricity, how obtainednbsp;Cufaion, on what principle it excitesnbsp;Exp. 9. The two ele�ric powers produced by thenbsp;bicaking a {lick cf fealing wax

Exp, II, 12 Effects of an infulated cufhion, and Dr. Franklin�s theory arifing from itnbsp;Difcbarging rod defcribednbsp;JointcQ dittonbsp;Univeiial dittonbsp;V-quot;ooden prefs

Kinncriley�s eledlrical air thermometer Quadrant electrometernbsp;'I'ownihend�s eledtrometer

CHAP. III.

The properties of eledtrical attradtlon and repulfion, illuftrated by experiments on light bodiesnbsp;Gcner. l properties of attraction and repulfionnbsp;Exp. 13. Feather tree

Repulfion and attraction of cork balls Ditto of threadnbsp;TsoHet�s hoop and thread

� 20. A leaf of brafs moving round a ball Electrical rope dancernbsp;Electric flftinbsp;Circulating glafs ballnbsp;EleCtrical bellsnbsp;Electric fpheroidnbsp;E lectric feather and tubenbsp;Bails and glafs tumblersnbsp;29. The attraction of the different pow-

CONTENTS.

CHAP. VI.

AttraAion and repulfion confidered, relative to the two ftates'of ele�ricitynbsp;nbsp;nbsp;nbsp;- -

Exp. 30, 3t, 32, 33, 34., 3?� 3^^� 37� 3^� 39-

Deiigned to point out the adtion of the two electric powers and their contrariety, amp;c.

Exp. 40, 41, 42. On eledtrical zones, or the alternate {irata of pofitive and negative eledtncity

--- 43. On the vibratory motion between the

air and the eledtric fluid nbsp;nbsp;nbsp;- ----

.� 44 to 52. On the attraftion, repulfion, and cohelion of excited filk ribbonsnbsp;nbsp;nbsp;nbsp;---

Exp. 6g, 70, 7 I, 72. Lucid appearances of points negatively or pofttlvely electrifiednbsp;nbsp;nbsp;nbsp;80

NTS.

�� 7^. The bending of a flame of a candle at condudlors, differently eleflrlfied

CHAP. VII.

98. A cork ball playing between two bottles 100 -�- 99. A cork ball playing between the knob ofnbsp;a jar and a brafs ball connedted with the outfidenbsp;coatingnbsp;nbsp;nbsp;nbsp;�

Defeription of an improved apparatus Exp. 103, 104, �05, io6. On the luminous appearances of pointed wires while a jar is charging

C H A P.

Wire made red hot Quire of paper perforatednbsp;Magnetifm communicatednbsp;Magnetic polarity deftroyednbsp;Wire meltednbsp;Wire melted in a tubenbsp;Marks left by a chainnbsp;To melt leaf goldnbsp;To break glafs

�� 143. Prifmatic colours produced Dr. Watfon�s experiments on the diftance to whichnbsp;the ele�ric fliock may be conveyednbsp;IVi�r, V(ilia�s remarks on Dr. Watfon�s experimentsnbsp;An hypothefis.

ExP. 144. Thunder houfe � 145, 146, 147, 148. Difeharges, amp;c. thro�nbsp;balls and points

CHAP. X.

CHAP.

CHAP. XI.

Mr, Volta on imperfedt infulation Condenfing apparatus, or micro-eledlrometer de-fcribed

Sparks produced from a difcharged Leyden phial Ditto, from a machine which gives no fenlible fignnbsp;of electricity

CHAP. XU.

Of atmofpherical eledlricity Beccaria�s apparatusnbsp;EfFedls of moifture in the airnbsp;Sign of the weather�s clearing upnbsp;Fogs eledlrical

Diurnal atmofpherical electricity Eledlricity of evening dewnbsp;Exp. 180. To iilullrate the eledlricity of dewnbsp;Obfervations on a kite for eledlricity and its apparatus

Portable atmofpherical eledlrometer Oeneral laws deduced from the experiments withnbsp;the kite

Mr. Achard on ele�lrical meteorology Defcriptioji of his atmofpherical electrometer

CHAP. XIII.

C PI A P. XIV.

Of the eleCtric light in vacuo ElxP. 193. With a tall receivernbsp;An obfervation of Mr. Wilfonnbsp;Exp. 194, To fhew that eleCtricity is not repul-five of itfelf

� nbsp;nbsp;nbsp;217. To perfotate aglafs tubenbsp;*' '� ' '* 218. Magic pidture

Mr. Brooke�s elecSlrometer defcribed ExP '22^. Colour of vegetable juices changednbsp;Experiments on different elaftic fluidsnbsp;ExP. 226. By Mr. Marlham

0 ...

ELECTRICITY.

C.-H A P. I.

'T mufi: appear furprifing to every fearcher after' truth, that Eledtricity, which is nownbsp;allowed to be one of the principal agentsnbsp;employed in producing the phcenomena ofnbsp;nature, fliould have remained fo long in obfcu-rity ; for, comparatively fpeaking, its exiftencenbsp;was not known to the ancients. They were not,nbsp;indeed, altogether ignorant of the peculiar properties of thofe bodies that we now term eledtricsnbsp;fer fe; neverthelefs their knowledge was cir-cumfcribed, becaufe the mode of acquiringnbsp;it was limited. Very little progrefs, therefore,

was made in this branch of natural hiftory, till the happy period arrived, when the philofophernbsp;Was emancipated from the chains of hypotheticnbsp;reafoning, and the uncertainties of vain conjecture.

The exiftence of this fubtle, and in molt cafes invifible, power, was then traced, and many of its properties developed ; its agency was dif-covered to be univerfal, and its extent unlimited.

Electricity has been dignified in a peculiar manner, by engaging the attention of the phi-lofophlc hiftorian ; who has delineated, in a verynbsp;pleafing manner, the gradual progrefs of itsnbsp;difcoveries. He has defcribed the differentnbsp;theories that have been invented to account fornbsp;its various phccnomena ; has communicated tonbsp;the public many valuable experiments of hisnbsp;own ; and pointed out the extenfive field whichnbsp;remains to be inveftigated.

Since the publication of Dr. Prieftley�s Hif-tory, the eleCtrical apparatus has been confidera-bly augmented, and many new experiments have been made. To deferibe the one, and tonbsp;arrange the other, under fuch heads as willnbsp;point out the connexion between the experiments and the received theory of eleCtricity,nbsp;was one of the principal views I had in com-poling this effay, I alfo vvifhed to put into the

hands

Kands of my cuftomers a trad;, which might enable them to ufe, with eafe and fatisfadion,nbsp;the eledtrical machines and apparatus which Inbsp;recommends

As eledlricit)^ is in its infancpj when con-fidered as a fcicnce, its definitions and axioms cannot be ftated with geometric accuracy. I fliallj therefore, erideavour to avoid, as muchnbsp;as pdffible, the ufe of pofitive expreffion ; Inbsp;wilh to invite the reader to examine the expe-fiments himfelf, to compare them one withnbsp;another^ and then draw his own conclufions.

Rub a dry glafs tube with a piece of dry filk, prefent light bodies, as feathers, pithnbsp;balls, amp;c. to it, they will be firli attracted,nbsp;and then repelleds

Rub a dry flick of fealing waXj it will firfl attrad and afterwards repel thofe light bodiesnbsp;that are brought near to in

The fridion in the two preceeding experiments has put in action an agent, or pow'er, which attracts and repels light bodies ; thisnbsp;power is called EleCtricity.

A certain quantity or natural fhare of the eleCtric fluid, is luppofed to be difl�eminated innbsp;B 2nbsp;nbsp;nbsp;nbsp;all

AN ESSAY

all bodies, in which ftate it makes no imprcflion on our fenfes ; but when, by the powers ofnbsp;nature or art, this equilibrium is deftroyed,nbsp;and the body becomes poflelTed of more or lefsnbsp;than its natural lhare, thofe effefts are producednbsp;which we term eleftrical, and the body is faidnbsp;fo be eleiflrified.

Any fubftance, that is made by friftion to exhibit eledric appearances, is faid to be excited.

Amber, lilk, jet, dry wood, and a variety of other fubftances, being excited, attrad and repel light bodies ; thefe are called eledrics.nbsp;Such fubftances, as metals, water, amp;c. thenbsp;�fridion of which w'ill not produce this powernbsp;of attradion and repulfion, are called non-eledrics.

When the excited glafs tube, or flick of feal-ing wax, is in good order, pencils of light will dart from them fpontaneoufly, in a beautifulnbsp;manner, and a fnapping noife will be heard onnbsp;the approach of any condudor.

Let a metallic cylinder be placed upon filk lines, or upon glafs, bring an excited eledricnbsp;near to it, and every part of the metallic cylinder will attrad and repel light bodies, a3 forcinbsp;I)ly as the excited eledric itfelf.

ON ELECTRICITY.

Support a dry glafs rod on fillc lines, or by glafs, bring an excited eledlric near it, and nonbsp;attraftion or repulfion will take place ; becaufenbsp;the eledlricity cannot be tranfmitted through it.

Thole metallic bodies which poffefs the power of tranl'mitting eledlricity, are called condu�lors.nbsp;Thofe fubftances, which are impervious to electricity, are called non-conduflors.

The moft important and fingular effefts of eledlricity would have reiTiained in obfcurity,nbsp;if this power in fome bodies, to refill the palfagenbsp;of eledlricity through their fiibHances and overnbsp;their pores, had not been difcovered. Inftancesnbsp;of the truth of this aflertion will occur in almoltnbsp;every page of this effay.

We learn, from the 3d and 4th experiments, that the eleflric powers mav be communicatednbsp;to infulated condudling fubdances by excitednbsp;eledlrics; that thefe will then attradl and repelnbsp;light bodies, amp;c. fimilar totheeledlricitfelf; withnbsp;this difference only, that a conduftor, which hasnbsp;received eledlricity, parts with it at once, when itnbsp;is touched by another condudlor that communicates with the earth; whereas the excitednbsp;B 3nbsp;nbsp;nbsp;nbsp;eledlric.

AN ESSAY

Eledrify, with excited glafs or fealing wax,, two infulated cork balls, fufpended by linesnbsp;about 6 inches Ipng, and the balls will feparatenbsp;from an4 repel each other.

Eledrify one ball with glafs, the other with fealing wax, and they will be mutually attrad^nbsp;ed.

Thefe two oppofite and remarkably diftind efFeds in the attradive and repulfive powers ofnbsp;eledricity, were difcovered at an early periodnbsp;of the hiftory of this fcience

The eledric power produced by the excita-. tion of glafs is called pofitive eledricity, andnbsp;the power produced by the excitation of fealingnbsp;wax is called negative eledricity. This differ-r,nbsp;cnce was at firft thought to depend on the elec-,nbsp;trie, and that the two kinds of eledricitiesnbsp;w'cre elTentially diftind ; but it is nownbsp;known, that each of thefe powers may be produced from the excitation of either glafs ornbsp;fealing wax,

The difeovery of thefe diftinguiftiing cha-* radcriftips in eledric fubftances, engaged phi-�

ON ELECTRICITY. 7

lofophers in an experimental enquiry into the eleftric properties of moft bodies, to afcertainnbsp;whether they poffeffed the pofitive or negativenbsp;powers. By this means the catalogue of eledlricnbsp;bodies, which originally was very faiall, is nownbsp;rendered exceedingly extenfive, aswillbe feen bynbsp;the following tables, which are taken from ^nbsp;Dr. Prleftley�s Hiftory and Mr. Cavallo�s Coui-pleat Treatife of Electricity.

Catalogue of conduding fubftances.

Lime-ftone and lime juft burnt are equally imperfedl condudors.nbsp;nbsp;nbsp;nbsp;^

Marbles conduft confiderably better than free-ftone, and there is found very little difterrnbsp;ence among any of the fpecimens of marblenbsp;that have been tried.

A large piece of white fpar with a tinge of blue and femi-tranfparent, will hardly condudnbsp;in the leaft degree : pretty ftrong fparks may benbsp;taken from the prime condudor, while it is innbsp;contad with it.

A piece of agate, femi-pellucid, receives the eledric fpark into its fubftance; thoughnbsp;it will pafs over about three quarters ofnbsp;an inch of its furface to reach the finger that

AN ESSAY

A piece of flate, fuch as is commonly ufed to write on, is a much better condudlor than anbsp;piece of free-ftone, which condufts but poorly.

A piece of gypfum and plaifter of Paris con-duft very well, only the latter having a fmoother furface takes a ftronger fpark.

A piece of afbeft from Scotland, juft as it is taken from its bed, will not condudf. Whilenbsp;in contadl with the condudlor, fparks may benbsp;taken at the diftance of half an inch with a moderate eledlrification.

Vitriol of copper and of iron condudl very well, though they will not tranfmit a ftiock.

Salt-petre does not conduff fo well as fal-ammoniac. If the eledlric explofion pafles over its furface, it difperfes into a great number

Rock-falt condufls, but not quite fo well as allum; the electric fpark upon it is peculiarlynbsp;red.

Sal-ammoniac exceeds rock-falt and allum in its condu(5ting powers, but will not take thenbsp;leatt fenfible fpark; fo that it feems made upnbsp;of � an infinite number of the fmeft points.

By allum the explolion is attended with a peculiar hiding noife, like that of a fquib.

A piece of pyrites, of a black colour, takes fparks at a cotifiderable diftance from the primenbsp;condudtor, like lome of the inferior pieces ofnbsp;charcoal.

Another piece of pyrites, which has been part of a regular fphere^ confifting of a fliiningnbsp;metallic matter, will not conduct near fo well,nbsp;though much better than any other ftony fub-ftance. It is a medium betwixt a ftone andnbsp;an ore.

Black-lead in a pencil conduces a lliock feem-ingly like metal or charcoal. A fmall lump of it takes as full and ftrong a fpark from thqnbsp;prime condudor as a brafs knob.

4. Metals

AN ESSAY

A piece of gold ore from Mexico is hardly to be diftinguiflied in this refpefl from the metal itfelf.

A piece of filver ore from Potofi, though mixed with pyrites, condu�ls very well,

Two pieces of copper ore, one the moft valuable that is known, and another of only half the value, are hardly to be diftinguifliednbsp;from one another in their conducting power.

Black-fand from the coaft of Africa, which is a good iron ore, and part of which is affeftednbsp;by the magnet as much as fteel filings, is foundnbsp;to conduct electricity, but not a fliock. Sepa-tating with the magnet all that will be eafilynbsp;attracted by it, it conduCts a Ihock very well;nbsp;the reft would hardly conduCl at all.

Even the ores in which the metal is mineralized with fulphur or arfenic, as the ores of lead, tin, and cinnabar, the ore of quickfilver, arenbsp;little inferior to gold and filver ore.

Ores that contain nothing but the earth of the metal, conduCt eleCtricity little better thannbsp;Other ftones.

ON ELECTRICITY. if

Snow, fmoke, the vapour of hot water, the vacuum produced by an air pump, charcoal, amp;c.

Amber, jet, pitch and fulphur; likewife all the precious ftones, as diamonds, rubies, garnets,nbsp;topazes, hyacinths chryfolites, emeralds, fa-phires, amethyfts, opals, and efpecially tour^nbsp;malins : all rcfins and refinous compounds,nbsp;wax, filk, cotton ; all dry animal iubflances,nbsp;as feathers, wool, hair, paper, amp;c. Whitenbsp;fugar, air, oil, chocolate, calxes of metals, drynbsp;vegetables, amp;c.

The real and intrinfic difference Ijetvveen eleftrics and non-elc�lrics, remain among thenbsp;electric defiderata ; for, nothing more is afcer-r,nbsp;tained, than, that the conducing power, innbsp;fome meafure, depends upon, or is governednbsp;by heat, Glafs, refin, and many other articles,nbsp;are made condutftors by heat; while on thenbsp;contrary, cold, if not attended with moifture,nbsp;renders every electric fubffance more eledtric.

Mr, Achard, of Berlin, has publifhed, in Rozier�s Journal de Phyfique, a very ingeniousnbsp;paper on this fubjedl; in which he proves, bynbsp;experiment, iff. That certain circumffances willnbsp;t;aufe a body to condu�t eledtrlgity which before .

was

AN ESSAY

was a non-condudor. 2d. That thefe circumftan-ces are the degrees of heat to which this body is fubjeded. He endeavours to Ihew, that thenbsp;principal changes which take place in any fub-flance from an increafe of heat are an augmentation in the fize of its pores, and an increafe of velocity in the igneous particles contained in, andnbsp;ading on, that body. He then proves, that thenbsp;' laft circumftance does not occafion the alterationnbsp;in the eledric properties; and infers, agreeablenbsp;to the lyftem of Mr. Euler, that the principalnbsp;difference between condudors and non-condud-ors of eledricity conlifts in the fize of the poresnbsp;of the conftituent parts of the bod)'.

In another interefling paper, which is pub-lifhed in the Memoirs of the Academy of Berlin for 1779, Mr. Achard. has flrewn the analogy between the produdion and effeds ofnbsp;eledricity and heat ; and alfo, between thatnbsp;property in bodies by which they condud thenbsp;eledrical fluid, and that which renders themnbsp;fufceptible of heat. He gives an account of anbsp;new inftrument, adapted to afcertain the quantity of the eledrical fluid which is condudednbsp;by bodies of different natures, placed in thenbsp;fame circumftances.

By means of this inftrument it is poffible to afcertain, with great accuracy, the quantitynbsp;of eledricity a body lofes in a given time, by

touching another body that is not eledlrified. He has not yet favoured the public with annbsp;account of the experiments he has made withnbsp;it ; only fays, that he has conftantly ob-ferved, that thofe bodies which lofe and receive with difficulty their prefent degree ofnbsp;heat, receive alfo, and lofe with difficulty, theirnbsp;eledtricity. A defcription of the inftrumentnbsp;will be given in this effay.

A lift of eledric fubftances, and of the different eledricities produced by them.

1 nbsp;nbsp;nbsp;f Every fubftan

gt; PoEtive lt; tried, except Jnbsp;nbsp;nbsp;nbsp;t a cat.

Pofitive gt;

^ metals.

f Woollen cloth,quills, wood. Negative lt; paper, fealing wax, whitenbsp;(, wax, the human hand.

Pofitive j Amber, air. �

I Negative ^ Diamond, human hand,

f Metals, filk, loadllone,

(Pofitive -J leather, hand, paper, C baked wood.

^Negative Other finer lurs.

� i. e. By blowing with a pair of bellows upon it. By this means many e'edlrics may be excited, and fome betternbsp;if the air blown is hot, although, in both cafes, very littlenbsp;eledricity can be obtained.

White

Many circumftanccs, apparently triflings will OGcafion an alteration in thefe contrary eledlrici-tiesi It has been faid, that of two equal fub-gt;nbsp;fiances rubbed together, that which fuffers thenbsp;greateft fri�tion, or is moft heatedj acquiresnbsp;the negative eledfricity* Though this in manynbsp;cafes holds true, with rcfpe�t to filk ribbons,nbsp;yet Mr. Bergman fays, that if the ribbon A benbsp;black, it will never become pofitive, unlefs B benbsp;black likewife. With pieces of glafs the effedfcnbsp;is contrary ; for, if they are both equal, thenbsp;piece A, which is drawn acrofs the piece B^nbsp;becomes negative ; and B, which fuffers thenbsp;greateft fridlion, becomes pofitive* Heatingnbsp;by fire produces the fame effedt as the greater

ON ELECTRICiTY. i$

friftion. If one piece of glafs be thicker than the other, the former becomes pofitive, thenbsp;latter negative. Coloured glafs, even whennbsp;heated, becomes negative, if rubbed with common white glafs. If a piece of blue glafs isnbsp;rubbed againft a green one, the blue glafs becomes ftrongly pofitive, amp;c. Bergmannbsp;Swedifh Tran. 1765.

The eledtricities produced by hair and glafs rubbed together feem to balance each Other,nbsp;and are therefore different according to the manner of rubbing and the quality of the hair.

Hair of a living animal, or hair newly cur, W'hen rubbed with a glafs tube lengthways, isnbsp;pofitive; and here, the glafs, which fufters thenbsp;greateft fridtion, is negative. But if the glafsnbsp;tube be drawn acrofs the animal�s back, ofnbsp;acrofs a fkain of hair newly cut, the glafs becomes pofitive. Old dry hair, rubbed on glafsnbsp;or on living hair, always becomes negative ;nbsp;but, if the hair is a little greafed with tallow,nbsp;the fame effedl is produced as vvlth living hair.inbsp;Wilke Swed. Tranf. 1769.

Eleftrics differ from each other with refpedt to the facility with which they arc excited,nbsp;their force when excited, and the pow'er w'ichnbsp;which they retain the effedts of the excitation,

i6 nbsp;nbsp;nbsp;ANESSAY

Silk feems preferable to any other electric fubftancc, for exhibiting a permanent and ftrongnbsp;attraflive and repullive power.

Glafs appears to have the advantage in exhibiting the electric light, attraftion and repul-fion in quick fucceffion, in a very vigorous though not a durable manner.

Negative eledtrics, as amber, gum-lac, ful-phur, refin, and all refinous fubftances, exhibit the eledric appearances for the greateft length of time. A Angle excitation is fuffici-ent to make them do fo for many weeks, innbsp;favourable circumftances. They are alfo remarkable for the ftrong eledlric powers whichnbsp;they communicate to conducing bodies thatnbsp;come in contadt wdth them ; and which theynbsp;will continue to communicate for a confiderablenbsp;time.

CHAP.

AS foon as the properties of elcclricity were in fome meafure developed, the philofo-pher and the artili concurred in contriving andnbsp;executing a variety of machines to excite and^nbsp;accumulate this extraordinary agent. Thenbsp;greater part of thefe have been laid afide, innbsp;proportion as the fcience advanced, and itsnbsp;boundaries were extended. I fhall, therefore,nbsp;only deferibe that eledrlcal machine which isnbsp;now in p-eneral ufe, whofe conftrudlion is lim-pic, and well adapted to produce the elcdtricnbsp;fluid in great quantities, and to tranfmit itnbsp;in full and continued llreams to the prime con^nbsp;duclor.

big. I and 2, Plate I, reprefent two cleffri-cal machines made on the moll approved con-flruiftion. They are both mounted and ufed m the fame manner, and differ only in the me-chanifm by which the cylinder is put in motion.

The cylinder of fig. 2 is turned round by means of the two w^heels a b, c d, which adtnbsp;on each other by a catgut band, part of whichnbsp;is feen at c and f; that of the machine repre-

AN ESSAY

fented in fig. i, is put in motion by a Ample winch, which is lefs complicated than thenbsp;other, and is not liable to be out of order.nbsp;Many practical eledlricians, however, prefer anbsp;machine which is moved by a multiplying wheel. They think that it fatigues thenbsp;operator lefs than that which is moved by anbsp;Ample winch ; and further, that a moderate in-creafe of velocity in the cylinder augments thenbsp;momentum of the electric Auid, and produces anbsp;greater quantity of it in the fame time, whichnbsp;prevents its being abfqrbed by the cuAiion.

As the two machines, which are reprefented in Ag. i and 2, plate I, are nearly Amilar, thenbsp;fame letters of reference are ufed in deferibine

Fig. 1 and 2. ABC reprefents the bottom board of the machine, the two perpendicularnbsp;fupports D, E, which fuftain or carry the glafsnbsp;cylinder F G H I, are Armly Axed to the boardnbsp;ABC; the axis by which the cylinder isnbsp;rnoved is Axed into two caps, which are madenbsp;fometimes of brafs, and fometimes of wood ;nbsp;one is cemented on each end of the cylinder.nbsp;K, Ag. I and 2, reprefents one of thefe caps.nbsp;The axis of the cap K paffes through the fup-port D; on the extremity of this axis a Amplenbsp;winch is Atted, as in Ag. i, or a pulley, as innbsp;Ag. 2. The axis of the other cap runs in a

�inall hole which is fitted into the top of the fupport E. O P is the glafs pillar to whichnbsp;the cufliion is fixed, T a brafs fcrew at the bottom of this pillar, which is to regulate thenbsp;prefiTure of the culhion againft the cylinder;nbsp;g h i a piece of filk that comes from the undernbsp;edge of the cufhion and paffes over the cylinder till it almoft touches the colleaing pointsnbsp;of the condudfor. Near the top of the glafsnbsp;pillar O P is an arm of Wood, to fupport a con-dudtor connedled with the cufliion, which isnbsp;called a negative c�ndudt�r* In both figuresnbsp;this is fuppofed to be fixed clofe to the cufhion,nbsp;and to lye parallel to the glafs cylindeti Innbsp;fig. I it is brought forwards, or placed too nearnbsp;the handle, in order that more of it may be innbsp;fight, as at R S; in fig. 2, the end R S onlynbsp;is feen-.

Y Z, figi I and 2, reprefents the pofitive prlnle condudfor, or that which takes the electric fluid immediately from the cylinder, L Mnbsp;the glafs pillar by which it is fupported andnbsp;infulated, and V X a wooden foot or bafe fornbsp;the glafs pillar. In fig. r, this condudtor isnbsp;placed in a dircdlion parallel to the glafsnbsp;cvlinder; in fig. 2 it ftands at right anglesnbsp;to the cylinder; it may be placed in cithernbsp;pofition oceafionally, as is moft convenient tonbsp;the operator,

Ca nbsp;nbsp;nbsp;If

AN ESSAY

If the negative condudtor is required to be placed at right angles to the cylinder, and parallel to the conduftor Y Z, fig. 2, it muftnbsp;be fixed on an infulating ftsfiid, and be conned;-cd with the cuflilon by a wire, whieh paflTesnbsp;under the c\Under.

Eut the machine in adlion, conned the cu-fliion by a chain with the ground, and thofc bodies which communicate wdth the pofitivcnbsp;condudor w'ill be eledrified pofitively. Conned the pofitive condudor with the earth by anbsp;chain, take off the chain from the cufhion,nbsp;and thofe bodies which communicate with thenbsp;negative condudor will be eledrified negatively.

The prineipal parts of an eledrical machine are, ift. The eledric, as the glafs cylinder.

Before the eledrical machine is put in motion, examine thofe parts of it which are liable to be injured by fridion, or by dirt or grit between the rublping furfaces, particularlynbsp;the axes which w^ork in the wooden fupporta

D and

ON ELECTRICITY. it

D and E, likewife the axes of the large wheel c d, fig. 2. When the culhion is taken off, the cylinder fhould move exceedingly free. If, onnbsp;turning it round, any grating or difagreeablenbsp;noife is heard, difcover the place from whencenbsp;it arifes, wipe it clean, and then rub over it anbsp;fmall quantity of tallow. Examine, in thenbsp;fame manner, the axis of the large wheel c d,nbsp;fig. 2. Put a drop of oil occafionally to thenbsp;axis of the cylinder; examine the fcrews thatnbsp;belong to the frame or cylinder, and if theynbsp;are loofe tighten them.

Wipe the glafs cylinder carefully, to free it from that moifture which glafs attra�fs fromnbsp;the air; be particularly attentive to leave nonenbsp;on the ends of the c\'linder, becaufe any dampnbsp;that remains on thefe will convey the eledlri-city from the c\'linder to the fupports, amp;c.

Take care that no dull, loofe threads, or any filaments adhere to the cylinder, its frame, thenbsp;condudtors, or their infulating pillars; becaufenbsp;thefe will gradually diffipate the electric fluid,nbsp;and prevent the machine from acting powerfully.

Rub the glafs cylinder firft with a clean, coarfe, dr^q warm cloth, or a piece of waflrnbsp;leather, and then with a piece of dry, warm,nbsp;foft filk ; do the fame to all the glals inlulatingnbsp;pillars of the machine and apparatus ; thefe pil-.

C 3 nbsp;nbsp;nbsp;lurs

AN ESSAY

lars muft be rubbed more lightly than th* cylinder,, becaufe they are varniflied,

A heated Iron is fometimes placed on the-foot of the condudlor, to evaporate the moiflure-which would injure the experiments..

In (prder to find out an effeftual mode of exciting powerfully an. electrical machine, it is neceffary to frame an idea of the mechanifm bynbsp;which the cylinder extracts the eledlric fluidnbsp;from the cufliion, and thofe bodies, which arenbsp;connedled with it; I have, therefore, fubjoinednbsp;thofe conjedtures on which I have worked, andnbsp;by which I have been able to, excite, in thenbsp;moil powerful manner, thofe machines whichnbsp;have paflfed through my hands..

It appears to me, that the refiftancc of tho air is leflened, or a kind of vacuum is produced, where the cufliion is in clofe contadl withnbsp;the cylinder. The eledlric matter, agreeablenbsp;to the law obferved by all other elaflic fluids,nbsp;is preflfed towards that part where it finds leaftnbsp;refiftance ; the fame inftant, therefore, that thenbsp;cylinder is feparated from the cufliion, the firenbsp;ilTues forth in abundance. The more perfectnbsp;the continuity is made, and the quicker the fo-^nbsp;lution of it, the greater is the quantity whichnbsp;will proceed from the cufliion. But, as thenbsp;fluid in this fituation will enter with aviditynbsp;^very conducing fubftance that is near it, if

any

ON ELECTRICITY.

�any amalgama lies above that part of the cu-fliion which is in contadl with the cylinder, it will abforb and carry back part of the eledtricnbsp;fire to the, refcrvoir from \Vhence ,it was extracted.nbsp;nbsp;nbsp;nbsp;...

If thefe conjedltires be true, to excite an cleftrical machine efFeclually, we muft,

ift. Find out thofe parts of the cufliion which are preffed by the glafs cylinder.

3d. Make the line of contadl between the cylinder and cufhion as perfedl as poffible.

About the year 1-772 I applied a loofe flap of leather to the front of the cufiiion ; the amalgama was fpread over the whole of the flap ; thenbsp;cufiiion was then put in its place, and the loofenbsp;flap of leather doubled down, or rather turnednbsp;in, more or lefs, till by fucceffive experimentsnbsp;the fituation was difcovered which producednbsp;the grcateft cffedl; for, by this means, thenbsp;quantity of amalgama afting againft the cylinder was leffened. I was naturally led tonbsp;contract the breadth of the cufiiion, and placenbsp;it in fuch manner that it might be eafily raifcdnbsp;or lowered.nbsp;nbsp;nbsp;nbsp;'

C 4 nbsp;nbsp;nbsp;The

AN ESSAY

The advantages gained by this method were confiderably improved by a very ingeniousnbsp;Gentleman. He glued a bit of leather on anbsp;large piece of cork, and placed his amalgamanbsp;on the leather ; with this he rubbed that zonenbsp;of the glafs cylinder which bears againft thenbsp;cufhion. By this excellent contrivance, thenbsp;line of contact between the cylinder and oufhionnbsp;is rendered very perfect, the fmaller pores ofnbsp;the glafs are filled with the amalgama, and thenbsp;fupcrfluous parts of it are depofited on the cti-fhion.

Beccaria fuggefts, that the amalgama thus depofited on th� furface of the glafs forms anbsp;continued feries of conducting particles, whichnbsp;carry the fire to the prime condu�tor, and,nbsp;under certain circumftances, back again to thenbsp;cufhion.

Another ingenious cledtrlcian afcertains the line of contaftj formed between the cylindernbsp;and cufliion, by placing a line of whiting,nbsp;which had been previoufly diffolved in fpirlts ofnbsp;wine, on the cylinder : on turning the cylindernbsp;this whiting is depofited on the cufhion, andnbsp;marks the places which bear againft the cylinder ^nbsp;the amalgama is to be placed only on thole partsnbsp;which are marked by the whiting.

Either of thefe modes will fucceed. If the firft is ufed, no ataalgama is to be placed on

the cufliion; that which is rubbed into the c\dinder, and depofited by it on the cufliionnbsp;in its revolutions, will produce an aftonifliingnbsp;quantity of fire. In either method, when thenbsp;cylinder is rubbed with the amalgamated leather, that part of the oil, or black filk, whichnbsp;lies above the cufhion, is to be turned back,nbsp;and it, by accident, any particles of amalgama flick to it, they muft be wiped off carefully.

If the cle�fricity of the cylinder grows lefs powerful, it is eafily renewed by turning backnbsp;the filk which lies over it, and then rubbingnbsp;the cylinder with the amalgamated leather.

A very fmall quantity of tallow placed over the amalgama, is obferved to give more forcenbsp;to the eledric powers of the cylinder.

When the cylinder is put into good adion, a number of circular lines of fire will iffue fromnbsp;the cufliion ; prefent a row of metallic pointsnbsp;towards thefe, and they wjH difappear. Thenbsp;conduding fubflance colleds the eledric fluidnbsp;before it can take thofe appearances, or be diffi-pated into the air.

Hence we learn, that to prevent a lofs of the eledric fluid which is excited, we muft prevent the air from ading on the fluid, which isnbsp;put in motion by the excitation ; for the air

not only refills the emiffion of the fluid, but alfo diflipates what, is colledled by means qfnbsp;the conducing fubftances, which are continually floating in it.- '

Thefe ends are efFcdlually anfvvered by letting a non-conducllng fubftance proceed from thenbsp;line of conta�l to the collecfling points of thenbsp;prime condudor, and placing thefe pointsnbsp;within its atmofphere. When no amalgama is put on the cufliion, a plain piecenbsp;of black filk, or one flightly Impregnated withnbsp;bees-wax, fixed to the under edge of thenbsp;cufhion, and proceeding from thence to thenbsp;collecting points of the condudtor, will anfwer.nbsp;exceeding well. If the amalgama is placed onnbsp;the cufhion, a piece of oil�d-filk feems to anfwernbsp;the beft.

I was Informed, by an Ingenious friend, that he had for many years ufed a piece of blacknbsp;filk, which was impregnated from one end tonbsp;the other with amalgama, in which a fmallnbsp;quantity of bees-wax had been mixed ; this henbsp;rubbed into the filk with a piece of fponge.nbsp;If the force of the machine diminiflied while itnbsp;W'as in ufe, he refreflied it by, holding an amalgamated fponge againft the cylinder when itnbsp;was revolving.

It is often very advantageous to dry the oil�d or black filk before the machine is ufed.

The operator- ought not to think his machine in good order till it pours forth the fire innbsp;great abundance, and ftrong denfe : fparksnbsp;are obtained in quick fuccefEon from thenbsp;conduclor. When the conductor is removed,nbsp;the fire fhould fparkle round the cylinder, quot;andnbsp;throw out many beautiful brufhes of light.

Two kinds of amalgama are much in requefi: at prefcnt. One is made of quickfilver fivenbsp;parts, zink one part, melted together with anbsp;fmall quantity of bees-wax : the other is thenbsp;aurum mufivum of the fliops. I find it difficult, after many trials, to fay which of thefenbsp;adt the bcft.

The following experiment feems to illuftrate and confirm the foregoing conjedfures on thenbsp;mechanifm by which the fluid is extradle'dnbsp;from the cuflrion, ancl the bodies connedtednbsp;with it.

Break a flick of fealing-wax in two pieces j thofe extremities that were contiguous will benbsp;found elcdlrified with contrary powers; one willnbsp;be pofitively, the other negatively, eledfrified.

Every eledlrical machine ought to be fur-niffied with an infulated cufhion and two prime condudfors, one for pofitive, the other for negative eledfricity ; as, by thefe,. either eledtri-

AN ESSAY

city is produced at pleafure, a greater number of experiments may be performed, and the properties of the ele�fric fluid more eafily explained.

Connedt the pofitive condudtor by a chain �with the table; turn the cylinder, and thenbsp;cufliion will be found to be. negatively eledlri-fied. Take the chain off from the pofitivenbsp;CCMidudlor, and both will exhibit figns of electricity ; but any eledlrified body which is at-tradled by the one, will be repelled by thenbsp;other. If they are brought fufficiently near tonbsp;each other, fparks will pafs between them, andnbsp;they will adj on each other ftronger than onnbsp;other bodies. If they arc connedled together,nbsp;the efedlricity of the one will deftroy that ofnbsp;the other; for, though it feems to proceednbsp;from the cufliion to the condudlor, the two,nbsp;when thus conjoined, will exhibit no figns ofnbsp;eledlricity, becaufe the fire is continually circulating from one to the other, and is keptnbsp;always in the fame ftate.

We fee, by this experiment, that eledtric appearances are produced both in the elcdlricnbsp;which is excited, and the fubftance by which it isnbsp;excited, provided that fubftance be infulated ;nbsp;but their eledlric powers are dircdlly reverfe of

ON ELECTRI Cl T Y. 29

If the culhion and the conduiflor are both mfulated, it is obferved, that the lefs cledlricnbsp;fluid is obtained, the more perfedl the infulationnbsp;is made.

The moifture which is at all times floating in the air, together with the fmall points, fromnbsp;which it is impoflible totally to free the cufhion,nbsp;do not permit It to be perfedlly infulated, fo asnbsp;to afford no fupply of eledric matter to thenbsp;cufliion.

If the air, and other parts of the apparatus^ are very dry, little or no ele�fricity will benbsp;produced in the above-mentioned circumftan-ces.

From this experiment It Is inferred, that the eledrlc powers do not exift in the eledricsnbsp;themfelves, but are produced from the earth bynbsp;the excitation of eledrics ; or, that the eledricnbsp;matter on the prime condudor is not producednbsp;by the fridion of the cylinder againft thenbsp;cufliion, but is colleded by that operation fromnbsp;it, and from thofe bodies which are connedednbsp;with it.

As Dr. Franklin feems to have fuggefted this idea firfl, that the clcdric fluid is colleded

AN ESSAY

from the earth, I have fubjoined his own ac^ count of the experiment which led him to thisnbsp;conclufion�

Let one perfon ftand on wax and rub a glafs tube, and let another perfon on wax take thenbsp;fire from the firft, they will both of thetn (provided they do not ftand fo near as to touchnbsp;each other) appear to be ele�lrified to a perfonnbsp;ftandinp- on the floor ; that is, he will perceivenbsp;a fpark on approaching either of them with hisnbsp;knuckle. 2. But if the perfons on wax touchnbsp;one another during the excitation of the tube,nbsp;neither of them will appear to be eleftrified;nbsp;3. If they touch one another after the excitingnbsp;the tube, and draw the fire as aforefaid, therenbsp;will be a ftronger fpark between them than wasnbsp;between either of them, and the perfon on thenbsp;floor. 4. After fuch a ftrong fpark neither ofnbsp;them difeover any eleflricity.

Thefe appearances he accounts for thus : he fuppofes the eledlric fire is a common element,nbsp;of which each of the three perfons has hisnbsp;equal fliare before any operation is begun withnbsp;the tube. A, who ftands upon wax knd rubsnbsp;the tube, colle�ls the eledlrical fire from himquot;nbsp;felf into the glafs, and his communication withnbsp;the common ftock being cut off by the wax,

ON ELECTRICITY.

Ills body is not again immediatel)^ lupplied. B, �who alfo ftands upon �wax, paffing his kn'ucklenbsp;along the tube, receives the fire which was col-ledled from A, and being infulated he retainsnbsp;this additional quantity. To C both appearnbsp;eledlrified ; for he, having only the middlenbsp;quantity of elcdtrical fire, receives a fpark onnbsp;approaching B, �who has an over qviantity, butnbsp;gives one to A, who has an under quantity.nbsp;If A and B approach to touch each other, thenbsp;fpark is ftronger, becaufe the difference betweennbsp;them is greater. After this touch there is nonbsp;fpark between either of them and C, bccaufe thenbsp;electrical fluid in all is reduced to the originalnbsp;equality. If they touch while eledtrifying thenbsp;equality is never deftroyed, the fire is only-circulating : hence we fay, that B is eledtrifiednbsp;pofitively, A negatively.

A defeription of forne parts of the eleclrkal apparatus, �which could not be regularlynbsp;introduced in the body of the work.

Fig. r, Plate II, reprefents a common dif-Charging rod ; it is generally made of brafs wire, with a ball at each of its ends. To difgt;nbsp;charge a leaden bottle with it, hold the femi-circular part in the hand, place one ball ofnbsp;the difeharging rod on the coating of thd

AN ESSAY'

phial, then bring the other to touch the knob o^ tire wire which communicates with the infide,nbsp;�when an explofion will enfue, and the phial willnbsp;be difcharged.

Fig. 2, Plate is a jointed difcharging rod �with a glafs handle, the legs of which may benbsp;moved and fet to any given diftance from eachnbsp;other by means of the joint G; the extremitiesnbsp;of the legs are pointed^ the points enter intonbsp;the balls a b, which fcrew on the legs, andnbsp;from v/hich they may be unfcrewed at plea^nbsp;fure ; fo that either the balls or the points maynbsp;be ufed as occafion requires^

Fig. 3, Plate II, reprefents the univerfid difcharger ; an inftrument which is of very ex-tenfive ufe in forming communications to di-teamp; or convey the ele�lric fhock through anynbsp;part of a given iubftance. Many examples otnbsp;the utility of this inftrument will occur in thenbsp;courfe of this efTay. When the univerfal difcharger is made on a large fcale, it is fuperior'nbsp;to any apparatus hitherto contrived to enable anbsp;perfon to eledrify himfelf. AB, fig. 3, ph H,nbsp;is the wooden bafe of this inftrument; on thisnbsp;are fixed two perpendicular glafs pillars c d,nbsp;on the top of each of thefe is cemented a brafsnbsp;cap, to which is fitted a double joint, or onenbsp;which has both a vertical and horizontal motion ; on the top of each joint is a fpring tube,

which

which receive the wires, E T, E F; thefe wires may be fet at various diftances from each other,nbsp;and turned in any diredlion ; the extremities ofnbsp;the wires are pointed, the points are eoverednbsp;Occafionally by the brafs bails, which are madenbsp;to fit on the wires by fpring fockets. G H is anbsp;fm'all Wooden table, on the furface of which anbsp;flip, of ivory is inlaid : this table is furnilhednbsp;with a cylindrical ftem, which fits into a cavitynbsp;of the pillar I; it may be raifed occafionally tpnbsp;various heights, and fixed at any one of themnbsp;by the fcrew K. ,

Fig. 4, Plate II,- is a little wooden prefs, fur-nifhed with a item, which fits the cavity in the pillar I, Fig. 3, into which it is to be placednbsp;occafionally, when the table GH is removed.nbsp;The prefs confifts of two boards, wEich arenbsp;brought clofe to each other by means of thenbsp;fcrews a a.

Fig. 5, Plate 11, is Mr. Kinnerfly�s eledtri-Cal air thermoineter ; ab is a glafs tube, on each �nd of which a brafs cap is Cemented ; cdnbsp;is a fmall glafs tube, open at both ends, whichnbsp;pafles through the upper, and defcends nearlynbsp;to the under plate; a box fcale, which is divided into inches and tenths of inches, is fittednbsp;to the upper part of this tube; g is a brafsnbsp;wire with a ball on it, which is fcrewed to thenbsp;under plate, a fimilar wire is made to pafsnbsp;Dnbsp;nbsp;nbsp;nbsp;through.

AN ESSAY

through a collar �f leathers on the upper jgt;lat:e^ and may be placed at any convenient diftancenbsp;from the lower wirei

Eledricians have Idng wilhed for an inftru-ment which would afcertaih^ in an exadf and invariable manner^ the degree of ele�lricitynbsp;excited when any exp�riment is made.- For thisnbsp;purpofe a great many contrivances have been!nbsp;propofed and executed, which, upon trial, arenbsp;all found to be very defeflivei

Mr. Achard, who has conlidered the fub-quot; jeft with attention, fays, that an eleclrometernbsp;ought to have the following properties-:

1. nbsp;nbsp;nbsp;That it lliould be limple in its conftruc-tion, and not compdfed of many parts^

2. nbsp;nbsp;nbsp;It flrould not be affedted by the variationsnbsp;of the atmofpherei-

The eledlric power fhould be expreffcd by a fixed and invariable force, as that of gravity.

6. That the obferver be enabled to read off the divifions at a diftance, which will preventnbsp;his weakening the influence of the eledricnbsp;powers.

This gentleman has publifhed a very ingenious paper on the nature and properties of eledtrome-

lers, which I meant to have introduced in this place if I had net been informed that he had made ftillnbsp;further improvements on the fubjeft which henbsp;defigned to communicate to the public.

Plate II, Fig. 6, reprefents the quadrant eledlrometer, the mod ufeful inftrument of thenbsp;kind yet difeovered, as well for meafuringnbsp;the degree of eleftriclty of any body, as to af-certain the quantity of a charge before an ex-plofion ; and to difeover the exad time thenbsp;eieftricity of a jar changes, when, withoutnbsp;making an explofion, it is difeharged by givingnbsp;it' a quantity of the contraty eleftricity; Th�nbsp;pillar L M is generally made of wood, thenbsp;graduated arch NOP of ivory, the' rod RSnbsp;is/made �f very light wood, with a pith ballnbsp;at the extremity; it turns upon the center ofnbsp;the femicifcle,' fo as always to keep hear itsnbsp;furface; the extremity of the ftem L M maynbsp;cither be fitted to the condpftor or the knob ofnbsp;ajar. When the apparatus is eledrified, thenbsp;tod is .repelled by the ftem, and moves alongnbsp;the graduated arch of the femicircle, fo as tonbsp;mark the degree to which the �ondudor is electrified, or the height to which the charge ofnbsp;the jar is advanced.

Beccaria recommends fixing the index between two femicircles, becaufe when it is placed over one only, the eledricity of this re-*nbsp;D znbsp;nbsp;nbsp;nbsp;pels

pels and counterafts the motion of the indent; Other improvements and variations have beennbsp;made in this inftrument, which will be de-feribed hereafter.

Plate 11^ Fig. 9, is an electrometer which was contrived many years fince by Mr. Townfend^nbsp;to afeertain the real force of the eledtric explo-lioii. a b is a fmall ivory plate, c a loofe conenbsp;of ivory to be placed on the plate ab, efg anbsp;circle which turns freely on two Centers, an arm,nbsp;d, of wood proceeds from this circle and lyesnbsp;oii the cOne of ivory. The difeharge is madenbsp;to pafs under the cone which throws up the armnbsp;d, the elevation of which is marked by thenbsp;index h ; a piece of filk ftring Is fixed at onenbsp;end to the bottom board at i,- and paffesnbsp;over the wheel, a weight k is tied to the othernbsp;end to regulate the ffiftion of the circle efg.

Fig. 8 is an infulating llool ; the feet are of glafs. When it is ufed, the infulation will benbsp;rendered more perfect by placing a fhe�t of pagt;nbsp;per well dried under the feet of the ftool*

e H A p. III.

Properties of E�eSiric AttraBion and Repulfon, illujlr(ited by Experiments onnbsp;light Bodies.

Natural phUofophers were originally incited to cpnfider the nature of eleftri-city from its ftrong attraftive and repulfivenbsp;powers. The phoenomena exhibited by thofenbsp;rpyfterious properties are fo various and fonbsp;pleafing, that they were led, as by enchantnbsp;rnent, to purfue the fubjedt; and have beennbsp;richly rewarded by the difeoveries, which arenbsp;b^oth interefting and important.

The powers of Genius have been exerted with induftrious ardour to inveftigate the caufesnbsp;of thofe properties ; but, we are forry to own,nbsp;they ftiil � remain involved in deep obfeurity,nbsp;and we are ftill totally ignorant of. thatnbsp;mechanifm by which light bodies, when electrified, approach or recede from each other.

To enter into a difeuffion of the difficulties which perplex this fubjedt, would lead me toonbsp;far from the defign of this effay; I ffiall, therefore, proceed to ftate thofe general properties,nbsp;or modes, of adtion which are obferved in elec-D 3nbsp;nbsp;nbsp;nbsp;trie

3^ nbsp;nbsp;nbsp;� AN � f �� A 't

trie attraftion and repulfion, and then deferibe the experiments from, which thofe propertiesnbsp;have been deduced, or by which they arenbsp;illufi:ra,ted.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;,nbsp;nbsp;nbsp;nbsp;,

1. nbsp;nbsp;nbsp;The ek�lric fluid, when in adtion, difjnbsp;pofes or places light bodies in fuch manner asnbsp;will beft facilitate its tranfmiflion through them,nbsp;with the greateft velocity ; and this in ptopor^nbsp;tion to the gravity of the body, its condudlingnbsp;power, and the ftate of the air.

3. nbsp;nbsp;nbsp;Bodies eledlrified negatively repel eachnbsp;other.

4. nbsp;nbsp;nbsp;Bodies cleflrified by contrary powers at-tradt each other ftrongly,

5. nbsp;nbsp;nbsp;Bodies that are eledlrified, attraft thofenbsp;fubftances which are not eledlrified.

6. nbsp;nbsp;nbsp;Thqfc fubftances that are brought withinnbsp;the influence of eledlrified bodies, become pof-feflfed of a contrary el.edlricity ; or, eledlrifiednbsp;fubftances, without parting with their ownnbsp;eledlricity, adt upon other bodies in their neighbourhood, producing in them an eledtricitynbsp;which is contrary to their own ; or, bodies

ON'ELECTRICITY. 39

which are iromerged in an elcftric atmofphere, always become poffclTed of a contrary eleftricitynbsp;to that of the body in \yhofe atniofphere theynbsp;are jmnicrged.

Fix the end A qf the wire A B, Fig. lO, in the fmall hole which is at the end of thenbsp;prime condudor .; turpthe cylinder^ and the feathers, which are connecled with the wire by^nbsp;linen threads, wdll feparate from each qthcr ;nbsp;the fibrous and downy parts will become turgid, and expand in a pleafing manner, in anbsp;variety of directions.

Prefent a metallic point, the finger, or any other conducting fubftance to the feathers, thenbsp;downy parts thereof will immediately co.llapfe,nbsp;the divergence of the feathers will ceafe, andnbsp;they will approach each other, and clingnbsp;^pund thc! non-eleftric body..

The feathers feparate from each other, and tend towards uneleftrified bodies, from thenbsp;effort made by the eledricity which is communicated to them to diffufe itfelf, and the refill-ance it meets with from the air.

Ex-

AN essay

Fix the end C of the wire CD, ifig. i r, into the hole at the end of the condudlor, put thenbsp;machine in a,�lion, and the two fmall balls cd

will recede from each other. Bring a conducl-itig fubftance within the fphere of their ajaion, and they will fly towards it; touch the con-dudor with a non-eledric, and they will immediately come together.

The balls do not always diverge fo much as might be cxpeded from the adion of their at-mofphere, becaufe they are influenced by thatnbsp;of the condudor.

The bails, or feathers, will feparate, amp;c. in the fame manner, if they are annexed to a negative condudor.

Prefent a fine thread towards an eledrificd condu�tor; when it is at a proper diftance it willnbsp;fly towards and flick to the condudor, andnbsp;convey the eledric fluid from it to the hand ;nbsp;remove the thread' to a liirall diftance from thenbsp;condudor, and It will fly backwards and forwards with great velocity, and in a very pleaf-ang manner ; prefent the fame thread towardsnbsp;ipne that hangs from the condudor, they will

it�tra�t and join each other. Bring a non-ele�tric bod)quot;, as a brafs ball, near thefe threads, the ball will repel that held by .the hand, andnbsp;attract that which is affixed to the conductor :nbsp;the tipper thread renders the brafs ball negative, and therefore goes towards it; while thenbsp;tinder thread, which is alfo negative, is repelled. Let the ball be brought near to thenbsp;lower part of the under one, and it will benbsp;attracted by it. The jundlion of the threadsnbsp;arifes from the effort the eledtric fluid makesnbsp;to diffufe itfelf through them.

To the edge of the brafs hoop bed, fig. 12, are fattened, at equal dittane'es from eachnbsp;other, fix or feven pieces of thread, aboutnbsp;four inches long ; a wire proceeds from thenbsp;hoop which fits into a cavity in the pillarnbsp;D ; ze, is a brafs wire, to one end ofnbsp;which are fattened feveral fmall pieces ofnbsp;thread; fit the plain end of the wire into thenbsp;hole at the end of the condudtor, place thenbsp;hoop b c d at right angles to the wire z e, andnbsp;diredly over the threads at the end z ; turnnbsp;the cylinder, and the threads tied to the hoopnbsp;will be attraded by thole which are fattenednbsp;to the wire z c, and will point towquot;ards each

AN ESSAY

other as fo many radii of a circle. The elec^f trie fluid pafles from the threads of the wirenbsp;Into thofe of the hoop, and thus occafions thenbsp;fceming attraiStion between them.

Place the hoop b c d on an infulating {land, and when it js faturated with the ekdlric mat?nbsp;ter, the threads which are tied to it will henbsp;repelled by thofe of the wire ; touqjr the hoop,nbsp;and they will be again attracted. If the handnbsp;is brought near the threads, they will quitnbsp;their central diredtion and move towards it,nbsp;The ends of the threads appear luminqus iivnbsp;the dark.

Sufpend the fmall metal plate F, fig, to the condudlor by J;he hook H ; place the�nbsp;ftand I direflly under it, and the largenbsp;plate G on the top of the fland; the uppernbsp;part of the ftand I is moveable, fo that thenbsp;diftance of the two plates from each othernbsp;rnay be occafionally varied. Lay fmall papernbsp;images, or any other light fubftances, on thenbsp;under plate, then put the machine in adlion,,nbsp;and the light bodies will be attradled andnbsp;repelled by each plate, and move fromnbsp;pne plate to the other with confiderable velocity.

. The light bodies placed on the under plate become pofleffed of an electricity whichnbsp;contrary to that of the upper plate, and arenbsp;therefore attracted by it, and acquire the famenbsp;electricity with it ; they are then repelled, andnbsp;part with this eleCtpcity to the Hand, and arenbsp;again in a proper ftate to be attracted by thenbsp;upper plate. That thefe bodies cannot be attracted by the upper plate till they have acquired a power contrary to it, or till the equi�Tnbsp;brium of the fluid in them is diflurbed, will benbsp;evident from the following experiment.

Remove the under plate and {land, hold in its dead, by one corner, a pane of glafs,nbsp;which has previoufly been made very cleannbsp;and dry ; now, as glafs does not tranfmitnbsp;electricity, no contrariety in the eleClric ftatesnbsp;of the conductor and the light fubftances caiinbsp;be occalioned, and therefore no attraction ornbsp;repulfion is obferved.

If a finger is prefcnted to the under fide of the glafs plate, the light bodies will be attracted and repelled; the caufe of this willnbsp;be feen when the nature of the Leyden phial isnbsp;(explained.

Mr. Eeles,* fpeaking of this alternate at~ traction and repulfion, fays, they may benbsp;agreeably varied, by wetting firft the head ofnbsp;the paper images, and when thefe are dry, wetting the feet.

� When you dry the head of one of thofe images, the powder thrown out from the con-�� dudor cannot enter the image with the lamenbsp;facility with which the contrary powder fromnbsp;� the table enters at the fee.t, which are npt fonbsp;�f dry ; this will therefore afcend to the uppernbsp;� plate and remain there. Reverfe the experi-� ment, dry the feet and wet the head, andnbsp;� the images wall fix themfelves to the low'ernbsp;� plate. If the image retains fo much more ofnbsp;� the attra�led power as will balance againft itsnbsp;weiglrt,' than there is of the contrary powernbsp;Inch proceeds from the ' conduidor, thenbsp;image wall be fufpended between the two.nbsp;� plates.�

This may be effeded by making the head, � of the image broad and round, which doesnbsp;� not admit the power coming out fo readilynbsp;as the feet, being fharp, admit the powernbsp;� o-oino- in ; a minute alteration will make thenbsp;images dance or remain fixed to one of thenbsp;plates.�

Place a fquare piece of leaf brafs or filver on the under plate, hold this parallel to the uppernbsp;one, at about five or fix inches from it, turnnbsp;the machine, and the leaf will then fife upnbsp;into a vertical fituation, and remain betweennbsp;the two plates without touching either of them;nbsp;Prefent a metal point towards the leaf, and i�nbsp;will immediately fall down.

Place a brafs ball at K, fig. 14, at the end of the conductor, and when the leaf of brafs isnbsp;flifpended between the plate and ball, movenbsp;the plate round the ball, and the leaf will alfonbsp;move round without touching either ball ornbsp;plate.

A glafs cylinder is occafionally placed between the two metal plates F G, fig. 13, to prevent bran, fand, or other light fub-ftances being thrown off*

Place tw'o wires diredbly under, and parallel to, each other, fufpend one from the con*nbsp;dudlor, let the other communicate with thenbsp;table ; a light image placed between thtfe will,

�when the condudlor is ele6lrified, apj^ear like d kind of eleftrical rope-dancer.�-See fig. 15.

Cut a piece of leaf brafs, with an obtufe angle at one end, and a very acute one at thenbsp;other, prefent the large end towards an eleftri-.fied condiidor,' and when the leaf brafs isnbsp;within its atm�fpher� let it go,' it will then fi^dnbsp;itfelf to the conductor tiy the apex of its obtufenbsp;angle, and, from its continual wavering motion, will appear to be animated.

The next experiment requires confiderable attention to make it fucceed; as a fmall difference in the apparatus, or in the force of thenbsp;machine, amp;ci will make it fail ; when it an-^nbsp;fwers, it generally affords pleafure to, and excites admiration in the fpcdators.

Fix the ring NOP, fig. 16, to the end of the condudor, place the plate G, fig. 13, on its*nbsp;ftand I under it, and at a little diftance from it,nbsp;put a very light hollow gkfs ball upon thenbsp;plate but within the ring, turn the cylinder,'nbsp;and the little ball will deferibe an orbit aboutnbsp;the ring, and turn at the fame time about its

owh

4bwn axis ; the poles of its rotation are nearly at right angles to the plane of its orbit;

Fig. 17 feprefents a finall fet of bells, the two exterior ones are conne�ted to the wire V Ynbsp;by a brafs chain, the middle bell and the clappers are fvifpended on lilk.

Hang the bells on the condudor by the hook R S, let the chain from the middle bell touchnbsp;the table, turn the cylinder, and the clappersnbsp;will fly continually from bell to bell as long asnbsp;the eleftricity continues^

The brafs chain, which connedts the two exterior bells to the condudlor, conveys the electric fluid to them, which attradts the clappers ; thefey when they have received the eledricnbsp;fluid.^ are repelled by the exterior bell, and at-traded by the middle one, on which they de-polit their eledricity ; they are thert again at-traded and repelled by the outer bells. Holdnbsp;up, by a filk thread, the chain X, which proceeds from the middle bell, and the ringingwillnbsp;ceafe, becaufe it cannot convey the eledrienbsp;fluid communicated by the clappers to thenbsp;grounds

Fig. 18 reprefents a more elegant form of jnountiiig the bells; When this is ufed, the

AN ESSAY

knob a, flibuld corhmunicate with the cot?� cluftor.nbsp;nbsp;nbsp;nbsp;, 'nbsp;nbsp;nbsp;nbsp;_

Fig. 19 reprefents another kind. In thi? ihe clapper is fufpended from the fly bed, thenbsp;axis of the fly refl:s in a fmall hole on the topnbsp;of the glaf� pillar ef, the upper part of thenbsp;axis moves freely in, and is flipported by, ^nbsp;hole in' the brafs piece g. Bells of differentnbsp;tones are placed round the board h IK. Remove the prime conduftor, and place this apparatus in its ftead near the cylinder, \lt;fhen this isnbsp;in adlion, it will caufe the fly to turn round, thenbsp;clapper will ftrike each bell in rotation, andnbsp;thus produce a pleafing and harmonious found.

Take ic�r 12 pieces of thread, each about ten inches long, tie them together at the topnbsp;and the bottom, as in fig. 20, then fufpendnbsp;them from the conduftor ; the threads, whennbsp;eleftrified, endeavour to recede from each other,nbsp;and the knot at the bottom rifing upwards asnbsp;the repulfion of the thread inereafes, will form'nbsp;them into a fpheroidal figure.

Bring a downy feather or lock of cotton near the end of an excited tube. Or the knob of A

ON ELECTRICITY. 49

oharged Leyden phialj the feather will at firft fly towards the tube, but when it is faturatednbsp;with the eledric matter, it will recede from it,nbsp;and may be driven about the room by the excited tube till it touches fome non-conduftornbsp;to which it can impart its ele�tricity- Thenbsp;fame fide of the feather is always turned towards the tube ; becaufe, the ele6tricity acquired by the feather, is forced by the a�rion ofnbsp;the tube to that fide W'hich is fartheft from it,nbsp;which is therefore repelled.

It is eafy to perceive, from this and the foregoing experiments, that it is not the mere matter which is attra�fed, but that the different phcEnomena are occafioned by the ftate of thenbsp;eleflric fluid, in thofe fubftances which arenbsp;influenced by the machine.

Put a pointed wire into one of the holes which are at the end of the conductor, hold a glafsnbsp;tumbler over th� point, then cle�trify the con-duflor, and turn the tumbler round, that thenbsp;whole interior furface may receive the fluidnbsp;from the point, place a few pith balls on thenbsp;table, and cover them with this glafs tumblerynbsp;(he balls will immediately begin to leap up and

AN ESSAY

down as if they were animated, and will continue to move for a long time.��See fig. 2 r.

This experiment may be agreeably varied with tw'o tumblers. Eledtrify the infide of onenbsp;pofitively, of the other negatively ; put thenbsp;balls in one tumbler, and then bring the mouthsnbsp;of both in conta�:, the balls will pafs from onenbsp;to the other, till the contrariety between themnbsp;is dcftroyed.

An eledtric fubdance contained between parallel furfaces, however difpofed, is called an eledtric plate.

Eledtrified fubftances will attraft thofe which are not eledtrified, although a thin eledlric platenbsp;be interpofed between them.

. Bodies eledtrified with contrary powers at-tradl each other ftrongly, although an eledtrie plate is interpofed between them.

CHAP. IV.

AttraSlion and Keputfion corijidered^ relative to the two Jiates of EleSlricity.

TH E experiments defcribed in this chapter are fimple, eafily performed, and certainnbsp;in their refults ; and, though they may at firftnbsp;fight appear to be trifling, yet, on an attentivenbsp;examination, they will be found of confiderablenbsp;importance, as they afford a clue to invefligatenbsp;and explain a variety of eledlric phcenomenajnbsp;and exhibit, in a ftrong point of view, fome ofnbsp;the contrary cffecls of negative and pofitivenbsp;eledricity.

Thefe experiments may all be made with a fmall and portable apparatus ; confifting generally of two brafs tubes, as A and fig. 22,nbsp;each of thefe is fupported on a giafs pillarnbsp;which fcrfcws into a wooden foot H, a pair ofnbsp;fmall pith balls fufpendcd on linen threads, asnbsp;I, K, fit upon each tube by means of a fmallnbsp;brafs ring ; thefe tubes, with a piece of fealingnbsp;wax or a giafs tube, are fufficient to illuflratenbsp;the greater part of the experiments in thisnbsp;chapter, as well as fome of the principal phoe-aomena in eledricity.

AN ESSAY

The apparatus will be rendered more complete by the addition of two more brafs tubes with their (lands, a fmall Leyden phial, andnbsp;a piece of varnilhed lilk.

Mr. Wilfon, in a mafterly fraft on this fubjefl, entitled, � A (hort View of Eledlricity,� has,nbsp;with a fimilar apparatus, explained and il-luftrated all its general principles

Touch a pair of infulated pith balls with an excited glafs tube, they will become eleftrlfied,nbsp;and will feparate from each other ; the balls arenbsp;ele�lrilietl politively, and are therefore attracted by excited wax, and repelled by excitednbsp;glafs.

Hold an excited glafs tube over one of the brafs tubes, but at forae diftance from^ it, partnbsp;of the natural quantity of eleClricity containednbsp;in the brafs tube will be driven into the pithnbsp;balls that are annexed to it, by the excited glafs,nbsp;the bails will diverge with pofitive eleftrieity;nbsp;remove the excited glafs, the balls will then,nbsp;return to their natural (late and clofe.

Ex-

Eledlrify the pith balls that are fufpended from the brafs tube A, fig. 27, then bring thenbsp;end of this tube in contaft with the end of thenbsp;]tube B, the balls of which are un-eleclrified;nbsp;the ftock of eledlricity given to the tube A will benbsp;erjually divided between ea.ch pair of balls,nbsp;thofe of the tube B will open, and thofe of Anbsp;will clofe a little.

Eledrify the tubes A and B, fig. 27, equally and with the fame power, put the ends of thenbsp;tubes together, and the divergence of the ballsnbsp;will not be altered.

Eleflrify the tubes equally, but with the different powers, one with glafs, the other with wax, bring the ends of the tubes in contadl,nbsp;and the balls will clofe.

We learn from thefe experiments, that the pofitive and negative powers countera�l eachnbsp;other ; whence, if both are applied at the famenbsp;time to any body, the electricity it acquires willnbsp;be only the difference of the two, and confe-qucntly that of the flrongefl.

Hold an excited glafs tube to one of the brafs tubes, touching this tube at the fame time withnbsp;your finger, part of the natural quantity of thenbsp;cledlrlcal fluid refident in it will be forced Iv'nbsp;the excited glafs tube into the finger, removenbsp;at the fame inilant the finger and glafs, and thenbsp;balls will remain negatively eledrified.

Place the brafs tubes, A and B, fig. 2 2, in a ftrait line with their ends in contad, hold thenbsp;. dxcited glafs over the tube A, part of the electric fluid naturally refident in this will be drivennbsp;into B; feparate the tubes, the balls of A will benbsp;negative, and thofe of B will be in a pofidvenbsp;flate.

Infulate a long metallic rod, fufpend a pair of pith balls from each end of it, place one ofnbsp;the ends at about two inches from the primenbsp;condudor, the other end as far from it as poffi-ble, eledrify the condudor, and the eledricnbsp;fluid in the rod will be driven to that end which,nbsp;is furthefl: from the condudor ; fo that one end

will

o N- E L E-C T R-I CITY.

�will be electrified negatively, the other end pofitively, as will be feen by the balls.

Apply a ftick of excited wax to the tube D, fig. 23, as at A, while it remains there thenbsp;balls open with negative electricity ; raife thenbsp;wax, as at B, and the balls will clofe ; raife itnbsp;ftill higher, and they will open with pofitivenbsp;electricity.

Excited glafs held over the middle of the tube A, fig. 24, forces fome part of the naturalnbsp;quantity of eleClricity of A into the balls, andnbsp;fome part out at the two ends into the air.nbsp;.During this experiment, the balls of A are repelled by glafs, and are therefore in a pofitivenbsp;ftate; but, after the excited glafs is removed,nbsp;they in a very little time change to. a negativenbsp;ftate, becaufe part of the natural quantity hadnbsp;efcaped from the pointed ends into the air,nbsp;while the glafs was held over the tube; but,nbsp;when the glafs is removed, the over-charge innbsp;the balls will of courfe return, and difFufe itfelfnbsp;equally in the tube, but as this is not fufficientnbsp;to balance the lofs fuftained, the tube, thread,nbsp;and balls muft be in a negative ftate. '*

AN ESSAY

Place three tubes. A, B, C, fig. 25, in a line near to, or in contaft with, each other ;nbsp;excited glafs held over A forces out part of thenbsp;natural quantity of fluid contained in A intonbsp;B and C; feparate A from B and C, A will benbsp;cleftrified negatively, B and C will be in a pofi-tive ftate. Put the three tubes into their formernbsp;fltuation, the equilibrium will be reftored, andnbsp;the balls vjqU Cjollapfe.^

Place four tubes, as A, B, C, D, fig. 26, in contafl with each other ; excited glafs heldnbsp;over A forces part of the fluid contained in Itnbsp;into B, the quantity received in B will forcenbsp;out a certain portion from C into D; the moment before the excited glafs is removed fromnbsp;A, feparate B and D from A and C, afternbsp;which it will be found, that A and C are in anbsp;negative, and B and D in a pofitive, ftate. *

Excited glafs held at about one inch diftance from the end B, of a folid cylinder of glafs B, D,nbsp;fig, 2S, PI. Ill, which is fix feet long, and

* Ibid, p. 8.

ON ELECTRICITY. 5?

atout half an inch diameter, will force part of the fluid at the end B towards the remote endnbsp;D ; but, in doing this, the natural quantitynbsp;belonging to the glafs will undergo feveral alterations, which are difcovered by the effeA annbsp;excited glafs tube has on a number of pith balls,nbsp;which are fufpended at equal diftances fromnbsp;each other between B and D ; in a little Ipacenbsp;of time the eletflricity of thefe is changed, thofenbsp;that were -pofitive will become negative, andnbsp;thofe that were negative w'ill become pofitive.

If the excited glafs is held in contaft with the end B, the additional quantity received atnbsp;B will, in going towards D, eaufe feveral alterations in the denfity of the fluid in B D, butnbsp;�hefe alterations will be converfe to the former,nbsp;and after a little time will alfo be reverfed.

It may be inferred from thefe experiments, that whenever the eleftric fluid in any bod}'- becomes fuddenly more denfe in any one part,nbsp;thg fluid in the neighbouring parts will benbsp;more rare, and vice veria. Thefe alternatenbsp;changes of rarity and denfit}- muft, from thenbsp;nature of an elaflic fluid, continue to ofcillatenbsp;many times backwards and forwards before thenbsp;fluid can be at reft ; tlxough, when thefe motions are weakened to a certain degree, they arenbsp;imperceptible to the obferver. *

* Ibid, p. 18.

It is not improbable that the attradivc and repulfive motions of eleftrlfied bodies, are owing to the alternate condenfation and delatationnbsp;of the ckftric fluid on the furface of thefe boshes, as they are naturally carried where theynbsp;meet with the leaf! refiftance.

That there is a vibratory motion, or ftruggle, between the eledric fluid, when in adion, andnbsp;the air, is evident from that fenfation which isnbsp;felt when a ftrongly excited eledric is broughtnbsp;near any part of the human body ; this is fuchnbsp;ns would be occafioned by a fplder�s web drawnnbsp;lightly along the Ikin, This circumflance isnbsp;rendered more clear by an experiment made bynbsp;Dr. Prieftley, in order to difcover whethernbsp;eledricity was concerned in the freezing ofnbsp;water.

He placed two difhes with water in the open, air in the time of a fevere froft, one of them he,nbsp;kept ftrongly eledrified, and could oblerve nonbsp;difference in the time when it began to freeze,nbsp;or in the thicknefs of the ice when it had beennbsp;frozen fome time; but he obferved, on eachnbsp;flde of the eledrified wire, the fame dancingnbsp;vapour which is feen near the furface of the

It appears, from feveral experiments of Be-caria, that if the air is thoroughly exhaufted from a glafs receiver, the . attraftion andnbsp;repulfion of eledlrificd bodies within the re-gt;nbsp;4:eiver, grow languid, and foon ceafc altogether.

J^xperiments on the attraSlion and repuljion

Put a black and a white ribbon together, and draw them through the fingers; by this operation the white ribbon will be eledtrified pofi�nbsp;lively, the black negatively, and will confernbsp;quently repel each other.

Lay either of the ribbons upon a quire of paper, and draw over it amber, fealing-wax,nbsp;or any other negative eledlric, the ribbons willnbsp;be excited pofitively.nbsp;nbsp;nbsp;nbsp;^

If pofitive eledtrics are drawn over the ribbons, they v,^iil be excited negatively.

AN ESSAY

Dry two w'hite lilk ribbons at the fire, extend ithem on any finootk plane, draw the edge of a Iharp ivory rule over them; while theynbsp;continue on the plane they do not feem to havenbsp;acquired any eledlricity, yet, when taken upnbsp;fcp':rately, they are obferved to be negativelynbsp;cleclritiedj and repel each other.

When they are feparated from each other ele�|ric fparks are perceived between them, butnbsp;when they are again put on the plane, no lightnbsp;is perceived without a fecond frid:ion,

Place the ribbons on a rough conducing fub-ftance, rub them as before, and they will, on their feparation, Ihew contrary eledricities,nbsp;which will alfo difappear w^hen they are joinednbsp;together.

If the ribbons are made to repel each other and then joined together, and placed on the

fore-mentioned rough fubftance, they will in a few minutes be mutually attracted ; the upper-moft being pofitively, the undermofl: negatively,nbsp;eledbrified.

When two white ribbons receive their friction on a rough furface, they always acquire contrary electricities j the upper one is negatively, the lower one pofitively, electrifierL

When two ribbons are made to repel each other, draw the point of a needle lengthwaysnbsp;down one of them, and they will rufn together.

Bring an eleClrified ribbon near a fmall infu-iated metallic plate, it will be attracted but feebly; bring a finger near the plate, a iparhnbsp;will be obferved between them, though bothnbsp;together fhew no figns of eleSrricity ; on thenbsp;feparation of the ribbon they again appear tonbsp;be electrified, and a fpark is perceived betweennbsp;the plate and the finger.

Lay a number of ribbons of the fame colour �pon a fmooth conducting fubftance, draw the

6i nbsp;nbsp;nbsp;A N E S S A Y

ivory rule over them, take them up fingly, afiii each will give a fpark at th� place where it i^nbsp;feparated from the other; the lafl: will do thenbsp;fame with the conduflor; they are all negativelynbsp;ele(5lrified. Take them from the plate together,�nbsp;they will all cohere in one mafs, which is negatively ele�lrified on b�th fides.

Let them be placed on a rough conducing fubftance, and then be feparated fingly, beginning with the lowermbft, fparks appear as before, but all the ribbons will be eledtrified po-litively except the uppermoft. If they receivenbsp;the friction upon the rough condu�lor, and arenbsp;all taken up at once, all the intermediate ribbons acquire the eledlricity of the highefi: ornbsp;loweft, according as the feparation is begunnbsp;with the higheft or the loweft.

The following very curious obfervations and experiments were made by Mr. Symmer. Henbsp;had been accuftomed to wear two pair of filknbsp;ftockings, a black and a white, when thefenbsp;were pulled off both together no figns of electricity appeared ; but, on pulling off the blacknbsp;ones from the white, he heard a flapping ornbsp;cracking noife, and in the dark perceivednbsp;fparks between them. To produce this and

the

ON ELECTRICITY. 63

tlie following appearances in great perfedtion,' It was only neceffary to draw his hand feverai'nbsp;times backward and forward over his leg withnbsp;the ftockings upon it;

When the {lockings were feparated and held at a diflance from each other, both of them appeared to be highly excited; the white {lockingnbsp;poiitively, the black negatively. While theynbsp;were kept at a diilance from each other, bothnbsp;of them appeared inflated to fueh a degree thatnbsp;they exhibited the intire ihape of the leg;nbsp;When two black or two white ftockings arenbsp;held in one hand, they repel one another withnbsp;confiderable force. When a white and a blacknbsp;flocking were prefented to each other they werenbsp;mutually attfadled, and rufh together, if permitted, with great violence. As they approach the inflation gradually fubfides, andnbsp;their attradlion of foreign objcdls diminlihcs,nbsp;but their attra�lion of one another increafes;nbsp;when they adlually meet, they become flat andnbsp;joined clofe together, like fo many folds ofnbsp;filk ; when feparated again, their eledlric virtuenbsp;does not feem to be in the leaft impaired fornbsp;having once met. The fame appearances willnbsp;be exhibited by them for a confiderable time.

When the dockings were fuffered to meet, they ftuck together with confiderable force jnbsp;at firfl Mr. Symmer found they required from

one to twelve ounces to feparate them. Ano* cher time they raifed i 7 ounces. Getting thenbsp;black {lockings new dyedy and the w'hite ones-walhedy and whitened in the fumes of new ful-phur, and then putting them one within thenbsp;other, with the rough fides together, they re-q^uired three pounds three ounces to feparatenbsp;them.

When the white Hocking was put within the black one, fo that the outlide of the w^hitenbsp;was contiguous to the infide of the black, theynbsp;raifed nine pounds, wanting a few ounces ;�nbsp;when the two rough furfaces were together theynbsp;raifed fifteen pounds, One penny weight, and anbsp;half.nbsp;nbsp;nbsp;nbsp;I

CHAP-

FI X the wire and ball B to the end of the conductor, as at A, fig. 29, turn the cy-('nbsp;Under, and then bring the krivicllt;le, of anothernbsp;metal ball, as c, towards B ; if the machinenbsp;IS powerfull, a long, crooked, brilliant, electric fpark, with the appearance of fire,nbsp;attended with a fnapping noife, will pafs between the two balls, or between the knuck!�;nbsp;and ball.

The experiments in the foregoing chapter lliow, that thofe fubftances which are broughtnbsp;within the influence of eledtrified bodies, willnbsp;become pofleffed of a contrary eledlricity, andnbsp;are confequently in a proper ftate to receive dnbsp;fpark from any body that is charged with electric matter; and, when brought near enough,nbsp;they will receive the fluid in one explofion. Ifnbsp;the condudtor is negative, it receives the fluidnbsp;from the approaching body. The fpark doesnbsp;not explode at the greatefl; diflance on a givennbsp;iiody, until it has firft been made to ftrike atnbsp;Fnbsp;nbsp;nbsp;nbsp;ifomc

AN ESSAY

fomc fmaller diftancc, which, as it were, entices the clifeharge gradually forwards.

The longell and moft denfe fparks proceed from that end of the conduftor which is far-theft from the cylinder, though long curvilinearnbsp;fparks may. alfo he taken near the infulatingnbsp;pillar which fupports the condudor.

The fpark, or quantity of eleclrlcity dif-charged, is nearly in proportion to the fize of the eondu�lor fo that larger and longer fparksnbsp;are obtained from a conductor which- has anbsp;conliderable furface, than from amp; fmall one..nbsp;This has been extended fo far,- that the forcenbsp;of the fpark from a conduftor has been equal-to a fcock from a good lized phiaL

The momentum or effort of the eleftric fluid feems to- be produced by the incumbentnbsp;preflure of the atmofpherc on the electric matter, and the preflure of one part of this matternbsp;upon another ; which muft be very great, ifnbsp;the parts of it are in contaft, or act immediately one on the other throughout the wide im-menfity of fpace.

When the quantity of eledricity is finall,^. and incapable of itriking at any confiderable-diftance, the fpark appears flrait; but,, when-it is Itrong, and capable of ftriking at a greaternbsp;diftance, it aflumes a crooked or zig-zag di-redion ; ^and this, probably, becaufe the more

fluid

ON E LEG t R I Ci T Y.

^uid electric matter has to pafs with great rapidity through the denfer and lefs fluid at-flrofphere, which reciprocally act upon each Other.

It will be feen, by a great variety of experiments, that the eleftric' fluid iS diflipated,-unlels it is refifled by the preffure of the at-mofphere, which keeps the fire together in a body,- and by concentrating it inctealcs� itsnbsp;fplendor. The fparic which exd'tl�des in thenbsp;air is vivid,- like lightening ; but, if the famenbsp;is tried in an' exhaufted receiver,- inftead of anbsp;fpark and expicfion, we have ohly a' filent,-faint, diluted ftream.'

Beccaria fays, that the air refifts the elcdtric fpark in proportion to its denfity, and the thlck-hefs of the ftratum it oppofes to the fpark, ornbsp;the length of the pafiage they open for them-felves through its fubifance. He alib fhews^nbsp;by a variety of experiments,- that the air isnbsp;driven in every dire�lioh by the eleflric fluidnbsp;with a force, the action of which docs not im-n�cdiatcly fubfldc.

The elcdtrie fpark appears of a diffl-rent colour according to its denfity : when it is rare,nbsp;it appears of a blueifli colour ; when more tlenle,nbsp;it is purple; when highly condenfed, it is clearnbsp;and white like the light of the fun.'

AN ESSAY

The middle part of an eleftric fpark often appears diluted, and of a red or violet colour, the ends are more vivid and white, probablynbsp;becaufe the fluid meets with the greateft rcfift-ance at its entrance and exit.'

The fpark is fometimes divided into' many parts, as in fig. 30. The rays of the pencilnbsp;concentrate where they ftrike the bally andnbsp;form upon it many denfe and Ihining fparks.-

Place an ivory ball on the conductor, take a ftrong fpark, (or pafs the charge of a Leydennbsp;bottle through the center of it) the ball willnbsp;appear perfectly luminous. If the charge isnbsp;not taken through the center, it will pafa overnbsp;and corrode the furface of the ball.-

' Take a fpark through a ball of box-wood, ��ild it will appear of a beautiful crimfon, or rather a fine fcarlet colour : or the fhock rnay; benbsp;pafFed . through pieces of wood, of differentnbsp;thicknefles and d�nfity, which will afford a'nbsp;very ample field for obfervation and. experiment.

The

ON electricity. %

The two foregoing experiments are fo analagous to the famous experiment of Mr.nbsp;Hauxfbee, and fome others which have beennbsp;made fince his time, that I have fubjoinednbsp;them, and hope they will lead to a further in-veftigation of this curious fubjedt.

Mr'. Hauxfbee lined more than half the infide �of a glafs globe with fcaling w'ax, he exhauftednbsp;the globe, and put it in motion, when, on applying his hand to excite it, he faw the fhapenbsp;and figure of it as diflin�lly on the concave fu-pcrficics of the wax within, as if only purenbsp;glafs had intervened between his eye and hisnbsp;hand. The lining of wax, wdtyre it was thln-cll, would but juil allow the light of a candlenbsp;to be feen through it in the dark. In fomenbsp;parts the wax was at leaft an eighth part of annbsp;inch thick ; yet, even in thofe places, the fhapenbsp;and figure pf his hand were as diflinguifhablijjnbsp;as any where elfe.

Beccaria difeharged an elcftrlc fliock through fome brafs dull fprinklcd between two plates ofnbsp;fealing wax ; the w'hole was rendered perfedllynbsp;luminous and tranfparent.

AN ESSAY

This extraordinary experiment was made by Dr. Pricfdeyj and is thus deferibed by hinunbsp;� laid a chain, which was in contad with thenbsp;outfide of a Jar, lightly on my finger, andnbsp;fornetimes kept it at a fmall diftance by meansnbsp;of a thin piece of glafs* If I made the dif-charge at the diftance of about three inches,nbsp;the eledlric fire was viftble on the lurface ofnbsp;the finger, giving it a fudden concuflipn, whieftnbsp;feemed to make it yibratc to the very bone ;nbsp;and, when it happened to pafs on that fide ofnbsp;the finger which was oppofite to the eye, thenbsp;whole fe.emedj ii) the dark, perfeeftly tranftnbsp;parent.

Connedl one end of a chain with the outfide of a charged jar, let the other end lye on thenbsp;table, place the end of another piece of chainnbsp;at about one quarter of an inch diftance fromnbsp;the former, then fet a decanter of water onnbsp;thefe feparated ends, and, on making the dif-charge through the chain, the water will appearnbsp;perfcftly and'beautifully luminous. This experiment was communicated to me by Mr. Haas, thenbsp;inventor of an improved air pump, which far

exceeds

Do not thefe experiments Indicate, that there is a fubtle medium both in eleftric andnbsp;non~eIcflric bodies that renders them tranfpa-jent when It is put in motion }

E F, fig. 31, is a glafs tube, round which, fmall but equal difiances from each other,nbsp;pieces of tin-foil are pafted in a fpiral form,nbsp;(hence it is called the fpiral tube) from end tonbsp;end; this tube is inclofed in a larger one, fittednbsp;w'ith brafs caps at each end, which are connedt-cd with the tin-foil of the inner tube. Holdnbsp;one end in the hand, and apply the other nearnbsp;enough to the prime conductor to takenbsp;from it, a beautiful and lucid fpot will then benbsp;feen at each feparation of the tIn-fo,il; thefenbsp;multiply, as it were, the fpark taken from thenbsp;condudtor, for,^ if there was no break in thenbsp;tin-foil, the eledlric fire would pafs off unper*'nbsp;ceived,

F 4 nbsp;nbsp;nbsp;Ex*

AN ESSAY

is exactly on the fame principles as the foregoing, The word is formed by the fmall fe.~ parations made in the tin-foil, which is pattednbsp;on a piece of glafs, that is fixed in a framenbsp;of baked wood, as is reprefented in fig. 32.nbsp;To make the experiment, hold the frame innbsp;the hand and prefent the ball G to the coi*-duftor, the fpark received on this will benbsp;communicated to the tin-foil, and follow it innbsp;^1 its windings, till it arrives at the hopk h,nbsp;and is conveyed from thence to the ground bynbsp;a chain : the lucid appearance at each breaknbsp;exhibits a word in charadlers of fire.

To take the eleiStric fpark with a metal point; ferew a pointed brafs wire into one end of anbsp;fpiral tube, and prefent it to the conductornbsp;while the machine is in action, when a ttrongnbsp;fpark will pafs between the conductor and thenbsp;point.

Take a clean dry glafs tube, of about a quarter of an inch bore, infert a pointed wire

o N * E L E C T R I C I T Y.

in this tube, keep the pointed end at fome dif-tance from the end of the tube, let the other end be connefted with the ground, bring thenbsp;former towards the prime conduftor, and ftrongnbsp;zig-zag fparks, attended with a peculiar noife,nbsp;will pafs between the condudtor and the point.

The feparation between the pieces of tin-foil, in experiment 62, forms a refiftance which hinders the immediate reception of the eleftricnbsp;fluid, and thus, in fome meafure, prevents thenbsp;common aftion of the point on the condudlornbsp;or, in other words, the power of a point to prevent an explofion, depends on its having a pier-fedl uninterrupted metallic communicationnbsp;with the earth; though this is not quite fufBci-ent, as may be feen by Ex. LXIII, where thenbsp;fluid is concentrated and coilcdted by the non-condufting fubftance, which furrounds thenbsp;point.

Let any perfon ftand on the infulating ffool, and conneft himfelf by a wire or chain with thenbsp;prime conduflor, he will then exhibit the famenbsp;appearances which are obtained from the con-dii�tor, and will attraft light bodies, give thenbsp;Ipark, amp;c. and thus afford a pleafing mode of di-rnbsp;verfifying every experiment. It is abfGlutely ne-

AN ESSAY

cdTary, to the complete fuccefs of this �rxpcri-* ment, that no part of the cloaths touch thenbsp;�oor, tabic, See. and that the glafs feet be�nbsp;carefully dried ; a Iheet of dry brown papernbsp;placed under the ftool, will be found of con-fiderable fervice, by gendering the infuktiotiinbsp;more epm pleat.

If the infulatcd pcrlbn lays his hand on the cloaths of one that is not lb, cfpccially if theynbsp;are woollen, they will both feel as k were manynbsp;pins pricking them, as long as the cylinder jgnbsp;in motion.

To fire fpirits of wine with the eiedrlc fpark, Heat the ladle, fig. 33, then pour a finallnbsp;quantity of fpirit of wine ipto it, and fix it bynbsp;its handle to the end of the prime conductor;nbsp;or, fire the fpirits, and blow them out a fewnbsp;minutes before the experiment is made; takenbsp;a fpark through the middle of the ladle with anbsp;Erafs ball, and the fpirits will be fired by it.nbsp;Or, let a perfon, flanding on an infulatingnbsp;fiool and connected with the prime conduftor,nbsp;hold the ladle with the fpirits in his hand, andnbsp;let a perfon on the floor take a fpark throughnbsp;them, and they will be fired. The experirnbsp;jnent anfwers equally weU, if the perfon on the

ON ELECTR I C I T Y. 75

If oil of turpentine is fet on fire in a veffel v;hich is placed on the conductor, and thenbsp;fmoke is received on a plate, held by a perfonnbsp;Handing on an infulated fiool, he will be elecrnbsp;trified thereby, and enabled tp fire fpirits ofnbsp;'ivine, amp;c, If the infulated perfon holds a brafsnbsp;wire at the top of the flame of burning Ijiiritsnbsp;of wine which is connedfed with the conductor,nbsp;he will alfo bepome electrified. Hence wenbsp;find, that eitlie? fmoke or flame condufls thenbsp;eleflrical fluid,

Mr. Volta has fucceeded in obtaining nn-r doubted figns of electricity from the Amplenbsp;evaporation of water, and from various chemical efiervefcences.

�nfulate a fmall crucible, containing three or four lighted coals, throw a fpoonful of waternbsp;on the coals, and in a fliort fpace of time, annbsp;electrometer, w'hich communicates with thenbsp;coals by means of a wire, will diverge withnbsp;pegatiye electricity.

From

AN ESSAY

From hence it would feem, that the vapour of water, and, in general, thofe parts of anbsp;body that are feparated by volatilization, carrynbsp;�\vay an additional quantity of eleftric fluid,nbsp;as well as of elementary heat; and that thenbsp;body, from which thofe volatile parts havenbsp;been feparated, remains both cooled and electrified negatively ; and, that thofe whichnbsp;arc refbivcd into a volatile elaftic fluid, havenbsp;their capacity for holding common fire, andnbsp;the cleclric fluid augmented.

A fpecies of air which is inflammable is frequently generated in coal mines : the air alfo emitted by ftirring the mud of fome flandingnbsp;waters, has been found to be inflammable.nbsp;Putrefeent animal matter alfo emits this fluid.nbsp;It may be obtained by ciiflillation from wax,nbsp;pitch, amber, coals, and other phlogiftic fub-flanccs. The following is the mofl: convenientnbsp;method of procuring it ; put fome fmall naiisnbsp;er iron filings into the bottle r, fig. 37, covernbsp;tbefe with water, then add to this a little cwlnbsp;�f vitriol, about one quarter of the quantitynbsp;there is of water, put the ground end of thenbsp;bent tube s into the mouth of the bottle, andnbsp;pafs the other end through the w^ater of thenbsp;bafon T into the neck of the bottle K, whichnbsp;is filled with water, and inverted in the bafon,nbsp;the bottle K mull be fupported during the operation ;

ration : in a little time the mixture will cffcr-vefce, and emit a fluid which will pafs through, the bent tube, go into the bottle K, and at iafi:nbsp;fill it totally, expelling the water; the bottlenbsp;is then to be removed, and corked as expediti*nbsp;oufly as poflible.

Fig. 39 reprefents a brafs piflol for inflammable air ; a b is a chamber of brafs, to the mouth a c of which a cork is fitted, a perforated piece of brafs ferews on to the bottom ofnbsp;this chamber, (this piece is reprefented by itfelfnbsp;in fig. 40) a glafs Uibe is cemented Into thenbsp;perforation of this piece, and a brafs wire isnbsp;alfo cemented into the glafs tube ; one end ofnbsp;this wire is furnilhed with a ball, the other extremity is bent, fo as to com.e within about anbsp;tenth of an inch of the brafs piece. Fig. 41 is anbsp;brafs cap, which ferews on the piflol, to prefervenbsp;the glafs tube from any accident. The air wdtlinbsp;which the piflol is to be charged Ihould benbsp;kept in a corked bottle : take out the cork,nbsp;and apply in the fame inftant the mouth of thenbsp;piflol to the opening of the bottle, and thenbsp;common and inflammable air will mix together,nbsp;becaufe the former being lighter than the latter will naturally defeend; keep the piflol lanbsp;this fituation about 15 feconds, then remove it,nbsp;and cork both the bottle and piflol with thenbsp;utmofl expedition.

^8 nbsp;nbsp;nbsp;AN E S S A t

If the piftol is held too long over the bottle,* and is intirely filled with inflammable air, ifnbsp;will not explodes

Bring the ball of the piflol, which is charged �with inflammable air, near the prime con-du�lor, or the knob of a chafged bottle,nbsp;the fparfc which paffes between the end ofnbsp;the wire f and the pieee g, fig. 40, will firenbsp;the inflammable air, and drive the cork tonbsp;a confiderable diftance. This air, like allnbsp;other, requires the prefence either of pure airynbsp;or the nitrous acid, to enable it to burn; but,nbsp;if it is mixed with a certain quantity of common air, an explofion will take place in paffing;nbsp;the ele�lric fpark through it.

Mr. Cavallo recommends a piftol made in the foUowdn'g manner, to tliofe who w'illr tonbsp;make experiments on the explofion of inflammable and dephlogiftiqated air, or with knownnbsp;quantities of common and inflammable air.*nbsp;It conflfts of a brafs tube, about one inch innbsp;diameter and fix inches long, to one extremitynbsp;of which a perforated piece of wwd is fecurelynbsp;fitted ; a brafs wdre, about four inches long,'nbsp;is covered, except its ends, firft with fealingnbsp;wax, then with filk, and afterwards with feal-'

rag

ON -electricity.

�ng wax again. This wire is to be cemented in the perforation of the wOodeii piece,- fo as to pro^nbsp;jedt about two inches within the tube, the reftnbsp;is on the outfide ; that part of the wire which/nbsp;is within, is bent fo as to be only about one-tenth of an inch from the infide of the brafinbsp;tube. *

To life this piftol � fill it with, and then invert it into a bafon of water ; make the re^* quired quantity of inflammable and commonnbsp;air in another velTel, by putting in known and�nbsp;proportionable meafures of each j introducenbsp;this mixture into the piflol, and then flop itnbsp;with a cork, take the piftol out of the water,nbsp;and pafs in the ufual manner the fpark of anbsp;charged jar through itj and the inflammablsnbsp;air will be fixed.

The inftruments for firinj; the inflammable air with the eledtric f|gt;ark, arc often made ianbsp;the lhape of a cannon.

CHAP.

CHAP. VI.

Of EleSlriJied Points.

PRESENT the pointed end of a wire towards a conductor which Is pofltively eledrified, a lucid globular point or ftar willnbsp;appear on the point, and the ele�lric fluid willnbsp;be evidently conveyed away and diflipated fromnbsp;the condudtor.

Prefent a pointed wire towards a condudor that is electrified negatively ; a lucid cone ornbsp;brufli will be feen diverging from the point, andnbsp;the quantity of fire will be increafed.

The lucid ftar is fecn on the colleding points of a pofitive condudtor, while a divergingnbsp;Cone will appear on a point placed at the end ofnbsp;fhc condudor.

Ex.

A lucid cone appears on the collector of a negative concki�lor, and a lucid itar on a pointnbsp;placed at the oppofite end of the condudlor.

The fufeeptibility of points to receive or part with the eleftric matter, and the differentnbsp;appearance of the light on. the point, innbsp;Various experiments, has led many eledriciansrnbsp;t6 conclude,' that thefe appearances determintd,nbsp;in a decilive manner, the di'edtion of the electric fluid. They fuppofe, that the appearancenbsp;of the globular light o'r ftar is an indicationnbsp;that the cleClric fluid is entering by thatnbsp;point; and that the fluid proceeds from thenbsp;point on which the lucid cone or brufh isnbsp;feen. This opinion is confirmed, by obfervingnbsp;that thefe appearances are conformable tonbsp;the laws obferved by �th'ef fluids, which di-�nbsp;verge from the reflftance they meet with fromnbsp;the air; as is the cafe when the eleflric fluidnbsp;iffucs from a point placed at the end of a pofi-tive conductor. To this it has been objefted,nbsp;that the .mys may be confidered as convergingnbsp;from fo many points in the furraunding air towards the metallic point. But it is difficult to�nbsp;fay, why a vifible ray fliould be fuppofed tonbsp;breakout from one poinf of theatmofphere rathernbsp;than another, as it is known to refill the paflage

AN ESSAY

of the fluid, and feenis to refill it equally ; ancT therefore, when it proceeds from the air to anbsp;point, it percolates flowly, invifibly, andnbsp;equally on all fides, till it comes fo near as to'nbsp;force its way through the intermediate fpace,nbsp;and fettle on the point, where it will appear as*nbsp;a luminous globule.

Bring an excited glafs mhe near a point that is fixed at the end of a pofitively eledlrified con-dudlor, and the luminous brulh will be turnednbsp;out of its dircdlion by the a�lion of the excitednbsp;tube ; if the tube is held dire�lly oppofite to'nbsp;the point, the brufli will vanifli.-

Fix the point to the end of the negative con-dudor, the lucid flat will turn towards the excited tube.

Thefe two experiments coincide with and confirm experiments 69, 7c, 71, 72, and kadnbsp;to the fame conclufion, viz. that the brufh is anbsp;fign of pofitive, and the flat an indication ofnbsp;negative, eledricity, which is flill further con-^nbsp;�firmed by the following experiment,-

Put a wire, which has a ball at one end, into the hole at the end of a pofitive condudor,

j')lace a lighted candle id that the middle of the flame may be even with the middle of the ball,nbsp;and about an inch from itturn the machine,nbsp;and place the fame wire at the end of the negative conductor, the appearance will be reveffedynbsp;and the knob will foon be heated by the flamenbsp;of the candle which is carrfed towards it.

Fix a pointed wire in the hole on thh uppC? fide of the conduflor, then place the center ofnbsp;the brafs crofs K, fig. 34, upon the point, thenbsp;ends �f which crofs are all bent one way ; electrify the condudlor, and the crofs will turnnbsp;upon its center with great rapidity. If thenbsp;room be dafkened, a circle of light will benbsp;formed by the elcftric fluid on the points ofnbsp;the wires,' The re-a�tion of the air on the di-verging cone of eleftric matter gives the retrograde motion to the points of the wire.

The fly turns round in the fame direftion,� �whether it is elcflrified negatively or pofitively ;�nbsp;though it will not move in vacuo, unlefs the'nbsp;finger, or feme other condud;or, is applied to�nbsp;the glafs receiver oppofite to one of the points,-it will then begin to move, and continue to do'nbsp;fo brilkly till the glafs is charged.'

Eleibrify the two infulatcd wires MN, oP�,-fig. 35, and the refiliance of the air againft the elc�tric ftream, from the point of the fly L,nbsp;(the axis of which rolls on the wires) will forcenbsp;the fly up the declivity of the inclined plan�nbsp;MN, oP.

Fig. 36 reprefehts a fmall crane, which wilt move from the fame caufe as the foreg�ihg,nbsp;and raife a {mall weight.

Several flyers may be made to turn at th� fame time, fee fig. 37, and many other pleafingnbsp;experiments may be contrived on the famenbsp;principle.

When the eieflric fluid percolates a wooden point, the* flream or cone -which iffues from itnbsp;feems diluted, and fomethittg finiilar to thenbsp;purple ele�ric light which is obtained in vacuo.nbsp;The adion of the eledric fluid on the air by an'nbsp;clcdrificd point, produces a fenfible aura, oTnbsp;wind, of fufficient force, as is feen above, tonbsp;^ut light bodies in motion, or difturb the flame

ON electricity. 85

of a candle, and occafion an undulation in fluids ; the aftion of the flviid is fo modifiednbsp;by points as to produce an agreeable fenfation,nbsp;refembling a gentle breathing; this fenfationnbsp;may be rendered more or lefs ftimulating, bynbsp;the refinance the fluid meets with in its adlionnbsp;on our bodies, an effedl which is produ�tive ofnbsp;great advantages in medical elelt;5lricity.

CHAP-

CHAP. VII.

Of the Leyden Phial.

TH E experiments upon the Leyden phial are fome of the moft interefting in electricity ; they excited the attention of the philo-fopher to this fubjeft more than any other experiment, and are ftill viewed with wonder andnbsp;furprize.

The phoenomena attending this very extraordinary experiment feemed totally inexplicable, till they were elucidated by the ingenious theory of Dr. Franklin; which, in a plain andnbsp;clear manner, accounts for moft of the difficulties which attend this intricate branch of electricity ; and accomodates itfelf fo eafily and fatif-faftorily to a variety of appearances, as tonbsp;make us almoh; lofe fight of the objeftionsnbsp;againft it.

Place the brafs ball of a coated jar in coatadl with the prime condudlor while the outfidcnbsp;communicates with the table, turn the cylinder, and the bottle will in a little time be

charged, or modify the eledlric fluid in a peculiar manner, To difchargc the jar, or reftore it to its natural ftate, bring one end of a con-dufting fubftance in conta�f with the outfidenbsp;coating, and let the other be brought near thenbsp;knob of the jar which communicates with thenbsp;inlide coating, a itrong explolion will takenbsp;place, the electric light will be vifible, and thenbsp;report very loud.

Charge the Leyden bottle, then touch the outfide coating with one hand, and the knobnbsp;with the other, the bqttle will be difeharged,nbsp;and a fiuklen pecqliaf fenfation will be perceived, That is called the eleftric fliock,nbsp;W'hlch, when it is taken in this manner, generally affects the wrifts, elbows, and breaft :nbsp;w'hen the fliock is flrong, it refcmbles an uni-verfal blow. This peculiar fenfation is probably owing to the two-fold and initantancousnbsp;aftion of the cleftric fluid, which enters andnbsp;goes out of the body and the various partsnbsp;through which it paffes at one and the famenbsp;inftant. It has been alfo obferved, that naturenbsp;has appointed a certain modification of thenbsp;eleftric fluid in all terreftrial bodies, which wcnbsp;violate in our experiments ; when this violationnbsp;G 4nbsp;nbsp;nbsp;nbsp;is

AN ESSAY

is fmall, the powers of nature operate in a gentle manner to reftore the diforder we have intro-ducv d; but, when the deviation is confiderable, the natural powers reftorc the original conftitu-tion with extreme violence.

If fcveral perfons join hands, and the firft touches the outfide of a charged jar, and thenbsp;laft the knob, the bottle will be difeharged,nbsp;and they wdll all feel the fnock at the fame in,-ftant ; but the greater the number of perfonsnbsp;that join hands to take a fhock, tjic weakernbsp;jt is.

The force of the Ihock is in proportion to the quantity of coated furfaccs, the thinnefs ofnbsp;the glafs, and the power of the machine ; or,nbsp;the efic(51: of the Leyden phial is increafed, innbsp;proportion as we deftroy the equilibrium onnbsp;the furfaces.

If a charged jar is coated very high, it will difeharge itfelf before it has received near thenbsp;charge it would take if the coating w^as lower.nbsp;If it is coated very lowq this part of the furfaccnbsp;may be charged very high, but a confiderablenbsp;part of the glafs is not charged at all.

When a jar is charged very high, it will often explode or difeharge itfelf over the glafsnbsp;from one coated furface to the other ; or, ifnbsp;the glafs is thin, it will make a hole throughnbsp;It, and fwell the coating on both fides,

ON ELECTRICITY.

the gLfs in the hole will be pulverized, and very often a variety of fiffures wiil proceed fromnbsp;it in various direftions.

A Leyden jar very often recovers its elcdlrlcity, in a fmall degree, after a dif-charge has been made ; this fecond explofionnbsp;is called the rcfiduum of a charge.

To avoid receiving the cleftric fhock, be carelul never to touch the top and bottom ofnbsp;the iar at the fame time, and never to enter anbsp;circuit formed between the infide and outfidenbsp;of a jar. By attending to this obfervation, jarsnbsp;' of any fize may be handled with fafety. Indeed,nbsp;the human frame makes fo little rcfiftance tonbsp;the free pafl'age of this fubtle agent, , that nonbsp;other inconvenience wall attend a Ihock from anbsp;common-fized charged jar, than a tranfient dif-agreeable fenfation.

Touch the knob of a charged jar, no fltock will enfue ; but the finger, or part that touchesnbsp;the ball of the jar, will be affedfed with a flrarpnbsp;fenfation, as ii it had been pricked with anbsp;ntedie.nbsp;nbsp;nbsp;nbsp;,

A charged phial, fet upon elediric fubftances, may be taken hold of without danger, cither bynbsp;the Coating or the wire ; a fmall Ipark only willnbsp;proceed from either.

AN ESSAY

Glafs IS fuppofcd to contain at all times, on its two furfaces, a large quantity of the ele�fricnbsp;fluid ; which is fo difpofed, that if you increafenbsp;the quantity on one fide, the other muft thrownbsp;off an equal proportion ; or, when one fide isnbsp;pofitive, the other mufl be negative ; now, asnbsp;no more of the eledlric fluid can be forced onnbsp;one fide than can go off on the other, there isnbsp;no more in the bottle after it is charged thannbsp;was there before ; the quantity is neither in-creafed or leffened on the whole, though anbsp;change may be made in its place and fituation ;nbsp;i- e, we may throw an additional quantity onnbsp;one of its fides, if, at the fame tirqe, an equalnbsp;quantity can efcape from the other, and notnbsp;otherwife. That this change is effected by liningnbsp;parts of its two furfaces wdth a non-eledtric;nbsp;through the mediation of which, we are enablednbsp;to convey the eledlric fire to every phyficalnbsp;point of the furface we propofc to charge,nbsp;where it exerts its adivity In repelling the electric particles naturally belonging to the othernbsp;fide ; all of which have an opportunity of efca-ping by the lining In contad with this furface,

.vrliich, for that purpofe, muft communicate with the earth : when the whole quantity belonging to this furface has been difeharged, innbsp;confequence of an equal quantity thrown uponnbsp;the other furface, the bottle is charged as muchnbsp;as it can poflibly be. Th� two furfaces are atnbsp;this time in a ftate of viqlence; the inner, ornbsp;pofitive fide, ftrongly difpofed to part with itsnbsp;additional fire, and the outer, or negative fide,nbsp;equally defirous to attrafl what it has loft, Jtutnbsp;neither of them capable of having a changenbsp;in its ftate effected, without the equal andnbsp;contemporary participation of the other. Thatnbsp;notwithftanding the vicinity of thefe two fur-faces, and the ftrong difpofition of the eledrienbsp;fluid contained in one of them to communicatenbsp;its fuper-abundance to the other, and^of thatnbsp;po receive it, yet there is an impenetrable barrier between them ; for, fo impermeable is glafsnbsp;to the eledtric fluid, (though it permits onenbsp;�fide of it to a6t on the other) that its two fur-faces remain in this ftate of contrariety till anbsp;communication is formed between them, ai?nbsp;extra, by a proper condudor, when the equilibrium is fuddenly and violently reftored, andnbsp;ihe eledric fluid recovers its original ftate ofnbsp;equality on the two fidcs of the glafs.

AN ESSAY

Experiments on charging and difcharging the Leyden Phial, intended to elucidatenbsp;atid conjirm Dr. Franklins Fheory.

Screw a Leyden phial, whofe coating is free from points, upon an infulated Hand, and placenbsp;it fo that its knob may be in conta�: with thenbsp;conditdor, (taking care that no condu�lingnbsp;fubftanfe is near the coating of the jar) turnnbsp;the cylinder rpund a fufficient number ofnbsp;times to charge the phial, then examine it withnbsp;a difeharging rod, and you will find it had received no charge ; which ihews clearly, thatnbsp;except the eledric fluid can efcape from onpnbsp;fide of the jar, it can receive none on thenbsp;other.

Place the fame infulated phial fo that its knob may be about half an inch from the condudqr,nbsp;and, while the pylinder is turning, hold a brafsnbsp;knob near the coating of the jar i this knobnbsp;will receive a fpark from the coating for everynbsp;one that pafiTes between the condudor and the

knob, and the jar will in i little time be charged, by adding eledtricity to one lide, antinbsp;taking it away from the other;

Screw the phial a, fig. 42, on the Infulated pillar d, and bring its knob in contaa with thenbsp;contiudior ; hold another bottle c, of the fiuncnbsp;fize with a, fo that its knob may be in contadlnbsp;with the outfide coating of the bottle a; turnnbsp;the cylinder, and when the bottle a is charged,nbsp;place c on the table, then unfcrew a from itsnbsp;Hand, and place it alfo on the table, but atnbsp;foine diftance from the other,; fit a brafs ballnbsp;to the bottom ftem of the quadrant eleftrcme-ter, and hold the elcdlrometer by a filk firing,nbsp;fo that the brafs ball may touch the knob ofnbsp;the bottle ; obfcrve at what height the irldex �fnbsp;the eledlroraeter ftands, and then remove it tonbsp;the other bottle, which will raife the index tonbsp;the fame height; Ihewing clearly, that thenbsp;bottle has thrown off from the outfide as muehnbsp;tieclricity as it received on the infide.

Place the knob of an infulated bottle in contact with a pofitive condudor, and conned thts

AN essay

outer coating with the cufhion, or a negative conduftor, turn the cylinder, and the bottlenbsp;will be charged with its own eleftricity ; thenbsp;fluid from the exterior coating being tranf-ferteef to the interior one.�

Charge the two bottles, fig. 43, pofitivcly ebnneft their outfide coatings by a wire ornbsp;chain, then bring their knobs together, therenbsp;will be no fpark between them, and the bottlesnbsp;will not be difehatged, becaufe neither fidenbsp;has any thing to coinmunicate to the other.

Charge the infulated bottle, fig. 43, negatively, and the other pofitively ; conneft the coating by a chain, and bring the knobs towards each other, an- cxplofion will take place,nbsp;and the bottles will be difeharged. If a lightednbsp;candle is placed between the knobs, the ex-plofion will be made through the flame in a'nbsp;beautiful inanner, aiwi at fome inches diftance.nbsp;See fig.- 44.

Fix a quadrant electrometer to the tall of a Leyden bottle^ and charge it negatively ;

ON ELECTRICITY.

when it has received a full charge the Index will ftand at degrees; then place the bottlenbsp;with its eleftrometer at the pofitive condudtoivnbsp;turn the cylinder, the electrometer will de^nbsp;Icend, and the bottle will be diicharged by tlicnbsp;contrary eledfricity.

Infulatetwo Le^�den bottles ; let their coatmgy be in contafl, and while you charge the iafidenbsp;�f one pofitively, let a perfon, ftandiag onnbsp;the floor, touch the top of the other with hisnbsp;finger, and it will be charged negatively.

L M, fig. 45, reprefents a Leyden jar,- whleh is furnilhed with moveable coatings of tin;nbsp;the inner one, N, may be removed by the fllknbsp;ftfings f, g, h ;� the jar may be taken from the-outer coating.

Charge the jar, and then remove the coat-ings, bring a pair of pith balls towards the jar,, and they will be llrOngly attra�led by it; replace the coatings, and the jar W'ill give a con-liderable fhock; w'hich Ihews, that the powernbsp;or force of the charge is refident in the glafs,nbsp;and not in the coatings.

T V, fig. 46, rcprcfents a bottle, whofc exterior coating is formed of fmall pieces of tin-foil, placed at a little diilance from each other. Chartre this bottle in the ufual manner, arid'nbsp;ftrong fparks of elcdricity will pafs from onenbsp;fpot of tin-foil to the other, in a variety of directions ;,the reparation of the tin-foil makingnbsp;the paffage of the fluid from the outfide to thenbsp;table vifible. Difcharge this bottle, by bringing a pointed wire gradually near the knob,nbsp;and the uncoated part of the glafs between thenbsp;fpots will be pleafingly illuminated, and fhcnbsp;noife will refemble that of fmall fired crackers.nbsp;If the jar is difeharged fuddenly, the wholenbsp;outfide furface appears illuminated. To produce thefe appearances the glafs mufl: be very-dry.

String a parcel of fliot on a filk firing, leaving A fmall fpacc between each of them; fufpendnbsp;this from the condudor, fo' that it may reach'nbsp;the bottom of a coated phial, which is placednbsp;on an infulated fiand ; coiined another firingnbsp;of fhot to the bottom of the jar and let it com-mumcate with the table, turn the maGhlne, ah'd

�N ELECTRICITY.

vivid fpark will be feen between each of the Ihoc, both w'ithin and without the bottle, as ifnbsp;the fire paffed through the glafs.

Hold a phial in the hand which has no coating on the outfide, and prefent its knob towards-an eleftrified cpndult;�torthe fire, while it ia charging, will pafs fr�m the outfide to thenbsp;hand, in a pleafing manner ; on the difchargeynbsp;beautiful ramifications will proceed from thatnbsp;knob of the difchargcr which is on the outfidenbsp;all over the jar.

Let a chain be fufpended from the condudlor and pafs into an uncoated bottle, fo that itnbsp;does not touch the bottom; put the machinenbsp;in aftion, and the chain will move round, innbsp;order, as it were, to lay the fire on the infidenbsp;of the jar, and thus charge it by degrees.

Fig. 4-7 reprefents two Leyden phials, placed cme over the other. Various experiments maynbsp;be made with this doubie bottle, which arenbsp;Hnbsp;nbsp;nbsp;nbsp;very

an e s s a t

Bring the outfide coating of the bottle A in' contadt with the prime condudtof, and turn thenbsp;machine till the bottle is charged, then placenbsp;one ball of the difcharging rod upon the coating of B, and with the other touch the knob ofnbsp;the jar A, which will caufe an explofion.' Nownbsp;place one ball of the difch'arger on the knob ofnbsp;A, and bring the other ball to its coatingnbsp;and you have a fecond difcharge. Again,nbsp;apply one ball of the difcharger on the coatingnbsp;of B, and carry the other to the coating of A,nbsp;and it will produce a third explofion. A fourthnbsp;is obtained by applying the difcharge from thenbsp;coating of A to its knob^

The outer coating of the tipper jar Commu-j nicating with the infide of the under one, conveys the fluid from the conduftor to the largenbsp;jar, which is therefore charged pofitively ; thenbsp;upper jar does not charge, becaufe the infidenbsp;cannot part with any of its eledlric fluid ; buvnbsp;when a communication is formed from thenbsp;outfide of A to the infide of B, part of the firenbsp;on the infide of A will be conveyed to the negative coating of B, and the jar will be dif-charged. The fecond explofion is occafionednbsp;by the difcharge of the jar A; but, as the outfide of this communicates by eonduding fnb-*

ftances

ON electricity.

ftances with the pofitive infide of the jar B, if the ball of the difcharging rod remains a fmallnbsp;time after the difcharge on the knob of A, partnbsp;of the fire of the infide of A will efcape, andnbsp;be replaced by an equal quantity on the outfidenbsp;from the jar B, by which means A is chargednbsp;a fecond time ; the difcharge of this producesnbsp;the third, and of B the fourth explofion.

'fhe contrary State of the two oppofite Sides of a charged Leyden Bottle^ Jhewn bynbsp;their refpeSlhe attraSlinoe and repuljwenbsp;Powers.

Screw the bottle H, fig. 49, with the belt fide-ways on the infulating Hand, as in fig. 48, and charge it pofitively, then touch the knob with anbsp;pair of pith balls, thefe will diverge with pofitivenbsp;elesSlricity ; hold another pair to the coating,quot;nbsp;and they will leparate with negative electricity.

�le�rify two pair of the pith balls which are fixed to the brafs tubes, as in fig. 22, PI. II,nbsp;by the knob of a pofitively charged bottle, andnbsp;H 2nbsp;nbsp;nbsp;nbsp;place

AN ESSAY

place them at a fmall diflance from each othe?y then pulh them together till the ends of thenbsp;tubes are in conta�t, and the balls will remainnbsp;in the fame ftate they were in before they werenbsp;brought together, becaufe their ele�lricity isnbsp;of the fame kind. The refult is the fame ifnbsp;both pair are elelt;5trified by the coating ; but ifnbsp;one pair is eledbrified by the coating and thenbsp;other by the knob, when they are brought innbsp;contafl they immediately clofe.

A cork ball, or an artificial fpider made of burnt cork with legs of linen thread, fufpendednbsp;by filk, will play between the knobs of twonbsp;bottles, one of which is charged pofitively, thenbsp;other negatively, and will in- a little time dif-charge them.

A ball, fufpended on filk, and placed between two brafs balls, one proceeding from the outfide, the other from the infide of a Leydennbsp;jar, when the bottle is charged, will fly fromnbsp;one knob to the other, and by thus conveying thenbsp;fire from the infide to the outfide of the bottle,nbsp;will loon difcharge it.

Ex-

An infulated cork ball, after having received a fpark, will not play between, but be equallynbsp;repelled by two bottles which are charged withnbsp;the fame power.

At fig. 58 a wire is fixed to the under part of the Infulated coated phial, b c another wirenbsp;fitted to, and at right angles with the former,nbsp;a brafs fly is placed on the point of thisnbsp;wire ; charge the bottle, and all the timenbsp;the bottle is charging the fly will turn romid;.nbsp;when the bottle is charged the needle flops.nbsp;Touch the top of the bottle wdth a finger, ornbsp;any other conducing fubftance, and the flynbsp;will turn again till the bottle is difeharged.nbsp;The fly will eledlrlfy a pair of balls politivelynbsp;while the bottle is charging, and negativelynbsp;�when difeharging.

Place a clean dry and excited pane of glafs, about one foot fquare, on an infulated box withnbsp;pith balls, it will caufe the balls to divergenbsp;H 3nbsp;nbsp;nbsp;nbsp;with

AN ESSAY

with pofitive eleftricity, and they will continue to repell each other upwards of four hours innbsp;dry air. When the balls come together, remove the glafs, and they will open with negative eledlrkit}�; replace the glafs, and theynbsp;will clofe; remove it, and they will opennbsp;again ; and thug alternately, as long as anynbsp;eleflricity remains in the glafs.

If the pane of glafs be placed in a frame of wood, and a light pith or cork ball be laid onnbsp;its furface, on prefenting towards it the end ofnbsp;a finger, or the poiirt of a pin, the ball willnbsp;recede from them with a very brilk motion,nbsp;and may thus be driven about on the furface ofnbsp;the glafs, like a feather in the air by an excitednbsp;tube. The ball being deprived of its eledlri-city by the pin, it inftantly flies to that partnbsp;of the glafs which attracts it mofl: forcibly.

To excite the pane of glafs ; lay it upon a quire of large paper, well dried, and then rubnbsp;it with a piece of clean dry flannel.

7he contrary States of the different Sides of a Leyden Phial, and the DireBion of thenbsp;EleBric Fluid in the Charge and Dif-charge thereof, invefigated by the Appearances of the EleBric Light.

In chapter 6 we obfervedj- that the different appearances of light on ele�lrified points was deemed a .criterion of the direftjon of thenbsp;eleclric fluid. That the luminous ffar, or globule, fhews the point is receiving the eledtricnbsp;matter, whilft the luminous brulh, or cone,nbsp;indicates that it is proceeding from the point.nbsp;We lhall now examine the ftates qf the difler-ent fides of the Leyden bottle by thefe appearances. For this, and many other purpofes,nbsp;the apparatus which is reprefented in fig. 49nbsp;will be found very convenient ; I have endeavoured to combine the parts of it in fuchnbsp;manner as will render the apparatus extenfivelynbsp;ufeful, without being complicated. A is annbsp;infulated pillar of' glafs, which is ferewed tonbsp;the wooden foot B ; all the different parts ofnbsp;the apparatus may be ferewed alternately onnbsp;this pillar, C is an exhaufted tube of glafs,nbsp;furnifhed at each end with brafs caps; at thenbsp;end D is a valve, properly fecured under thenbsp;H 4nbsp;nbsp;nbsp;nbsp;brafs

AN ESSAY

brafs plate, a brafs wire with a ball projeftf from the upper cap, a pointed wire proceedsnbsp;ffom the bottom plate ; this tube is called thenbsp;luminous copduclor. The flalk, reprefentednbsp;at E, is called the Leyden vacuum. It is fur-nilhed with a valve under the ball E j this ballnbsp;unfcrews, in order to come' more readily at thenbsp;valve; a wire, with a blunt end, projects anbsp;little below the neck of the flalk, the bottomnbsp;of the flalk is. coated with tin-foil^ a femalenbsp;fcrew is cemented to the bottom, in order tonbsp;fcrew it on the pillar A.

F is a fyringe to exhauft the air occafionally either from the luminous condu�ior, or thenbsp;Leyden yacuum. To do this; unfcrew thenbsp;ball of the Leyden vacuum, or the plate of thenbsp;luminous condu�tor, and then fcrew the fyringenbsp;in the place of either of thefe pieces, beingnbsp;careful that the bottom of the female fcrew Gnbsp;bears clofe againft the leather which covers thenbsp;Ihoulders ab, cd, then work the fyringe, andnbsp;in a few minutes the glalTes wjll be fufficientlynbsp;exhauftecl. H and I are two Leyden bottles,nbsp;each of which has a female fcrew fitted to thenbsp;bottom, in order that they may be convenientlynbsp;�rewed on the pillar A. The bottle H is fur-pilhed with a belt, that it may be ferewed fide-ways on the pillar A. K and L are two fmallnbsp;wires, which are to fcrew occafionally into

either the ball E, the knobs e or f, the cap C, or the focket g, on the top of the pillar ; thenbsp;balls may be unfcrewed from thefe wires,nbsp;tvhich will then exhibit a blunt point, M is anbsp;wooden table, to be fcrewed on the glafs pillarnbsp;occafionally.

Screw the jar I on the infulating pillar, and the pointed wire into the hole g, place anothernbsp;pointed wire at the end of the condudor, bringnbsp;the knob of the jar near this wire, and thennbsp;turn the cylinder, a pencil of rays will divergenbsp;from the pointed wire in the conductor to thenbsp;knob of the jar, at the fam.e time another penrnbsp;cil of rays wall diverge from the point at thenbsp;bottom into the air. See fig. 50.

Repeat this experiment -with the negative condu6tor, and a luminous ftar will appear onnbsp;the end of each wire.

Screty a pointed wire into the knob of tlie jar, (fee fig. 51) charge the bottle pofitively,nbsp;the fire will be received from the condutdor bynbsp;the pointed wire, and appear there as a luminous ftar, while th-e wire on the outfide of thenbsp;jar will throw off a diverging cone.

Fjg-

Fig. 52 reprefents the foregoing appearance� Teverfed, by charging the jar negatively -at thenbsp;pofitive condnftor.

This experiment may be further varied, by applying the bottle to a negative con-dudornbsp;nbsp;nbsp;nbsp;�nbsp;nbsp;nbsp;nbsp;-

After the jar is charged, as in the foregoing experiments, turn that wire from the cylindeynbsp;which before w'as neareft to it, then put thenbsp;machine in a�lion, and the afflux and effluxnbsp;will be more apparent than before ; one pointnbsp;throwing off, and the other receiving the fluidnbsp;with extreme avidity, which will in a littlenbsp;time difcharge the jar.

Charge the jar as before, then touch the wire which is connefted with the negativenbsp;fide, and the oppofite wire will throw off ^nbsp;diverging cone ; but, if the pofitive fide isnbsp;touched, a luminous cone only wdll be feen onnbsp;the other wire.

53 is an eledlric jar, B B the tinT-foil iipoating, C a ftand which fupports the jar, D a

focket of metal which carries the glafs rod E; 5. curved metallic wire, pointed at each end,nbsp;is fixed to the end of the rod G, which rod isnbsp;moveable at pleafure in a fpring tube N, thatnbsp;tube being fixed by a focket upon the top ofnbsp;the glafs rod E, the charging wire communicates with the different divifions of the infidenbsp;coating of the jar by horizontal wires.

Place the jar as ufual, and put the machine in adfion, a fmall luminous fpark will appearnbsp;upon the upper point of the wire F, (a plainnbsp;indication that the point is then receiving electricity from the upper ring of the coating onnbsp;the outfide of the jar) a fine ftream or pencil ofnbsp;rays will at the fame time fly off, beautifullynbsp;diverging from the lower point of the wire Fnbsp;upon the bottom ring of the coating on the jar;nbsp;when thefe appearances pcafe, wdiich they willnbsp;as foon as the jar is charged, let a pointed wirenbsp;be prefented towards the prime conduftor,nbsp;this will foon difeharge the jar filently, duringnbsp;w'hich, the lower point will be illumined withnbsp;fmall fpark, while the upper point of the wirenbsp;w'ill throw off a pencil of rays, diverging to-w'ards the upper ring of the coating.

Take a Leyden phial, the neck of which amp;ould not be very broad, fet the coating on

AN ESSAY

the Gonduftor, and charge it negatively; when charged, if not too dry, the upper edge of thenbsp;coating will throw off one or more brulhes ofnbsp;light into the air, which will vifibly incline towards the charging wire of the bottle, andnbsp;fometimes aclually reach it. Prefent the knobnbsp;to the prime condu�tor, and charge the jar po-fitively, a fmall fpark of light will firft appearnbsp;on the edge of the cork in the neck of the bottle, through which the wire paffes after a fewnbsp;turns of the cylinder; this fpark becomes anbsp;bruin, darting out from the cork, and graduallynbsp;lengthening till it forms an arch, the end of itnbsp;extending downwards till it reaches and touchesnbsp;the end of the coating. If the bottle be dry,nbsp;it will in both cafes be difcharged fpontaneoufly.nbsp;See fig. 54 and 55.

An infulated pofitively charged bottle will give a fpark from its knob to an excited flicknbsp;of wax, while no fpark will pafs between it andnbsp;�an excited glafs tube.

this on the infulatcd ftand, wkh' the pointed wire from the bottom. Fig. 56 reprefents thenbsp;appearance of the fluid on the points when thenbsp;bottle is charging negatively, at a conduftornbsp;loaded with poiitive electricity.

Fig. 57 the appearances it difplays when It is charging pofitively at the fame conduftor.

Fig.' 59 is the fame bottle charging pofitively at a negative conduClorlt; Figj 60 it is chargingnbsp;negatively at the fame conduftor.

Fig. 61 reprefents the luminous conductor on the infulating ftand. Set the collectingnbsp;point near the cylinder, and place the knob ofnbsp;an uncharged phial in contact with the ball,nbsp;or hang a chain from it to the table, and, onnbsp;working the machine, the ball will be enveloped in a denfe electric atmofphcre. if thenbsp;point be brought in contact with an infulatcdnbsp;rubber, and a communication is made from thenbsp;ball to the table, the atmofphere will be on thenbsp;point in the tube. If a bottle, pofitivelynbsp;charged, be prefented, the appearances in thenbsp;tube will be as delineated in fig. 62. But,nbsp;if a bottle negatively charged be thus applied, the appearance will be as in fig. 61.

AN ESSAY

This tube, when mounted on its infulating ftand, may be ufed inftead of the prime con-du�lor, and all the common experiments maynbsp;be performed with it ; the tube will be lumi-hous during the whole of the operation.

Place a charged jar on a fmall glafs flanJ under the receiver of an air pump as the receiver is exhatffting the cleClric fire will iffuenbsp;from the wire of the phial, in a very luminous pencil of rays, and continue flalliing to*nbsp;the coating till the air is exhaufted/ when thenbsp;jar will be found to be difeharged.�

If the phial is charged negatively, the cur� rent of fire will app�ar fo have a different di-feftion from that which it had before.

From this experiment we may infer the effe�ts of the atmofpheric preflure upon thenbsp;charge of the Leyden phial, and learn that it isnbsp;the natural boundary to every charge of electricity we can give ; and, confequently, that anbsp;phial would contain double the charge, in airnbsp;doubly condenfed, as it does in th� common

atmofphere, fiiice it wotild incfeafe the intenfity of the ele�tric atmofphere.

Place a fmall lighted tapet between the twa balls of the 'univerfal difcharger, then pafs anbsp;very fmall charge of a pofitiye phial throughnbsp;them, and the flame of the taper will be at-fradtcd in the dire�tion of the fluid towards thenbsp;Coating. See fig^ 63.

In both thefe experiments it is neceflary tct ufe the leaft charge that can be given, juftnbsp;fufficient to leap the interruption in the circuit.

� Place a card oh the table of the Univerfat difcharger, and bring one of the points undernbsp;the card, then conned this point with the coating of a jar politively charged, place the othernbsp;point on the top of the card, and at ahoQt annbsp;ihch and a half from the formernow eompleatnbsp;the circuit, by bringing a difcharging rod

AN ESSAY

fro�n the lail; wire to the top of a bottle, anti the eleftricity will pafs through the upper wireynbsp;along the furface of the card, till it comes tonbsp;the point which is underneath, where it willnbsp;make a hole in the card, and pafs throughnbsp;the wire to the coating of the bottle. Secnbsp;fig. 64.

Four cork balls. A, B, C, D, being placed at equal diftances from each other, from thenbsp;balls of the dlfoharging rod, and from thenbsp;coating of a pofitively charged bottle ; on making the difcharge, the ball A next the rodnbsp;W�^as repelled to B, which w'as again repelled to�nbsp;G, C remained immoveable^ but D' flew to thenbsp;coating of the bottle.

Take a card, and paiAt both fides with cln-' nabar about the breadth of tl^ finger, fix thisnbsp;card vertically by a little w'ax on the table of.nbsp;the univerfal difcharger, let the pointed ends ofnbsp;one of the wires touch one fide of the card,nbsp;and the end of the other wire the oppofite fide ynbsp;the diftance of the points from each other- muflnbsp;be proportioned to the ftrength of the charge

ON ELECTRICITY. 1x3

difcliarge a jar through the wires, and the black mark, left by the explofion on the coloured band, fhews that the ele�ric fluid paflednbsp;from the wire, communicating with the infidenbsp;of the bottle, to that which communicatesnbsp;with the outfide, againft W'hich it makes anbsp;hole.

, Let the furfaces of an �ledlric plate be very flightly charged and infulated, let an interrupted circuit be formed, the two powers willnbsp;be vifibje, illuminating the points of the inter-tupted circuits, and each power will appear tonbsp;extend farther from the furface contiguous tonbsp;it, the ftronger the charge is communicated tonbsp;the plate ; but, if the illuminations on eachnbsp;fide meet, there will immediately follow annbsp;explofion of the whole charge.

If a cylindrical plate of air, contained in the receiver of an air-pump, be charged, it is ob-

AN ESSAY

ferved, the more air that is exhaufted from between the furfaces the more eafily the powers will unite.

If an exhaufted receiver be made part of the ele�tric circuit, and the charge Ihould not benbsp;fufEcient to caufe an explofion, an eleftric lightnbsp;will appear to proceed in oppofite directionnbsp;from the parts communicating with the negative and pofitive furfaces.

Let a coated phial be fet on an infulating ftand, and let its kziob be touched by the knobnbsp;of another phial negatively ele�rified, a fmallnbsp;fpark will be feen between them, and bothnbsp;lides of the infulated phial will be inftantlynbsp;negatively eledtrifted.

* Fatten a pith ball cledrometer by a little wax to the outfide coating of a jar, chargenbsp;the jar ftightly with pofitive eledricity, and fet

it on an infulated ftand, the ball will either not diverge, or only a very little j bring the knobnbsp;ot a bottle which is flrongly charged withnbsp;pofitive eledtricity near the knob of the former,nbsp;and the loalls will diverge with pofitive elec'snbsp;tricity. .

Let the fame phial, with the pith balls ttL fixed to its outfide coating, be flightly chargednbsp;negatively, and then infulated, bring the knobnbsp;of a phial, which is ftrongly electrified negatively, to that of the infulated one, and thenbsp;pith balls will diverge with negativ^e eleflrLnbsp;eitv.

Charge a jar pofitlvely, and then infulate it, charge anoth�r ftrongly with negative eledtri-^nbsp;city, bring the knob of the negative bottle nearnbsp;that of the pofitive one, and a thread will playnbsp;between them ; but, w'hen the knobs touchnbsp;each other, the threads, after being attracted,nbsp;will be repelled by both. The negative electricity is fome-how fuperinduced on the pofitive,nbsp;and, for a few minutes after they are feparated,nbsp;I 2nbsp;nbsp;nbsp;nbsp;both

CHAP. VJII.

Of the EleBrical Batteryt and the lateral Explofon of charged fqrs.

TO increafe the force of the elelt;5lric ex-plofion, feveral Leyden phials are connected together in a box ; this collection is termed an eleCtrical battery. Fig. 65 repre-fents one of the moft approved form.

The bottom of the box is covered with tin-foil, to connect the exterior coatings ; the in-fide coatings of the jars are connected by the wires b, c, d, e, f, g, which meet in thenbsp;large ball A ; C is a hook at the bottom ofnbsp;the box, by which any fubftance may be connected with the outfide coating of the jars;nbsp;a ball B proceeds from the inlide, by whichnbsp;the circuit may be conveniently compleated.nbsp;The following precautions are neceliary to benbsp;attended to by thofe who make ufe of an electrical battery.

To keep the top and uncoated part of the jars dry and free from dull, and after the ex-plofion to connect a wire from the hook to thenbsp;ball, which fliould be left there till the batterynbsp;I 3nbsp;nbsp;nbsp;nbsp;is

AN ESSAY

is to be charged again, which will totally obviate the inconveniencies that have occafi-onally happened from the refiduum of a charge.

If one jar in a battery is broke it is impoffi-ble to charge the reft till the broken jar is removed.

To prevent the jars of a large battery breaking at the time of the explofion, it has been recommended not to difcharge a battery throughnbsp;a good condud'or, except the circuit is at lead:nbsp;five feet long; but what is gained on one handnbsp;by this method is Ipft on the other, for, bynbsp;lengthening the circuit the force of the ftiocknbsp;is weakened proportionably.

I have been informed, that it is very difficult to break by an explofion the jars which are made of green gl^s, fabricated at Newr-caftle, but have had no opportunity to makenbsp;^ny experiments on this glafs myfelf.

The force of a battery may be confiderably increafed by concentrating the fpark from thenbsp;explofion, which is effefted by caufing it tonbsp;pafs through fmall circviits of non-condudtingnbsp;fubftances. By this means the refifting medium, through which the fpark is to pafs, maynbsp;be fo prepared as to augment its power. If thenbsp;fpark is made to pafs through a hole in a platenbsp;of g�afs, one twelfth or one fixth part of an

inch in diameter, it will be lefs diflipated, niore compa�t and powerful. If the part roundnbsp;the hole is wetted with a little water, the fpark,nbsp;by converting this into vapour, may be conveyed to a greater diftance, with an increafe ofnbsp;rapidity, attended with a louder noife thannbsp;common,

Mr. Morgan, by attending to thefc and fome other circumftances, has melted wires, amp;c.nbsp;with fmall bottles. I hope he will be inducednbsp;to communicate this, as well as the reft ofnbsp;his important difeoveries, to the public.

Pafs the charge of a ftrong battery through two or three inches of fmall wire, it will fome-times appear red hot, firft at the pofitive fide,nbsp;and the rednefs will proceed regularly towardsnbsp;the other end.

Difcharge a battery through a quire of paper, a perforation will be made through it ; each of the leaves is protruded by the ftrokenbsp;from the middle towards the outward leaves, asnbsp;if the fire darted both ways from the center.nbsp;If the paper is very dry, the fire meets withnbsp;I 4nbsp;nbsp;nbsp;nbsp;more

more difficulty in its paffage, and the hole js fmall. Ifnbsp;nbsp;nbsp;nbsp;of the paper,. through

�which the explolion is made, is wet, the hole is larger, the light more vivid, and the ex-plofion louder.

The difcharge of a battery through a fmall Heel needle will, if the charge is fufficient,nbsp;communicate magnetifm to the needle.

The difcharge of a battery through a fmall and flender magnetic needle, will generally de-ftroy the polarity of the needle, and fometimesnbsp;invert the poles thereof. To fucceed in thisnbsp;experiment, it is often neceffary to pafs feveralnbsp;ftrong charges through the needle before it isnbsp;removed from the circuit.

It appears, from Beccaria�s experiments, that the magnetic polarity, which is communicated to the needle by eledrricity, dependsnbsp;on the pofition of the needle when the chargenbsp;is font through it, and is not regulated by thqnbsp;direftion of the eledlric matter in entering th^nbsp;needle.

Difcharge a batter}? through a Hender piece of wire, ex. gr. one 50th of an inch in diameter,nbsp;the wire will be broken to pieces, or melted,nbsp;fo as to fall on the table in glowing balls.

When a wire is melted in this manner, the fparks fly frequently to a confiderable diftance,nbsp;being fcattered by the explofion in all directions.

If the force of the battery is very great, the w'ire will be entirely difperfed by the force ofnbsp;the explofion. Small particles of fuch fub-Itances as cannot be eafily drawn into wire, asnbsp;platina, grain gold, ores, amp;c. may be placednbsp;in a groove of wax, and then put into the circuit, if a difcharge of fufficient ftrength isnbsp;patfed through them they will be melted.

The force by which wires are melted by a battery varies with the length of the circuit, asnbsp;the fluid meets w'ith more refiftance in proportion as the paffage through which it is to pafsnbsp;is longer. Dr. Prieftley could melt nine inches of fmall iron wire at the diftance of 15nbsp;fc.ct, but at twenty feet diftance he could onlynbsp;make 6 inches of it red hot.

AN ESSAY

Inclofe a very flender wire in a glafs tube, difcharge a battery through this wire, and itnbsp;will be thrown into globules of diiFerent fizes,nbsp;which may be collefted from the inner furfacenbsp;of the tube : they are often found to be hollow,nbsp;sad little more than the fcoria of the .metal.

Many experiments have been made, in order to try the different conducing pov/ers of metals, by paffing the difeharge of a batterynbsp;through them ; but it has not yet been determined, whether the greater facility with whichnbsp;fome metals are exploded depends on the cafenbsp;with which the fluid pafl.es through them, ornbsp;whether it proceeds from the degree of refift-ance they make to its paffage, or from a wantnbsp;of dudtility in the metal, which is thereforenbsp;iefs capable of expanfion.

Difeharge a battery through a chain which is laid on paper, and black marks will be leftnbsp;on the paper in thofe places where the rings ofnbsp;the chain touch each other ; the rings will benbsp;more or lefs melted at thofe places.

Take two pieces of window glafs, of about 3 by 2 inches, place a flip of brafs or gold leafnbsp;between them, leaving the metallic leaf out beyond the glafs at each end ; then place the twonbsp;pieces of glafs in the prefs of the univerfal dif-charger, bring the points of the wires ET,nbsp;E F, fig-33, to touch the ends of the leaves,nbsp;and pafs a difeharge through them, which willnbsp;Ibrce part of the metal into the glafs, and ftainnbsp;it with a colour which differs from the metalnbsp;that is made ufe of. The metallic leaf fliouldnbsp;be made narrovveft in the middle, becaufe thenbsp;force of the eledfric fire is in proportion to itsnbsp;denfity, which is increafed when the fame quantity of fire is compelled to pafs through fewernbsp;conducing particles.

The explofion in melting the ftripes of leaf-gold, amp;c. renders them non-conduding, and lefs capable after each difeharge to tranfmitnbsp;another. Some particles of the metal are drivennbsp;into the glafs, which is really melted ; thofenbsp;parts of the metal which lye contiguous to thenbsp;glafs are the moft perfedlly fufed. The piecesnbsp;pf glafs which cover the flip of metal are gene-, rally broken to pieces by the dlfcharger.

AN ESSAY

Place a thick piece of glafs on the ivorjr plato of the univerfal difcharger, fig. 3^ PI. II, andnbsp;a thick piece of ivory on the glafs, on which anbsp;weight from one to fevcn pounda is to be placed ; bring the points of the wires EF ETnbsp;againfl; the edge of the glafs, and pafs the dif-charge through the wires, by conneding one ofnbsp;the wires, as EF, with the hook C of the battery, fig. 65, PI. IV, and forming a communication, when the battery is charged, from thenbsp;other wire ET to the ball, and the glafs willnbsp;be broken, and fome part of it firivered to annbsp;impalpable powder. When the piece ot glafsnbsp;is ftrong enough to refift the fhock, the glafsnbsp;is often marked by the explofion with the moilnbsp;lively and beautiful colours, I have been informed by Mr. Morgan, that if the glafs isnbsp;cemented down the effed is the fame as wheq,nbsp;it is preffed by the weights ; and this mode isnbsp;in various experiments more convenient.

If the difeharge is paiTed under the piece of ivory with the weights upon it, without anynbsp;glafs between the piece of ivory and the table

ON ELECTRICITY. 1I5

GH of the univerfal difcharger, the weights will be lifted up by the lateral force of thenbsp;charge; the number of weights mufl benbsp;portioned to the force of the explofion.

Fig. 66, a, reprefents an infulated rod, nearly touching a charged jar d, b is another infulatednbsp;rod, placed in a line with and near to the former ; make the difeharge by the rod e, froninbsp;which a chain hangs that does not touch thenbsp;bottom of the jar, and the rod b wnll receivenbsp;an cledlric fpark, which quits it again almoU isnbsp;the fame inftant, becaufe the fineft threadsnbsp;hung upon it wnll not be eledlrified by thenbsp;fpark.

This ele�trical appearance, without the circuit of a difeharging jar, is called the Lateral Explofion.

If pieces of cork, or any light bodies, be placed near the explofion of a jar or battery,nbsp;they will be moved out of their place in all di-re�ions from the center of the explofion ; andnbsp;the greater the force of the expiouon, fo muchnbsp;greater will the diftance be to which they arenbsp;removed. It is not furprifing, therefore, thatnbsp;heavy bodies fhould be removed to confiderablcnbsp;diftances by a ftrong flailt of lightening. Dr.

AN ESSAY

Prieftley apprehends, that this fpecies of lateral force is produced by the explofion of the airnbsp;from the place through which the ele(flric dif-cliarge paffes-

This lateral force is not only exerted in the neighbourhood of an explofion, when it isnbsp;made between pieces of metal in the open air,nbsp;but alfo when it is tranfmitted through piecesnbsp;of wire that are not thick enough to condu�: itnbsp;perfeclly. The fmaller the wire is, and thenbsp;greater the fufion, the greater is the difperfioiinbsp;of light bodies near it.

If circuits, different in length and of different fubftances, form a communication between two charged furfaces of an electric plate, it isnbsp;obferved, the difcharge will be made throughnbsp;the beft eOndudtors, whatever be the length ofnbsp;the Others.

2. nbsp;nbsp;nbsp;If circuits of the fame fubftance be different in length, the difcharge. will be madenbsp;through the fliorteft of them.

3. nbsp;nbsp;nbsp;If the circuits be the fame in every re-fpefl;, the difcharge will be made through many of them at the fame time.

I have been informed by a gentleman, that it was his cuftom to make a variety of circuits

for

ON ELECTRICITY. 127

for the dlfcharge of a large jar or battery ; and, that having a fuiEcient number of thefe, benbsp;could introduce himfelf into one of them, andnbsp;take his part of the Ihock without inconvenience, it even was not difagreeable ; and henbsp;could by this means IclTen the fcnfation almoilnbsp;to nothing.

Mr. Henly made a double circuit, the firll by an iron bar, one inch and a half in diam�*nbsp;ter, and half an inch thick ; the fecond, bynbsp;four feet and h half of fmall chalru On dlf-charging a jar, containing five hundred fquarenbsp;inches of coated furface, the electricity pallednbsp;in both circuits, fparks being vifible on thenbsp;fmall chain in many places. On making thenbsp;difeharge of three jars, containing together fix-teen fquare feet of coated furface, throughnbsp;three dilferent chains at the fame time, fig, 67,nbsp;bright fparks were vifible in them all. Thenbsp;chains were of iron and brafs, of very different lengths ; the Ihorteft ten or twelve inches,nbsp;the longed; many feet in length. When thofenbsp;jars w'ere difeharged. through the iron bar before-mentioned, together with a fmall chain,nbsp;three-quarters of a yard in length, the wholenbsp;chain was illumined, and covered throughout

AN � S S A Y

with beautiful rays, like briftles, or golden hair. Having placed a large jar in contadtnbsp;with the prime condudtor, and affixed to thenbsp;coating of it an iron chain, which was alfo con-nedted with a plate of metal, on which wasnbsp;made the difcharge by the difcharging rod jnbsp;this done, he hooked another chain, muchnbsp;longer, and of brafs, to the oppofite fide ofnbsp;the jar, and brought the end of it within eightnbsp;inches and an half of the metal plate. In con-tadt with this end a fmall oak Hick was laid,nbsp;eight inches long, which was covered withnbsp;faw-duft of fir-wood. On making the difcharge upon the plate, both the chains werenbsp;luminous through their whole lengths, as wasnbsp;alfo the faw-duft, which was covered by anbsp;ftreak of light, making a very plcafiDg appearance.

At the glafs-houf� there is generally a great number of folid fticks of glafs, about one quarter of an inch diameter ; if thefe be examinednbsp;narrowly, feveral of them will be found tubular a confiderable length ; the diameter of thenbsp;cavity feldom exceeds the aooth part of annbsp;inch. Seledt and break off the tubular part,nbsp;which may be filled ivith quickfilver by fucking, care being taken that no moifture previ-oufly infinuates itfelf; the tub'e will then bbnbsp;prepared for the experiment.

Ex-

Pafs the ihock through this fmall thread of tjuicklilver, which will be inllantly difplodedjnbsp;and will break or fplit the tube in a curiousnbsp;rhannet. *

Take a glafs tube, the bore �f which is about one quarter of an inch, fill it with water,, andnbsp;ftop the ends with cork, infert two wdresnbsp;through the corks into the tube, fo that theirnbsp;ends may nearly touch, make the ends of thefenbsp;part of a circuit from a battery ; on the dif-eharge, the water will be difperfed in every di-reftion, and the tube blown to pieces by thenbsp;difcharge.

The eleftric fluid, like common fire, converts the water into an highly elaftic vapour. Dr. Franklin, on repeating this experimentnbsp;with ink, could not find the leafl; ftain uponnbsp;the white paper, on which the tube had beennbsp;placed. Eeccaria paffed the ftiock through anbsp;drop of water, which was fupported,, in thenbsp;center of a foiid glafs ball, between the endsnbsp;of two iron wires, and the ball was fliivered innbsp;pieces by the explofion. On this- principle henbsp;Knbsp;nbsp;nbsp;nbsp;contrived

AN ESSAY

contrived what he calls an eleftrieal mortar, wdrich will throw a fmall leaden ball to the dif-tance of twenty feet. It is clear, from feveralnbsp;of the foregoing experim^ents, that the ele�lricnbsp;�nid endeavours to explode in every direftionnbsp;the parts of the refilling fubftances throughnbsp;which it paffes.

Place a building, which is formed of feveral loofe pieces of wood, on a wet board in the middle of a large bafon of water, let the electric flafli quot;from a battery be made to pafs overnbsp;the board, or over the water, or over bothnbsp;the water will be llrongly agitated, and thenbsp;building thrown down. The report is loudernbsp;than when the explofion paffes only throughnbsp;the air. The eledric fluid endeavours to pafsnbsp;near the furface of the water where it meetsnbsp;with more refiftance, than if it is forced to pafsnbsp;through it. This partly arifes from the powernbsp;the eledric fluid has of raifing an expanfivenbsp;vapour front the furface of the w�ater, whichnbsp;drives off the refilling air.

A difcharge palled over the furface of a piece of ice will leave on it fmall unequal cavities,nbsp;exhibiting the fame appearance as if a hotnbsp;chain had been placed on it.

ON ELECTRICITY. 13!

A difcharge fent through a green leaf tears the furface in various directions, leaving annbsp;iniao-e in miniature of fome of the effects ofnbsp;lightening. A difcharge will pafs to a certainnbsp;diftance over fpirit of wine, without inflamingnbsp;it ; but, if the diftance is increafed, it will fetnbsp;it on fire. From hence it appears, that thenbsp;facility with which the eleCtric fire Is tfanfmit-ted over the furface of moifl fubftanees, depends on the eafe with which the)gt; are turnednbsp;into vapours.

The difcharge, in melting the particles of metals, drives into its paflage the conductingnbsp;vapours which arif� from them ; and, in proportion as the parts of any body are more readily driven into vapour or duft, the fpark willnbsp;fun to a greater diftance;

Experiment CXL�.

If a wire is ftrctched by weights, and S ilrock is fent through it that will render it rednbsp;hot, it is found to be confiderably lengthenednbsp;after the difcharge. When the wire is loofe,nbsp;if is faid to be flrortehed b'y the explofion.

If a long narrow trough of water is made part of the circuit in the difcharge of a battery,nbsp;K 2nbsp;nbsp;nbsp;nbsp;and,

AN ESSAY

and a perfon�s hand be immerged in the trater at the time of the explofion, he will feel an oddnbsp;vibration in the water, very dilFerent from annbsp;eledrical Ihock. The quick ftroke from thenbsp;repercuffion of the air and the vapour, is communicated to the hand by the water, and thenbsp;hand receives a fliock limilar to that receivednbsp;by a fliip at fea during an earthquake.

Place a plain piece- of metal between the points of the univerfal difeharger, pafs feveralnbsp;cxplofions of a battery through the wires,, andnbsp;the difeharges will gradually form on thenbsp;metal different circles, beautifully tinged: withnbsp;the prifmatic colours. The circles appearnbsp;fooner, and are clofer to each other, the nearernbsp;the point is to the furface of the metal. Thenbsp;number of rings, or circles, depend on. thenbsp;lharpnefs of the point; the experiment therefore fucceeds better if a diarp needle is faft�enednbsp;to one of the points of the difeharger.

Several very curious experiments were made by Dr. Watfon and others, to afeertain the dif-tance to which the ele�tric fhock might benbsp;conveyed, and the velocity with which it moves.nbsp;In his firft experiment, the Ihock was givennbsp;and fpirits fired by the eledric matter which

had been conveyed through the river Thames. In the next experiment, the eleftric fluid wasnbsp;made to pafs through a circuit of two miles,nbsp;croffing the New-river twice, going over feveralnbsp;gravel pits, and a large field. It was afterwardsnbsp;conveyed through a circuit of four miles. Itnbsp;pafled over thefe fpaces inftantaneoufly as tonbsp;fenfe. This fenfible inftantaniety in the motionnbsp;of the electric fluid, w^as afccrtained by an ob-ferver, who, though in the room with thenbsp;charged phial, was, at the fame time, in thenbsp;middle of a circuit of two miles, and felt him-lelf flrocked at the fame inftant he faw the phialnbsp;difeharged.

may be touched fo quickly, even by the beft conduflors, that all the eledtric matter has notnbsp;time to make the circuit, and the phial willnbsp;remain but half difeharged; and there are fe-vcral inftances where its motion appears flow,nbsp;and not eafily reconcilable with this immealura-ble velocity ; and it is alfo certain, that, thisnbsp;fluid is refifted in its paflhge through, or over,nbsp;every fubftance.

The wonderful part of the foregoing experiments will vanilh, if we admit the reafoning of Mr. Volta on this fubjedl; and the reader willnbsp;find his reafoning confiderably ftrengthened bynbsp;K 3nbsp;nbsp;nbsp;nbsp;experi-

experiments n 8, 119,120 of this effay, whicl^ were originally made by Mr. Atwood ; thoughnbsp;it muft be owned, thefe experiments feem. tqnbsp;lead much further, and give an idea of the direction of the eleftric fluid in the difcharge of thenbsp;Leyden phial, which differs altogether from thenbsp;received theory.

The following account is extradled from a very long paper of Mr. Volta, in the Journalnbsp;de Phyfique for 1779 ;

Let us fuppofe that a, I, c, d, e, �, g, h, if k, I, m, n, 0, hold hands j let a grafpnbsp;the outfide of a charged Leyden phial, and 0nbsp;touch the knob ; at the inftant 0 receives thenbsp;fire dlfcharged from the infide by the knob, anbsp;will furnifh from his natural flock to the outfide, without waiting till the fire arrives to himnbsp;from 0, by Uf to m, amp;c. in the mean whilenbsp;the lofs of a is compenfated from by and bnbsp;is furnifhed with frclh matter from c, and fonbsp;on. It is ftill true, that there is but onenbsp;flream, if we confider only the direftion of thenbsp;fluid, which is excited fimultaneoufly at thenbsp;two extremities, and moves at the fame inftantnbsp;of time; though, to fpeak more accurately,nbsp;it is not one ftream, but two united in one.nbsp;If the extream rapidity with which the firenbsp;paffes, did not prevent our perceiving the fuc-ceffive commotions received by the perfons who

form the chain, we fliould find they did not follow the order 0, n, I, but v.'ere feltnbsp;fimiikaneoufly, firft at the two extremities 0nbsp;and a, then at n and b, m and r, amp;c. advancing towards the middle of the chain. Agreeable to this, if the bottle is fmall, the longernbsp;the circuit is made, thofe who are furthefl;nbsp;from the extremities find the Ihock w'eaker.

To render this account more clear, feparate the circuit, and form on a dry floor two rows,nbsp;(i, b, c, d, � Cy f, g, h, interrupted in thenbsp;middle ; let d grafp the bottle by the outfide,nbsp;and a excite the difeharge by touching thenbsp;knob of the bottle; now, if the eledric fire wasnbsp;obliged to take the fliortefl; courle to come tonbsp;the exterior and negative furface, it ought tonbsp;defeend to the feet of e, pafs over the boardsnbsp;to the feet of d, and then through him to thenbsp;outfide, without ading on �, g, h, whichnbsp;wmuld be out of its circuit. But, contrary tonbsp;this, the fluid goes out of the direct courfe, tonbsp;follow that of the conducting perfons, whichnbsp;afford if a proper receptacle, and comes to thenbsp;outfide by another fource. The fire which goesnbsp;from the infide from e to �, g, h, gives themnbsp;a fenfible fenfation in their hands and theirnbsp;heels, fhewing itfelf by a fpark, if the handsnbsp;and the feet are feparated a little from eachnbsp;other, and finifhes by diflipating itfelf in thenbsp;K 4nbsp;nbsp;nbsp;nbsp;common

AN ESSAY

common rcfervoir- In the fame manner d, �who firft gives the fire to the outfide, receivesnbsp;it fucceflively from c, h, a, who all draw itnbsp;in from the floor. The ftream therefore whichnbsp;proceeds from the knob of the bottle, paflingnbsp;through the conducting fubftance, lofes itfeifnbsp;in the general fource; while, from the famenbsp;fource, a fufflcient qiiantity is taken to fupplynbsp;the deficiency of the exterior furface.

If �, g, h, do not form a chain, but are irregularly placed round e, tfle pofitivc part ofnbsp;the fluid may be feen to fpread itfeif on different fides, and divide itfeif in different branchesnbsp;to reach the floor. The fluid will in the famenbsp;manner rife from the floor to reach d, ' if a, bynbsp;and c, are irregularly placed round him ; fonbsp;that each furface excites its own ftream ; onenbsp;that enters the bottle, the ether proceedingnbsp;from it. Thus alfo, in the foregoing experiments of Dr. Watfon, where it has been fup-pofed that the eledlric fluid has made fuchnbsp;amazing circuits through rivers, over fields,nbsp;amp;c. The fluid from the infide was difperfednbsp;in the river, at the inftant that the outfide col-lefted, from the fame fource, fupplies for itsnbsp;own deficiency.

It appears alfo, from other experiments, that one fide of a charged ele�trie may contain

ON ELECTRICITY.

�more of one power than is fufficient to balance the contrary power on the other fide. For, ifnbsp;a charged jar is infulated, and the difeharge isnbsp;made by a difeharger with a glafs handle, afternbsp;the explofion, the difeharger, and both fides ofnbsp;the jar, will pofTefs a contrary power to thatnbsp;which obtained on the fide of the jar, whichnbsp;W'as touched the lafi: before the difeharge.

It may not be improper to introduce here an hypothefis which has been ofiered to the pubii#.nbsp;inftead of the received theory.

2. nbsp;nbsp;nbsp;Since they counterad each other whennbsp;united, they can be made evident to the fenfesnbsp;only by their feparation.

3. nbsp;nbsp;nbsp;The two powers are feparated in non-elec-itrics by the excitation of eledrics, or by thenbsp;application of excited eledrics.

5. nbsp;nbsp;nbsp;The two eledricities attrad: each othernbsp;llrongly through the fubftance of eledrics.

7. Either power, when applied to an unelectrified body, repels the power of the fame fort, and attrads the contrary power.

CHAP,

AN ESSAY

C II A P. IX.

TH E importance of eledlricity, as well as its univerfal agency, becomes more con-,nbsp;fpicuous, in proportion as our acquaintancenbsp;xvith it increafes. We find no fubftance in nature which is not acted on by it, either as anbsp;conductor or non-condu�tor ; and dilcover, thenbsp;fvirprizing phoenomena of thunder and lightening owe their origin to and are of the fame nature with it. Very little progrefs had beennbsp;made in eledlricity when the analogy betweennbsp;the electric fpark and lightening was difeovered;nbsp;but the fublime idea of realizing thefe conjectures, and proving, that the fire which flafhesnbsp;in the Iky is the fame agent which explodes andnbsp;gives a ihock in our experiments, was given tonbsp;Dr. Franklin ; who alfo firft fuggefted the utilitynbsp;of pointed conductors of metal, to prelervcnbsp;buildings from the dreadful effedts of lightening ; an idea which was received with generalnbsp;applaufe and approbation. Since this period,nbsp;many electricians have been induced tonbsp;change their opinion relative to the utility

ON ELECTRICITY. i

pf thele conduftors ; and, among thofe who underftand the fubjedt well, it has been dif-pntcd, whether the preference fliould be givennbsp;to a condudtor v/ith a pointed end, or to onenbsp;which has an obtufe termination.

The experiments which have been made on this fubjecl: are very numerous, but the greaternbsp;parr appear to me very inconclufive, and prelent only a very partial view of the fubjeft.

A pointed condu�lor, which communicates Wi� h the earth, has not any particular power ofnbsp;attiadilng eledtricity, and acts only as any othernbsp;eoncli.cting fubftance which does not refift thenbsp;paffage of the eleftric fluid.

It is true, that eleflricity palTes with more eafe from an eledrified body to a condudor which isnbsp;pointed, than to one which is flat or globular;nbsp;becaufe, in this cafe the elafticity of the eledricnbsp;fluid, and its power to break through the air,nbsp;are weakened by the flat furface, which acquiresnbsp;a contrary electricity, and compenfates the di-minilhed intenfity more than a point can ; thenbsp;point being eafily rendered negative, while thenbsp;effort of the fluid to efcape from the eledrifiednbsp;body is greater than when it is oppofed by anbsp;flat furface. So that it is not the particularnbsp;property of a point, or flat, but the differentnbsp;ftate of the eledrified body, which caufes itnbsp;to part with its eledricity eafier, and from anbsp;greater diftance, when a pointed conduding

AN ESSAY

The capacity of condudors to hold eledri-city is in proportion to the furfaces which arc free, or uninfluenced by a fimilar atmofphere jnbsp;a circumftance which will, more or lefs, affedtnbsp;thofe condudlors which are applied to buildings,nbsp;according to the ftate of the clouds and theirnbsp;atmofphere, the time their influence has beennbsp;exerted, the nature of the condufting ftrata ofnbsp;the earth, and its eleftric fituation.

Fig. 68 reprefents the gable end of a houfe, fixed vertically on the horizontal board F G jnbsp;a fquare hole is made in the gable end at h i,nbsp;into which a piece of wood is fitted ; a wire isnbsp;inferted in the diagonal of this little piece ; twonbsp;wires are alfo fitted to the gable end ; the lowernbsp;end of one wire terminating at the upper corner of the fquare hole; the top of the othernbsp;wire is fixed to its lower corner ; the brafs ballnbsp;on the wire may be taken off, in order that thenbsp;pointed end may be occafionally expofed to receive the explofion.

Place a jar with its knob in contadl with the conductor, conneift the bottom of the jar with thenbsp;hook H, then charge thejar, and bring the ball

under the conduftor, and the jar will be dif-charged by an explofion from the conduftor to the ball of the houfe. The wires and chainnbsp;being all in connexion, the fire will be conveyednbsp;to the oLitfide of the jar without affefting thenbsp;houfe ; but, if the fquare piece of wood isnbsp;placed fo that the wires are not connecled, butnbsp;the communication cut off, the elcdlric fluid,nbsp;in palling to the outfi.de of the bottle, willnbsp;throw out the little piece of wood to a confi-derable diftance by the lateral force of the ex^nbsp;plofion. See fig. 68.

Unfcrew the ball, and let the point which is underneath be prefcnted to the condu(3:orj andnbsp;then you will not be able to charge the jar; fornbsp;the fharp point gradually draws the fire fromnbsp;the condudlor, and conveys it to the coatingnbsp;on the ourfide of the jar.

The prime condudtor is fuppofed to reprefent a thunder cloud difcharging its contents on anbsp;weather cock or any other metal, at the t�p ofnbsp;a building. From this experiment many havenbsp;inferred, that if there is a conneftion of metafnbsp;to conduft the ekaric fluid down to the cartli.

where the conneaion is imperfea, it will flrite from one part to another, and thus endangernbsp;the whole building.

AN ESSAY

Mr. Henly affixed to the top of a glafl Hand a wire, three-eighths of an inch in diameter, terminated at one end by a ball, three-fourths of an inch in diatnetcr, and at the othernbsp;end by a very lharp point; (fee fig. 69) roundnbsp;the middle of this wire hung a chain, 12 inchesnbsp;long ; he conneffed the chain with the coatingnbsp;of a charged bottle, and brought the knob ofnbsp;it very gently towards the ball on the infulatednbsp;wdre, in order to obferve precifely at what dif-tance it would be difchatged upon it, whichnbsp;conftantly happened at the diftance of half annbsp;inch, with a loud and full cxplofion. Thennbsp;charging the bottle, he brought it in the famenbsp;gradual manner towards the point of the infulated wire, to try alfo at what diftance it wouldnbsp;be ftruck; but this, in many trials, never happened at all ; the point being approached innbsp;this gradual manner, always drew off the chargenbsp;imperceptibly, leaving fcarce a fpark in thenbsp;bottle.

The fame gentleman connected a jar, containing 509 fquare inches of coated furface, with the prime conductor, (See fig. 68) If

the jar was fo charged as to raife the ekftrome-ter to 6o�j by bringing the ball on the wire of the thunder houfe to half an inch didance fromnbsp;that conne�ted with the prime conduftor, thenbsp;jar would be difcharged, and the piece in thenbsp;thunder houfe thrown out to a confiderablcnbsp;didance. Ufing a pointed wire as a condudlornbsp;to the thunder houfcj inftead of the knob,- thenbsp;charge being the fame, the jar was difchargednbsp;filently, though fuddenly, and the piece wasnbsp;hot thrown outlt;

Me afterwards made a double circuit to the thunder houfe ; the fird by a knob, the fccondnbsp;by a lharp pointed wire, at an inch and a quarternbsp;didance from each other, but exactly the famenbsp;height. The charge being the fame, the knobnbsp;was fird brought under the prime conduclor*'nbsp;\lt;^hich was half an inch above it,- and followednbsp;by the point at an inch and a quarter didanccynbsp;yet no explofion fell upon the ball, as the pointnbsp;drew off the charge filently, and the piece 1�nbsp;the thunder houfe remained unmoved.

He infulated a large jar, and conneded, by chains, with the external coating, on one fide, a

AN ESSAY

knob, on the other a iharp pointed wire,- boA being infulated, and ftanding five inches fromnbsp;each other, (fee fig. 70) and placed an�infulated copper ball, eight inches in diameter, fonbsp;as to ftand exa�tly at half an inch diflance bothnbsp;from the knob and the point; the jar was thennbsp;charged, and the difeharge made by the dif-charging rod on the copper ball, from whencenbsp;it leaped to the knob A, which was three-quarters of an inch in diameter, the jar wasnbsp;difeharged by ^ loud and full explofion, andnbsp;the chain was very luminous.

Mr. Henly fufpended by a filk firing from One end of a wooden bar, which turned freelynbsp;in a horizontal diredion upon the point of anbsp;needle, a large bullock�s bladder, gilded withnbsp;leaf copper ; the bladder was balanced by anbsp;weight at the other end of the arm ; (fee fig.nbsp;71) he gave a ftrong fpark from the knob of anbsp;charged phial to the bladder, he then prefentednbsp;towards if a brafs ball, two inches diameter,nbsp;and obferved that the bladder would come towards it at the diftance of three inches, andnbsp;when it got within an inch, would throw offnbsp;its eleftricity in a full fpark. He then gave itnbsp;another fpark, and prefented a pointed wire to--

\'?ards the bladder, which never approached tfi the point, nor ever gave any fpark, the electricity being carried off.

Take two or three fine locks of cotton ten one of them to the conduftor by a finenbsp;thread, another lock to that, and a third to thenbsp;fecond, put the machine in' adtion, and the locks,nbsp;of cotton will expand their filaments, and willnbsp;extend themfelves towards the table. Prefent anbsp;fliarp point under the lowed and it will flirinknbsp;up towards the fecond, and this tow^ards thenbsp;firft, and altogether tow^ards the prime conductor, where they will continue as' long as' thenbsp;point remains under them.

Fallen a number of fine threads or hair to 'the end of the prime condudlor ; when the cylinder is turned, thefe will diverge like raysnbsp;proceeding from a center : continue turning thenbsp;cylinder, and prefent a point towards �ne fidenbsp;of the conduftor, and the threads on one fidenbsp;will hang down, and lofe their divergence, butnbsp;thofe on the other fide will ftill continue to' di- �nbsp;verge ; which fitews, that the power of points

an essay

to draw off eledricity does not extend round the eledrificd body when means are ufed to keepnbsp;up the fupply of eleftricity.

Fig. 72 reprefents an oval board, three feet Ir.ng and two feet broad, coated on both fidesnbsp;with tin-foii, and fufpendcd by filk lines fromnbsp;the double hook, this turns on an axis, whichnbsp;is fattened to one arm of a nice balance, andnbsp;counter-poifed at the other arm by a weight,nbsp;part of the table underneath the board is to benbsp;covered with tin-foil, and communicate to thenbsp;floor by a chain.

Connedl the pendulous board with the prime conductor by a fmall wire, a few turns of thenbsp;machine will elcdrify the apparatus. Whennbsp;this experiment was made, the board wasnbsp;attraded by the table at 15 inches dittance, andnbsp;difeharged itfelf with a ttrong fpark ; the famenbsp;happened to a metal ball which was placed onnbsp;the table, the board approaching till it wasnbsp;about one inch from the ball, and then dif*nbsp;charging itfelf by a fpark. If a point isnbsp;fixed on the board inftead of the knob, thenbsp;pendulous board, though it begins to approach,nbsp;flops at about four or five inches from the table,nbsp;and it will not approach nearer or give a fpark :nbsp;a fmall light is feen upon the point in the dark,

ON ELECTRICITY. ,47

A Leyden phial was then conneded with the prime conductor ; it now required more turnsnbsp;of the machine to charge the apparatus, thenbsp;�ffeft was the fame as before^ The counter-poife was now held, that the board might notnbsp;defcend till it had received a full charge; whennbsp;fet at liberty, it was not only attra�led by, butnbsp;alfo gave a loud explofion on the point, info^�nbsp;much, that the tin-foil round it was ftained bynbsp;the overflowing of the fire,

An Account of Experiments made at the Pantheon on the Nature and Ufe of Con-dudlors,� by Mr, Wilfon. It was made innbsp;order to point out what he deemed erroneousnbsp;in an experiment of Mr. Henly, which is thenbsp;148*^ of this eflayi

A bent rod of brafs, with a ball of the fame metal, three quarters of an inch in diameter,nbsp;fcrewed on to the upper extremity of it, andnbsp;a copper ball, five inches in diameter, fcrewednbsp;on to the lower end, forms one of the parts.nbsp;This part was fupported by a Hand of woodnbsp;that had a cap of brafs at the top, into whichnbsp;the brafs rod was occafionally fcrewed,

L 2 nbsp;nbsp;nbsp;^nbsp;nbsp;nbsp;nbsp;The

AN ESSAY

The other part of the circuit confifleti of a brafs rod alfo; one end of which branched outnbsp;in the form of a fork, with two prongs thatnbsp;pointed towards the center of the copper ball ;nbsp;and thofe prongs were fo conftru�tedj that eithernbsp;of them could, be made longer or fliorter, juftnbsp;as the experiment required^ On the end ofnbsp;one of the prongs was fixed a ball- of brafs,.nbsp;three quarters of an irtch in diameter,- and onnbsp;the other a fharp fteel point or needle. Thenbsp;fltoulder of this fork fcrewed into a fmall platenbsp;of iron, that was fixed on the infide of a wooden veflel, which contained the greateft part ofnbsp;a ct'lindrical glafs jar, twelve inches threenbsp;quarters high, and about four inches- in diameter. This glafs was rather thick than other-wife, and the coating of it- (which was tin-foil)nbsp;meafured nearly 144 fqiiare inches on each fur-face. Befides this coating, part of the infidenbsp;of the wooden veflel was coated alfo with tin-foil, for the purpofe of making a fecure communication between the iron plate and the outward coating of the jar. Within the jar itfelfnbsp;was fitted a cylinder of wood, that was coverednbsp;with tin-foil alfo, to make a communicationnbsp;between the infide coating of the glais and anbsp;brafs rod, that vvas fixed upright in the centernbsp;of the wooden cylinder. This upright rodnbsp;having a ball of brafs at the end, three quarters

ters of ail inch in cliameter, was bent towards the firft part of the circuit; fo that the twonbsp;balls A and B, in fig. yj, being upon a level,nbsp;looked towards each other, but were placednbsp;from time to time at dilFerent diftances, as oc-cafion required ; and thus anfwered the purpofenbsp;of an electrometer.

Mr. Wilfon began the experiments where the cleftrometer was {truck at the greateftnbsp;diftance, and then adjufted the diftances of thenbsp;ball accordingly ; fo that if the point wasnbsp;{truck when they were adjufted, the moving ofnbsp;the ball the thirty-fecond part of an inch wouldnbsp;occafion the ball to be ftruck in preference tonbsp;the point, and vice verfa. Afterwards he lelTen-ed the ftriking diftance of the electrometer, innbsp;every experiment, till he attained the leaftnbsp;diftance.

Upon reverfing part of the apparatus, and fixing the ball to the battle, and the fork tonbsp;the ftand, all thofe experiments were repeatednbsp;again ; the copper ball being put nc'areft to thenbsp;glafs, in the place of the forked part, and thenbsp;forked part in the place of the copper balknbsp;This fet of experiments being compleated, henbsp;made others, where the ball only was oppofed jnbsp;and after them, where the point only was op-^nbsp;pofed to the copper ball.

TABLE I.

EXPERIMENTS made at Dr. HrcoiNs�s, June 19, 1778, with the LEYDEN PHIALnbsp;and forked Apparatus.

N. B, The meafures exprelTed in the following tables were taken from a fcale containing 32 parts in one inch.

The number oppofite the word eleftrometer, denotes the dillance between the balls which conftituted the eleftro*nbsp;meter; and the numbers oppoftte to the words ia//a.n(inbsp;point, ihew the greatell dillance at which the^ were re�nbsp;fpeiaively ftruck.

� nbsp;nbsp;nbsp;� 25 f26 f26

� nbsp;nbsp;nbsp;� 334334 �

~ � 46 L� (.64

� nbsp;nbsp;nbsp;� 20 r 20 f 20

---2I-j23lt;j-

� nbsp;nbsp;nbsp;� 50 w� c6o

f E. nbsp;nbsp;nbsp;� i6fi6fi6

V. 4 B. � � 2i4 islt; �

CP.--5S�-^S3

-E. � � 13 ri3 ri3 lt; B. -r- � l6lt; Il4 �nbsp;t-P. � �- 44 C.� 442

rE. � � 10 f 10 { 10

VII4b.--

B. P. only only-

� nbsp;nbsp;nbsp;f28 f28

{25 r 26 f26

2^ 274 � 37c �f 37

20 r 20 ( 27

r l6rl6

alter.

p. _ nbsp;nbsp;nbsp;_nbsp;nbsp;nbsp;nbsp;4^

E, � .� 10 11

CP. � � 38

f Eleftrometer � 21.J nbsp;nbsp;nbsp;f23

Ball .� nbsp;nbsp;nbsp;�nbsp;nbsp;nbsp;nbsp;24 Apparatus reverfed. lt; 25

64J nbsp;nbsp;nbsp;C30

� Ever

Ever fince the difcovery of the identity of ele�tricity and lightening, it has been allowednbsp;by all parties, that condudlors of fome kind arenbsp;in a manner neceflary for the fafety of build*nbsp;ings in thofe countries where' thunder ftormsnbsp;are very frequent. The principle on whichnbsp;they adl is this : that the elediric fluid, whennbsp;impelled by any power, always goes to thatnbsp;place where it meets with the leaft refiftance,nbsp;Noxv, as metals are found to give the leaft re-iiftance to its paflage, it will always choofe tonbsp;run along a metaline rod, in preference to anbsp;paflage of any other kind. But it is neceflarynbsp;to obferve here, that eledlricity never ftrikesnbsp;a body merely for the fake of the bodynbsp;itfelf, but as by means of that body it cannbsp;arrive at the place of its deftination. Whennbsp;a quantity of elcdlricity is colledled fromnbsp;the earth, by means of an eleftric machine, anbsp;body communicating with the earth will receivenbsp;a ftrong fpark from tlie prime conductor; itnbsp;receives this fpark not becaufe it is capable ofnbsp;.containing all the eiedtricity of the cylindernbsp;and condudor, but becaufe the natural fituationnbsp;of the fluid being difturbed by the motion ofnbsp;the machincj a ftream of it is fent olf from thenbsp;earth. The natural powers, therefore, makenbsp;an effort to fupply what is thus drained off from

AN ESSAY

which comes out is moft proper for fupplying the deficiency, as not being employed for anynbsp;natural purpofe, there is always an effort madenbsp;for returning it to the earth. No fooner, therr,nbsp;is a conducing body, communicating with thenbsp;earth, prefented to the prime condudtor, thannbsp;the w�hole effort of the eleftricity is directednbsp;agalnfl that body; not merely becaufe it is anbsp;condudtor, but becaufe It leads to the placenbsp;where the fluid is directed by the natural powers by v.'hich it is governed, and at which itnbsp;would find other means to arrive, though thatnbsp;bcKly were not to be prefented. That this isnbsp;the cafe we may eafily fee, by prefenting thenbsp;fanie conducting fubltance in an infulated flatenbsp;to the prime condudor of the machine, whennbsp;we fhall find only a fmall fpark will be produced. In like manner, when lightening ftrikesnbsp;a tree, a houfe, or a thunder-rod, it is not becaufe thefe objects are high, or in the neighbourhood of the cloud, but becaufe they communicate with fome place below the furface ofnbsp;the ground, againft which the impetus of thenbsp;lightening is direCled, and at that place thenbsp;lightening' would certainly arrive, though nonenbsp;of the above-mentioned objeCts had been inter-pofed.

� When the atmofphere begins to be electrified, either negatively or pofitively, the earth,

ON ELECTRICITY.

by means of the inequality and moifture of its furface, but efpecialiy by the vegetables whichnbsp;grow upon it, abforbs that eledtricity, andnbsp;quickly becomes eledlrified in the fame manner with the atmofphcre; this abforption, however, ceafes in a very Ihort time, becaufe itnbsp;cannot be continued without fetting in motionnbsp;the whole of the eledric matter contained innbsp;the earth itfelf. Alternate zones of pofirivenbsp;and negative eledricity will then begin to takenbsp;place below the furface of the earth, for rea-fons given in the courfe of this eflay. Ee-tween the atmofphere and one of thefe zonesnbsp;the ftroke of lightening will always be. 1'hus,nbsp;fuppofing the atmofphcre is pofitively eledtri-fied, the furface of the earth will, by meansnbsp;of trees, amp;c. quickly become pofitively elcdri-fied alfo, we will fuppofe to the depth of tennbsp;feet : the eledricity cannot penetrate further,nbsp;on account of the refiftance of the elcdric matter in the bowels of the earth. At the depthnbsp;of ten feet from the furface a zone of negative!}�nbsp;eledrified earth begins, and to this zone thenbsp;eledricity of the atmofphere is attraded; butnbsp;to this it cannot get, without breaking throughnbsp;the pofitively eledrified zone, which lies up-permoft, and fliattering to pieces every badnbsp;f ondudor which lies in its way. We are therefore fure, that in whatever place the outer zone

AN ESSAY

of pofitively eleamp;ified earth Is thinneft, there the lightening will ftrlke, whether a condu�lornbsp;happens to be prefent or not. If there is anbsp;condudtor, either with a knob or fharp pointed,nbsp;the lightening will infallibly ftrike it: but itnbsp;would alfo have ftruck a houfe htuated on thatnbsp;fpot without any condudlor ; and if thenbsp;houfe had not been there, it would have ftrucknbsp;the furface of the ground Itfelf. Again, if wenbsp;fuppofe the houfe with its condudlor to ftandnbsp;on a part of the earth where the pofitivelynbsp;eleflrified zone is very thick, the eondudornbsp;will neither filently draw off the eledricity,nbsp;nor will the lightening ftrike it; though, perhaps, it may ftrike a much lo,wer objed, ornbsp;even the furface of the ground itfelf at no greatnbsp;diftance ; the reafon for which undoubtedly is,nbsp;that there the pofitively eledrifed zone is thinner than where the eondudor was.

To fuppofe that a pointed eondudor will exhauft a thunder cloud of its eledricity, muffnbsp;at firft fight appear trifling, to infift on it, ridi-eulous. Innumerable objefts are all confpiringnbsp;to drav/ off the eledricity as w^ell as the con-dudor, if it could be drawn off; but of ef-feding this, there is an impoflibility, beeaufenbsp;they have the fame kind of eledricity with thenbsp;clouds themfelves.

ON ELECTRlCiTY.

Befides, Becaria has obferved, that during fhe progrefs and increafe of the ftorm, thoughnbsp;the lightening frequently ftruck to the earth,nbsp;yet the fame cloud was the next moment readynbsp;to make a greater difcharge, and his apparatusnbsp;continued to be as mvich afFedted as ever.

� The condu�lor has not even the power of attraSling the lightening a fev^ feet out of thenbsp;diredtion it would choofe rtfelf: of this w� havenbsp;a moil decifive inftance in what happened tonbsp;the magazine at Purfleet,- in Efl'ex- Thatnbsp;houfe was fumiilred with a condudtor, raifednbsp;above the higheft part of the building; never-thelefs, a fiafl� of lightening ftruck an ironnbsp;cramp in the corner of the wall of the building, confiderably lower than the top of thenbsp;conduflor, and only forty-fix feet in a dopingnbsp;line diftant from the point.

� The condudbor, with all Its power of drawing cff the eleftric matter, Wfas neither ablenbsp;to prevent the flafli, nor to turn it forty-fix feetnbsp;out of its way. The matter of fa�f is, thenbsp;lightening was determined to enter the earthnbsp;at the place where the Board-houle ftands, ornbsp;near it 5 the condudtor, fixed on the houle,nbsp;cdfered the eafieft communication, but forty-fixnbsp;feet of air intervening between the point of thenbsp;condudtor and the place of the explofion, thenbsp;refiftance was lefs through the blunt cramp of

AN ESSAY

iron, and a few bricks moiftened with the rair! to the fide of the metalline condudor, thannbsp;through the forty-fix feet of air to its pointynbsp;for the former was the way in which the lightening a�tually paffedw

� The ziz-zag kind of lightening is the mod dangerous, becaufe it mull overcome a verynbsp;violent refinance of the atmofpher�, and wTerc-ever that refiftancc is in the fmalleft degreenbsp;leflened, there it will undoubtedly ftrike, andnbsp;even at a confiderable eliftance* It is otherwifenbsp;with that kind whieli appears in llafltcs of no'nbsp;determinate form : the eledric matter of whichnbsp;is evidently diffipated in the air by fomc con-duding fubftances which arc prefent there, andnbsp;they are therefore rendered lefs powerful^

� The mod dellrudive kind of lightening is that which affunles the form of balls. Thelenbsp;are produced by an exceeding great power ofnbsp;eledricity, gradually accumulated till the refinance of the atmofphere is no longer able tonbsp;confine it. In general, the lightening breaksnbsp;Out from the eledrified cloud by means of thenbsp;approach of fome.conduding fubftance ; butnbsp;the fire-balls feem to be formed not becaufenbsp;there is any fubftance at hand to attrad thenbsp;elcftric matter from the cloudj but becaufe thenbsp;eledricity is accumulated in fuch a quantitynbsp;that the cloud can no longer contain it. Hence,

fuch balls fly off flowly, and have no particulaf deflination ; their appearance indicates a pro-^nbsp;digioiis commotion and accumulation of electricity in the atmofphere, without a proportion^nbsp;able difpofition in the earth to receive it. Thisnbsp;difpofition is however altered by a thoufandnbsp;circumllances, and the place which firfl: becomesnbsp;mofl; capable of admitting eleftricity will amp;rftnbsp;receive a fire-ball. Hence this kind of lightening has been known to move llowly backwards and forwards in the air for a conflderablenbsp;time, and then fuddenly fall in one or morenbsp;houfes, according to their being more or lefsnbsp;affedted with an eledtricity oppofite to that �fnbsp;the ball at the time. It will alfo run along thenbsp;ground, break into feveral parts, and producenbsp;feveral explofions at the fame time.-

� It is very difficult to imitate this kind of lightening in our eledlrical experiments- Thenbsp;only cafes in which it hath been done in anynbsp;degree are thofe in which Dr- Prieftley madenbsp;the explofion of a batttery pafs for a confidera-ble way over the furface of raw flefli, water,-he. In thefe cafes, if, while the electric flaflinbsp;paired over the furfaces of the flefli, it hainbsp;been poffible to interrupt the metallic circuit bynbsp;taking away the chain, the cledlric matter discharged would have been precifely In the fitua-tion of one of the -above-mentioned fire-balls ;.

A ESSAY

f.e. it would have been at a lofs for a condu^lor; The negative �de of the battery was the placenbsp;of il^s deftination, but to that it could not eafilynbsp;have got, becaufe of the great quantity ofnbsp;atmofphere which lay in.its way, and the incapacity of the neighbouring-bodies to receivenbsp;it. But, wdrife the cledfric matter was thugnbsp;Sationary for want of a cond�dtor,- if any onenbsp;Sanding near, or touching the negative fide ofnbsp;the battery, prefented a finger to this feeminglynbsp;inolFcnfive luminous body, he would be inftantlynbsp;Stuck very violently, becaufe a free communication being now made by means of his body,'nbsp;the powers by which the electric fluid is impelled from one place to another would urgenbsp;it upOn him. But if we fuppofe a perfon, whonbsp;has no communication with the battery, tonbsp;prefent his finger to the fame body, he maynbsp;perhaps receive a flight fpark from it, but notnbsp;a fliock of any confequence^

� We may now account for the feemingly capricious nature, of all khlds of lightening,�nbsp;but efpecially of that kind which appears innbsp;the form of balls. Sometimes it will ftrikenbsp;trees, high houfes, amp;c, without touching cot-tages, men, or other animals, who are in thenbsp;neighbourhood ; in other inftances, low houfes'nbsp;and cattle have been ftrufk, while high treesnbsp;and fteeples in the neighbourhood have

cfcapecLquot;^� The reafon of this is, that In thun-der-ftorms th�re is a zone of earth confiderably under the furface, which the lightening defires to ftrlkc, (if we may ufe the expreffion)nbsp;becaufe it has an electricity oppofitc to thenbsp;lightening itfelf. Thofe objciSs, thercforCjnbsp;which form the ntoft perfe�t conduftors betweennbsp;the cleclrificd clouds and that zone of earthnbsp;will be flruck by lightening, whether they arenbsp;high or low. Let us fuppofe a pofitively electrified cloud is formed over a certain part ofnbsp;the earth�s furface ; the eleClric matter llo-wsnbsp;out from it firft into the atmofphere all round,nbsp;and while it is doing fo, the atmofphere isnbsp;elc�trificd negatively. In proportion, however,nbsp;as the current pervades greater and greaternbsp;portions of the atmofpherical fpace, the refift-ance to its motion iftcreafes, till at laft, thenbsp;air becomes pofitively elciflrified as well as thenbsp;cloud, and they both act as one body. Thenbsp;�furface of the earth then begins to be electrified, and it filcntly receives the eleClric matternbsp;by means of the trees, grafs, amp;c. which grownbsp;Mnbsp;nbsp;nbsp;nbsp;upon

* Of ;hi3 two rcinarkablc iiifiances have been adduced, ill a paper read by Mr. Achard at the Berlin Academy of Sciences. And Beccaria cautions perfonsnbsp;from depending on a higher, or, in ail cafes, a betternbsp;�^onduflor than their own body.

AN ESSAY

upon its furface, till' at laft, it becomes alfd pofitively eledlrilied, and begins to fend off anbsp;current of eledlricity from the furface downwards.

� The caiifes which firft prodticed the elec-i tricity ftill continuing to aCt, the power of thenbsp;eleftric current becomes inconceivably greatinbsp;The danger of the thunder-ftorm now begins ;nbsp;for, as the force of the lightening is diredted tonbsp;fome place below the furface of the earthj itnbsp;will certainly dart towards that place, and fhat^nbsp;ter every thing to pieces which refifts itsnbsp;paflage*

� The benefit of conduding-rods will now alfo be evident. For we are fure, the eleftricnbsp;matter will� in all cafes, prefer that way wherenbsp;it meets with the leaft refiftance, and this isnbsp;over the furface of metals* In fuch a Cafe, therefore, if there happen to be a houfe furnilhecinbsp;with a conductor direCtly below the cloud, andnbsp;at the fame time a zone �f negatively electrifiednbsp;earth not very far below the foundation of thenbsp;houfe, the conductor will almoft certainly banbsp;ftruck, but the building will be fafe^ If thenbsp;houfe wants a conductor, the lightening willnbsp;neverthelefs Hrike in the feme place, in ordernbsp;to get at the eieCtrificd zone above-mentioned ;nbsp;but the building will be now damaged, becaufe

ON ELECTRICITY. 163

CH A P-

That the eleftric matter, which forms and animates the thunder-clouds, ifiues from places far below the furface of the �arth, and buries itfclf there, is probable, from the deep holes that have been made innbsp;many places by lightening, by the violent inundationsnbsp;that have accompanied thunder-ftorms, hot occafionednbsp;by rain, but by water burftin^ from the bowels of thenbsp;earth, from which it muft have been dillodged by fome

AN ESSAY

charge a Plate of Air.

AS air is an eledtric, it will receive a charge like all other ele�lric lubftances. To thisnbsp;property may be afcribcd many of the phoeno-mena which are obferved in the courfe of thenbsp;common cieftrical experiments ; for the airnbsp;which furrounds an eledtrificd non-elcdlric isnbsp;always in fome degree charged with the fluid,nbsp;and thus acts upon the atmofphere of the electrified condudlor, not only by its prefTvtre, butnbsp;alio by its acquired eledlric powers; and thatnbsp;it pervades tJie air to a confiderable diftance isnbsp;evident, from the different methods by ivhichnbsp;the air of a room may be eledlrified.

Cover two large boards with tin-foil; fufpend one by filk firings from the cieling, and thennbsp;connedl it with the condudlor; place the othernbsp;board parallel to the former, on an infulatingnbsp;Hand that may be eafily raifed or lowered, tonbsp;regulate the diftance of the plates from eachnbsp;other. Or place the boards in a vertical fitu-ation, on infulating ftands of the fame height.nbsp;In moft cafes this form will be found the rnoftnbsp;convenient. Thefe boards may be confidered

ON ELECTRICITY. ,65

Conned the upper board with the pofitive condudor, and the other with the ground;nbsp;turn the cylinder, and the upper one will benbsp;eledrified pofitively, and the under one negatively ; the fpace of air between the two platesnbsp;ads as a plate of glafs, it feparates and keepsnbsp;afunder the two electric powers. Touch thenbsp;negative plate with one hand, and the uppernbsp;one with the other, and a lliock will be receivednbsp;fimilar to that from the Leyden phial.

The eledric lliock will always be felt whenever a quantity of the fluid paffes through any body in an inftantaneous manner, and the forcenbsp;of the Ihock will be proportional to the quantitynbsp;of eledricity accumulated, and the eafe withnbsp;which it can efcape ; for the whole energy ofnbsp;the eledricity depends on its tenfion, or thenbsp;force with which it endeavours to fly off fromnbsp;the eledrilied bodiv

The two plates, when in contrary Hates, ftrongly attrad each other, and wdll come together, if they arc not kept afunder by force.nbsp;A fpark will fometinies pafs between the plates,nbsp;and deftroy the eledricity of each. If an emi-

AN ESSAY

ncnce is placed on the under plate, the fpark, in the fpontaneous difcharge, will ftrike it.nbsp;The experiments with thefe boards will benbsp;more plealing, if one furface of the upper boardnbsp;is covered with gik leather,. The two plates,�nbsp;when charged, are fuppofed to reprefent thenbsp;ftate of the earth and the clouds in a thunder-llorm. The clouds being in one ftate, andnbsp;the earth in an oppolite .gne, while the plate ofnbsp;air adts. as ~ the eledtric, and the fpontaneousnbsp;difcharges exhibit the phoenomena of lightening.

An obferyation has been made on this experiment, which feems to affedl one of the principal fupports of the received theory. I have fubjoined it, in order to invite thofe who arenbsp;converfant with eledlricity to a clofer invefti-gation of the fubjefl-

In this experiment it feems impoflible to deny, that the air is penetrated by the eledfric Huid. The dlftance between the plates is fonbsp;fmall, that it muft appear abfurd to fay thatnbsp;this fpace is penetrated only by a repullivenbsp;power, when in other cafes we fee the fluidnbsp;pervading much greater fpaces of air. But ifnbsp;one eleclric fubftance is penetrable by the electric fluid, me muft be led ftrongly to fufpedl:nbsp;at leaft that all the reft are fo too. If glafs wasnbsp;altogether impenetrable to the fliiid, it is natural

ON electricity. 167

tural to think that it would run over its fur-face very eafily. But inftead of this, fo great is its prQpenfity to enter, that a Ihock fentnbsp;through between two glafs plates, if they arenbsp;prefled pretty clofe together, always breaksnbsp;them to pieces, and even reduces part of themnbsp;to a powder like fand. This iafl; effedl cannotnbsp;be attributed to any other than the elcdtricnbsp;fluid entering the pores of the glafs, and meeting with refinance, the impetus of its pro-greffive motion violently forces the vitreousnbsp;particles afunder in all dirc(5lions.

Turn that fide of the upper board on which the gilt leather is pafled towards the lower one ;nbsp;place one or two fmall metal hemifpheres onnbsp;the lower board ; connedl the upper boardnbsp;rvith the pofitive condudtor, and the lowernbsp;one with that which is negative, put the machine in aftion, and the upper board will dif-charge the whole of its contents on one of thenbsp;hemifpheres, in a ftrong flafii, attended with anbsp;fmart explofion ; vivid corrufeations of eleftricnbsp;light will be feen darting in various diredfionsnbsp;on the furface of the gilt leather. This experiment, fays Mr. Becket, is more than a refem-M 4nbsp;nbsp;nbsp;nbsp;blauce

168

Conneft a coated phial with the pofitive con-dudto'-, fo that it may be difcharged with the boards, and the flaflres of light will extendnbsp;further, and the explofion will be louder.

Place the wire, fig. lO, with the feathers tied to it in the middle of one of thefe largenbsp;boards, their divergence will not be near fonbsp;much in this fituation as when they are at thenbsp;edge of the board. If a piece of down or anbsp;feather is placed near the edge of the board, itnbsp;will fly off to the neareft non-ele�rified body ;nbsp;but, if it is placed in the middle, it will be anbsp;confiderable time before it will move, and itnbsp;will fearcely fliow any figns of attradtion.

Place bran, or fmall pieces of paper, near the center of the lower board ; when the machine is put in adlion thefe wdll be alternatelynbsp;attracled and repelled with great rapidity, andnbsp;agitated in an amazing manner. A pleafingnbsp;variation is made in this experiment by takingnbsp;off the chain from the lower board, and now

ON ELECTRICITY. i6^

and then touching it with the hand; touch both boards at tlie fame time and the motionnbsp;ceafes. But the moft furprizing appearance innbsp;this experiment is, that fometimes, when thenbsp;cledlricity is ftrong, a quantity of paper or brannbsp;will accumulate in one place, and form a kindnbsp;of column between the boards, it will fuddenlynbsp;acquire a fwift horizontal motion, moving likenbsp;a whirling pillar to the edge of the boards,nbsp;and from thence fly off, and be fcattered aboutnbsp;the room to a confiderable diftance.

Take two phials, the one charged pofitively, the other negatively,, place them on the infu-lated board, but as far from each other as thenbsp;board will permit; infert a range of candlesnbsp;in a piece of wood, about two inches diftance from each other, fo that the flame ofnbsp;each may be exadtly parallel; w'hen thefe candles are quickly introduced between the knobsnbsp;of the phials, the fpark will be feen to dartnbsp;through all of them, and will have the appearance of a line of -fire, variegated in a thoufandnbsp;different curves.

CHAP.

CHAP. XI.

Of the EkSlrophorous.

Fig. 73 reprefents an eledlrpphorous. This inilrument was invented by Mr. Voltajnbsp;p� Coma in Italy.¹ Jt confifts of two platesnbsp;of a circular form, the under plate is of brafsnbsp;covered over with a ftrafum of an eledtricalnbsp;fubftance, generally of fome negative eledlric,nbsp;as wax, fulpliur, amp;c, the upper plate is ofnbsp;brafs, wdth a glafs handle fcrewed on the centernbsp;of its upper furface.

Refinous eledlrics generally fucceed better, for an eledfrophorous than thofe made only ofnbsp;glafs, not only as they are Icfs afFeded by thenbsp;humidity of the air, but as they feem to havenbsp;the power of retaining longer the eleflricitynbsp;which is communicated to them.

To ufe this apparatus, firft excite the under plate c, by rubbing its coated fide with a piecenbsp;of clean dry flannel, or hare-lkin ; when thisnbsp;plate is well excited, it is to be laid on thenbsp;table with the eledlric uppermofl. Secondly,

Mr. WHck, in Angufl, 1762, contrived a refinoijs apparatus, to which he gave the name of a perpetual eleftio-phorous. Sec Scripta Academise Suec. 1762.

place the metal plate upon the eleflric, as in fig. 74 and 75. Thirdly, touch the metal plate withnbsp;the finger, or any other condudlor. Fourthly,nbsp;feparate the metal plate from the eledtric bynbsp;the glafs handle. This plate, when raifed tonbsp;fome .difiance from tire tinder one, will be foundnbsp;firongly eledlrlfied with the power which isnbsp;contrary to that of the eleflrip plate, and willnbsp;give a fpark to any condudlor that is broughtnbsp;near it. By repeating this operatioti, i. e. bynbsp;fetting the metal plate on the eleflric, and thennbsp;touching it with the finger, a great number ofnbsp;fparks may be fucceffively obtained without anbsp;freflr excitation of the eledlric,

The following experiments, which -were made with a view to analyfe this curious littlenbsp;inftrument, are extrafted from a paper of Mr,nbsp;Achard�s, in the Memoirs de TAcademie Rpj-de Berlin for 1776,

Mr. Achard placed horizontally a circular plate of glafs, which was about two tenths ofnbsp;,an inch in thicknefs, and one foot in diameter,nbsp;pn a tin plate, which only touched the glafs innbsp;a few places ; having excited the upper furfaccnbsp;of the glafs, it produced all the effefts of thenbsp;cledtrophorous i from whence he infers, that

AN ESSAY

it is not Tieceflary that the inferior metallic plate feould touch exactly in all its furface thenbsp;ek�tric coating-

He infulatedy in a horizontal pofition, a plate of glafs of one foot diameter, he excited this,nbsp;and then applied the upper plate in the ufualnbsp;manner, and obtained a fuccelEve number ofnbsp;weak fparks ; but in order to procure them, henbsp;was obliged to let the finger remain fometimenbsp;on the upper plate. If, inftead of infulatingnbsp;the plate of glafs by glafs, be infulated it bynbsp;was OF pitch, he conftantly found that thenbsp;fparks were ftrongcr. From this experimentnbsp;he concludes, that the inferior plate is not ne-ccffary to the production of the efiefts obfervednbsp;in this inftrument, and that when deprived ofnbsp;it, retains all its properties.

Having excited the upper furface of an elec-trrophorous of wax, he placed the upper plate on it, and after fome time lifted it off by itsnbsp;infulating handle, without previoufly touchingnbsp;it with the finger; it gave no fpark, and wasnbsp;nut poflelicd of the leall: power of attraflion and

ON ELECTRICITY,

repv�fion; which proves, that the eieftropli�� rous cannot render the upper plate ele^^Iiic,nbsp;unlei's it is touched bv a body which is capable of giving or taking elcdiricity from it.

Place the upper plate on an excited eledt�-phorous, bring a finger near the upper plate, and a fpark will pafs between them; Now .asnbsp;the eleftric fluid never appears as a fpark, except when it paffes with rapidity from one bodynbsp;to another, and as the upper plate exhibits nonbsp;eledtric appearance, if it has not been previouslynbsp;touched by a condudlor, wc may conclu/ic,nbsp;that the eledlrophorous only renders the uppe-rnbsp;plate elefliric when it has received or loll anbsp;quantity of eledlricity.

Place one of the fmall brafs condutflors witli its pith balls on the upjgt;er plate, and then putnbsp;them both on the elc�rophorous, the balls willnbsp;immediately feparate a little ; touch the uppernbsp;plate with the finger and the divergence ceales ;nbsp;but on lifting this plate from the eledropho-rous by its glafs handle the balls diverge with

AN ESSAY

taking a fpark from the plate thej^ immediately clofe. The reparation of the balls Ihews clearlynbsp;that the upper plate cither abforbs a quantitynbsp;of elcdtricity, or imparts a portion of its naturalnbsp;lliare to the under one } it alfo iltews^^ that thenbsp;former, as foon as it is laid on the eleftrophorous,nbsp;acquires a fmall degree of eledtricity, which itnbsp;lofes on being touched with the finger ; but itnbsp;again becomes eledlrical when it is feparatednbsp;from the eledlrophorous^

Infulate an eledlrophorous, and fufpend a pith ball by a linen thread, in fuch mannernbsp;that it may be about one quarter of an inchnbsp;from a piece of metal which is connedled withnbsp;the bottom plate ; the ball does not move whennbsp;the upper plate is laid on the eledlrophorous,nbsp;but when this is touched by the finger the ballnbsp;is attradledi As foon as the upper plate is taken off, the �nf�rior metallic coating attraflsnbsp;the ball, but quits it if tlie chafing is touchednbsp;by the finger. It is alfo attrafted' irhthe uppernbsp;plate is put on before the fpark has been takennbsp;from it, though it lafts longer and is ftrongernbsp;if the fpark is taken before it is placed on thenbsp;eleftrophoTous.

Ex-

ON ELECTRICITY.

Eleftrify the under fide of the eleflropho^ rous, by connedling the under plate with th�nbsp;conduftor of a machine ; the upper plate willnbsp;give ftrong fparks to the hand, or any othernbsp;non-eledlric. Touch the upper plate tvith onenbsp;hand, and the under one with the other, anbsp;fiiock will be received. The fame efiect' isnbsp;produced if the upper plate is electrified by thenbsp;machine. See fig. 74.

Infulate an electrophor��s which is not excited, and place the upper plate upon it, then electrify the under plate by a chain from thenbsp;prime conductor, take a fpark from the chain,nbsp;and the eleCtrophorous acquires all the properties which are given to it by exciting thenbsp;upper furfacci

Connect the upper plate by a chain wfith the prime conductor, and eleClrify it, then take anbsp;l|)ark from the chain, and the eleCtrophorousnbsp;will acquire as before the fame powers which,nbsp;it gains when the upper furface is rubbed^^

AN ESSAY

The fame effect is produced by placing a Leyden phial on the upper plate of an unexcitednbsp;eleclrophorous, then charging and difchargingnbsp;it on the plate.

From the three lall experiments we learn, that the eleflrophorous may be put in aftiorcnbsp;by communication as well as by fridlion.

Mr. Achard placed the upper plate on an excited eledtrophorous, and a cube of metal, furnifli-cd with a glafs handle, on this plate ; on taking the cube by its handle from the upper plate,nbsp;without previoufly touching it, it attracted anbsp;light ball. On repeating this experiment, andnbsp;touching the upper plate before the cube wasnbsp;taken off, it did not appear in the leaft electrical.

, I. That as foon as the upper plate is placed on an electrophorous of wax it acquires a weaknbsp;pofitive eledtricity ; and the contrary, if placednbsp;on an eledtrophorous of glafs.

ON ELECTRICITY.

i. That when th'� upper plate is touched by the finger it lofes all its eleclricity,

3. When the upper plate is to-uched by the finger and removed from the eleflrophorous, itnbsp;acquires a ftrong negative electricity, if thenbsp;elcCbrophorous is of glafs^ and a pofitive electricity if it is of wax.

The eleClrophorous may be conlidered as formed of fcveral horizontal ftfata ; fb thatnbsp;W'hen th� Upper one is excited, either by friction or .conimunication, it is infulated by thenbsp;rnfetioT flrata : now all infulated eleCtrics pre-ferve their eledtricity a confiderable time, andnbsp;ft is from that caufe that the clcCtricity of thenbsp;� elcdlrophorous continues fo long.

Infulated and excited glafs induces the negative electricity on bodies broirght within the fphere of its adlion,* while negative cleClrics,nbsp;in fimilar circumftances, produce the pofitivenbsp;electricity. Therefore the furface of the elec-trophorous ought to communicate immediatelynbsp;a pofitive electricity if it Is of wax, the negative if it is made of glafs, which is perfectlynbsp;conformable to experiments., But when thenbsp;upper plate is touched by the finger, the uppernbsp;furface of th� eleCtr�phorous- ceafes to be inlVnbsp;lated, and gives the negative cleCtricity to thenbsp;upper plate, if it is of glafs, and the contrary

if of wax, agreeable to the different experiments which are defcribcd in Chap; IV.

Elelt;flric bodies do not put the fluid in that degree of motion which is neceflary to producenbsp;the fpark, or exhibit the phoonomena of at-tra�lion and repulfion, while they arc in contact with conducing fubflances, which is thenbsp;reafon why the upper plate exhibits no figns ofnbsp;eledricity while it remains in contadl with thenbsp;under one, though they become fenfible thenbsp;inftant it is removed from it*

As the theory of this inftrument has beert deemed very intricate, I have fubjoined anothernbsp;explanation of it, which is given by the editors'nbsp;of the Monthly Review*

� Therefore, (in the cafe of a glafs elcdro-phorous) as it is a cafe which admits of a fome-what eafier illuftration, the excited plate a�ls upon the eledtrlc matter naturally contained innbsp;the upper brafs plate, fo as to repel a part of itsnbsp;natural quantity from it in form of a fpark, atnbsp;that part where the finger is applied to it. Ifnbsp;the brafs plate in this ftate is lifted up by itsnbsp;handle, it will receive a fpark from the finger.nbsp;On being replaced, and the fame operation taking place, the fame refult will be obtained ;nbsp;which may be continued for a great length ofnbsp;time, without diminiflilng the virtue of thenbsp;excited ele(51:ric, which in fafl does not part

With any of its own eledrlci'ty, bnt only repels a part of what is in the upper plate, which isnbsp;repeatedly reftored to it from the earth by thsnbsp;perfon who makes the experiment.�

Place a piece of metal ort an excited cleflr�� phorous, it may be of any lhape ; a pair of triangular compaffes are very convenient for thisnbsp;purpole. Eledlrify the piece of metal with th�nbsp;power which is contrar^f to that of the eleftro-inbsp;phorous, and then remove it by means of fonlenbsp;eledlric, and afterrvards lift upon the eledtropho-rous f�m� finely powdered tefin, which willnbsp;form on its furface curious radiated figures.nbsp;When the plate is negative, and the piece of metal pofitive, the pcfwder forms itfelf principallynbsp;about thofe parts where the metal was placed ;nbsp;but if the plate is pofitive, and the fpark is negative, the part where the metal touched willnbsp;be free from j)owderi and the other parts morenbsp;Goveredi

Infulatc a metal quart mug, and fufpend a pair of fmall pit.h balls by filk, fothat the wholenbsp;of the elcftromctcr may he within the mug,nbsp;N 2nbsp;nbsp;nbsp;nbsp;elec-

AN ESSAY

elc�rify the mug, and the eledrometer will not be in the leaft affe�ted. The fimilar atnio-fpheres counteraft each other ; and as no contrary power can take place in the eleftrometer,nbsp;it will remain uneleftrified. Touch the mugnbsp;with fome conducting fubftance, and it willnbsp;immediately attraCl the balls.

Sufpend a fmall cylinder of gilt paper by tin-foil, and then touch the electrified and infulated mug with it, a fpark will pals between them,nbsp;and the eleClricity will be diffufed in each innbsp;proportion to their capacity. Now plunge thenbsp;infulated cylinder to the bottom of the mug,nbsp;and it will rellore to it the cleCtricity it had received, and does not give the leaft fign of electricity when taken out.

Conned a pair of pith balls with an infulated metal veftel, in which a metal chain is placed,nbsp;raife the chain by means of a filk thread, andnbsp;the divergence of the balls will diminilli in proportion as the chain is raifed and difplayed;nbsp;fhewdng, tliat the eledricity is rarlficd, and itsnbsp;denfity is diminilhed, in proportion as it fpreads

kfelf from the furface of the veffel on the ex-tended chain ; which is confirmed by the balls diverging again when the chain is let downnbsp;into the veffel. This experiment affords annbsp;eafy folution for many of the pheenomena of at-mofpheric eledfricity, as why the vapour ofnbsp;eledlrified w'ater gives fuch fmall ligns of elec-'tricity, arid why the eledlricity of a cloud is in-creafed by being compreffed or cpndenfed.

Excite a flip of white flannel or a fllk ribbon, and take as many fparks from it as it will give,nbsp;tlren double or roll it up, and the contra�lednbsp;flannel will be ftrpngly eledlrical, give fparks,nbsp;and thrpw put bruflres of light.

Of the advantages �which may be derived from an imperfeSl Infulatmi^ and of rendering very fenfible very Jmall degrees ofnbsp;natural and artificial Elediricity, by Mr.nbsp;Voita.

A condudlpr, properly conftructed for making obfervations on atmofpherical elcftricity, willnbsp;feldpm affedt the moft fenfible eleftrometer whennbsp;thefky is freefrom eleiflrical clouds; butby meansnbsp;N 3nbsp;nbsp;nbsp;nbsp;'nbsp;nbsp;nbsp;nbsp;'nbsp;nbsp;nbsp;nbsp;' of

AN ESSAY

of tlie apparatus now to be defcribed It wilt appear, that thele conduftors are always electrical, and confeqvjently the air which furroundsnbsp;them muft be at all times eleftrified. This method not only determines the exiftence, but alfenbsp;the quality of the eledtricity, whether politivenbsp;or negative, and that, even when the conduflornbsp;W'ill not attract the fineft thread ; but if a verynbsp;fmall attraction is vilible in the conductor, thennbsp;the apparatus will give long {parks.

The eleCtrophorous ufed for this purpofe may with propriety be termed a micro-elcCtrometer,nbsp;or condenfer of eleClricity.

Whenever the atmofpherical conductor gives fufficient hgns of eleCtricity, then the condcn-iing apparatus becomes ufelefs. For when thenbsp;ele�tricity is Itrong, it often happens that partnbsp;of the eieCtricity .of the metal plate is impreffednbsp;upon the other, in which cafe the apparatusnbsp;afts as an eleCtrophorous, and becomes unfitnbsp;for our purpofe.

The apparatus adaptal for this purpofe con-liits of the upper plate of an eleCtrophorous, and a femireleCtric, or an imperfeCt conductingnbsp;plane, which will only hinder in a certain de*nbsp;gree the paflage of the fluid. Many conductors of this kind may be formed; fuch asnbsp;a clean dry marble llab, a plate of wood, covered with a coat of varnifh, amp;c. The furfacenbsp;of thofe bodies not contrafting electricity, or

ON ELECTRICITY. ,83

if any fliould adhere to them It foon vaniflies, on account of their femi-condufting nature;nbsp;for which reafon they cannot anlwer the end ofnbsp;an eleflrophorous, but aye fit to be ufed asnbsp;condenfers of eledtricity.

Care fliould be taken however in choofing this plane, that it be not of too free a condudt-'nbsp;ing nature, nor likely to become Co by ufe, itnbsp;being abfolutely necelTary that the eledricitynbsp;fliould find a confiderable refiftance in pervadingnbsp;its furface. In preparing fuch a plane, by drying, or otherwife, it is much better to comenbsp;too near than too far from a non-conduftor.nbsp;A marble flab or board, properly dried, an-fwers well, and is preferable to any othernbsp;plane; otherwife the plate of the eleftropho-rous is preferable to all bodies unprepared.

The worft fort of marble, if coated with copal, amber, or lac-varnifli, and then kept in an oven for a fliort time, will anfwer very well,nbsp;even without previqufly wai^-ming for the experiment.

This, in fadf, it may b? faid, is returning to the eledtrophprous; as marble, wood, amp;c.nbsp;varnifhed, if they are hot, may be excited by.nbsp;a very flight fridlion, and fometimes by onlynbsp;laying the metal plate on them; to preventnbsp;which, they fliould be ufed without w^arming.

N 4 nbsp;nbsp;nbsp;The

AN ESSAY

The advantages plates of this kind have over the common eledrophorous are, i. That thenbsp;varnilh is always thinner than the commonnbsp;refinous ftratum of an eledrophorous ; and,nbsp;2. That the varniflt acquires a fmoother andnbsp;plainer furface : hence the metal plate can withnbsp;more advantage be adapted to it.

Any fort of plane, covered with dry and clean oil-cloth, or oiled-filk, or fattin, and anynbsp;other lilk fluff that is not very thick, may benbsp;ufed with equal advantage, if it is flightlynbsp;w^armeci. Silk �fluffs anfwer better for thisnbsp;purpofe than thofe made of cotton or wool, andnbsp;both better than linen. Paper, leather, wood,nbsp;ivory, bone, and every other fort of imperfednbsp;condudors, may be made to anfwer to a certainnbsp;degree, if they are previoufly dried, and keptnbsp;hot during the experiment.

This apparatus is rendered more fimple by applying the filk, amp;c. to the upper platenbsp;e�f metal, which is fixed to the glafs handle,nbsp;inflead of the marble or other plate, whichnbsp;now becomes ufelefs; for in its ftead, a plane ofnbsp;any kind may be ufed, as a common woodennbsp;or marble table, even not very dry ; a piece ofnbsp;metal, a book, or any other condudor, with anbsp;fiat furface.

Nothing more is requilite in thefe expetj? CaentS, than that the eledricity, which tends

jjo pats from one furface to the other, fhould meet with fome reliftance or oppofition in onenbsp;ef the furfaces, as will be evident in the fecondnbsp;part.

It is immaterial whether the non-condu�ling or femi-condufling ftratum be laid upon onenbsp;or the other of thofe planes ; all that is neceffarynbsp;is, that they Ihould coincide together, whichnbsp;r-enders it proper to ufe two planes that havenbsp;been ground together, and one of them var-nilhed. A fingle metal plate, covered withnbsp;filk, with three fiik ftrings faftened to it bynbsp;way of handle, may be conveniently ufed fornbsp;ordinary experiments.

To ufe the apparatus, the upper metal plate mull; be placed upon the unelectrified plate,nbsp;and in perfedt contadt with it.

The plates being, thus placed, let a tviire, eommunicating with the condudtor, be broughtnbsp;to touch the metal plate of the eledtrophorous,nbsp;and that only.

The apparatus being left in that fituation a certain time, will acquire a fufficient quantitynbsp;of eledlricity, though but very Howdy.

Remove the communicating wire from the metal pRte, and, by means of its Infulatednbsp;handle, feparate k from the under one ; fit wdllnbsp;now attradt a thread, cledfrify an cledlrometer,nbsp;and, if it is ftrong, will glv.e fparks, amp;c.

AN ESSAY

It is not eafy to determine the exaft time neceflary for this apparatus to remain in contact with the condudtor, as it will depend onnbsp;many circumftances ; for, if there are no fignsnbsp;tgt;f electricity in the condudtor, it will requirenbsp;eight or ten minutes, but if it' attracts a finenbsp;thread, as many feconds will be found fufh-cient.

It is difficult alfq to determine the prccife degree to which the eledtricity. may be con-denfed, or how much the eiedlricai phoeno-mena may be increafed by this apparatus, as itnbsp;depends on various cirpumflances, The augmentation is, however, greater in proportionnbsp;as the body which fupplics the metal plate ha^nbsp;a greater capacity, and is larger in proportionnbsp;3S the eledlricity is weaker. Thus, thoughnbsp;the atmofpherical cpndudlor has fcarcely powernbsp;fufficient to attradl a fine thread, it is neverthe-,nbsp;lefs capable of giving fuch a quantity of electricity to the metal plate of the electrophorous,nbsp;as not only to aftuate an eledtrometer, butnbsp;even dart ftrong fparks. But if the eledlricity ofnbsp;the atmofpherical condu(fl:or is ftrong enoughnbsp;to afford fparks, or to raife the index of thenbsp;eledlrometer to 5 or 6 degrees, then the receiving plate of the eledlrpphorous, according

ON ELECTRICITY. 187

to this method, will raife its index to the high-jeft degree, and give a ftronger fpark ; yet it may be plainly perceived, that the condenfationnbsp;is proportionably lefs in this than in the othernbsp;cafe ; Ipr this reafon the eledlricity cannot benbsp;accumulated bej�ond the greateft degree ; thatnbsp;is to fay, v/hen it is increafed fo much as to henbsp;diffipated every way. Therefore, as the electric power, which fupplies the condenfer, isnbsp;neared; to the highell; degree, the condenfationnbsp;is proportionably lefs; but in this cafe the condenfer is ufelefs; its principal ufe being tonbsp;colledt and render fenhble that fmall quantity ofnbsp;eledtricity which would othervvjfe remain im,-perceptiblc and unobferved.

Hitherto tve have adapted our condenfer tc? the detedting weak atmofpherical eledlricity,nbsp;as brought dqwm by the condudlor ; but this,nbsp;though the principal, is not the only ufe tpnbsp;which it may be applied. It will likewife dif-cover artificial .eledlricity, when it is fo weak asnbsp;not to be difeoverabie by any other means.

A Leyden phial charged, and then difehar-ged by touching, its coated fides with the dif-charging rod or the hand, appears to be quite deprived of its electricity ; yet, if you touchnbsp;the knob of it with the metal plate of the condenfer, (fituated upon an imperfedl: condudlingnbsp;plane) and immediately take up the plate, it

AN ESSAY

will be found to give very confpicuous figns of cledricity. But, if juft fufficient charge is leftnbsp;in the phial to attraft a fine thread, and the metal plate is then brought to touch the knob fornbsp;a moment, it will, when lifted up, give anbsp;ftrong fpark, and if touched again, a fecondnbsp;Icarce fmalicr than the former; and thus, fparknbsp;after fpark may be obtained for a long time.

This method of producing fparks, by means of a phial which is npt charged fo high as tonbsp;give fparks of itfelf, is very convenient fornbsp;varipus plealing experiments; as to fire or lightnbsp;the inflammable air-piftol, or lamp ; efpeciallynbsp;xvhen a perfon is provided with one of thofenbsp;phials contrived by.Mr. Cavallo, which, whennbsp;charged, may be carried in the pocket a lon^nbsp;time. Thefe phials, as they retain a fenfiblenbsp;charge for feveral days, will retain an infenfiblcnbsp;one, for weeks and months ; or, fueh a one asnbsp;cannot eafily be difeovered without the con-denfer, in which cafe It becomes more thannbsp;fenfible, and fufficient for the experiments ofnbsp;the inflammable air-piftol, amp;c.

* Secondly. If you have an cleftrical machine fo far out of order that Its condudtor will not give a fpark, nor attraft a thread,nbsp;then let this conductor touch the metal platenbsp;of the condenfer, and continue In that fituationnbsp;a few minutes, (the machine being flill in motion)

Thirdly. If the eleftrical machine a6ls well^ but the conductor is fo badly infulated that itnbsp;will not give a fpark, either from its being con-nedled with the walls of the room, or by havingnbsp;a chain from it to the table, let the condu�tornbsp;in this Hate touch the metal plate of the co-n-denfer while the machine is in adfion, ' thenbsp;plate will afterwards give fufficient ftrong lignsnbsp;of electricity; which proves the great powernbsp;this apparatus has of drawing and conuenfingnbsp;tire ele�tricity.

Fourthly. Where the eleftrometers are fufficicntly fenfible to difeover the.,quantities'nbsp;of excited ele�tricity, thofe quantities maynbsp;be readily explored by the condenfer. For thisnbsp;purpofe, rub thofe bodies with the metal platenbsp;of the condenfer, which for this purpofc muftnbsp;be naked, and if the plate be then prefcnted.tonbsp;an eledlrometer, it v/ill be found confiderabiynbsp;eJeftrified, although the body rubbed may havenbsp;acquired little or no eledricity- The quality,nbsp;whether pofitive or negative, may be eafrly al-ccitained, ftnee the eledricity of the metalnbsp;plate muft be the contrary of that body onnbsp;which it was rubbed. Mr. Cavailo made ufenbsp;of this method to difeover the elc�lricky ofnbsp;many bodies. But a ftill better method may

AN ESSAY

te ufed in cafe the bodies to be examined cannot eafily be adapted to the metal plate, viz. The metal plate being laid on the imperfe�tnbsp;GonducSling plane, the body to be tried isnbsp;rubbed againft, or repeatedly ftroaked upon it,nbsp;which done, the plate is taken up and examined by an eledlrometeri If the body tried isnbsp;leather, a firing, cloth, velvet, or other im-perfed condudtor of the like fort, the platenbsp;�will certainly be found eledrified, and incona-parably more by this means than if it werenbsp;ftrOaked by the fame bodies, whilfl ftandingnbsp;iiifulated in the air. In fhort, by either ofnbsp;thofe methods you will obtain eledlricitv fromnbsp;tjodies which could hardly be expeded to givenbsp;any, even when they are not very dry; Indeed,'nbsp;foals and metals excepted, every other bodynbsp;will afford fome eledrieity; Eledricity may_nbsp;often be obtained by ftroaking the plate withnbsp;the naked hand;

Tlie metal plate has a much greater power to retain eledricity when it lies upon a propernbsp;plane, as mentioned in the foregoing experiments, than when quite infulated;

It is eafy to comprehend,- that where the capacity of holding eledricity is greateft, there the intenlity of the eledricity is proportionablynbsp;lefs, for it will then require a greater quantitynbsp;to raife it to a given degree of intenfity; fo that

the

ON ELECTRICITY. *9,

the capacity is inverfely as the ifitenjity ^ by which we mean, that endeavour, by whibh thenbsp;cleftricky of an electrified body tends to' efcapcnbsp;from all parts of it; to whi�h tendency or endeavour, the ciedtrical phoenomena of attradtionnbsp;and repulfion, and efpccially the degree of elevation of an electrometer, cortefpond.

That the interjily of eleCtricity muft be in-' verfely proportional to the capacity of the bodynbsp;electrified will be clearly exemplified by thenbsp;following experiment.

Take two metal rods of equal diameter, the one a foot, the other five feet longj let thenbsp;firft be electrified till the index of the electro'--meter rifes to 60�, then let it touch the othernbsp;rod ; and in that cafe it is evident, that the in-tenlity of the electricity being diffufed betwecitnbsp;the two rods, will be diminifiied as the capacitynbsp;is increafed; fo that the index of the electrometer, which before was elevated to �c�, willnbsp;now fall to 10�, viz. to one fixth of the former intenfity. For the fame reafon, if the likenbsp;quantity of eleCtricity was communicated to anbsp;rod 60 feet long, its intenfity would be dltni-nilhed to one degree; and on the contrary,nbsp;if the electricity of the long conductor was

AN ESSAY

contradled into the 60'** part of that capacity^; its intenfity would be increafed to 60.

Conductors of different bulk have not only different capacities for holding eleCtricity, butnbsp;alfo the capacity of the fame conductor is in-,nbsp;creafed and diminiflr�d in proportion as its lur-face is enlarged and contracted ; as is fhewn irrnbsp;Dr. Franklin�s experiment of the can and chain,nbsp;hci from which it has been concluded, that thenbsp;capacity of conductors is in proportion to theirnbsp;furface, and not to their quantity of matter.

This conclufion is true, but does not comprehend the whole theory, fince even the exten-fion contributes to increafe the capacity. In fliOrt, it appears from all the experiments' hitherto made, that the capacity of conductorsnbsp;is not ill proportion to the furfa�es in gehefal,-but to the furfaces which are free,- or uninfluenced by fimilar or homologous atmofpheres ;nbsp;and further, that the capacity of a' conductor,nbsp;neither altered in its form or furface, is increafed, when inftcad of remaining quite infu-latcd, it is prefented to another not infulated ?nbsp;and this increafe is more confpicuous, as thenbsp;furfaces of the conductors are larger and approach nearer to each other.-

The above-mentioned circumftances, by which the natural capacity of conductors isnbsp;greatly augmented, has been overlooked, and

tKerefore no advantage has hitherto been deduced from it. The following experiment will fliew this increaied capacity in the fim-gt;nbsp;pleft manner.

Take the metal plate of an eleclrophoroiis^ hofd it by its handle in the air, and eleftrify itnbsp;fo high that the index of an eledtfometer annexed to it may be elevated td 6o�, then lowernbsp;the plate by degrees to a table, or other planenbsp;fcondufting furface, the index will graduallynbsp;fall from 6o� to 50�, 40�, 30�, and yetnbsp;the quantity of eleftricity in the plate remainsnbsp;the fame, except it is brought fo near the tablenbsp;as to occafion a tranfmiffion of the eleftricitynbsp;from the former to the latter ; at lead, it willnbsp;remain as near the fame as the dampnefs of thenbsp;^Av, amp;c. will permit! The decreafe of intenfitynbsp;is Owing to the increafed capacity of the plate,nbsp;which is now not infulatcd, or folitary, butnbsp;conjugate^ or communicating with another con-duaor : for, let the plate be gradually removed from the table, the eledromcter willnbsp;rife again to its former ftation, namely, to 60^ ;nbsp;excepting the lofs that the air, amp;c.- may havenbsp;occafioned during the experiment,

AN ESSAY

The reafon of this phoenomenon is eafily derived from the aftion of electric atmofpheres. The atinofphere of the metal plate,- which fornbsp;the prefent I lhall fopjacfe ele�frified poliiivehquot;,nbsp;acts upon the table, or other conduftor, tonbsp;which it is prefented ; fo that the eledlric fluidnbsp;in the table, retiring to the remoter parts of it,nbsp;becomes more rare in thofe parts which are ex-pofed to the metal plate, and this rarefadtionnbsp;increafes, the nearer the elcdlrified metal isnbsp;brought to the table. If the metal plate isnbsp;eledlrified ncgativel3^, the contrary effecls takenbsp;place. In flrort, the parts which are immerfednbsp;in the fphere of aflion of the eledtrined plate,nbsp;by contradbing a contrary elcdtricity, give thenbsp;electricity of the metal plate an opportunity to'nbsp;expand itfelf, and will thus diniinifli its inten-fity, as is fltewn by the depreffion of thenbsp;eledfrometer.

The two following experiments will throw more light upon the reciprocal adtion of thenbsp;eledtric atmofpheres.

Eledlrify two flat condudtors, either both pofitively or negatively, then bring them gradu-�nbsp;ally towards each other, and it will appear, by-two annexed eledtrometers, that the nearer they

� N ELECTRICITY. 195

approach each other, the more their denfities will incrcafe, as all elaftic bodies re-adt itl proportion as they ate afted on; which flicws jnbsp;that cither of the two conjugate powers has anbsp;much lefs capacity to receive more fluid nownbsp;than when fingly infulated, and out of the influence of the other; This experiment explains,nbsp;whv the tenfiom of the elediric atmofphere ohnbsp;an elcdlrified cOndudlOr is greater when it isnbsp;fcontradled into a fmallcr bulk # and alio, why anbsp;long extended conductor will fhew lefs intenlitynbsp;than a more compafl: one, fuppofing theifnbsp;quantity �f iurface and electricity to be thenbsp;lame; becaufe the homologous atmofpheres ofnbsp;their parts interfere lefs with each other in thenbsp;former than in the latter cafe, and of courfe,nbsp;sis their action is Iels, the re-a(5tion is alfonbsp;lefs;

Elcctrii'i' one of thefe flat conduiflors poli; tlvelv, the either negatively, and the effeCts willnbsp;then be iuft the reverfe of the preceding; viz.nbsp;the intcnfity of their electricities will be dum-nilhed, becaufe their capacities, or their powernbsp;and facilitv of expanding arc increafed the nearer the conductors come to each Other.

O 2 nbsp;nbsp;nbsp;Apply

AN ESSAY

Apply the explanation of this laft experiment to that mentioned before, vizj the bringing thenbsp;ekdrified metal plate towards a conducingnbsp;plane which is not infulated ; for, as this plan�nbsp;acquires a contrary eleflricity, it follows, thatnbsp;the inteniity of eledlricity in the metal platenbsp;muft be diminiflied, and the annexed eleiStrome-ter is depreffed according as the capacity �fnbsp;the plate is increaled, or as the denflty of itsnbsp;atmofphere is diminiflied; and confequently,nbsp;the plate in that fltuation is capable of receivingnbsp;a greater quantity of eleflricity.

Infulate the conducling plane whilfl; the other eleftrified plate is upon it, and afterwards feparating them, both the metal platenbsp;and conducing plane, which may be called thenbsp;inferior plane, will be tounJ eleflrificd, butnbsp;poflefled of contrary electricities, as may benbsp;afeertained by eleclrometcrs.

If the inferior plane is infulated firft, and then the clcclrltied plate is brought over it,nbsp;then, the latter will caufo an endeavour in thenbsp;former to acquire a contrary eieflricity, whichnbsp;tlie infuiation prevents from taking place; hence

ON ELECTRICITY.

the intenlity of the elcftricity of the plate is not diminilhed, at leaft, the elc6trometer will flicwnbsp;a very little, and almoft imperceptible deprel-fion, which fmall depreffion is owing to thenbsp;imperfedrlon of the infulation of the inferiornbsp;plane, and to the fmall rarcfaiftion and conden-fation of the cit'dric liuid which may take placenbsp;in different parts of the faid inferior plane.nbsp;But if, in this fituation, the inferior plane benbsp;touched fo as to cut off the infulation for a moment, then it will acquire the contrary clec-tricltv, and the intenfity in the metal plate willnbsp;be diminiflied.

If the inferior plate, inftcad of being infu-lated, were itfelf a non-conduding fubftance, then the fame phoenomcna would happen, viz.nbsp;the intenfity of the electrified metal plate laidnbsp;upon it w ould not be diminllhed. This, however, is not ahvays the cafe, for if the fiiid inferior non-coadudlng plane is very thin, andnbsp;is laid upon a condudor, then the intenfity ofnbsp;the electrified metal plate will be diminiflied,nbsp;and its capacity will be increafed by being laidnbsp;upon the thin infulating ftratum ; as innbsp;that cafe, the conduding iubltancc, whichnbsp;ftands under the noii-conduding llratum, acquiring an clcdricity contrary to that of thenbsp;metal plate, will diniinifh its intenfitv, amp;c.nbsp;and then the infulating ftratum will only dimi-O 3nbsp;nbsp;nbsp;nbsp;mfli

AN ESSAY

iiiili the mutual a�lion of the two atniofplicres more or lefs, according as it keeps them atnbsp;greater or fmaller diftances from each other.

The intenfity or ele�rric action of the metal 2gt;Iate, which diminiihes gradually as it

nd nearer to a condudling plane not infulated, becomes almaft nothingnbsp;when the plate is nearly in contafl: with thenbsp;plane, the compeniatlon or natural balancenbsp;being nearly perfect, tience, if the interiornbsp;plane pnly oppofes a fmall refiftance to the paf-fage of the eledricity, (whether fuch reftilanccnbsp;is occafioned by a thin eleftric llratum, or bynbsp;the plane�s imperfehl conduding nature, as isnbsp;tlie cafe wnth dry m^ood, marble, amp;c) that rc-tiftance, joined to the interval, however final!,nbsp;that is between the two plates, cannot he overcome by the weak intenfity of the electricity ofnbsp;the metal plate, vchich on that account willnbsp;not dart any fpark to the inferior plane, (except its electricity were very powerful, or itsnbsp;pdges not well rounded) and wdll rather retainnbsp;its eledricity ; fo that being removed from thenbsp;inferior plane, its eledrometer will nearly recover its former height. Befidcs, the elcdtri-�fied plate may even come to touch the imper-feftly condudlng plane, and may remain innbsp;that lituation for fome time ; in which cafe,nbsp;the intenfity being reduced almoft to nothing,

ON ELECTRICITY. ,99

the ele�lricity will accordingly pafs but flowly to the inferior plane. , But the cafe is dilferent,nbsp;if, in pcrfonning this experiment, the elcdri-fied metal plate touches the inferior plane edg-wife, for then its intenlity being greater thannbsp;when it is laid flat, as appears by the eleftrome-ter, the eledlricity eafily overcomes the fmallnbsp;refinances, and paffes to the inferior plane,nbsp;even acrofs a thin ftratum, bccaufe the eledlricity of one plane is balanced by that of thenbsp;other, only in. proportion to the quantity ofnbsp;furface which they oppofe to each other withinnbsp;a given diftance ; fo that when the metal platenbsp;touches the other plane in flat and ample con-tadt, its eledlricity is not diffipated. This apparent paradox is clearly explained by the theory of eledlric atmofphercs.

�Tis flill more like a paradox, that neither touching the metal plate with a finger or piecenbsp;of metal will deprive it of all its cledlricit}qnbsp;while Handing upon the proper plane ; fo thatnbsp;it generally leaves it fo far electrified, thatnbsp;when feparated from the plane it will give anbsp;fpark. Indeed, this phocnomenon could notnbsp;be explained on the fuppofition, that the fingernbsp;or metals were perfedt condudtors. But, fincenbsp;we do not knov/ of any peiiedl condudlor, thenbsp;metal or finger oppofe a fufficient rcfiftance tonbsp;retard the immediate difliparion of the eledlri-O 4nbsp;nbsp;nbsp;nbsp;city

An essay

city of the plate, which is in that cafe a�tuatcd by a very fmall degree of intenfity, or powernbsp;of expanlion ; fo that, fuppofe for inftance,nbsp;the piece of metal or finger touching the plate,nbsp;took off fo much of its eleftricity as to reducenbsp;the intenfity of the remainder to the 50th partnbsp;of a degree , this remaining eledtricity wouldnbsp;be then nothing; but when the plate, by beingnbsp;feparated from the inferior plane', has its capacity fo far diminifhed as to render the intenfitynbsp;of its eledtricity 100 times greater, then thenbsp;intenfity of that remaining eledlricity would become of two degrees or more; viz. fufficientnbsp;to afford a fpark.

Having confidered in what manner the adlion of eiedtnc atmofpheres modifies the electricitynbsp;of the metal plate in its various fituations, wcnbsp;fhall now confider the effedts which take plgcenbsp;when the eledtricity is communicated to thenbsp;metal plate, whilft Handing upon a metalnbsp;plane. As the whole bufinefs has been provednbsp;in the preceding pages, it is eafy to deducenbsp;the applications from it ; neverthelefs, it willnbsp;be ufeful to exemplify it by an experiment.

Suppofe a Leyden phial or a condudtor, fo y/eakly eledtrified that its intenfit)' is on half a.

degree, or even lefs : if the metal plate of the condenfer, when Handing upon its propernbsp;plane, was to be touched with that phial ornbsp;conductor, it is evident, that either of themnbsp;W'ould impart to it a quantity of its eleftricity,nbsp;proportional to the plate�s capacity, viz. ibnbsp;much as flrouid make the intenficy of the electricity of the plate equal to that of the eleftrl-city in the condudtor or phial, viz. half a dc-gree ; but the plate�s capacity, now it liesnbsp;upon a proper plane, is above i oo times greater than if it Hood infulated in the air ; or,nbsp;which is the fame thing, it acquires loo timesnbsp;more eledlricity from the phial or conductor.nbsp;It naturally follows, that when the metal platenbsp;is removed from the proper plane, its capacitynbsp;being leffened fo as to remain equal to the loothnbsp;part of what it was before, the intcnfitv of itsnbsp;eledlricity muft become 50�, fince the intcnfitynbsp;of the eleftricity in the phial or conduftor wasnbsp;half a degree.

If a fmall quantity of eledlricity, applied to the metal plate of the condenfer, enables It tonbsp;give a ftrong fpark, it may be alkcd, Whatnbsp;would a greater quantity do ? Why nothingnbsp;more. Becaufe, when the elediricity communicated to the metal plate is fo flrong as tonbsp;overcome the fmall refiftance of the inferiornbsp;plane, it tvill be diffipated.

AN ESSAY

It js eafy to imderftand, that if the metal plate pf the condenfer can receive a good lharcnbsp;of eledlricity from a Lej�-den phial or amplenbsp;eondoctor, however weakly cleflrified, it cancer receive any conficlcrabie quantity of it fromnbsp;e eondudor of fmall capacity ; for this conductor eannoc give what it has not, except it werenbsp;ccnfinually receiving a ftream, however fmall jnbsp;as is the cafe with an atmofpherigal condudlor^nbsp;or with the condudor of a machine which adsnbsp;very poorly bur continues in adion. In thofenbsp;cafes it has been obferved, that a conliderablenbsp;time is required before the metal plate has ac-jnbsp;quired a fuiheient quantity of elcdricity.

As an ample condudor, w'eakly eledrified, imparts a conliderable quantity of eledricity tqnbsp;the metal plate of the condenfer, fo, when thisnbsp;plate is afterwards feparated from its plane, thenbsp;eledricity in it appears much condenfed andnbsp;vigorous ; fo, when the fame plate contains anbsp;fmall quantity of eledricity, fuch as cannotnbsp;give a fpark or affed an eledrometer, that electricity may be rendered very confpicuous bynbsp;communicating it to another frnall plate or condenfer.

Mr. Cavallo firft thought of this improvement, by reafoning on Mr. Volta�s experiments. Henbsp;made a fmall metal plate not exceeding the fizcnbsp;of a Ihilling. This fecond condenfer is of great

ON ELECTRICITY.

iifc In many cafes where the elcftricity is fo finall as not to be at all, or not clearly, obfer-vable, by one condenfcr only, as has been fullynbsp;provecL Sometimes the ulual metal plate ofnbsp;my condenfer acquired fo fmall a quantity ofnbsp;clcdlricity, that being afterwards taken fromnbsp;the inferior plane, and prefented to an ex-trcamly fenfiblc electrometer, made by Mr. Ca-vallo, it did not affedl it. In this cafe, if thenbsp;faid'plate, thus weakly eledrilied, was made tonbsp;touch the other fmall plate properly fituated,nbsp;and was afterwards brought near an eleClromcTnbsp;ter, the eledricity was then generally flrongernbsp;than was fafficient merely to afeertain itsnbsp;quality.

Now if, by the help of both condenfers, the intenfity of the eledricity has been augmented 1000 times, which is by no means annbsp;exaggeration, how weak muft then be the electricity of the body examined ! how fmall thenbsp;quantity of clcdriclty that is produced by rubrnbsp;bing a piece of metal with one�s hand ! fince,nbsp;when it is condenfed by both condenfers, andnbsp;then communicated to an eledrometer, it willnbsp;hardly affed that inlfrumcnt, and yet is fuffi.nbsp;cient to afford convidioh that the metal cannbsp;be eledrified by the fridion of a perfon�s hand.

Before the difeovery ot the condenfer and Mr* Cavallo�s very fcnflble eledrometer, we

CHAP. xn.

For the fubjeifl of this chapter we are prlii-cipally indebted to P. Beccaria, who has fdr niany years abcurately obferved the variousnbsp;changes in the elcdtricity of the atmofpherc,nbsp;and their relation to the other phoenomena ofnbsp;the weather. His apparatus was admirably wellnbsp;adapted for this purpdfe, and faperior to anr*nbsp;thing that wc arc at prefent acf[hainted with,nbsp;for intimating eafily and at all times the electricity of the air, It not being at fird fui|ib�l:cdinbsp;that elcftricity was fo intimately blended withnbsp;every operation of hature, as it is now knownnbsp;to be. The labourers in this part arc of courfenbsp;very few, the principal are P. Beccaria, Mr.nbsp;Ronayne, and Mr. Cavalio.

I have cxtraflcd and methodized the refults of the obfervatibns made by P. Beccaria, introducing occafionally thofe made by Others, thatnbsp;the reader might be in poffcffion of the moftnbsp;material fads, and excited to invcftigatc andnbsp;purfue with attention this delicate and important iubjed; for, indeed, little certainty cannbsp;be expeded from any fyftem of meteorologynbsp;where the adion of the principal agent is cotnbsp;particubrly confidered and attended to^

The

The apparatus ufcd by P.gt; Beccaria, for in-veftigatlng the eledlricity of the atmofpherc, was an iron wire, which he tetms an exploringnbsp;wire, one hundred and thirty-two feet long.-It was fixed at one end to a pole raifed over thenbsp;chimney, the other end was faftened to the topnbsp;of a cherry�. The extremities of the wirenbsp;were infulated, arid coveted with a fmall umbrella of tin. Another wire was brought frontnbsp;this, (through a thick glafs tube, coated withnbsp;fealing-wax) into the room ; by which meansynbsp;continual information of the flate of the electricity in the exploring wire was obtained. � Henbsp;connedlcd with this wire a fatall flip ot metal,nbsp;on each fide of which was a fmall pith ballynbsp;one line diameter; the balls were fufpended bynbsp;(ilk threads, fixteen lines long.

The eleflricity, in ferene weather, generally makes each of the balls diverge about 6 lines ;nbsp;when it is very ftrong, they will diverge 15 ornbsp;20 degrees from the metal pktc; when weakynbsp;the divergence is very fmall.-

In ferene weather, the wire, after being touched, will take a minute or longer bctorc itnbsp;again fliews figris of elcflricity ; though, atnbsp;other times, it will become cleftrified in thenbsp;fpace of a fecond.

The eleftricity dvtring fcrcnc weather is al-�^ays pofitive. There are few inftances in

ON ELECTRICITY, 207

which it is negative, and then it is brought over by the wind from fomc part of the atmo-fphere, (perhaps very diftant from the plate ofnbsp;obfervation) w'here there is either fog, faow,nbsp;rain, or clouds- The w'hole feries of o'bfer-vations which P,. Beccaria has made-connrmnbsp;this pofition. He feems to have met with onlynbsp;three or four inftances to the contrary-

Dr. Franklin has obfervedj that the clcnds are Ibmetimes negative, which is certainlynbsp;true ; becaufe they will at times abibrb at, andnbsp;through the apparatus, a large and full bottle ofnbsp;pofitive cledricity, of which the apparatusnbsp;could not have received and retained the loothnbsp;part. And it is eafy t� conceive, how a ftronglvnbsp;charged large pofitive cloud may reduce fmailernbsp;clouds to a negative ftate.

The elclt;5lricity of the atmofpherc Is very niuch conneded with the ftate of the air, as tonbsp;moifture and drynefs; fo that it Is ncceffary tonbsp;attend to the hygrometer, in order to form anbsp;proper judgment of the different degrees ofnbsp;cledricity at different times. That invented bynbsp;Mr. Coventry, which is made of hatters� jrapetjnbsp;will anfwer beft ; it Is very fcnfible, abl'orbsnbsp;moifture foon, and parts with it eafily. Comparative obfervations may alfo be made withnbsp;it, It is alfo neceflary to place a thermometernbsp;near the hygrometer, to afeertam what quantity

AN ESSAY

of moillure the air can keep in folution vyitlt a given degree of heat. Though this objeftnbsp;will more probably be obtained by obfervin^nbsp;accurately the quantity of moifture evaporatednbsp;Irom a given furface at dilFerent times. It isnbsp;alfo to be obferved, that the different degreesnbsp;of denfity in the air will affefl the quantity ofnbsp;moillure W'hich is retained in the air.

The moillure in the air is the conllant con-tluftor of the atmofpheric electricity during clear weather; and the quantity of cledlricitynbsp;is proportioned to the quantity of moillurenbsp;which furrounds the exploring wire ; exceptnbsp;there Is fo much as to lelTcn the exadnefs ofnbsp;the iniulation of the wire and of the atmo-fj)herc. In a dry Hate of the air it will fome-times be above a minute before the balls willnbsp;manifell any eledrlcity after the wire has beennbsp;touched; though in a damper {late, a fccondnbsp;�will fcarce elapfc before rapid ofcillations ofnbsp;the balls may be obferved between the fingernbsp;and the plate oi brafs to which tliey arcnbsp;affixedh-^

The eleflricity, when the weather clears up, }S always pofitive. When the weather is clear-becomes dry quickly, the electricity rifes to a great degree of iritenfity, andnbsp;affords frequent opportunities for repeating thenbsp;obfcrvations. It fometimes happens, that thenbsp;eledricity, eaufed by the clearing up of thenbsp;weather, continues in its ftate of intenfity for anbsp;long while ; and alfo, after being interrupted, it begins afrefli. Thefe accidents feemnbsp;to be owing to the eledricity being broughtnbsp;over by the wind from great diftances.

P. Beccaria fays, that whenever he obferved that the thick low clouds which were overnbsp;his head began to break, and the rare evennbsp;clouds, which ^e above the former, becamenbsp;dilated, that the rain ceafed, and the ballsnbsp;diverged with pofitive eledricity, he\ alwaysnbsp;wrote down certain tendency to clear weather.

Prior Ceca faySj that a ftrong pofitive elec-^ tricity after rain is an indication that the weather will continue fair for feveral days. If thenbsp;eledricity is weak, it is a fign that the fairnbsp;\veathef will not laft the whole day, but that itnbsp;will foon be cloudy, and even rain.

If, when the Iky grows clouded over the place of obfervatlon, and a high cloud isnbsp;formed, without any fecondary clouds undernbsp;Pnbsp;nbsp;nbsp;nbsp;it.

AN ESSAY

it, and that it is not an extenfion of a cloud which drops rain elfewhere ; either no eledtri-clty takes place, or it is pofitivci

If the clouds which are gathering are lhaped like locks of wool, and keep moving firft nearernbsp;to, and then feparating from each other; or, ifnbsp;the general cloud which is forming lies verynbsp;high, and is ftretched downwards like defcend-ing fmoke, then politive eledfricity commonlynbsp;takes place, which is more or lefs ftrong innbsp;proportion to the quicknefs with which thisnbsp;cloud forms ; and it foretells the greater or lefsnbsp;quantity and velocity of the rain or fnow whichnbsp;is to follow. ^

When a thin, even, and extenfive cloud is forming, which darkens the fky, and turns itnbsp;into a grey colour, a flrong and repeated poli-tivc eledricity takes place; but in proportionnbsp;as the gathering of the cloud flackens, thisnbsp;cleclricity leflens, or even fails. On the contrary, if the rare extenfive cloud is graduallynbsp;formed of fmaller clouds, like locks of wool,nbsp;which are continually joining to, and partingnbsp;from each other, the pofitive eleflricity commonly continues.

Low and thick fogs, (efpecially when aS they rife the air above them is free from moif-ture) carry up to the exploring wire an electricity

ON ELECTRICITY.

tricity which will give fmall fparks repeatedly, and produce a divergence of the balls from

iluggilh, and continues round the exploring wire, the eledricity foon fails ; but, if it continues to rife, and another cloud fucceeds, itnbsp;electrifies again the wire, though lefs than before. Sky-rockets fent through fueh thick,nbsp;low, and continued fogs, often afford figns ofnbsp;eleftricity. P. Beccaria, under any one of thenbsp;circumftances above deferibed, never met withnbsp;an inftance of negative ele�tricicy ; except,nbsp;perhaps once, when he fent a Iky-rocket, tonbsp;which a firing was fixed, through a low thicknbsp;fog; though he had afterwards every reafonnbsp;to think that he had mifiaken a fdlfe littlenbsp;Jlar for a true one.

Mti Ronayne obferved, that the air in Ireland was generally electrified in a fog, and even in a mifi, and that both day and night, butnbsp;principally in w'inter; feldom in fummer, except from pofitive clouds, or cool fogs. Thenbsp;electricity of the air in a frofi or fog is alwaysnbsp;pofitive. He fays, that he has often obferved,nbsp;during what feemed the palEng of one cloud,nbsp;fucceffive changes from negative to pofitive,nbsp;and from pofitive to negative,

AN ESSAY

Mr. Henly has Ihewn, that fogs arc mote ftrongly eleftrified in, or Immediately after anbsp;froft, than at other times ; and that the electricity in fogs is often the ftrongeft foon afternbsp;their appearance.

Whenever there appears a thick fog, and at the fame time the air is iharp and frofty, thatnbsp;fog is ilrongly eleftrified pofitively.

Though rain is not an immediate caufe, yet he is inclined to think it was always a remote confequence, of eledtricity in the atmo-fphere; and he generally found, that in twonbsp;or three days after he had difeovered the air tonbsp;be ftrongly ekdlrified, we had rain, or othernbsp;falling weather.

If, in clear weatlrer, a low cloud, which moves flowly and is confiderably diftant from anynbsp;other, pafles over the wire, the pofitive electricity generally grows very weak, but doesnbsp;not become negative; and when the cloud isnbsp;gone, it returns to its former Hate. Whennbsp;many wEitilh clouds, like locks of wool, keepnbsp;over the wire, fometimes uniting with, andnbsp;then feparating from, each other, thus formingnbsp;a Ix)dy of confidcrable extent, the pofitivenbsp;electricity commonly increafes. In all thenbsp;above circumftances the pofitive electricitynbsp;Bever changes to a negative one.

The

The clouds which leflen fhe eleflricity of rhe exploring wire are thofc which move; thoughnbsp;thofe that are low feem alfo to have the famenbsp;efTedl.

Of the Diurnal Atmofpherical EkSlridty.

In the morning, when the hygrometer indii cates a degree of drynefs equal to, or little lefsnbsp;than that of the preceding day, an eleftricitynbsp;takes place before the fun rifes ; whick isnbsp;manifefted by jundlions, adhefions, or even anbsp;divergence of the balls, and is proportional tonbsp;the drynefs of the air, and the fmallnefs of itsnbsp;difference from that of the preceding day. Ifnbsp;this ftate of drynefs does not obtain, no dif-eernable eledfricity will be perceived before, ornbsp;even for a little while after, the riling of thenbsp;fun. As the air is generally damp in the night,nbsp;ele�tricity is feldom obferved before the funnbsp;rifes. During three months obfervations P. Bec-caria found the eleftricity before the fun rpfenbsp;only eighteen mornings ; and from the wholenbsp;of his numerous obfervations it appears, thatnbsp;the appearance of eledlricity in winter beforenbsp;fun-rife is more frequent than in the lumnier^nbsp;cfpecially if the dampnefs from hoar-froft is,nbsp;prevented from affedfing the apparatus.

P 3 nbsp;nbsp;nbsp;la

AN ESSAY

In the morning, as the fun rlfes higher, the elefcricity, whether it begun before fun-rife ornbsp;only after, gradually increafes. This gradualnbsp;increafe of the morning eleftricity begins foonernbsp;if the hygrometer continues after fun-rife tonbsp;indicate a greater degree of increafing drynefs.nbsp;The intenfity and the rife of the eledricitynbsp;(after it has been annihilated by touching thenbsp;exploring wire) lafts in ferene days, in whichnbsp;no impetuous wdnd takes place, and the hy-^nbsp;grometer is ftationary at the higheft degree itnbsp;has attained that day, till the fun draws nearnbsp;the place of its fetting. When the fun is nearnbsp;fetting, and in proportion as the hygrometernbsp;abforbs the moifture, the intenfity of the dailynbsp;eledtricity leflens.

Though the hygrometer may indicate equal degrees of drynefs at twelve o�clock, in different days, yet the eledricity will appear foonernbsp;after being deftroyed on fome days than onnbsp;others; and this is in a great meafure proportioned to the increafe of heat. The eledricitynbsp;moreover commences on fuch days later in thenbsp;morning, and falls fooner in the evening.

The fridion of winds againft the furface of the earth is not the caufe of atmofpheric electricity, Impetuous winds leffen the intenfity ofnbsp;the eledricity in clear weather. If they arenbsp;damp, they leffen its intenfity in proportion to

the diminution they caufe in the exactnefs of the infulation, both -of the wire and atmofphere.

OJ the l^leStricity produced by the Evening Dew.

In cold feafons. If the iky Is clear, little wind, and a great degree of increafing drynefs, annbsp;eledlricity of CQnliderable intenlity arifcs afternbsp;fun-fet, as foon as the dew begins, The frequency of fuch eledlriclty is moreover greaternbsp;than that of the daily eledlricity, and it vanilhesnbsp;Ilowly,

In temperate or warm feafons, if the fame circumftances as above take place, an electricity intirely fimilar to the former arlfes as foonnbsp;as the fun has fet; only its intenfity is not fonbsp;conftant, it begins with greater rapidity, andnbsp;ends fooner,nbsp;nbsp;nbsp;nbsp;,nbsp;nbsp;nbsp;nbsp;�

If, under the above circumftances refpeftive-ly, the general drynefs of the air happens to be lefs, the eledlricity that arifes in the even-''nbsp;ing, when the dew begins, is lefs in proportionnbsp;to the diminutions of the exaflnefs of the inlu-lation of both the exploring wire apd the atmofphere ; but correfpondently to the greaternbsp;quantity of dew, the frequency of the clcdricity

AN ESSAY

'The electricity of dew feems to depend on the quantity of dew, and to follow in its variousnbsp;changes proportions fimilar to thofe which takenbsp;place between the eledtricity of calm mild rain,nbsp;and that of rainy and ftormy weather, and variesnbsp;alfo according to the feafons.

As rain, Ihowers, the Aurora Borealis, and the zodiacal light, have a tendency to appearnbsp;for feveral fucceffive days with the fame ch^-radteriftic accidents, fo the eledtricity of dewnbsp;feems to have as it were an incUnation to appearnbsp;for feveral evenings fucceflively with the famenbsp;charadlers.

Let the air in a well-clofed room be eledtri-fied; that is to fay, the moifture and other vapours dilFufed in it: then let a bottle, filled with water colder than the air in the room, and in-fulated on a tube of glafs, be raifed prettynbsp;high in this room. Care muft be taken to prefer ve the infulation of the glafs with warmnbsp;cloths. The eledtric figns that will arife in twonbsp;threads fufpended to fuch bottle will exactlynbsp;reprefent the eledtricity of dew ; and they w'illnbsp;exhibit the different manner after W'hich thisnbsp;eledtricity takes place, according as the elec-jirified vapours in the room are more or

left rare, as the difFerence between the heat pf the air in the room, and that of the water innbsp;the bottle, is lefs or greater, and the infulationnbsp;of the bottle is more or lefs exadt.

In a thunder-ftorm Mr. Rona3'ne obfervcd, that the flafhcs would caufe fudden changes.nbsp;Sometimes the elcftricity would be extended,nbsp;fometimes diniiniflied ; at other times increafed,nbsp;and fometimes even changed to the contrarynbsp;again, though none was perceived before ;nbsp;it would come on fuddenly with a flalh of lightening. A large thunder-cloud, when it darkens the hcmilphere, c)oes not produce fo muchnbsp;eleflricity as a branch of it, or even as a common fhower ; that a ftorm does not go in anbsp;regular current of tjie wind, but obliquely andnbsp;zig-zag; viz. it rains in that region fromnbsp;whence the ftorm is to proceed.

Thcfe were principally made with an cledlri-cal kite, which will colledl eledlricity from the 'air at any time. The power of this inftrumentnbsp;refides in the firing. The beft method of making the firing is by twifting two threads of common twine with one of that copper thread whjchnbsp;is ufed for trimming : a fchool-bo)ds kite with

AN ESSAY

this firing anfwers the purpo-fe as well as any other, When a kite, conftrufted in this manner, was raifed, Mr, Cavallo fays he alwaysnbsp;obferved the firing to give figns of eleflricity,nbsp;except once ; the weather was warm, and thenbsp;wind fo weak, that the kite was raifed withnbsp;difficulty, and could hardly be kept up for anbsp;few minutes : afterwards, when the wind in-creafed, he obtained as ufual a flrpng pofitive.nbsp;cleiflricity.

If the kite was raifed at a time when there �was any probability of danger from the greatnbsp;quantity of ele�lricity, Mr. Cavallo conne�lednbsp;one end of a chain with the firing, and let thenbsp;other end fall on the ground, and placed him-felf alfo on an infulating ftooL Except the kitenbsp;is raifed in a thunder-florm, there is no greatnbsp;danger that the operator will receive, a fhock,nbsp;Although he raifed his kite hundreds of timesnbsp;without any precaution whatever, he feldomnbsp;received even a few flight fhocks in. the arms.nbsp;But it is not advifeable to raife it while flormynbsp;clouds are overhead. This is alfo lefs neceffary,nbsp;as the eledricity of the atmofphere may thennbsp;be eafily obferved by other means. When thenbsp;kite was raifed, he often Introduced the firingnbsp;through a window into a room of the houfe,nbsp;and faftened it by a ftrong filk lace to a heavynbsp;chair in the room. Fig. 78, AB reprefents part

of the firing of the kite which comes within the room, C the filk lace, DE a fmall primenbsp;conductor, which, by means of a fmall wirenbsp;is conne�led with the firing of the kite; F anbsp;quadrant eleClrometer, fixed upon an infulatingnbsp;Hand, and placed near the prime condudor;nbsp;G a glafs tube about 18 inches long, gn a ballnbsp;and wire of brafs, which are fixed to the glafsnbsp;tube. This fmall inflrument is ufeful to determine the quality of the elcdricity when itnbsp;is not fiife to come near the firing. This isnbsp;effeded by touching the firing with the wire,nbsp;which takes a fufficient quantity from it to af-certain thereby the quality of the eledriclty,nbsp;either by the attradion and repulfion of lightnbsp;balls, or the appearances of the eledric light; ornbsp;it may be afeertained by a Leyden phial, whichnbsp;wjll retain a charge for a confiderable time;nbsp;and then the kite need not be kept up anynbsp;longer than is neceffary to charge the phial,nbsp;by which the quality will be fhewn even atnbsp;fome days diflance.

If a charged phial is carefully kept from any of thofc means by w'hich it is known to benbsp;difeharged, it will retain its charge for a longnbsp;time. On this principle the above-mentionednbsp;phial is conflruded ; the bottle is coated in thenbsp;nfual manner, the uncoated part of the glafsnbsp;is covered with wax, or elfe well varnifhed.

AN ESSAY

A glafs tube, which is open at both ends, is cemented into the neck of this phial, having anbsp;piece of tin-foil connedled with its laweft extremity, which touches the inhdc non-electricnbsp;coating, A glafs handle is fixed to the ball onnbsp;the wire which palTes into the foregoing glafsnbsp;tube; the wire is of a proper length to touchnbsp;the tln-fpil which is at the bottom of the tube.nbsp;Charge this bottle in the uf�al manner, andnbsp;then take out the wire from tire glafs tube bynbsp;means of the glafs handle. This may be donenbsp;without difcharging the phial, and, as the firenbsp;cannot now efcape eafily, the charge of a phialnbsp;may be preferved for many weeks.

Fig, 8o reprefents a very fimple inftrument (contrived by Mr. Cavallo) for making experiments on the eledricity of the atmofphere, andnbsp;which, on feveral accounts, appears to be thenbsp;bell for the purpofe. A B is a common jointednbsp;filhing rod, without the lafl; or fmallefl; joint ;nbsp;from the extremity of this rod proceeds a fmallnbsp;glafs tube C, covered with fealing-wax, a corknbsp;D is fixed at the end of it, from which an electrometer with a pith ball is fufpended. HGInbsp;is a piece of twine, faftened to the other extremity of the rod, and fupported at G by anbsp;fmall firing F G. At the end of the twine Tnbsp;a pin is faftened, which, when pulhed into thenbsp;cork Dj renders the elciflrometer E unlnfulated.

ON ELECTRICITY.

When the eledricity of the atmofpere is ob-icrved with this inftrument, thruft the pin T into the cork D, and hold the rod by the lowernbsp;end A ; place it out of a window at the uppernbsp;part of the houfe, raifing the end of the rodnbsp;with the eleClroraetcr, lb as to make an anglenbsp;of 50 or 60 degrees with the horizon- Keepnbsp;the inlirument In this fituation for a few fe-concls, then pull the twine at H, and the pinnbsp;will be difengaged from the cork D; whichnbsp;operation caufes the firing to drop in the dotted fituation K L, and leaves the electrometernbsp;Infulated, and electrified with an eleClrieitynbsp;contrary to that of the atmofpliere; This beingnbsp;done, you may draw the electrometer into thenbsp;room, and examine the quality of the electricity, without obfiruCtlon either from wind ofnbsp;darknefs.

Figi 81 is an eleCtrometer for rain, con-trived by Mr. Cavallo. A B C F is a firong glafs tube, about two feet and a half long,nbsp;having a tin funnel D � cemented to its extremity, which funnel defends part of thenbsp;tube from the rain. The outfide furface of thenbsp;tube from A to B is covered wfith fealing-wax,nbsp;and fo is the part of it which is covered bynbsp;the funnel. F D is a piece of cane, roundnbsp;which brafs wires are tvvifted in dificrent di-feclions, fo as to eateh the rain eafily, and at

AN ESSAY

the fame time to make no refiftance to the winc�i This piece of cane is fixed into the tube, andnbsp;a fmall wire proceeding from it goes throughnbsp;the tube, and communicates wdth the ftrongnbsp;wire A G, which is thruft into a piece of corkynbsp;faftened to the end A of the tube^ The end Gnbsp;of the wire AG is formed into a ringy fromnbsp;which a fenfible pith ball eledlrometer is to benbsp;Ibfpendedi This irtflrument is faftened to thenbsp;fide of a window framcy where it is fupportednbsp;by ftrong brafs hooks at C B j which part ofnbsp;the tube is covered with a filk lace, in order tonbsp;adapt it better to the hooks. The part F L isnbsp;out of the window, with the end F elevated anbsp;little above the horizon; The remaining partnbsp;of the iirftrument comes through a hole in onenbsp;of the lights in the fafh, within the room, andnbsp;no more of it touches the fide of the windownbsp;than the part C B; When it rains, efpeciallynbsp;in palling fhowers, this inftrument is frequentlynbsp;eleffrified ; and by the divergence of the eledtro-meter, the quantity and quality of the rain maynbsp;be obferved, without any danger of a miftakc;nbsp;With this inftrument, in rainy weather, Mr.nbsp;Cavallo has been able to charge a fmall coatednbsp;phial at the wire AG. It fliould be fixednbsp;in fuch a manner that it may be eafilynbsp;taken off from the window, and replaced again,nbsp;as occaljon requires; as it will be neceffary

The principal part of this inftrument Is a glafs tube C D M N, cemented at the bottomnbsp;into the brafs piece A B, by which part thenbsp;inftrument is to be held when ufed for the at-mofphere; and it alfo fetves to fcrew the inftrument into its brafs cafe ABO, fig. yb.nbsp;The upper part of the tube C D M N is lhapednbsp;tapering to a fmall extremity, which is Intirelynbsp;covered with fealing-wax; into this taperingnbsp;part a fmall tube is cemented ; the lower extremity G being abb covered with fealing-wax, projefts a fmall way within the tubenbsp;CDMN; into this fmaller tube a wire Is cemented, w'hich, with its under extremity,nbsp;touches the flat piece of ivory H, faftened tonbsp;the tube by means of a cork; the upper extremity of the wire projects about a quarter ofnbsp;an inch above the tube, and fcrews into thenbsp;hrafs cap E F, which cap Is open at the bottom, and ferves to defend the waxed part ofnbsp;the inftrument from the rain, amp;c.

AN ESSAY

T M and K N are two narrow flips of titt-foil, ftuck to the infide of the glafs C D M K, and communicating with the brafs bottom AB.nbsp;They ferve to convey that clcdricity which,nbsp;when the corks touch the glafsj is commfuni'nbsp;cated to it, and, being accumulated, mightnbsp;difturb the free motion of the corks.

To ufe this inflrument for artificial eleftri-city, eleftrify the brafs cap by an eleflrified fubiiance, and the divergence, of convergencenbsp;�f the balls of tlie �le�lrometer, at the approach �f an excited electric, will fliew thenbsp;quality of the eledlricity. THe beft manner toinbsp;�leftrify this inftrumerit is, to bring excitednbsp;fvax fo hear the cap that one or both of thenbsp;corks may touch the fide of the bottle CDMN,nbsp;after which they will foon collapfe and appearnbsp;iineledrified. If now the wax is removed^nbsp;they will again diverge, and remain elcdtrifiednbsp;pofitively.

When this ele�tf�meter is to be ufed to try the electricity of the fogs, air, clouds, amp;Clnbsp;the obfetver is to do nothing more than to un-ferew it from itS cafe, and hold it by the bot-t�m A B to prefect it to the air a little abovenbsp;his head, fo that he maiy conveniently fee thenbsp;corks P, which wdll immediately diverge ifnbsp;there is any eleClricity; i. e. whether pofitiv'enbsp;or negative may be afOefCained, by bringing an

1. nbsp;nbsp;nbsp;The air appears to be eledlrified at allnbsp;times. Its eledtricity is conftantly pofitive',nbsp;and much ftrbnger in frbfty than in warih weather ; but it Is by no means lefs in the nightnbsp;than in the day time;

2. nbsp;nbsp;nbsp;The prefence of the clouds generalistnbsp;leflens the eledricity of the kite : fometimesnbsp;It has no effed upon it, and it very feldom in'!-creafes it,

3. nbsp;nbsp;nbsp;When It rains the eledriclty of the kitenbsp;is generally negative, and feldom pofitive.

5; The eledrical fpark, taken from the firing of the kite, or from any infulated con-dudor conneded with it, efpecially when itnbsp;does not rain, is feldom longer than a quarternbsp;of an inch, but it is exceedingly pungent.nbsp;When the index of the eledrometer is notnbsp;higher thannbsp;nbsp;nbsp;nbsp;the perfpn who takes the '

AN ESSAY

appears more like the difcharge of an electric jar, than the fpark taken from the prime condudior of an eledrical machine.

6. nbsp;nbsp;nbsp;The eledricity of the kite is in generalnbsp;ftronger or weaker, according as the firing isnbsp;longer or fhorter, but it docs not keep any exadnbsp;pro^rortion to it. For inftance ; the eledricitynbsp;brought down by a firing of a hundred yardsnbsp;may raife the index of the eledrometer to 20�,nbsp;when with double that length of firing the index of the eledrometer will not go highernbsp;than 25�.

7. nbsp;nbsp;nbsp;When the weather is damp, and thenbsp;eledricity is pretty flrong, the index of thenbsp;eledrometer, after taking a fpark from thenbsp;firing, or prefentlng. the knob of a coatednbsp;phial to it, rifes furprizingly quick to its ufuainbsp;place, but in dry and warm weather it rifesnbsp;exceedingly flow%

It appears, from the obfervatlons which have been made on the eledricity of the atmofphcre,nbsp;that Nature makes great ufe of this fluid innbsp;promoting vegetation. ,

I. In the fpringj when plants begin to grow, then temporary elcdrical clouds begin to appear, and pour forth eledric rain. The electricity of the clouds and of the rain continuesnbsp;to Increafe till that part of the autumn i�nbsp;which the lafl fruits are gathered.

2. nbsp;nbsp;nbsp;It is this fluid which fupplies commonnbsp;�re with that moifture by the help of whichnbsp;it aduates and animates vegetation; it isnbsp;the agent that colleds the vapours, forms thenbsp;clouds, and is then employed to dilbrder ahdnbsp;cliffipate them ih rain.

3. nbsp;nbsp;nbsp;From the fame principle may Be explained the proverb, that No watering givesnbsp;the country fo fmiliiig' d 'look as rain. Thenbsp;clouds �f raiiij by extending their ekclric at-mofphere to the plants, difpofe the pores of thenbsp;latter to receive with greater facility the waternbsp;which is impregnated with this penetrating andnbsp;dilating fluid. Befides, it is natural to fup-pofe^ that the pofitive eledricity which continually prevails in ferene weather^ will contribute to promote vegetation, fince this hasnbsp;been found to be the effed of even artificialnbsp;�'�ledricityi

lt;0

AN ESSAY

Of the IvtperfeBion of Meteorology, fo long �S Barometrical, T^hermofnetrical, and Hy-grometrical Ohf�rvati�ns are not accompa-Tiled with the regular Objervation of thenbsp;EleBricity of the Atmofphere, of the Electricity of Rain, Snow, Mijls, and aqueousnbsp;Meteors in general. By Mr, Achard.

As it 19 now clearly afcertained^ that electricity is a caufe of various meteorological pliocnomena, it is rather furprizing that philo-fophers have not perceived the abfolute neceffitynbsp;of joining an inllrument by which obfervationsnbsp;may be made on the ele�tricity of the atmofphere, to thofe which indicate its weight,nbsp;heat, and humidity.

Without conlidering in this place the different proofs of the influence of eledfricity on meteors, it will be fufficient to remark, thatnbsp;we cannot attain to an adequate knowledge ofnbsp;any phoenomena, occafioned by the concurrence of various caufes, without being acquainted with them all; for if any one isnbsp;negleded, it will be abfolutely impoffible thoroughly to explain the phoenomena. If electricity is not the foie caufe of feveral meteorological appearances, it is undoubtedly concerned

ON ELECTRICITY. 2?^

cerned more or iefs in their formation ; fo that by neglefling to obferve it, as well as the barometer, amp;c. we lofe the fruits of other, evennbsp;very exadt, meteorological obfervations.

The influence of eleftricity on vegetation is ^)roved by a fet of obfervations made by different philofophers; but it evidently appears,nbsp;that the botanic meteorological obfervationsnbsp;alone will never be fo ufeful as might be ex-pcdled, till we unite thofe made by an inftru-ment which will Indicate the eledlric flate ofnbsp;the atmofphere, to thofe made with other in-ftruments. It is otving to this caufc, perhaps,nbsp;that it is impoffible tb draw any conclufionnbsp;from the botanical meteorolQgical obfervationsnbsp;of Meflfs. Gautier and Duhamel, which werenbsp;continued from 1751 to 1769.

Mr. Achard has had an opportunity of making only a few obfervations, but they were fufflci-ent to convince him of the intimate connexionnbsp;that fublifts between the formation of the mod:nbsp;part of meteors, and atmofpherlcal eleftri-city.

To difeover if the atmofphere was eledtrical, he made ufe of a pair of light pith balls whichnbsp;were attached to a refinous rod. This eledtro-nieter, from its fimplicity, is almofl: preferablenbsp;to any other for merely difeovering that electricity exifts in the atmofphere.

During

AN ESSAY

During the month of July, 177S, Mr. Achard obferved daily the eledricity of thenbsp;atmofphere in the morning, at noon, and innbsp;the evening, with a pair of fmall pith balls,nbsp;which were placed above the roof of the houfc,nbsp;above 40 feet high, and fufficiently diftantnbsp;from buildings, trees, amp;c. During the^wholenbsp;time there was only 10 days which gave no fignsnbsp;of eledfricity ; 17 days, including the foregoingnbsp;I o, in which he could obferve no eledlricity innbsp;the morning, though it became very fenfible atnbsp;noon, and was very much increafed towardsnbsp;the fetting of the fun. Every other day henbsp;found the air eledlrical during the whole da)',nbsp;but alwa^'s ftrongeft a little before fun-fet, anbsp;Ihort time after which it began again' to dimi~nbsp;nilh.

If in ferene weather the fky became fuddenly cloudy, the electrometer indicated continualnbsp;changes in the cledtricity of the atmofphere ;nbsp;fometimes increafing, then difappearing, thennbsp;re-appearjng j in which cafe, it had generallynbsp;changed from pofitive to negative, or vicenbsp;verfa. In windy weather he found it difficultnbsp;to obferve with the eledtrometer, on account ofnbsp;the continual motion of the balls, It feemednbsp;to vary confiderably when the air was heavy,nbsp;but not windy. When the weather was verynbsp;palm, and the |ky withput clouds, the eledlro-

ntetey

nieter did not alter in tlie leaft, except towards fun-fet, when it increafed in a fmall degree.

It is remarkable, that in thofe days in which he obferved no electricity in the air, there wasnbsp;no dew at night; while on the other nights, itnbsp;fell in greater or lefs quantities. He does notnbsp;think thofe obfCrvations are fufficient to determine that the dew is occalioned by eleCtricitynbsp;but it may, he thinks, be fairly inferred, thatnbsp;the elevation and fall of the dew is obftruClednbsp;or promoted by the eleClricity of the air. Itnbsp;is eafy to point out in what manner electricitynbsp;may produce the effedt. Let us fuppofe thenbsp;air to be either pofitively or negatively eleftri-fied, but the furface of the globe where we arenbsp;not to be fo; the aqueous and volatile partsnbsp;of the vegetables exhaled by the rays of thenbsp;fun, and fufpended in the air, will becomenbsp;cledtric by communication. The air coolingnbsp;by the abfence of the folar heat, will not, after the fetting of the fun, retain the aqueousnbsp;particles with the fame force ; and thefe beingnbsp;attradted by the non-eledlric bodies which arenbsp;on the furface of the earth, their fuperliciesnbsp;will be covered with dew^ Again, let us fuppofe that the furface of the earth is eledltical,nbsp;but that the air is not eledtfical, and the efiedtnbsp;will be fimilar to the preceding caie. If thenbsp;air and the earth are both electrified, hut with'nbsp;0.4nbsp;nbsp;nbsp;nbsp;contrary

AN ESSAY

contrary powers, the attradlion will be ftronger and the dew more abundant, but no dew willnbsp;fall if they are both poffeffed of the fame power, and in the fame degree. It is known thatnbsp;the dew does not fall with the fame facilitynbsp;upon all bodies^ and that elediric bodies arcnbsp;thofe on which it falls with the greateft abundance. This fadt admits of an eafy explana-rnbsp;tion, if we fuppofe eledtricity to be tjip caufpnbsp;of the dew ; for the electric bodies do not readily receive eledtricity from the medium* whichnbsp;furrounds them ; there is, therefore, always anbsp;greater difference between the eledtricity of thenbsp;air and that of the eledtrics which are placed innbsp;it, than between the eledtricity of the air andnbsp;the condudting bodies which it epvelopes.nbsp;Now it is in the ratio of this difference thatnbsp;the power of eledtric attradtion adts, and con-fequently thefe bodies ought to be coverednbsp;more abundantly with dew.

As eledtricity is often, if not always, the caufe of dew, no one will doubt the neceffitynbsp;of attending to it in the botanical meteorology,nbsp;as every one is acquainted with the influence of^nbsp;dew on the growth of vegetables.

In the Phil, Tranf, for 1773, are obferva-tions on the eledtricity of fogs, which prove that they are generally eledtrical. Mr. Achardnbsp;has made feveral obfervations, the refults of

which

�which correfpond entirely with thofe, for he con-ftar.tly found that the air was more or lefs elepr trifled by a fog. Twice he obferved, that innbsp;the fpace of a few minutes the fog ceafed altogether, and fell in form of a fine rain i andnbsp;though it was very thick, difappeared in aboutnbsp;fcvcn minutes. It is alfo very probable thatnbsp;rain is occafioned by eleftricity ; and of thisnbsp;we fhall be convinced, if we confider the attractions and repulfions that the terreftrial or atmo-fpheric eieflricity mufl: occafion, as well between the furface of the globe and the vapoursnbsp;contained in the air, as between the particlesnbsp;of vapour which always neceffarily tend to dif-perfe or unite the aqueous particles wdiichnbsp;fwim in the atmofphere, and to bring themnbsp;nearer, or carry them farther from, the earth-Having proved the neceffity of combiningnbsp;obfervations on the eleftricity of the atmofpherenbsp;with other meteorological obfervations, Mr-Achard proceeds to defcribe the properties re-quifite in a good atmofpherical eleftrometer,nbsp;the want of which accounts for the negled andnbsp;fuplneneis of philofophers on this fubjcd.

Necejfaiy

AN ESS A Y

EleSirometer.

2. nbsp;nbsp;nbsp;It fliould not only indicate that the air ijnbsp;electrical, but in what degree*

4. nbsp;nbsp;nbsp;That the obferver Ihould be in no dangernbsp;in florray weather.

The number of difficulties w'hich oppoffi the conftruClion of an Inftrument which will unitenbsp;all thefe advantages are very confiderable.nbsp;The greatefl is to infukte the metal which receives the electricity from the air, fo, that rain,nbsp;may not eftablift a communication between itnbsp;and the earth, and that the iirfulation is fuffi-ciently perfedl to prevent too quick a diffipationnbsp;of the electricity received by the metaL_ Mr.nbsp;Achard does not pretend that he has furmount-ed all thefe difficulties, but after feveral trialsnbsp;he has contrived an inftrument fufficientlynbsp;portable, eafy to obferve with, and that without danger.nbsp;nbsp;nbsp;nbsp;gt;

Defcrlption of the portable Atmojpherical

Eleblrometery contrPced for the Furpofs

This inftrument is compofed of a hollov/ and truncated cone of tin, whofe upper end is open,nbsp;and which is clofed at bottom by a plate of thenbsp;fame metal. This plate is covered, in the in-fide of the cone, with a layer of rofm twonbsp;inches thick : to the lower furface of this layernbsp;of rofin a tube of tin is cemented, w'hich,nbsp;w'hen it is placed on a wooden pedeftal, fup-ports the cone in fuch a manner, that the greatnbsp;bafe is horizontal, and turned downwards; thenbsp;rolin infulates the cone perfedtly, and, whennbsp;the latter becomes eledlric, prevents the lofs ofnbsp;its eledlricity by tranfmiffion. The cone rnuftnbsp;be high enough, and its inferior bafe muff exceed far enough, in diameter, its fuperior extremity, to prevent the rain, even though itnbsp;ftiould fall in an oblique direction, from wetting,nbsp;either in its fall, or by rebounding from thenbsp;pedeftal, the lower furface of the rolin-la)'er,nbsp;with which the bottom of the truncated cone is^nbsp;internally covered : otherwife the cone wouldnbsp;ceafe to be infulated, and the eledlrometernbsp;would be changed into a condudtor. On thenbsp;jruncated part of the cone Mr. Achard faftens

a fquare iron branch, on which he place? 3 thermometer and two ele�lron�eters ; the onenbsp;very light, and thus capable of being fet in motion by fmall degrees of eledricity; the othernbsp;heavier, and which, confequently, only rifesnbsp;when the eleflricity becomes too ftrong to benbsp;meafured by the light cle�frpmeter. Befidesnbsp;thefe two eledlrometers, Mr. Achard tied to thenbsp;iron bar a thread, which indicates, by its fifing,nbsp;the fmalleft degrees of ele�lricity ; the wholenbsp;is inclofed in a receiver of glafs, open abovenbsp;and below; the bafe of this receiver is alfpnbsp;infulated with rofin, that it may not derive anynbsp;eledlricity from the tin cone; the remainingnbsp;fpace of the upper part of the receiver, betweennbsp;the bar of metal, which pafles through it, andnbsp;the glafs, is likewife filled with rofin, to prevent the communication of ele�lricity to the receiver ; to preferve this rofin from rain, which,nbsp;by moiftening it, would form a communicationnbsp;between the receiver and the bar, it is coverednbsp;over with a glafs funnel, through which thenbsp;bar paffes, and which hinders the rain fromnbsp;falling on the rofin. This receiver is alfo in-difpenfably neceffary to prevent the a�lion of thenbsp;wind upon the eleftrometers, which wouldnbsp;render the accurate obfervation of them impoffi-ble. At the end of the metal bar, which paffesnbsp;through the receiver, hollow tin pipes may be

placed, of a fmall diameter, to render them as iight as poffible, and they may be raifed tonbsp;the height of lOj 20, or 30 feet. The uppernbsp;end of the pipe terminates in an iron point, extremely lharp and well gilt; the gilding is ne-ceflary to hinder the point, which mull: be always even and fmooth, from contralt;5ling ruft.^nbsp;With refpedt to the elevation that it may be proper to give to the tin-pipe, this muft vary withnbsp;the height of the buildings or trees in the different places where obfervations are made ; fornbsp;the height of the pipe mull always exceed, atnbsp;lead by fix feet, the elevation of all the bodiesnbsp;that are near it. Mr. Achard joins a thermometer to this machine, which may be obferved atnbsp;the fame time, and be the means, perhaps, ofnbsp;difeovering the relation, if any there be, between eledricity and the temperature of thenbsp;A barometer and hygrometer may, with

In order to know whether the cle�lrlcity of the air be pofitive or negative, Mr. Achardnbsp;fufpends a ball of cork, by a linen thread, onnbsp;the wire which communicates with the ironnbsp;bar, and which paffes through the rofin, withnbsp;which the bafe of the truncated cone is covered. The wire muft be of fuch a length, thatnbsp;bodies, pofitively or negatively eledrical, may

AN ESSAY

te commodiovifly brought near the cork ball, which is fufpehded bn it; and it is according aSnbsp;thefe bodies attradt or repel the ball, that thenbsp;obfetver learns, whether the eledlricity whichnbsp;the infirunient has received from the air, benbsp;pofilive or negative.

That the obferver may be In no danger from fudden accumulations of ele�ricity, whichnbsp;femetimes happen, Mr- Achard fallens to thenbsp;bafe of the pedeftal an iron bar, which not onlynbsp;communicates with, but even enters into, thenbsp;ground, feveral feet deepi This bar, whofenbsp;upper part terminates In a roulid knob or ball,nbsp;muft be only at the diftance of an inch fromnbsp;the cone. When the eledtrical fluid is fo ac-Cumula'ted that the inftrument can no longernbsp;contain it, It will difeharge itfelf againft thisnbsp;metal bar^ which will conduft it under ground.nbsp;The fame thing would, if the lightening fellnbsp;upon the Inftrument^ and the obferver v/ouldnbsp;be in no fort of danger, even at the diftancenbsp;of a few feet. When the inftrumeiit is placednbsp;in a garden, this method of forming a communication v/ith the ground Is fubje�l to no incon-venlency ; but if it thould be judged proper tonbsp;employ the inftrument in a houfe, (which maynbsp;be done by making the tin pipe pafs through anbsp;hole in the roof, and placing the inftrument Irinbsp;a garret) the manner above-mentioned of form-'

ing

ON ELECTRICITY.

ing its communication with the earth would not be fo eafily executed : in this cafe, thenbsp;communication mull be elFefted by means ofnbsp;a bar of metal defcending from the garret to anbsp;depth of fome feet under ground; and fornbsp;greater fecurity againft the too great proximitynbsp;of a thunder-ftorm, it would be proper to placenbsp;the metallic bar in contaft with the cone of tin ;nbsp;thus the inftrument would become a real con*nbsp;dudor, which, inftead of expoling ^he houlenbsp;to danger, would, on the contrary, preferve itnbsp;from all the accidents that are occafioned bynbsp;lightening.

When the inftrument Is placed in a garretj or on the platform of a houfe, no inconvenience is to be apprehended from afcending dews;nbsp;but when it is placed in a garden, the dew adheres to the rofin which covers the truncatednbsp;bafe of the cone, and forming thus a communication between the cone and the earth, makesnbsp;the Inftrument lofe the eletftrlcity with which Itnbsp;may have been charged. To prevent this accident, it is neceflary to pave the ground onnbsp;rvhich the inftrument is placed^ and that in fuchnbsp;a manner, that the pavement may extend itfelfnbsp;on ail fides, at leaft two or three feet beyondnbsp;the circumference of the lower bafe of the cone :nbsp;the rifing of the dew, which by adhering to

AN ESSAY

When the air is electrical,- it muft neceffa-rily communicate its ele�ricity to the vapours which it contains. This is evident from thenbsp;formation of lightening, which is not producednbsp;by the difeharge of the electrical matter of thenbsp;air, but by that of the vapours which float Innbsp;that atmofoherCi Hence it follows^ that rainjnbsp;fnowi hail, mill, and dew, mufl; be very oftennbsp;eleCtric. As it appears to Mr. Aehard a matternbsp;of great confequence to know and obferve ex-inbsp;aClIy the eleCtricity of thefe meteors, he hadnbsp;conftruCted a machine that is adapted to dif-covOr both its nature arid degree. This machine is compofed of a truncated tin cone^nbsp;clofed at the top, open at bottom, and infula-ted upon a pedeflal, like that of the machinenbsp;employed to meafure the eleClricity of thenbsp;air. In the center of the upper truncatednbsp;part of the cone, Mr. Aehard fixes an iron barnbsp;terminated by a ball; he covers the whole withnbsp;� an infulated glafs receiver, high enough tonbsp;have its fummit at the diftance of three inchesnbsp;from the ball, which terminates the iron bar,nbsp;to, which he faftens a very fenfthle eledtrometerynbsp;and alfo a linen thread to difeover the fmallefl:nbsp;degrees of eledlricity. As this inftrument isnbsp;but little elevated, and has no pointed extremity,

ON �L�CtR�CiTY.

hilty, it is not �afily charged with the eleftri-city of the air, which, at fuch a degree of proximity to the earth, is always impercepti-inbsp;hie ; but rain, fnow, hail, mift, and deW,' ifnbsp;they are eledfrical, will render it alfo eleclricalnbsp;by falling upon the cone ; the degree of electricity is afcertaifled by the eleArometer,nbsp;which is under the receiver; and in order tonbsp;know whether it beipoiitive or negative^ thenbsp;obferver has only to employ the method indicated above, in our account of the inftrumentnbsp;Ufed to meafure the eleAricity of the air. Be-fides the ufe of this inftrument in difcoveringnbsp;the eleAricity of aque�Us meteors, it fnay ftillnbsp;ferve farther putpofes : it may be highly ufe-ful to compare it with the atmof|)hefical electrometer,' in order to difcern th� tru� principle,nbsp;of the eleAricity with whieh k is charged,nbsp;and to fee whether it proceeds immediatelynbsp;from the air, or from the heterogeneous bodiesnbsp;that are fufpended in the atmofphere ; for thenbsp;atmofpherical eleArometer may alfo becomenbsp;eledtricil by rain, fnow, haily or mift; andnbsp;the comparing thefe two inftruments is thenbsp;Only method that occurs to Mr; Achard bynbsp;which wc can know, whether it receives itsnbsp;eledtricity difedlly from the air, or by the intervention of bodies (indued with a conducingnbsp;power) which are diffufed in it. If, duringnbsp;Rnbsp;nbsp;nbsp;nbsp;rain.

AN ESSAY

rain, hail, fnow, or mift, the atm�fpherical eie^ilrometer is eleSriml, while that which indicates the elelt;5lricity of aqueous meteors isnbsp;mot fo,. we may conclude, with certainty, thatnbsp;the ekftricity, of the former proceeds onlynbsp;�from the air; if, on the contrary, they arcnbsp;both electrical,: it muft then be inquired, whether they be fo in the fame degree; if this benbsp;the cafe,, it is only to the rain, or fnow, See.nbsp;that the �le�ricity muft be attributed. I neednbsp;not obferve (concludes Mr. Achard) that whennbsp;there.is neither rain, fnow, hail, or mift, thenbsp;atm�fpherical eleflrometer will always indicatenbsp;the eledlricity of the air.

CHAP.

c H A p. xin.

ֻ the lyiff'ujion and Subdivijion of Fluids, by EleSlriciiy.

WE are eliiefly indebted to the Abb� Nollet for what is known on the fub-jeft of this chapter, which was inveftigatcdnbsp;by him with incredible induAry and patience.'nbsp;I have only fnbj�ined the principal refult of hisnbsp;experiments, and muft refer the reader, for anbsp;more ample account, to the Abbe�s own writings, or Dr, Prieftley�s Hiftory of Electricity.

Eledtricity augments the natural evaporation of fluids; finee, excepting mercury and oil,nbsp;all the Others which were'tried fulFered a diminution that could not be afcribcd to any othernbsp;caufe than eledfricitv.

It increafes the evaporation of thofe fluids moft which naturally tend to evaporate readily. Volatile fpirits of fal-ammoniac loftnbsp;more than fpirits of wine, this more than wa-jnbsp;ter, amp;c.

Eleftricity aSs ftrongeft upon fluids when the veffels which contain them are non-eleftrics,nbsp;R 2nbsp;nbsp;nbsp;nbsp;The

AN ESSAY

The evaporation wras greateft in the moft open Veflels, but (}id not increafe in proportion tonbsp;their apertures. It does not make anp liquornbsp;evaporate through the pores either of metal ornbsp;of glafs.

To extend thefe principles further, the Abbe made a great variety of experiment on eleftri-fied capillary tubes, and found,, that the ftreamnbsp;would be fub-divided, but is not fenfibly accelerated, if the tube is not lels than one tenthnbsp;of an inch diameter in the infide.

Under this diameter, if the tube is wide enough to let the fluid run in a ftream, eledri-city will accelerate its motion in a fmall de-gree.

If the tube is fo far capillary that the water' only iflties from it in drops, the eledrified jetnbsp;becomes a continued ftream, it wdll even benbsp;divided into feveral fmaller ones, and its motionnbsp;is confiderably accelerated j the fmaller the diameter of the tube, the greater is the acceleration. When the furface is wider titan onenbsp;tenth of an inch, eledricity feems rather tonbsp;retard the motion of the fluid.

Fig. '77, reprefents a metal phial, to which a capillary tube is adapted, which will only

mil

mit water to pafs through it in interrupted drops. Fill the pail with water, and fufpendnbsp;it from the prime conduflror, then turn the cylinder, and the w^ater will pafs through thenbsp;tube in a continued ftream, this will feparatenbsp;into other ftreams, that will appear luminousnbsp;in the dark,

Sufpend one pail from a pofitive conduflor, and another from a negative one, fo that thenbsp;end of the tubes may be about three or fournbsp;inches from each other, and the ftream proceeding from one will be attrafled by that w^hichnbsp;iflues from the other, and form one ftream,nbsp;w^hich will be luminous in the dark.

If the pails are fufpended on two pofitive, or two negative conduflors, the ftreams willnbsp;recede from each other.

Place a metal bafon on an infulating ftand, and connedt it with the prime conduftor; thennbsp;pour a fmall ftream of w'ater into the bafon,nbsp;which in the dark will have a beautiful appear--R 3nbsp;nbsp;nbsp;nbsp;ance.

AN ESSAY

Dip a fponge in water, and then fufpend it from the conduclor ; the water, which beforenbsp;only dropped from it, will now fall fail, andnbsp;appear in the dark like fiery rain

Hold a pail, which is furnilhed with feveral capillary tubes placed in various diredlions, nearnbsp;an eleftrified conduSpr, and the water willnbsp;dream out of thpfe jets near the condudtor,nbsp;while it will only drop at intervals from thqfenbsp;which are oppofite to it,

The knob of a charged jar will attract a drop of water from a faucer, amp;c. This drop, thenbsp;moment the bottle is removed from the faucer^nbsp;aflumes a conical firape, and if it is broughtnbsp;near any conducing fubftance, it is driven for^nbsp;jcibly away in fmall ftreams, w'hich are luminous in the dark,

It appears by this experiment, that the cleflrie fire not only tends to feparate the particles Of water, and to, diffipate them into vapour as common fire, but that it effedls thisnbsp;with uncommon rapidity.

Difcharge a battery through a drop of water, previoufly placed, on the knob of one of itsnbsp;bottles, the whole will be inftantly explodednbsp;into vapour; the fparks will be much longernbsp;than common, and more compaft,

Beccaria obferves, that by fending a difcharge to a greater or lefs diftance, through one or more drops of quickfilver, the difchargenbsp;diffufes itfelf into drops, and drives them intonbsp;vapour; part of it rifing into, the air in thenbsp;form of fmo,ke, the other part remains on thenbsp;glafs,,

A drop of water, hanging from the con-, denfing ball pf an eleftrified conduflor, wdllnbsp;ftretch to.wards, water placed in a cup under it,nbsp;lengthening and lliortening itielf according tonbsp;the force of the eledricity.

AN ESSAY

Place a drop of water on the prime conduct tor, turn the machine, and long zig-zag fpark?nbsp;may be taken from it; the drop will take anbsp;conical figure; the body that receives the fparknbsp;will be wetted, and the fparks will be confider-ably longer than can be obtained from the con-dudtor without the water.*

Stick a piece of fealing-w'ax on the conduftor, in fuch manner that it may be ealily fet on firenbsp;by a taper ; w'hile it is flaming turn the cylinder, the wax will become pointed, and fliootnbsp;out an almoft invilible thread into the air, tonbsp;the length of a yard and more. If the filaments that are thrown out by the wax are received on a fheet of paper, the paper will benbsp;covered by them in a very curious manner, andnbsp;the particles of the wax will be fo far fub-divided as to refemble fine cotton. To fafl:eqnbsp;tEe piece of wax conveniently to the conduflor,nbsp;flick it firfl: on a fmall piece of paper, thennbsp;twift the end of the paper fo as to fit one of thenbsp;holes which are made in the prime conduftor;

ON ELECTRICITY- 249

Infulate a fountain, made by condenfed air, and which emits only one ftream; ele�trify thenbsp;fountain, and the ftream will be feparated intonbsp;a great number; thefe will diffufe themfelvesnbsp;equally over a large fpace of ground. By laying a finger upon the condudlor, and taking itnbsp;off again, the operator may command eithernbsp;the Angle ftream or the divided one, at plea-fure.

Eleftrify two fmall infulated fountains with the different eleftric powers; the ftreams ofnbsp;both will be difperfed into very minute partl-cleSj which will run together at the top, andnbsp;cpme down in heavy drops, like a fliower ofnbsp;jfain.

AN ESSAY

Of the FjleBric Light in Vacuamp;.

TA K E a tall dry, receiver, and infert la the top, with cement, a wire with anbsp;rounded end, then exhauft the receiver, andnbsp;prefent the knob of the wire to the conductor,nbsp;aad every fpark will pafs through the vacuumnbsp;in a broad ftream of light, vifible the wholenbsp;length of the receiver. The ftream often divides itfelf into a variety of beautiful rivulets,nbsp;W'hich are continually dividing and uniting innbsp;a moft pleafing manner. If the veffel is grafpednbsp;by the hand, at every fpark a pulfatiop is felt,nbsp;like that of an artery, and the fre bends itfelfnbsp;tow'ards the hand. This pulfation is even feltnbsp;at feme diftance from the receiver, and in thenbsp;dark, a light is feen between the hands and thenbsp;glafs.

From fome experiments made feveral years fince by Mr. Wilfon, with an excellent air-pump of Mr, Smeaton, he obferved, that verynbsp;fmall differences of air occafioned very materialnbsp;differences in the luminous effedts produced bynbsp;the eledric fluid ; for when all the air was taken

ON ELECTRICITY,

put of the receiver, which this pump at that time was capable of extracting, no eleClricnbsp;light was vifible in the dark. Upon letting innbsp;a little air by a flop-cock, a faint cleClric lightnbsp;was vifible, and by letting in a little more airnbsp;jncreafed the light, which again decreafed onnbsp;letting in more air ; till at laft, on admittingnbsp;great quantities, it intirely vanifhed. By thisnbsp;experiment it appeared, that a certain limitednbsp;quantity of air was neceffary to occasion thenbsp;greateft luminous effeCl.

Fig. 82, reprefents an exhaulled receiver, /landing on the plate of an air pump, ab annbsp;electrified wire difcharging a flream b c oi thenbsp;.eleCtric 'fluid on the plate of the air pump. Ifnbsp;the flratum of air on the outfide of the receivernbsp;Be lefTened by the application of the finger tonbsp;the receiver, and by this means an opportunitynbsp;be given to the fluid on the outfide to efcapc,nbsp;the fluid within will be impelled to that part,nbsp;as at defy It has been inferred from thisnbsp;^experiment, that no repulfive power exifls be-,nbsp;tween the particles of the eleClric fluid; be-caufe, if it _yvas in itfelf really elaftic, or endowed with a repulfive power of its own, it isnbsp;not probable it could pafs in an uninterrupted

AN ESSAY

ftream, as' at^ b c, when the refiftance was taken off; it would then Ipread wider, and difplay its elaftic power.

It is more confiftent, fays Dr. Watfon, to fuppofe, that the repulfion of thefe particlesnbsp;which is feen in the open air, is occafioned bynbsp;the refiftance of the air, and not to any naturalnbsp;tendency of the eledtricity itfelf.

The following experiment of Beccaria con-Te)^s a clear idea of the refiftance the air makes to the paflage of the elelt;5tric fluid, and of thenbsp;diminution of that refiftance in an exhauftednbsp;receiver.

Before the air was exhaufted from the receiver, if the wire at the top of it was electrified, a diverging brulh proceeded from it, about an inch long. On exhaufting the receiver the.nbsp;following changes took place : firft, the raysnbsp;of the brufli became longer ; feeondly, thenbsp;rays diverged lefs, were fewer in number, andnbsp;the fize of the remaining rays was increafcd ;nbsp;thirdly, they all united at laft, and formed anbsp;continued column of light, from the wire tonbsp;the plate of the air pump.

From this experiment it is clear, that the air is the agent by which, with the afliftance or

other eledrics, we are able to communicate eledricity bn eleftrics, as well as non-eledrics ;nbsp;for when this is removed, the fluid pervadesnbsp;the vacuum, and. flies off to a confiderablcnbsp;diftance.i

To diftinguilh with great accuracy the changes in the form and length of the electric fpark when it is paffing through a receiver, thenbsp;air of which is more or lefs rarefied ; fix a ballnbsp;to the rod, let another proceed from the platenbsp;of the air pump; the balls are to be placednbsp;about one inch from each other. When thenbsp;vacuum is good, a Angle uniform ray, of anbsp;purple colour, paffes from one ball to the other ;nbsp;but in proportion as the air is admitted^ thenbsp;ray acquires a quivering motion, which indicates that a refiftance to its motion then begins,nbsp;and this interruption is followed by a divifion ofnbsp;the ray or ftream ; the ray now acquires a morenbsp;vivid light; and, laftly, it becomes the common;nbsp;fpark, which is emitted with greater or lefsnbsp;facility, in proportion to the power of the machine, and the refiftance of the air.

AN ESS A Y

i^refentatbinexhauftedflafk, fimilarto that re-prefented at E, fig. 49, but without any coating on the outfide, to the conduftor, and the bottlenbsp;will be luminous from end to end,, and whennbsp;taken from the conductor, will continue lumi-movine in various curvilinear directions

for a confiderable time, flalhing at intervals in a manner which very much refembles the Auroranbsp;Borealis. The light may be revived by pallingnbsp;the flalk through the hand. The ftrOke of thenbsp;fluid againft the glafs is very fenfibly heard andnbsp;felt in this experiment

The flexuous motions of the eledtric fluid in an exhaufted receiver may, in fome degree,nbsp;be produced at pleafure. By wetting the outfide of the receiver, the fire will follow the direction of the wetted line, as the refiftance isnbsp;now lefiened on one fide; and the fire can adhere and accumulate itfelf On the infide of thenbsp;receiver, becaufc, by means of the dampnefs,nbsp;it can expel a portion from the outfide.

This experiment may be exhibited very pleaf-ingly,� by making a toricellian vacuum in a glafs tube about three feet long, and then fealednbsp;hermetically. Hold one end of this tube innbsp;{he hand, and apply the other to the conduClory

and

ON ELECTRICITY. 255

and immediately the whole tube will be illuminated from end to end, and will continue fo fiot a confiderable time after it is removed from thenbsp;tonduftor,. flafhing at intervals for manynbsp;hours.

Experiment CXCVIII.

Another beautiful appearance may be produced in the dark, by inferting a fmall Leyden phial into the neck of a tall receiver, fo thatnbsp;the outward coating may be expofed to thenbsp;vacuum. Exhauli the receiver, and then chargenbsp;the phial, and at every fpark which paffes fromnbsp;the conduflor to the inlide, a flafli of light isnbsp;feen to dart from every part �f the externalnbsp;f�rface of the . jar, fo as to fill the rcceivcT.nbsp;Upon making the difeharge, the light is feeanbsp;to return in a clofc body.

A very perfed: vacuum for the palTagc of the eledric fluid may be made by a double barometer, or long bent tube of glafs filled with mercury and inverted, each leg Handing in a bafoanbsp;of mercury; the bent part of the tube abovenbsp;the mercury forms a compleat vacuum. If a

�l N ESSAY

bottle is difcharged through this fpace, ths light appears uniform through the whole fpaceJnbsp;but is molt vivid when the difcharges are ftrong�nbsp;Dri Watfon irifulated this apparatus, and thennbsp;made one of the bafons of mercury communicate with the condudlor, and touched the othernbsp;with a ncm-eleftfic ; the �l�dtric fluid pervadednbsp;the vacuum in a continued flame, without any divergence : when one of the bafons was connectednbsp;with the infulated cufhion, the fire appeared tonbsp;pervade the vacuum in a diflerent diredion.

Fig. 83, reprefents a glafs tube, fuch as is generally ufed for barometers; on the end h anbsp;fleel cap is cemented, from which a w'ire andnbsp;ball_f d proceed into the tube* Fill this tubenbsp;with quickfilver, and then, by fending up anbsp;large bubble of air, and repeatedly inverting thenbsp;tube, free the quickfih'Cr and iron ball fromnbsp;air, according to the ordinary mode of fillingnbsp;barometers ; then place a fmall drop of sethernbsp;on the quickfilver, and put the finger on thenbsp;end of the glafs tube, invert the tube, and thennbsp;infert the end/ in abafon of quickfilver, takingnbsp;care not to remove the finger from the end ofnbsp;the tube, till the end is immerged hdf arr tnc\i

tncler the filver. When the finger is removed, the quickfilver will defcend, and the ethernbsp;will expand itfelf, leffen the vacuum, and de-prefs the mercury in the tube; now prefentnbsp;the metallic top of the tube to a large chargednbsp;conduflor, and a beautiful green fpark will pafsnbsp;from the ball to the quickfilver. By admittingnbsp;a fmall quantity of air into the vacuum,; annbsp;appearance fomething fimilar to a falling ftarnbsp;is obtained^ I an indebted for this valuablenbsp;experiment to Mr. Morgan, of the Equitablenbsp;Affu ranee Office.

See alfo Ex. iro, iii, T19, 120, of this ElTay, for further obfervatloiis on the appearance of the eledric light in vacuo.

CHAP. XV.

Of Medical 'EleBricity.

The Abb� Nollet fa^'Sj that he received more pleafure when he dlfcovered that thenbsp;motion of fluids in capillary tubes, and thenbsp;inlenfible tranfpiration of animated bodies werenbsp;augmented by eledlricity, than by any othernbsp;difeovery he had made; bccaufe they feemed tonbsp;promlfe fuch abundant advantages to mankind,nbsp;tvhen properly applied by a Ikilful hand. Butnbsp;how much would this pleafure have been augmented, if he had lived, to fee his hopes realized, and this branch of eledtricity obtain thenbsp;feme medical certainty as the bark in inter-mittents!

It is true, that like every other Ample medicine which has proved beneficial to mankind. Electricity met with much oppofitlon from thenbsp;interefted views of foine, and the ignorancenbsp;of others; has been treated with contempt, andnbsp;injured by mifplaccd caution. I fliall recommend to thofe who thus oppofe it, not ' tonbsp;condemn a fubjedt of which they are ignorant,nbsp;but to hear the caufe before they pafs fentence;nbsp;C0 take fomc pains to underftand the nature of

ON ELECTRICITY.

eledtricity; to learn to make the eledrical machine ad welt, and then apply it for a few weeks to fome of thofe diforders in which it hasnbsp;been adminiftered with the greatefl fuccefs;nbsp;and there is no doubt but they would foon benbsp;convinced that it deferves a difiinguiflied ranknbsp;in medicine, which is the offspring of phl-lofophy.

The fciencc of medicine and its praditioners have been reproached with the inflability andnbsp;fluduations of pradice; at one time cold as thenbsp;ice at Zembla, at another hot as the Torridnbsp;Zone ; that they are led by fafhion, and influenced by prejudice. On this ground it hasnbsp;been predided, that however great the benefitsnbsp;which may be derived from eledricity, itnbsp;would ftill only laid for the day of fafltion, andnbsp;then be conligned to oblivion. I muft confefs,nbsp;that I cannot be of this opinion, nor eafily lednbsp;to think a fet of men whofe judgment has beennbsp;matured by learning and experience, will evernbsp;negle�t an agent, which probably forms the mofl:nbsp;irriportant part of our conftitution. Eledricitynbsp;is an adive principle, which is neither generatednbsp;nor deftroyed; which is every where, and always prefent, though latent and unobferved ;nbsp;and is in motion by night and day, to maintainnbsp;an equilibrium that is continually varying. Tonbsp;give one inftance, among many, it has beennbsp;S 2nbsp;nbsp;nbsp;nbsp;fliewn.

AN ESSAY

fhewn, that the rain which defcends in a ftotm is ftrongly impregnated with electricity, andnbsp;thus brings down what the heated vapours carried up into the air, till the deficiency of thenbsp;earth is fupplied from the fuperfluity of thenbsp;heavens. A variety of other caufes concur tonbsp;vary continually the equilibrium of this fluid;nbsp;as the perpetual inteftine and ofcillatory motion,nbsp;which contributes fo much towards carrying onnbsp;the operations of nature, further, if a particular portion of this fluid is diftributed to everynbsp;fubftance, then every alteration of its capacity,nbsp;which is continually changing by heat or cold,nbsp;muft move and operate on it.

As heat, or fire in a�tion is the firfl: mover in the animal machine, and the chief adlivenbsp;principle during its exiftence, and as eledlricitynbsp;exhibits fo many pheenomena, which cannotnbsp;be diftingulfhed from thofe of fire, we are naturally led to conceive high ideas of the importance of this fluid to medicine. Thoughnbsp;the vital flate of it is not to be eflimatednbsp;by the degree of heat, abftradledly confider-ed, becaufe the degree of heat only afeer-tains the quantity which is adbing in a peculiarnbsp;manner.

It is known, that this vivifying principle hallens the vegetation of plants. Myrtle-trees,nbsp;which were eleiflrificd, budded fooner than

others of the fame kind and lize, and in the fame green-houfe. Seeds, daily eleftrified,nbsp;have fhot up, and grown more in three or fournbsp;ilays, than others of the fame kind, and alikenbsp;in all other circumftances, have done in elevennbsp;or twelve days. In the fame manner Mr.nbsp;Achard has fliewn, that it may be ufed as anbsp;fupplement for heat, to hatch the chicken fromnbsp;the egg. The fuppofition of an ingeniousnbsp;writer is by no means improbable, that the vegetating power w'hich is operating during thenbsp;whole year in ever-greens, may arife from thefcnbsp;trees having more refin in their compofitionnbsp;than thofe whole leaves fall in autumn, bynbsp;which they are enabled to attrafl and retainnbsp;thole juices wdiich give them their continualnbsp;verdure, and fupply, in fome degree, the ab-fence of folar heat. This may be inferrednbsp;from their natural properties, and is confirmednbsp;by the ftrong elcdlric power pofl'efled by theirnbsp;leaves. The fame writer thinks, that the fluidnbsp;collefled in our eledlrical experiments is onlynbsp;thofe foiar rays that have been difperfed in, andnbsp;are arrefted by the earth ; an idea which isnbsp;ftrongly corroborated by the obfervations madenbsp;on atmofpherical eleftricity, and by the de-d'uftions which have been made from the relative affinities of fire, light, and heat.

The agency of this fluid, and its exiflence in animated nature, has been fully proved bynbsp;the experiments that have been rhade on thenbsp;Torpedo and the gymnotus ele(ftricus ; for thenbsp;flmilitude eftabliihed between the electrical fluidnbsp;of the Torpedo and that of nature at large, isnbsp;fuch, that, in a phyflcal fenfe, they may benbsp;confidered as precifely the fame, Mr. Hunternbsp;has well obferved, fays Sir J. Pringle, and Inbsp;think he is the firft who has made the obferva-tion, that the magnitude and number of thenbsp;nerves bellowed on thofe eleCtric organs, in proportion to their fize, mull appear as extraordinary as their effedts ; and that, if we exceptnbsp;the important organs of our fenfes, there is nonbsp;part, even of the moll perfeCl animal, which, fornbsp;its fize, is more liberally fupplied with nerves thannbsp;the Torpedo; nor yet do thefe nerves of the electric organs feem neceflary for any fenfation thatnbsp;can belong to them : and with ref^reCt to aCtion,nbsp;Mr. Hunter obferves, that there is no part ofnbsp;any animal, however ftrong and conftant itsnbsp;action may be, which enjoys fo large a portionnbsp;of them. If then it be probable, that thefenbsp;nerves are unneceflary for the purpofe either ofnbsp;fenfation or aCtion, may we not conclude, thatnbsp;they are fubfervient to the formation, collection,nbsp;and management of the eleCtric fluid ? efpe-cialljj as it appears from Mr. Wallh�s experiments.

ON ELECTRICITY. 263

ments, that the will of the animal commands the elcftric powers df its organs. If thefe reflexions are juft, we may with fome probability foretell, that no difcdvery of confequenccnbsp;will ever be made by future phyfiologifts concerning the nature of the nervous fluid, without acknowledging the lights they have borrowed from the experiments of Mr, Walfhnbsp;upon the living Torpedo, and the difleXion ofnbsp;the dead animal by Mr. Hunter, *

A variety of curious faXs clearly evince, that the cleXric fire is eflentially conneXednbsp;with the human frame, and is continually ex-rnbsp;erring its influence upon it, Mr. Brydone mentions 3 lady, who, on combing her hair innbsp;frofty weather in the dark, had fometimes ob-ferved fparks of fire to. ifTue from it; this madenbsp;him think of attempting to colleX the eleXricalnbsp;fire from hair alone, without the afliftance ofnbsp;any other eleXrical apparatus, To this end, henbsp;defired a young lady to ftand on wax, andnbsp;comb her fifter�s hair, who was fitting in a chairnbsp;before her ; foqn after fhc had begun to comb,nbsp;the young lady on the wax was furprized tonbsp;find her whole body eleXrified, and darting outnbsp;fparks of fire againft every objeX that approached her. The hair was ftrongly eleXrical,nbsp;and affeXed an eleXrometer at a confiderablenbsp;diftance. He charged a metallic conduflor fromnbsp;S 4nbsp;nbsp;nbsp;nbsp;k,

AN ESSAY

it, and in the fpace of a few minutes coll�d|.e4 a fuiEcient quantity of fire, to kindle commoi)nbsp;fpirits, and by means of a fmall jar, gave manynbsp;fmart ftrokes to all the company.

Mr. Cavallo obtained, by means of a fmal} condenfing plate, very fenfible figns of ele�tri-city from various parts of his own body, andnbsp;the head of almofl; any other perfon.

When the difoveries in this fcience, fays Mr. Brydone, are further advanced, we may find,nbsp;tltat what we call fenfibility of nerves, and many .other difeafes, which are Jcnown only bynbsp;name, are owing to the bodies being poffelTednbsp;of too large or too fmall a quantity of this fubtlenbsp;fluid, which is perhaps the vehicle of all ournbsp;feelings- It is knowm, that in damp and ba?ynbsp;weather, when this fire is blunted and abforbednbsp;by the humidity, its aflivity is leflened, andnbsp;what is colledled is foon diffipated; then ournbsp;fpirits are more languid, and our fenfibility isnbsp;lefs acute. And in the fierce wind at Naples,nbsp;when the air feerns totally deprived qf it, thenbsp;whole fyfiem is unflrung, and the nerves feemnbsp;to lofe both thcjr tenfion and elallicity, till thenbsp;north-weft wind awakens the aftivity of the animating power, which foon reftores the tone,nbsp;and enlivens all nature, which feemed to droopnbsp;and languifh in its abfence : nor can this aprnbsp;pear furprizing, if it is from the different ftate

ON ELECTRICITY. 2%

pf this fire in the Iiuman body, that the ftriftun* |ind laxum proceeds, and not from any alteration in the fibres themfelves, or their beinjgnbsp;more or lefs braced up, (among which bracersnbsp;cold has been reckoned one) though the muf-cular parts of an animal are more braced whennbsp;they are hot, and relaxed when they are cold.

Mr. Jalabert and Profeflbr Sauffure, when pacing the Alps, were caught amongft thunder-clouds, and found their bodies full ofnbsp;eleftr^cal fire; fpontaneous flafhes darting fromnbsp;tjaeir fipgers, with a crackling noife, and thenbsp;fenfations they felt were the fame as whennbsp;jftrongly eleftrified by art. It feems pretty evident, that thofe feelings were owing to their bodies containing too great a fhare of electricalnbsp;fire; and it is not improbable, that many ofnbsp;pur invalids owe their feelings to the pppofirpnbsp;caufe.

Pafs the charge of a large jar, or battery^ ,ftom the head to the back of a moufe ; this,nbsp;if the Ihock is fufficiently ftrong, will kill thenbsp;animal. After its death, make the difchargenbsp;in the fame manner, and the fluid will pafs vi�nbsp;flbly over the body, and not through it t. evincing.

cingf, that the power or medlam which tranP mitted the fliock through the animal, is loftnbsp;with its life. This experiment is taken fromnbsp;Mr. Cavallo'^s treatife on medical eieftricity*nbsp;Its importance is felf-evident, and it certainlynbsp;merits a iurther invelligation, by thofe who arenbsp;acquainted with the animal cEconomy, as wellnbsp;as ekdlricity.

The following experiment fliews, that the ��elt;ilrIc fluid paflTcs through that feries of muf-cles which form the Ihorteft pafiage for it, andnbsp;whofe conducing power, or eledlric capacity,nbsp;is. mofi; favourable to it. ,

Let A grafigt; a Leyden phial with his right hand, and touch, with a brafs rod held in hisnbsp;left hand, the naked right foot of B; let thenbsp;left foot of B communicate by a brafs rod wnthnbsp;the right foot of C ; let D with his right handnbsp;hold the left ear of C, and touch the knob of thenbsp;bottle with his left hand ; A will feel the fliocknbsp;in the mufcles of the right hand and arm, ofnbsp;the thorax, and of the left hand and arm ; Rnbsp;will feel the commotion in the mufcles of hisnbsp;right foot, right leg and thigh, and thofenbsp;which are connedted with the left thigh, leg,nbsp;and foot, while C will perceive it in that feries

wliich goes from the leg to the ear by whicb he communicates with D. The aftion of thenbsp;fluid on the human body in the fhock, is thenbsp;fame when it paffes through fimilar parts withnbsp;the fame denfity. Its aftion is more extenfiv�nbsp;when the fire is denfeft, and therefore moft in-*nbsp;tenfe when it meets with any rcfiftance.

Affifted by a furgeon, Beccaria made feveral experiments upon the effedls produced by electricity on the mufcles in the left leg of a cock.nbsp;The mufcles rvere ftrongly contradled when anbsp;fhock was paffed through them, and the con-tradion w^as always accompanied by a fuddennbsp;and proportional fwelling of the mufcles, excepting at the part where the membrane is inferred, which feparates one mufcle from another, w'hich was always depreffed. The membrane w'hich invefted that part of the mufclenbsp;dirough which the fluid paffed, became drynbsp;and wrinkled, and a vapour arofe from thatnbsp;part ; when one mufcle was contracted, a general contradlion took place in thofc that werenbsp;contiguous to it, and they were a little con-vulfed after the fliock.

In another inftance, where the mufcle was relaxed and parted from the thigh, on pallingnbsp;the Ihock through it the mufcle contracted it-fclf, and wa^,drawn back into its natural place,nbsp;and could not be again difplaced but by force;

AN ESSAY

a circumflance which ftrongly manifefts the power of eledtricity to give tone to a flaccidnbsp;fibre. Indeed, when we confider, fays a verynbsp;fenfible writer, that the mufeles have beennbsp;brought into adlion by the eleftric Are, that itnbsp;has rendered palfied limbs plump, and reflorednbsp;a power of adtion and motion to many, whofcnbsp;palfies did not arife from the ^fpinal marrow.nbsp;Is it not a convincing proof, that the vital firenbsp;is the caufe of mufcular motion, and that thisnbsp;is the fame with that which is colledted by thenbsp;eledlrical machine ?

As the fcience of medicine knows of no f]iecific, fo we are not to fuppofe, that eledlri-city will triumph over every diforder to whichnbsp;it is applied. Its fuccefs will be more or lefsnbsp;extenfive, according to the difpofition of thenbsp;fubjedf, and the talents of thofe who diredl it jnbsp;it cannot therefore appear furprifing, that manynbsp;diforders have been refraflory to its powers,nbsp;and others have only yielded in a fmall degree;nbsp;or, that the progrefs of the cure has often beennbsp;flopped by the impatience, or prejudice, of thenbsp;difeafed : but, at the fame time, it muft be acknowledged, that even in its infancy, whennbsp;it had to combat againft fear, prejudice, andnbsp;intereft, its fuccefs was truly great: we havenbsp;furely then the higheft reafon to expeft a con-fiderable increalc of fuccefs, now that it is cultivated

ON electricity.

This experiment Ihews, that the elcftrlc powers may be put in adtion by heat and cokbnbsp;It was originally made by Mr. Canton. Henbsp;procured fome thin glafs balls, of about annbsp;inch and a half diameter, with ftems or tubes,nbsp;of eight or nine inches in length, and electrified them, fome pofitively on the irifide,nbsp;others negatively, and then fealed them hermetically ; foon after li� applied the naked balls tbnbsp;his eieftrometer, and could not obferve thenbsp;leaft fign of their being eledlrical; but holdingnbsp;them at the fircj at the diltance of five or fixnbsp;inches, they bedame ftrdngly eleftrical in 3nbsp;a fhort time, and more fo when they were cooling. Thefe balls would, every tim.e they werenbsp;heated, give the eleClric power to, or take itnbsp;from, other bodies, according to the plus ornbsp;minus ftate of it within them. Eleating themnbsp;frequently diminiflied their power, but keepingnbsp;one of them under water a week did not in thenbsp;leaft impair it. The balls retained their virtuenbsp;above fix years. The tourmalin, and rttariynbsp;other precious ftones, are alfo known to acquirenbsp;eleflricity by heat. The tourmalin has always,

AN essay

at the fame time, a politive and negative electricity ; one fide of it being in One ftate, the other in the oppofite. Thefe powers may benbsp;excited by friftion and by heat; nay, even bynbsp;plunging it in boiling water.

Infulate a fenfible mercurial thermometer, and place the bulb between two balls of wood,nbsp;one affixed to the condudtor, the other communicating with the ground, and the eledtricnbsp;fluid, in paffing between the two balls, will raifenbsp;the mercury in the thermometer confiderably.nbsp;With a cylinder, of about feven inches and anbsp;half in diameter, the fluid paffing from a ballnbsp;of lignum vitie to a ball of beech, and thencenbsp;to the ground, elevated the quickfilver in thenbsp;thermometer from 68� to 110�, repeatedly tonbsp;105. The thermometer was raifed from 68�nbsp;to 85�, by the fluid paffing from a point of boxnbsp;to a point of lignum vitte ; from 67� to ico�,nbsp;from a point of box to a ball of box ; ftomnbsp;66� to ico�, from a ball of box to a brafsnbsp;point; from 69� to 100�, from ball to ball;nbsp;the bulb of the thermometer covered vvithnbsp;flannel.

A of dlfeafes, in which it has been fuc-cefsful, is given by fomc vvriters, but I refrain from following their example, becaule I under-Hand there is a rational fyftem formed on thenbsp;experiments of the laft four years ; to comprehend which, a knowledge of difeafes, theirnbsp;caufes and fymptoms, is requifite.

In this fyflem, it is ranked as an anti-fpafmo-dic, is confidered as the moft powerful external application to dlfeafes, and, from the various'nbsp;manners in which it is ufed, ferves the purpofcsnbsp;of a fedative, a ftimulant, and a deobilruent.nbsp;In medicine, it becomes then applicable tonbsp;palfics, rheumatifms, intermktents; to fpafm,nbsp;obftruflion, and inflammation. In furo-ery itnbsp;has confiderable fcope for aftion ; where con-tradiions and fpralns, tumors, particularly ofnbsp;the glandular fort, w^afting of the mufeies,nbsp;and other incidents, form a catalogue of vifiblenbsp;difeafes, as diftreffing to the fight of others asnbsp;to the patiertts themfelves. The gout, and thenbsp;fcrophula, or king�s evil, two difeafes whichnbsp;have tormented mankind, and been the difgracenbsp;of medicine to the prefent time, are rankednbsp;among thofe to w^hich this remedy is applicable ; and in the commencement of the complaints, I am informed, has been wonderfullynbsp;fuccelSituL To remove ill-placed fits of thenbsp;gout, it flmuld feem to be a more rational application

s s Ar

plication tha� inj medicine, for it applies dP reftly to the feat of the dif�afe, with� a powernbsp;and rapidity unlendwn in phyfic, and perfcdll/nbsp;manageable at difcretion i and, as it is a remedy which applies t� the underftahding as wellnbsp;as to the feelings, I Ihould think it better worthnbsp;the attention and contemplation of men of liberal education, than the compoundifig a medi-in which they place, little faith, ornbsp;applying a piaiHer, in. which they have nonenbsp;at alii

The fuccefs of ele�lrieity, ifi relieving the fnlFerings of mankind, has been Conliderablynbsp;promoted, and its operations rendered inorenbsp;rapid, fenfibie, and efficacioits, by applying itnbsp;in different manners and quantities to the human frame. The modes formerljt tifed werenbsp;the fhock, the fpark, and fbmetimes, thoughnbsp;very feldom, fimple electrification. Thefenbsp;modes are now varied, and their number augmented.' The ftream of the eleClrlc fluid may,nbsp;without a Ihock, be made to pafs through anynbsp;part of the body j it maty alfo be thrown upon,nbsp;or extracted from any part, and its aCtion innbsp;each cafe varied, by Caufing the fluid to pafs?nbsp;through materials which refill its paflage in different degrees; it may be applied to the nakednbsp;Integuments, or to the ffcin covered with different refifting fubitances; and its power may

fee rarefied or condeiifed, confined to one fpot, or applied in a more difFufive nnanner, at thenbsp;difcretion of the Q|rcra�or.nbsp;nbsp;nbsp;nbsp;i

The apparatus neceffary for this purpofe is fimple, and confifts of the following articles*

1. nbsp;nbsp;nbsp;An eleftrical jnachine, with an infulatecinbsp;cufhion, properly conftruded to afford a continued and ftrong ftream of the eleftricalnbsp;fluid.

2. nbsp;nbsp;nbsp;A ftool with infulating feet, �r rather annbsp;firm chair fixed on a large infulating fiool*nbsp;The infide part of the back of the chair Ihouldnbsp;move on a hinge, that it may oecafionally letnbsp;down to ele�lrify conveniently the back of thenbsp;patient : the arms of the chair ihould alfo be'nbsp;made longer than is uflial*

5. nbsp;nbsp;nbsp;A few glafs tubes of different bores^ fomenbsp;of them with capillary points.

To thefe may be added, an univerfal dif-eharger on a large fcale, a pair of fmall directors with filver wires, and a pair of infulating forceps.

Fig. 93 r�prefents the directors, the handles are of glafs. A is a brafs wire with a ball opnbsp;its end. The wire of one is bent, for the movenbsp;�onveniently throwing the eledric fluid on the

AN ESSAY

eye, See. The balls may be iinfcrewed from the wires, and the wooden point B ferewed innbsp;its place, or the pointed end of the brafs wirenbsp;may be ufed. The diredors fhould always benbsp;held by that extremity of the glafs handlenbsp;which is fartheft from the brafs, and carenbsp;fhould be taken that the heat of the hand doesnbsp;not make them molft.

Fig. 85 is the medical bottle, furnilhed with an eledrometer, to limit the force of thenbsp;fhock, and enable the o^rerator to give a fuc-ceffive number of them of the fame force. Cnbsp;is a bent piece of glafs, on the upper part ofnbsp;W'hich is cemented a brafs focket D, furnifliednbsp;wdth a fpring tube E; the wire F moves innbsp;this tube, lb that the ball G may be fet at anbsp;convenient diftance from the ball H. Thenbsp;end I of the bent piece of glafs is alfo furnifliednbsp;with a fpring tube, which Hides upon the wirenbsp;K, communicating with the infide of thenbsp;bottle.

To ufe this bottle, place the ball H in contad with the condudor, or conned them toeethernbsp;by a wire, and then charge it in the ufual manner. Now, if a wire proceeds from the ballnbsp;L to the outlide coating, the bottle will be dif-charged whenever the fluid has acquired fuffici-�nt force to pafs through the fpace of air between the two balls; confequently the fliock

It is obvious, that when the eledfometer is thus connedted, it adls in the fame manner as anbsp;common difcharging rodj and forms the communication from the outfide , to the infide ofnbsp;the bottle ; with this difference onlv, that thenbsp;diftance of the end which is to communicatenbsp;with the infide may be limited and regulated.nbsp;The fhock may be given to any part of the human body by introducing that part of the bodynbsp;into the circuit which is made between the outfidenbsp;and infide of the bottle. This is convenientlynbsp;effedled, by connedling oile diredtor by a piecenbsp;of wire with the eledlrometer, and the other tonbsp;the outfide of the bottle j then hold the directors by their glafs handles, and apply the ballsnbsp;of them to the extremity of the parts throughnbsp;W'hich the fhocks are to be paffed.

The force of the fhock, as we have already obfefved, is augmented or diminifhed by in.nbsp;creafing or leffening the diftance between thenbsp;two balls, which muft be regulated by the operator to the ftrength and fenfibility of the patient.

The handles of the dirediors fhould be carefully dried, as alfo the bent piece of glafs C, and thofe parts of the bottle which are abovenbsp;the coating. It is iikewife neceflary to prefsnbsp;T 2nbsp;nbsp;nbsp;nbsp;the

AN ESSAY

the ends of the dire�tors agalnil the party to convey the flrock more readily.

Some gentlemen have thought the eleftric forceps a very convenient inftrtiment to conduitnbsp;the fhock through any particular part of thenbsp;body. Their ufe is evident from an infpeftionnbsp;of fig. �8 6.

The following mode of extrailing the con-denfed fluid from the infide of a charged Leyden jar has been found, in certain circumftances, peculiarly advantageous. Connedl a direitor,nbsp;by nteans of a wire, with the ball of a Leydeanbsp;jar, charge the jar, cither compleatly or partially, and then apply the ball or point of thenbsp;conduilor to the part intended to be eleitrified,nbsp;and the fluid, which was condenfed in the phial,nbsp;will be thrown on the part in a denfe flownbsp;flream, attended with a pungent fenfation,nbsp;which produces a confiderable degree ofnbsp;warmth. If a wire, that communicates withnbsp;the ground, is placed oppofite to the end ofnbsp;the direitor, the paflage of the fluid will benbsp;rendered more rapid, and the fenfation ftronger.nbsp;It is obvious, that in this cafe the circuit between the infide and the outfide of the jar ianbsp;not compleated, therefore the fliock will not benbsp;felt. The condenfed fluid paflTes in a denfenbsp;flow flream through the required part, whilenbsp;the outfide acquires a fulfleient quantity, from

the

the conducing fubftances near it, to reftore the equilibrium.nbsp;nbsp;nbsp;nbsp;'

To pafs a ftream of the eleftric fluid through any part of the human body, conned: one di-redtor by a wire with the pofitive conduclor,nbsp;and another diredor with the negative conductor, or infulated cufliion, then place the end ofnbsp;the diredlors at the extremities of the part, andnbsp;turn the cylinder, the fluid will pafs throughnbsp;the part from one direftor to the other.

To throw the fluid on any part of the body, conned the diredor with the pofitive conductor, turn the cylinder, and then prefent thenbsp;brafs end of the diredor towards the patient,nbsp;and the fluid will pafs between the ball and thenbsp;patient. Or you may infulate the patient, andnbsp;then draw the communicated ekdricity fromnbsp;him by the diredors. In this cafe, a ware fhouldnbsp;pafs from the brafs part of the diredor to thenbsp;ground, or to the hand of the operator.^ Innbsp;either of thefe cafes, the quantity of the fluidnbsp;and its mode of adion may be varied, bynbsp;making the fluid pafs through points or ballsnbsp;of metal or of wood, or by covering thenbsp;fkin w'ith flannel ; whenever the flefli-bruflinbsp;is advifcd, it is highly probable that coveringnbsp;the affeded part w'ith flannel, and then rubbingnbsp;it with the ball of a diredor, conneded withnbsp;the machine, would have a fuperior effed.

AN ESSAY

The refinance to the fluid�s motion may be va^ ried by increafing the thicknefs of the cover*nbsp;ing, or the nature of the fubftance throughnbsp;which it is to pafs.

Some peculiar efFeds have taken place from the application of the interrupted fpark; that is,nbsp;a fpark received from a fecond condudor,nbsp;placed within the ftriking diftance of the primenbsp;condudor. It is not improbable, that in thisnbsp;cafe the condenfation and expanfion of thenbsp;fpark may be more rapid than when it is received from the prime condudor alone. Thenbsp;diredor, when the interrupted fpark is required,nbsp;Ihould be conneded with the fecond condudor,nbsp;and then ufed as in other cafes.

Fig. 87 reprefents an univerfal difehargcr upon a large fcale, with a patient fitting be*nbsp;tween the two pillars, one ball refting at A, thenbsp;other being placed at B. The convenience of thisnbsp;apparatus is obvious, from an infpedion of thenbsp;figure; for as the joints have both an horizon*nbsp;tal and vertical motion, and the wires pafsnbsp;through two fpring fockets, they may thereforenbsp;be placed in any diredion, and the balls fixednbsp;in any required fituation. Hence, by conned-ing one wire with a pofitive condudor, and thenbsp;other with a negative one, or one with the bottom of a Leyden bottle, and the other withnbsp;the elcdrometer ; the flrock or ftream may be

ON ELECTRICITY.

conveyed to any part, with the greatefl facility. It is alfo evident, that a perfon may, by meansnbsp;of the two joints of this fimple apparatus,nbsp;eledrify himfelf with eafe, (or any patient, conveniently) without the affiftance of any othernbsp;perfon ; that is, he may turn the machine withnbsp;one .hand, while he is receiving the fluid, ornbsp;the fliock, by means of this univerfal dif-charger. But this may alfo be readily effected,nbsp;by faftening a wire to one of the condudlors,nbsp;and pinning the other end of it to one extremitynbsp;of the part through which you intend to pafsnbsp;the fliock, or convey the fluid ; then connefl; anbsp;diredlor with the other conductor, and hold itnbsp;to the other extremity pf the part. If the fitu-ation is fuch as to ocqafion the wires to touchnbsp;the table, pafs a fmall glafs tube over them,nbsp;which wdll prevent a diflipation of the fire.

L and M, fig. 84, reprefent glafs tubes, through which fmall wires are made to pafs,nbsp;to convey the fluid diredtty to the ear ornbsp;throat.

Fig. 88 reprefents another glafs tube, of a larger fize, the end of which is capillar} ; anbsp;fmall quantity of rofe-water, or any other fluid,nbsp;is to be poured into this tube, then connect it with the prime condu6lor by a wire;nbsp;turn the cylinder, and a fubdivided, gentle,nbsp;T 4nbsp;nbsp;nbsp;nbsp;and

AN ESSAY

It is in all cafes moft advifeable to begin with the more gentle operations, and proceed gradually to increafe the force, as the ftrength andnbsp;conftitution of the patient, or the nature ofnbsp;the diforder requires. The ftream from a wooden point, a wooden ball, or brafs point, maynbsp;be firft ufed ; fparks, if neceflTary, may then benbsp;taken, or fmall fliocks given.

In rheumatic cafes, the eledric fridion is generally ufed. If the pains are local, fmallnbsp;fhocks may be given. To relieve the tooth-ach,nbsp;very fmall fliocks may be paflTed through thenbsp;tooth ; or, cover the part affeded with flannel,nbsp;and rub it with a diredor, communicating withnbsp;the machine.

In inflammations, and other diforders of the eyes, the fluid fliould be thrown from a wooden point : the fenfation here produced, is thatnbsp;of a gentle cooling wind ; but, at the famenbsp;time, it generates a genial warmth in the partnbsp;affed:ed.

In palfies, the eledric fridion and fmall fliocks are adminiftered. Streams of the fluidnbsp;fliould always be made to pafs through the,nbsp;afFeded part.

The only Treatife we have yet had from the Faculty, on the fubjed of Medical Eledricity,

ON electricity. 28*

is a pamphlet intitled, � Confiderations on the Efficacy of Eledtricity in removing Female Ob^nbsp;ftrudlions,� by Mr. Birch; to whom I am indebted for a variety of important obfervationsnbsp;and pradlical remarks on the different branchesnbsp;of ele�ricity ; and if its merits were to be confined to this difcafe alone, (in which it may benbsp;reckoned a fpecific) it would be entitled to thenbsp;attention of praflitioners ; but we have reafon tonbsp;expedt much more from it, fince the prejudicesnbsp;of the Faculty feem removed, and the pradliecnbsp;1$ becoming more general every day.

C H A p.

CHAP. XVI.

MtJcelJaneous 'Experiments and Ohfervations^

HE difpute coneeming tlie preferable i utility of pointed or knobbed conductors,nbsp;for fecuring buildings from lightening, occafi--cned the fettiiig up a more magnificent appara-fas than had ever appeared before. An immenfenbsp;conductor was conllruCted, .at the expence o�nbsp;the Board of Ordnance, and fufpended in thenbsp;Pantheon, under the direction of Mr.,Wilfon..nbsp;It confifted of a great number of drums, covered with tin-foil, which formed a cylinder ofnbsp;about 155 feet in length, and more than 16.nbsp;inches in diameter ; and to this vafl conductornbsp;was occafionally added 480a yards of wire..nbsp;The eieClric blaft from this machine fired gun--powder in the mofl unfavourable circumftan-ces, namely, when it w'as drawn off by a fharp.nbsp;point. The method of doing it waamp;, as follows : upon a flaiff of baked wood a flem ofnbsp;brafs was fixed, which terminated in an ironnbsp;point at the top; this point was put into thenbsp;end of a fmall tube ot India-paper, made fome-what in the form of a cartridge, about an inchnbsp;and a quarter long, and two tenths of an inch

ON ELECTRICITY. 283

in diameter, when the cartridge was filled with common gunpowder unbruifed; a wire,nbsp;communicating with the earth, was then faf-tened to the bottom of the brafs item. Thenbsp;charge of the great cylinder being continuallynbsp;kept up by the motion of the wheel, the topnbsp;of the cartridge was brought very near thenbsp;drums, fo that it frequently touched the tin-foil with which they were covered. In this fitu-afion, a fmall, faint, luminous ilream wasnbsp;frequently obferved between the top of the cartridge and the metal. Sometimes this ilreamnbsp;would fet fire to the gunpowder the moment itnbsp;was applied, at others, it would require half anbsp;minute or more before it took effeft. This difference in time was fuppofed to be owing tonbsp;fome fmall degree of molilure in the powder,nbsp;or the paper.

Gun-powder may alfo be fired by a ftream from a large charged Leyden jar, in the foi-Jowing manner :

Fix a fmall cartridge on a metallic point, which is fitted to a wooden or glafs handle ;nbsp;make a communication from the wire to thenbsp;ground, then prefent the cartridge to the knob

AN ESSAY

of the phial, and the gun-powder will be fired by the paflage of the eledtric ftream throughnbsp;the cartridge. Tinder, or touch-wood, placednbsp;in a metal cup, may be lighted, by pallingnbsp;the ftream from the inlide of the jar throughnbsp;them, as in the foregoing experiment, withoutnbsp;compleating the circuit.

As it therefore appears, that the eledlric fluid, when it moves through bodies, either with greatnbsp;rapidity, or in great quantities, will fet themnbsp;on fire, it is fcarce difputable, that this fluid iss-the fame with the element of fire.

To fife the fmall ekflrical cannon, charge it. �with gun-powder in the ufual manner, then,nbsp;fill the ivory touch-hole with gun-powder,nbsp;ram it well down, and pulh the brafs pin down,nbsp;fo that the end of it may be neajr the bottom ofnbsp;the hole; make a communication between thenbsp;outfide of a large charged jar or battery andnbsp;the body of the cannon, by placing one end, ofnbsp;the difcharging rod on the pin which paflesnbsp;down the touch-hole, and bring the other endnbsp;to the knob of the jar, and the difcharge will-fire the powder.

Fig. 89 is a perfpetflive vkw of the powder-* iioufe; the fide of the roof next the eye beingnbsp;omitted, that the infide may be more conveniently feen. The front of this model is fittednbsp;tip like the thunder-houfe, and is ufed in thenbsp;fame manner; the fides of the houfe, the back,nbsp;and fore-front, are joined to the bottom bynbsp;hinges; the roof is divided mto two parts,nbsp;which are alfo fattened by hinges to the fides ;nbsp;the building is kept together by a ridge on thenbsp;roof; when the roof is blown up, it will fallnbsp;down w'ith the fides, the back, and fore-front.nbsp;To life this model, fill the fmall tube a withnbsp;gun-powder, and ram the wire c a finall way innbsp;the tube, then connefl the hook e with the bottom of a large jar or battery ; when the jar isnbsp;charged, form a communication from the hooknbsp;d to the top of the jar ; the difeharge will firenbsp;the powder, and the explofion of the gun-powder will throw off the roof, and the fides, thenbsp;fore and back fronts will then all fall down.

Fig. po reprefents a wooden pyramid, de-figned to Ihew the experiments which are made with the thunder-houfe, and is ufed in thenbsp;fame manner. When the piece a is thrown

AN ESSAY

Fix the ladle I, fig, 33^ into the hole at the end of the condu(B;or, place a fmall piece ofnbsp;camphor in the ladle, fet the Camphor On fire,nbsp;and then put the machine in aftion ; thenbsp;camphor will throw out a variety of fmallnbsp;flioots, and have the appearance of an imperfe�fnbsp;vegetation.

ExperiaIent CCIX.

Wrap fome loofe cotton, which has been pre-vioufly rolled in fine powder of yellow refin, round One of the balls of a difeharging rod,nbsp;and hold the other end to the outer coating ofnbsp;a charged jar; then bring the knob with thenbsp;refin towards the ball of the jar, and the ex-plofion will fire the refin, and this will communicate the. flame to the cotton.

Fig. 91 reprefents the inflammable air lamp^ invented by Mr. Volta. A is a glafs globe tonbsp;contain the inflammable air, B a glafs bafon, ornbsp;refervoir, to hold water ; D is a cock, which isnbsp;to form occafionally a communication betweeir

ON Electricity.

the refervoir of water B, a�d that of air A 5 the water palTes into the latter through the metal pipe gg, which is fixed to the upper partnbsp;of the refervoir A ; j is a finall lt;;ock, to cut offjnbsp;or open a communication with, the air in thenbsp;ball, and the jet K. N is a fmall pij^gt;e to holdnbsp;a piece of wax taper, L a brafs pillar, on thenbsp;top of which is a brafs ball ; a is a. pillar ofnbsp;glafs, furniflred at top with a locket; a wirenbsp;Aides in this locket, a ball is fcrewed on to thenbsp;end of the wire. F is a cock, by which thenbsp;ball A is filled with inflammable air, and whichnbsp;afterwards ferves to confine the air and tbsnbsp;water that falls from the baibn B into th�.nbsp;ball A.

To ufe this inftrument, after having fiUed the refervoir A with pure Inflammable air, andnbsp;the bafon B with water, turn the cocks D andnbsp;S, and the water which falls from the baibn Bnbsp;will force out fomc of the inflammable air, andnbsp;caufe it to pafs through the jet K into the air.nbsp;If an eledlric fpark is made to pafs from thenbsp;brafs ball m to the brafs ball k, the inflammablenbsp;jet, which palTes through the pipe K, will benbsp;fired. To extingullli the lamp, fhut firfl; thenbsp;cock S, and then the cock D.

To fill the refervoir Aa with inflammable air, which is to be made in the ufual manner, andnbsp;with the ufual apparatus, having previoufly

AN ESSAY

filled A with water, place the foot R undct water, on a board or ftool in a large tub ofnbsp;water, that the bent glafs tube, through whichnbsp;the inflammable air paflesy may pafs commodi-oufly under the foot of the lamp ; when th�nbsp;air has nearly driven out all the water, turn thenbsp;cock F, and the apparatus is ready for ufe.nbsp;This inftrument is convenient to preferve anbsp;quantity of inflammable air ready for any occa*nbsp;fional ejjperiment, as charging the inflammable air piflol, amp;c. It Is alfo convenient tonbsp;light a candle for oeconomical purpofes, as thenbsp;fmallefl; fpark from an eledlrophorous, or anbsp;fmall bottle, is fufficient to fire the air.

A fmall battery of Inflammable air piftols rs OCCafionally made, that affords eonliderablenbsp;amufement; as either one piflol, or the wholenbsp;together, may be fired at the pleafure of thenbsp;operator.

The following experiment was tirade by Mr. Kinnerfly with his eledlrical thermometers^nbsp;which is deferibed in page 33 of this ElTay. -

Having put fome tinged water into the'large tube, he placed the two wires within the tubenbsp;in contadt, and pafled a large charge of electricity

ON � L � C T R I C I T Y.

tricity from above thirty fquare feet of coated glafs, which produced no rarefaftion in thenbsp;air, and Ihewed that tile wires w^ere not heatednbsp;by the fire paffing through them. When thenbsp;rvires were about two inches afunder, the chargenbsp;of a three^pint bottle, darting from one to thenbsp;other, rarefied the air very evidently. Thenbsp;charge of a jar, which contained about five gallons and a half, darting from wire to wire,nbsp;occafioned a very confiderable expanfion in thenbsp;air; and the charge of a battery of thirty fquarenbsp;feet of coated glafs, would raife the water innbsp;the fmall tube quite to the top : upon the co-alefcing of the air, the column of water inftantlynbsp;fubfided, till it was in equilibrio with the rarefied air; then gradually defeending as the ait-cooled, fettled where it flood before. Bynbsp;carefully obferving at what height the defeending w^ater firft flopped, the degree of rarefadlionnbsp;might be eafily difeovered.

Take a glafs tube, about four inches long, one quarter of an inch in diameter, and opennbsp;at both ends; moiflen the infide of the tubenbsp;with oil of tartar per deliquium, then fix twonbsp;pieces of cork into the ends of the tube, andnbsp;jiafs a wire through each cork, fo that the ends

U nbsp;nbsp;nbsp;of

.AN ESSAY

of the wires, which are within the tube, may be about three quarters of an inch afunder.nbsp;Connedl one wire with the outfide coating of anbsp;large jar, and form a communication from thenbsp;other to the ball of the jar, fo as to pafs thenbsp;difcharge through the tube; repeat this feveralnbsp;times, and the oil of tartar will very often givenbsp;inanifcfl; figiis of cryftalizadon. *

Charge a Leyden phial, (the top of which is cemented into the bottle) place it upon annbsp;infulated ftand, and then take hold of it by thenbsp;ball, and prefent the coated furface towards thenbsp;condenfing ball of a prime conduftor while thenbsp;cylinder is charging, and a large brnlh andnbsp;fpark W'ili pafs between the coating of the bottle and the ball of the conductor, from four tonbsp;twelve inches and upwards in length.

Take fome of the powder of Canton�s phof-phorus, and by means of a little fpirit of wine, dick it all over the infide of a clean glafs phial,nbsp;then flop the bottle, and keep it from the

Difcharge a jar over a thin piece of wood, which is cut in the fhape of a crcfeent, andnbsp;covered with this phofphofus, and the cref-cent will be luminous in the dark.

Place a fmall key on the phofphorus, and difcharge a Leyden phial over the phofphorus, and then throw the key off from it, andnbsp;when it is exhibited in the dark, the form ofnbsp;the key and all its wards will be perfedlynbsp;feen.

As the experiments on phofphorus are In themfelves exceedingly curious, and appear tonbsp;me to be intimately connedled wdth the naturenbsp;of eledtricity, I hope I fliall not be thought tonbsp;have deviated too far from the fubjedt of thisnbsp;effay by Introducing fome experiments of Mr.nbsp;Wilfon on this fubjedl j the more fo, as thenbsp;producing the prifmatic colours is by no meansnbsp;difficult, as little more is required than a fewnbsp;U 2,nbsp;nbsp;nbsp;nbsp;oyftcr*

oyfter-faells, and a good fire of any kind. Fof, if thole lliells are thrown carelefsly into thenbsp;middle of the fire, and continued there for anbsp;proper time, (which may be from ten minutes,nbsp;a quarter, half, or three quarters of an hour,nbsp;to one, two, or three hours, according to thenbsp;thickneft and compadlnefs of the Ihells, and thenbsp;degree of fire thej^ are expofed to) they willnbsp;exhibit lively prifnratic colours, after they arcnbsp;removed from the fun into the dark fuddenly,nbsp;and the tyes have been previoufly prepared anbsp;little to receive them. Mr. Wilfon excited allbnbsp;the light of thefe lltells with ekflricity, in thenbsp;following manner.

He placed upon a metal fland, which was rounded at top, and about half an inch in diameter, a prepared Ihell, that would exhibitnbsp;the prifmatic colours very lively ; on the uppernbsp;fvirface of this fliell, and near the rriiddle, wherenbsp;the colour-making parts predominated, henbsp;brought the end of a metal rod, and then connected the two metals properly with the coatings of a charged phial, in order to difehargenbsp;the fluid. In this circuit there was left, de-fignedly, an interval of about three inches, unoccupied by metal, and next one fide of the

glafs ; the difcharge was made by compleating the circuit with metal where the interval wasnbsp;left. The Ihell, at; that inflant, v^as lighted upnbsp;to an exceeding, great advantage, fo that alinbsp;the colours appeared perfcdly diftinft, and innbsp;their relpeftive places, anfwering to their different colour-making parts. Thefe colours continued vifible feveral minutes, and when theynbsp;ceafed to appear, a white purplifti light occupied their places, which lafted for a confiderablenbsp;time. And notwithftanding this experimentnbsp;W'as repeated with the fame and other fliells,nbsp;.the colours continued in their refpedtive places,nbsp;and nearly of the fame degree of brilliancy;nbsp;excepting, that in or near thole parts wherenbsp;the explofion took place a few fcales were driven off.

Which proves, that bodies of the �me nature, but of different volumes and different maffes, are charged with eledlrical matter onlynbsp;in proportion to their furface, without�any influence or coneurrehee, of their mafles 'hl' thisnbsp;cafe.

The following experiment, which we fhall give in Mr. Achard�s own wmrds, feems to de-U 3nbsp;nbsp;nbsp;nbsp;cidc

cide this queiiion, on which philofophers have) entertained very different opinions.

I cledtrificd (fays he) a cylindrical, hollow brafs condudfor, feven inches long, and onenbsp;and a half in diameter : when it had acquirednbsp;forty degtees of eledtricity, I drew from' it anbsp;fpark, with a condudlor-�f hollow brafs^ ofnbsp;feven'inches long, and one and a half diameter,nbsp;which weighed eight ounces, and was carefullynbsp;infulated. The firft cOndudlor loft fifteen degrees of its eledlricity, - I repeated the famenbsp;experiment, wTen the condudtor had thirty degrees of eledlricity, and then it loft ten degrees.nbsp;Finally, when the condudlor had twenty degrees of eledlricity, it loft only feven by its in nbsp;ftantancous contadl with the fame cylinder.nbsp;After having filled this cylinder with' icad^nbsp;which produced an addition of five pounds to itsnbsp;weight, and confequently to its mafs, I repeatednbsp;the fame experiments, and obtained from themnbsp;the very fame rcfults,

This is followed by other experiments, W'hich are a farther confirmation of Mr, Achard�snbsp;opinion,

Thefe experiments fliew, ift. That bodies of an equal furface, but different in mafs,nbsp;when they are placed in the fame clrcumftahces,nbsp;are charged with an equal quantity of eledlri-cal matter; and adly. That bodies equal in

mafs.

ON electricity. 295

mafs, but different in extent of furfacc,. when they are placed in fimilar circumftances, arenbsp;charged with an unequal quantity of eie6tricalnbsp;matter, and that the body, whofe farface is larger, receives more than .that whofe furface isnbsp;Icfs. Therefore, it is in, proportion to theirnbsp;furfaces, and not to their mafs,, that bodies arenbsp;charged with a greater'or kfs quantity of thenbsp;eledrical fluid.

Before thefe experiments were made it had been obferved, that the extreme fubtilty, and,nbsp;in moft cafes, invifibility of the eleftric fluid,nbsp;render all reafoning about its motion precarious.nbsp;It is however incredible, that this fluid flrouldnbsp;pafs through the very fuhflance of. metallic bodies, and not be retarded by their foHd particles, In thofe cafes, where the fblid parts ofnbsp;metals are evidently penetrated, i. e. when'nbsp;�wires are exploded, there is a manifefl; refift-ance, for the- parts of the wire are fcatterednbsp;about with violence in all direflions.

The like happened in Dr. Prieftley�s circles, made on fmooth pieces of metal. Part of thenbsp;metal was alfo difperfed and thrown off, for thenbsp;circular f|X)ts were compofed of little cavities.nbsp;If therefore the fluid-was difperfed throughoutnbsp;the fubftance, and not over the furface of thenbsp;metal, it is plain, that a wire, whofe diameternbsp;is equal to one of thofe circular fpots, oughtnbsp;U 4nbsp;nbsp;nbsp;nbsp;alio

295 AN ESSAY

alfo to have been deftroyed by an explofion of equal ftrcngth fent through it; whereas, a wire,nbsp;xvhofe diameter is equal to one of thofe fpots,nbsp;would without injury condud: a Ihock muchnbsp;greater than any battery hitherto conftrudednbsp;could give. It is moft probable, therefore,nbsp;that though violent flaflies of.eledricity, whichnbsp;ad alfo as fire, will enter into the fubftance ofnbsp;metals and confume them, yet it immediatelynbsp;difpe�fes irfelf over: their furface, without entering-their .fubftance any more, till being forced to coiieot itfelf into a narrow compafs, itnbsp;again ads as fire.

In many cafes the eledric fluid will be con-duded very well by metals reduced to a mere furface. A piece of white paper will not con-dud a Ihock, without being torn to pieces, asnbsp;it is an eledric fubftance ; but a line drawn onnbsp;k with a black lead pencil, will fafely conveynbsp;the charge of feveral jars. It is impoflible wenbsp;can think, that the fire here paflfes through thenbsp;fuhftance of the black lead flroke, it muft runnbsp;over its furface; and if we confider amp;me ofnbsp;the properties of metals we fliall find, that therenbsp;is great reafon to fuppofe that their condudingnbsp;power lies at their furface.

Fig. 92 reprefents a fmall glafs tube, flopped at one end with a piece of cork ; k Is a wirenbsp;�which palTes through a piece of cork, fitted into

ON ELECTRICITY.

the other end of the tube, the upper part of the wire is furnifhed with a brafs ball, the endnbsp;of the wire within the tube is bent at right angles to the reft of the wire.

Tajte out the upper cork and wire, pour fome fallad oil into the tube, and then fit innbsp;the cork, and pulh dovvngt;the wire, fo that,nbsp;the end of it may be near or rathernbsp;below the furface of the oil ; prefent thenbsp;ball towards a prime conduftor, holding thenbsp;finger or any other non-condudor oppofite thenbsp;bent end of the wire, and when a fpark paflesnbsp;from the condult;5lor to the brafs ball, anothernbsp;will pafs from the end of the wire, and perforate the glafs, the oil will be curioufly agitated.

This experiment appears more beautiful when it is made in the dark. After the firftnbsp;hole is made, turn the end of the wire roundnbsp;towards another part of the glafs tube, and anbsp;fecond hole may be made in the fame manner.nbsp;This experiment w'as communicated to me bynbsp;the Rev. Mr. Morgan, of Norwich, who hasnbsp;carried it much farther, by filling final] bottlesnbsp;with cement, and then pafling the fnock in a

A N� ESS A -Y

limilar mode through themv ' The 'perforation may be made with water in the tube infteatfnbsp;of oil. -nbsp;nbsp;nbsp;nbsp;.

Mr. Lullen produced very cohfidcrable eifedls by pal��ng the fliock through wires that werenbsp;inferted in tubes, filled with oil. The fparknbsp;appears larger in its paffage through oil, thannbsp;when it paffes through water.

Mr. Vilette �filled i difli of 'metal with oil, dnd when he had elcdtrified the dilh, he plungednbsp;a needle into thC oil, hnd received a very ftrongnbsp;Ipark as foon as the point of it came wnthin anbsp;fmali diftance of the difh. A fmall cork ballnbsp;being made to fwim in this oil, upon the approach of the thick end of the ftalk of a lime,nbsp;it plunged to the bottom, and immediately rofenbsp;up again.

Analogous to this experiment of Mr. Morgan are fome obfervations of Dr. Prieftley, who conftantly found, that v/henever he hadnbsp;covered the fradtured place of a jar with anynbsp;kind of cement 'or varnifli, it always broke atnbsp;the place where the cement terminated ; therenbsp;the glafs was perforated, and a new fraiturenbsp;W'as made, which had no communication withnbsp;the former. The jar always broke at the firftnbsp;charge, generally before it had received halfnbsp;its charge. Struck with this phosnomenon, the

ON ELECTRICITY.

a Jar which was not broke, and whofe ftrength he had previoufly afcertained by repeated dif-charges ; he took off a little of the outfidenbsp;coating, and put on the glafs a patch of cement, about an inch in diameter, then drawingnbsp;the coating over it, he charged the jar, butnbsp;before it had received half its charge, it burftnbsp;by a fpontaneous explofion, not indeed at thenbsp;termination, but at the middle of the patch ofnbsp;cement, where the glafs was thinned:. He covered anotlier entirely with cement, and itnbsp;broke near the bottom, where the glafs Is generally thickeft, A jar that was covered entirely both infide and outfide with cement, andnbsp;then coated with tin-foil, burft at the very firltnbsp;attempt to charge it.

The magic pilt;5lure is a f��/ei pane of glafs, proper to anfwer the purjxife of the Leydennbsp;experiment; over the coating on one fide isnbsp;palled a picture, on the other fide a piece ofnbsp;white paper is pafted, fo as to cover the wholenbsp;glafs j it is then put into a frame, with thenbsp;pifture uppermoft, and a communication isnbsp;formed from the tin-foil of the under fide tonbsp;the bottom rail of the frame- of the pidilre,nbsp;ivhich rail is covered with tin-foil.nbsp;nbsp;nbsp;nbsp;n

Lay

AN ESSAY

Lay the picture on the table, with the print eppeumoft, and a piece of money on it, let anbsp;chain foil from the conduftor to the print,nbsp;ram the cylinder, and the plate of glafs willnbsp;foot! be charged ; now take hold of the piclurcnbsp;by the top rail, and let another perfon takenbsp;hold of the bottona rail and endeavour to takenbsp;off the piece, of money, in doing this they wVl�nbsp;receive a Ihock, and generally fail inquot; the at-gt;nbsp;tempt.

Put a quantity of brafs dull into a Coated jar, and w-ben-k is charged invert it, and thrownbsp;fome of the dull out, which will be fpread in annbsp;equable and uniform manner on any flat furface,nbsp;and fall juft like rain or fnow. May it not benbsp;queftioned, fays an ingenious writer, whethernbsp;water, foiling from the higheft region of thenbsp;clouded atmofphere, would not meet the earthnbsp;in much larger drops, or in cataradls, if thenbsp;coalefcing power of the drops w'^as not counter*nbsp;aded by their eledric atmofphcres ?

Place a piece of finoaking wax-taper on the prime cendudor, turn the cylinder, the vo*

ON electricity. 301

lumc of fmoak will become more contraSed, and its motion upward accelerated- Take offnbsp;the eie�lricity of the condudor, and fufpend anbsp;pair of pith balls over ic, and about �ve fec^tnbsp;cliftance from it, turn the machine, and in anbsp;few feconds the balls will open half an inch ;nbsp;remove the taper, and the balls will not fo-parate.

This experiment, therefore, clearly evinces, that fhioke is a conductor of eledricity.

Take around board, well varnidied, and lay on it a chain in a fpiral form, let the interiornbsp;end of the chain pafs through tire board, a�dnbsp;conned it with the coating of a large jar ; fixnbsp;the exterior end to a difchaxglng rod, and thcanbsp;difeharge the jar ; a beautiful fpark will benbsp;feen at every link of the chain. The illuminations to be produced by a chain arc capable ofnbsp;an infinite variety of modifications.

Place fpots of tin-foil, at equal diftanccs from each other, on a piece of bent glafs, andnbsp;let the ends of the glafs be furuiflied with brafs

302 balls, and a glafs handle be fixed to the middle'nbsp;�f the bent glafs* This inftrum�nt will fervOnbsp;as a difeharger, and at the fame time exhibit,nbsp;at each feparation of the tin-foil, the eleftricnbsp;light*

I made feveral of thefe luminous dilcharging rods, many years fince, in order to Ihew, thatnbsp;the eicftric fluid iflues from the negative andnbsp;pofitive coating erf each difeharge, agreeable tonbsp;the idea conveyed by Mr* Atwood�s experiments, fee EXp. iiS, up, 120, of this Efla]^nbsp;But I foon found, that the circuit of a dif-charging rod was not fuffieiently extenfive fornbsp;the purpofe*

Fig, 98 reprefents feveral fpiral tubes, placed round a board, a glafs pillar is fixed to thenbsp;board, and on this pillar is cemented a metalnbsp;cap, carrying a fmall fleel point; a brafs wire,nbsp;furniflicd with a ball at each end, and nicelynbsp;balanced, is placed on this point: place thenbsp;middle of this wire under a ball proceedingnbsp;from the conduftor, fo that it may receivenbsp;a continued fpark from the ball, then give thenbsp;W'ire a rotative motion, and the balls in revolvingnbsp;will give a fpark to each ball of the fpiral tube,

jviiich will be communicated from thence t� the board ; forming, from the brilliancy ofnbsp;the light and its rapid motion, a very pkafingnbsp;experiment.

All thefe experiments on the interfuptcd fpark may be plealingly and beautifully varied,nbsp;and the fpark made to appear of different colours, at the pleafurc of tlie operator.

Sufpend a light cork ball, which is covered x)ver with tin-foil or gold-leaf, by a pretty longnbsp;filk thread, fo as juft to touch the knob of anbsp;charged jar placed on a table; k will be firftnbsp;attradfed and then repelled to foine diftance,nbsp;where, after a few vibrations, it will remainnbsp;at reft. If a lighted candle is now placed atnbsp;fome diftance behind it, fo that the flame ofnbsp;the candle may be nearly as high as the knobnbsp;of the phial, the cork will inftantly be agitated,nbsp;and, after fome irregular motions, will deferibenbsp;a curve round the knob of the phial, and thisnbsp;it will continue to do for fome time.

Fig, 96 and 97 reprefent an eleftrometer, nearly fimilar to that contrived by Mr. Brooke.nbsp;The tw'o inftruments arc fometimes combinednbsp;in one, or ufed feparately, as in thefe figures.

The arms FH fh, fig. 97, when in ufe, arS to be placed as much as poffible out of the at-mofphere of a jar, battery, prime condu�tor,nbsp;amp;c. The arm F H and the ball K are madenbsp;of copper, and as light as poffible. The divi-fions on the arm F A are each of them exadlynbsp;a grain. They are afeertained at firfl; by placingnbsp;grain weights on a brafs bail which is withinnbsp;the ball I, (this ball is an exact counterbalancenbsp;to the arm F H and the ball K when the fmallnbsp;fiide r is at the firft divifion) and then removing the Hide r till it, together with thenbsp;ball K, counterbalances the ball I and thenbsp;weight laid on it.

A, figf^^, is a dial-plate, divided into 90 equal parts. The index of this plate is carriednbsp;once round when the arm BC has movednbsp;through 90 degrees, or a quarter of a circle.nbsp;The motion is given to the index by the repul-five power of the charge ading between thenbsp;ball D and the ball B.*

The arm B C being repelled, firews when the charge is increafing, and the arm F Hnbsp;fhevvs what this repulfive power is between twonbsp;balls of this fize in grains, according to thenbsp;number the weight refts at when lifted up bynbsp;the repulfive power of the charge : at the famenbsp;time the arm B C points out the number of degrees

grees to which the ball B is repelled ; fo that, by repeated trials, the number of degrees, an-fwering to a given number of grains, may benbsp;alc�rtained, and a table formed from thef� experiments, by which means the rfeclrdmetcr,nbsp;fig. 96, may be ufed without that of fig. 97.

Mr. Brooke thinks that no glafs, charged (as vve call it) with ele�lricity, will bear anbsp;greater force, than that whofe repuliiYe power,^nbsp;between two balls of the fize he ufed, is equalnbsp;to 60 grains: that in very few inftancesf itnbsp;will ftand 60 grains weight ; and he thinks;nbsp;it hazardous to go more than 45 grains.

Hence, by knowing' the quantify of coated furface, and the diameter of the balls, we maynbsp;be enabled to fay, fo much coated furface,nbsp;with a repulfion, between balls of fo manynbsp;grains, will melt a wire of fuqh a fize, or killnbsp;fuch an animal, amp;c.

Mr, Brooke thinks, that he h' not acquainted with all the advantages of this electrometer j but that it is clear, it fpeaks a' language which may be u-niverfally underft�od,'nbsp;r^'hich no other will dcr;' for though other electrometers will Ihew whether a charge is greaternbsp;or lefs, by an index being repelled to greaternbsp;or fmaller diftances, or by the charge explodingnbsp;at different diftances, yet the power of th�nbsp;charge is by m means afeertaiped: but-thisnbsp;-Xnbsp;nbsp;nbsp;nbsp;eleo

AN ESSAY

ele�rometer fhews the force of the repulfive power in grains; and the accuracy �f the inftru-*nbsp;ment is ealily proved, by placing the weightnbsp;on the internal ball, and feeing that they coincide with the divifions on the arm F H, wheitnbsp;the Hide is removed to them.

Obfer'uations and Experiments made by Dr, Friejlley on the EffeSis of EleSlricity onnbsp;different elajlic Fluids,

To change the blue colour of liquors,, tinged with vegetable juices red. The apparatus for this purpofe is feen in fig. 94. A Bnbsp;is a glafs tube, about four or five inches long,nbsp;and one or two tenths of an inch diameter innbsp;the infide ; a piece of wire is put into one endnbsp;of the tube, and fixed there with cement ; anbsp;brafs ball is placed on the top of this wire;nbsp;the lower part of the tube from a is to be fillednbsp;with w^ater, tinged blue with a piece of turn-fole or archal. This is eafily efFefted, by fet-ting the tube in a veflfel of the tinged water,nbsp;then placing it under a receiver on the plate ofnbsp;the air pump j exhaufl; the receiver in part,

and then, on letting in the air, the tinged liquor ivill rife in the tube, and the elevation wdll benbsp;in proportion to the accuracy �f the vacuum,nbsp;now take the tube and veffel from under the'nbsp;receiver, and throw ftrdng fparks on the brafsnbsp;ball from the prime conductor.

When Dr. Prieftley made this experiment, he perceived, that after the eledlric fpark hadnbsp;been taken, between the wire h and the liquornbsp;at a, about a minute, the upper part of it began to look red j in two minutes it was mani-feftly fo, and the red part did not readily mixnbsp;with the liquon If the tube was inclined whennbsp;the fparks were taken, the rednefs extendednbsp;twice as far on the lower fide as on th�nbsp;upperlt; In proportion as the liquor becamdnbsp;red, it advanced nearer to the wire, fo thafnbsp;the air in which the fparks were taken was di-minithed; the diameter amounted to aboutnbsp;one fifth of the whole fpace; after which, anbsp;continuance of the cleftrificatlon produced nonbsp;fenfible effeft.

To determine, whether the cauTe �f the change of colour was in the air, or in the electric matter, Dr. Prieftley expanded th� air iilnbsp;the tube, by means of an air purnp, till it expelled all the liquor, and admitted frefii blue'nbsp;liquor in its place; but after this, electricitynbsp;produced no fenfible effedt on the air or on th�nbsp;X 2nbsp;nbsp;nbsp;nbsp;liquor 5

liquor; fo that it was clear, that the eleftrie matter had decompofed the air, and made itnbsp;depofit fomcthing of an acid nature. The re-fult was the fame with wires of different me-gt;nbsp;tals. It was alfo the fame when, by means ofnbsp;a bent tube, the fpark was made to pafs fromnbsp;the liquor in one leg, to the liquor in thenbsp;other. The air, thus diminiflied, was in thenbsp;higheft degree noxious.

In palling the eletSric fpark through different elaftic fluids it appears �f different colours. In fixed air, the fpark is very white; in inflammable and alkaline air, it appears of anbsp;purple or red colour. From hence we may infer, that the conducing power of thefe airs isnbsp;different, and that fixed ait is a more perfeftnbsp;non-condudlor than inflammable air.

The fpark was not vifible in air from a cauf-tic alkali, made by Mr.i Lane, nor in air from fpirit of fait; fo that they feem to benbsp;more perfeil condudors of eledrieity than water, or other fluid fubftances.

The elcdric fpark, taken in any kind of oil, produces inflammable air. Dr.. Prieftley triednbsp;it with ether, oil of olives, oil of turpentine,nbsp;�gtd eflential oil of mint, taking the eledrienbsp;fpark in them without any air to begin wfithnbsp;inflammable air was produced in them all.

Dr. Prieftley foimd, that on taking a fiiiall eiedric explofion for an hour, in the fpace ofnbsp;an inch of fixed air, confined in a ^lafs tubenbsp;one tenth of an inch diameter, when water wasnbsp;admitted tp it, only one fourth of the air wasnbsp;imbibed. Probably the whole would havenbsp;been rendered immifcible in water, if the electrical operation had been continued a fufficientnbsp;time.

The eleftric fpark, when taken in alkaline air, appears of a red colour ; the cledtric ex-plofions, which pafs through this air, increafenbsp;its bulk ; fo that, by making about 200 ex-plofions in a quantity of it, the original quantity will be fometimes increafed one fourth. If

vyater is admitted to this air, it will abforb the original quantity, and leave about as muchnbsp;elaftic fluid as was generated by the eledtrlcity,nbsp;and this elaftic fluid is a ftrong inflammablenbsp;air.

Dr. Prieftley found, that when the eletftric fpark was taken in vitriolic acid air, that titcnbsp;infide of the tube in which it was confined wasnbsp;covered with a blackifh fubftancc. He leemsnbsp;to think, that the whole of the vitriolic acidnbsp;air is convertible into this black matter, not bynbsp;rneans of any union which it forms with thenbsp;ele�lric fluid, but in confequence of the con-cuflion given to it by the explofion ; and that,nbsp;X 3nbsp;nbsp;nbsp;nbsp;if

AN ESSAY

if it be the calx of the metal which fupplied the phlogifton, it is not to be diftinguilhednbsp;from what metal, or indeed from what fub-ftance of any kind, the air had heen ex-traded.

Dr. Prieftley made 150 explofions of a common jar in about a quarter of an ounce mcafure of vitriolic acid air from copper, by whichnbsp;the bulk was diminilhed about one third, andnbsp;the remainder feemingly not changed, beingnbsp;all abforbed by water. In the courfe of thisnbsp;procefs, the air was carefully transferred threenbsp;times from one veflel to another ; and the laftnbsp;veffel, in which the explofions were made in it,nbsp;was, to all appearance, as black as the firft ;nbsp;fo that the air feems to be all convertible intonbsp;this black fubfiance.

Thinking this diminution of the vitriolic acid air might arife from its abforption by the cement, with which the glafs tubes employed innbsp;the laft experiment were clofed, he repeated itnbsp;with the air from quickfilver, in a glafs fyphonnbsp;confined by quickfilver, ^nd the refult was thenbsp;fame.

That this matter comes from the vitriolic acid air only, and not from any combination ofnbsp;fhe eleftric matter with it, will appear from thenbsp;following experiment,

ON ELECTRICITY. 311

He took the fimple eledric fpark from a con-(dudor of a moderate fize, for the fpace of five minutes without interruption, in a quantity ofnbsp;vitriolic acid air, without producing any changenbsp;in the infide of the glafs; when immediatelynbsp;after, making in it only two explofions of anbsp;common jar, each of which might be producednbsp;in lefs than a quarter of a minute with the famenbsp;machine in the fame ftate, the whole of thenbsp;infide of the tube was compleatly covered withnbsp;the black matter. Now had the eledric matternbsp;formed any union with the air, and this blacknbsp;matter had been the refult of that combination,nbsp;all the difference that w^ould have arifen fromnbsp;the fim^ple fpark or the explofion, could onlynbsp;have been a more gradual, or a more fuddennbsp;formation of that matter,

A large phial, about an inch and a half wide, being filled with this air, the explofionnbsp;of a very large jar, containing more than twonbsp;feet of coated furface, had no effed; upon it;nbsp;from which it fhould feem, that in thefe cafes,nbsp;the force of the Ihock was not able to give thenbsp;quantity of air fuch a concuffion as was ne-ceflary to decompofe any part of it.

He had generally made ufe of copper, but afterwards he procured this air from almofl;nbsp;every fubftance from which it could benbsp;obtained; the eledric explofion taken in itnbsp;X 4nbsp;nbsp;nbsp;nbsp;pro-

312 nbsp;nbsp;nbsp;AN ESSAY

produced the fame effeift. But, as fome of ths experiments were attended with peculiar cir-cumftances, he briefly mentions them, as fol-�otvs.

when he endeavoured to get vitriolic acid air from lead, putting a quantity of leadennbsp;{hot into a phial containing oil of vitriol, andnbsp;applying only the ufual degree of heat, a conrnbsp;flderable quantity of heat was produced ; butnbsp;afterwards, though the heat was encreafed tillnbsp;the acid boiled, no more air could be got. Henbsp;imagined therefore, that in this cafe the phlo-gifton had, in faff, been fupplied by fomenbsp;thing that had adhered to the fliot. However,nbsp;in the air fo produced, he took the ele�lric ex-ploflon ; and in the firft quantity he tried, anbsp;whitifh matter was produced, almoft coveringnbsp;the infide of the tube; but in the fucceedingnbsp;experiments, with air produced from the famenbsp;{hot, or from fomething adhering to it, therenbsp;was lefs of the whitifli matter ; and at laft, nothing but black matter was produced, as in allnbsp;the other experiments. - Water being admittednbsp;to this air, there remained a conliderable refi,nbsp;duum, which was very flightly inflammable.

Vitriolic acid air is eafily procured from fpirit of wine, the mixture becoming black before any air is yielded. The eledtric explofion

The experiments made with ether feem to throw moft light upon this fubjeft, as thisnbsp;air is as eafily procured from ether as anynbsp;other fubftance, containing phiogifton. 1�nbsp;the air procured by ether the eledlric explo-fion tinged the glafs very black, more fo thannbsp;in any other experiment of the kind ; and,nbsp;when water had abforbed what it could of thisnbsp;air, there was a refiduum in which a candlenbsp;burned with a lambent blue flame. But whatnbsp;was iiiofl: reniarkable in this experiment was,nbsp;|:hat,befides the oil of vitriol becoming verynbsp;black during the procefs, a black fubftance,nbsp;and of a thick confiftence, w^as formed, whichnbsp;fwam on the furface of the acid.

It is very poflible, that the analyfis of this fubftance may be a means of throwing lightnbsp;upon the nature of the black matter, formednbsp;by eledfric exploftons, in vitriolic acid air,nbsp;as they feem to refcmble one another verynbsp;much.

The eleiftric fpark or explofion, taken in common air, confined by quiekfilver in a glafsnbsp;tube, covers the infide of the tube with a blacknbsp;matter, which, when heated, appears to benbsp;pure quiekfilver. This, therefore, may be thenbsp;cafe with the black matter into which he fyp.i

poftd

AN ESSAY

pofed the vitriolic acid air to be converted by the fame procefs, though the effeft was muchnbsp;more remarkable than in the coinmon air�nbsp;The explofion will often produce the dimintwnbsp;tion of common air in half the time that fimple;nbsp;fparks will do it, the machine giving the famenbsp;quantity of fire in the fame time ; alfo, thenbsp;blacknefs of the tube is much fooner producednbsp;by the fhocks than by the fparks, When thenbsp;tube confiderably exceeds three tenths of annbsp;inch in diameter, it will fometimes become very black, without there being any fenfible di-miputicm of the quantity of air.

This curious experiment was made by Mr, Marfham, originally with a view to melt wiresnbsp;with a fmall Eeyden bottle, The effedts arenbsp;curious, and feem to open a new field for in-vcftigating the force and diredtion of the electnbsp;trie fluid. He fixed a fmall piece of wax uponnbsp;the outfide coating of the Leyden bottle,, thenbsp;head of a fmall needle was ftuck in the wax,nbsp;fo as to be at right angles to the coating i op-pofite to the point of this needle, and at halfnbsp;an inch diftance another needle was fixed,nbsp;by being forced through the bottom of a chipnbsp;box, this was connedled with the difeharging

rod by a wire. On difcharging the bottle, the needle with the wax was driven from the coat-,nbsp;jng of the bottle, and fixed into the box op,tnbsp;pofed to it. The diftance between the needlesnbsp;was then increafed to two inches and a half,nbsp;which was the greateft ftriking diftance, Thenbsp;head of the needle, which was fixed to thenbsp;bottle, was evidently melted in two or threenbsp;places. If the charge w^as ftrong* and the waxnbsp;was not ftuck fall to the coating of the bottle,nbsp;both the wax and the needle would be drivennbsp;fome inches from the bottle. On placing anbsp;ball of wax on the point of each needle, andnbsp;paffing the difcharge through them, the ballnbsp;was thrown from that conneded with the bottlenbsp;full two feet. Repeating this again, he couldnbsp;not produce the fame effeft.

Mr. Marfham now fixed the needle, oppofed to that on the bottle, with wax on a brafs plate.nbsp;On paffing the charge through them, when thenbsp;needles were half an inch diftance from eachnbsp;other, the needle was thrown fix inches frorrinbsp;the brafs plate, while the other remained in itsnbsp;fituation. On increafing the diftance, the effedsnbsp;were the fame, till it came to one inch and anbsp;half, when neither were thrown off. In manynbsp;inftances, both were thrown off, leaving thenbsp;wax behind them,

AN ESSAY

The needles in all thefe experiments palTcc! through the wax, fo as to touch the coating ofnbsp;the bottle and the brafs plate, both the coatingnbsp;and plate were beautifully fufed at each ex^nbsp;plofion.

Mr. Marlham then fubftituted fmall pieces of putty inftead of wax; when, on making thenbsp;difeharge with the points, at only three-eighthsnbsp;of an inch, the needle was driven from the bottle-, and the putty forced up the needle. Thenbsp;points were then placed as near each other asnbsp;was poffible; when, on making the difeharge,nbsp;the putty of both needles was blown to pieces,nbsp;and the needle thrown at a confiderable diAnbsp;tscnce ; the brafs plate was. alfo curioufly melted,nbsp;and the bottle btokegt;

On the Analogy between the Produblion and EffeSis of EleAricity and Heaty and alfonbsp;between the Power by which Bodies con--du�i Eledlrittty and receive Heaty withnbsp;the Defcriptioh of an Infrument to mea-Jure the ^antity of the EleSlrical Fluidsnbsp;which Bodies of a different Nature wilinbsp;condudiy when placed in the fame Circuni-*nbsp;fiances. By Mr. Achard.^

Every kind of friftion produces heat and eleftricity. It may be objcfted to this, that innbsp;order to render the analogy pcrfeft, it would bonbsp;neceflary that the fri�tion of every body fliouldnbsp;produce eledtricity, which appears contrary tonbsp;experience, as metals and other conduftingnbsp;fubftances do not become eledrical, but bynbsp;the contad of eledric bodies, and that thnnbsp;immediate fridion of thefe fubftances will notnbsp;tender them eledrical.

To this it may be anfwered, that when aq eledric body is excited by friclion againft anbsp;non-eledric, that the laft, if it is infulated,nbsp;gives as ftrong figns of eledricity as thofenbsp;of the elcdric itfelf. This eledricity is not

AN ESSAY

eommimicated by the eledtric^ fince it is of an oppofite kind : negative, if the elelt;5lric isnbsp;pofitive; and the contrary.

This obfervation proves, not only that th� conducing bodies become electrical by fridlion,nbsp;as well as eleftric bodies, but alfo, that to produce eleftricity it is neceffary that the equilibrium between the elcflricity of the rubbingnbsp;bodies Iho�ld be deftroyed ; if each fubftancenbsp;is equally adapted to receive and tranfmitnbsp;the electrical fluid, it is clear, that the equilibrium of the fluid between them cannot be deftroyed ; becaufe, that at the inflant one receives from the other any given quantity, itnbsp;willgt; by its elaflicity, be again divided betweennbsp;them : we may therefore conclude,

j. That the eleCtricity produced by the frid-tion of two bodies is greater, in proportion to the increafe of the difference between thenbsp;conducting power of thofe bodies*

2. That where tw'o bodies are equally adapted to receive and tranfmit the eleCtric fluid, theynbsp;give no fign of eleCtricity; not becaufe theynbsp;cannot become eleCtrlfied by friCtion, but becaufe the electricity, which is difturbed by thenbsp;friction, is at thq fame inflant reflored, on account of the facility with which it penetratesnbsp;each fubftance. For a reafon nearly fimilargt;

It feems therefore, that we may conclude from this theory, which is founded on fa6t, thatnbsp;in all cafes, and whatever is the nature of thenbsp;fubftance, the fridion always produces electricity ; and when the effedl is not fenfible, itnbsp;is only becaufe eleAricity is loft as foon as produced.

That there are no fubftances, that are tubbed againft a body, which tranfniit the ele�bric fluidnbsp;with greater or lefs difficulty, but what givenbsp;figns of eleftricity: that metals are as electrical by themfelves as glafs and wax.

That as fridtiofl always, and in all cafes, produces eleftricity, there is a perfeiSt analogy between the produftion of heat and ekdfricity.

^he effeSls which are produced by ele�ricity, are fimilar to thofe produced by heat�

Heat dilates all bodies. The adloit of the cleftric fluid on the thermometer fliews itsnbsp;dilating power alfo; �apd if we do not generallynbsp;perceive it, it is becaufe the force with whichnbsp;bodies cohere together exceeds the dilating,nbsp;power of eledtricity.

Heat promotes and accelerates vegetation as well as germination : Eledtricity, whether po-pofitive or negative, does the fame.

Heat and eleftricity accelerate the motion of the blood. Leaft fear, conftraint, or the*nbsp;attention to the experiment, might accelerate'nbsp;the pulfe and this be attributed to eledlricity,nbsp;Mr. Achard made the experiment on a dognbsp;when a fleep, and always found, that the number of pulfations was increafed when the animal v/as ele�lrified.

The experiment made b^f Mr. Achard on the eggs of a hen, and by othets on the eggs ofnbsp;moths, prove that eledtricit)'�, as well as heat,nbsp;favour the developement of thofe animals. The�nbsp;eleftric fluid, in common with fire, will thrownbsp;metals into fufion.

If fubftancesy with unequal degrees of heat,' touch each other, the heat is difFufed uniformlynbsp;between them. In the fame manner, if twonbsp;bodies with unequal degrees or different kindsnbsp;of eledlricity, touch each other, an equilibriumnbsp;will be eftablifiicd.

'Tlrere is an exa�i analogy between the faculty with which bodies conduSi the elebiric fluid and rc'nbsp;ceivt heat.

If bodies of different kinds, and of equal degrees of heat, are placed in a medium of a different temperature, they will all acquire, at tb�

end

ON ELECTRICITY. 321

end �f a certain time, the fame degree of heat. There ft a confiderable difference, however, innbsp;the fpace of time in which they acquire thenbsp;temperature of the medium, ex. gr. metalsnbsp;take lefs time than glafs, to acquire or lolbnbsp;an equal degree of heati

-On�uan attentive examination of the bodies which receive and lofe their heat fooneff, whennbsp;they are placed in mediums of different temperature, they will be found to be the fanfe^tiichnbsp;receive and lofe their eledtricky wkhe.tHenbsp;greateft facility. Metals, which become,nbsp;or grow cool the quifckeft, are the fttbllancesnbsp;Ivhich receive and part with their electricitynbsp;fooneft. Wood, which requires more, time tonbsp;be heated or cooled, receives and lofes eledlri-city flower than metals. Laftly, glafs and re-finous fubftancesy which receive and lofenbsp;flowly the elecftric fluid, acquir-e with difficultynbsp;the temperature of the medium which fur-rounds them.

If one extremity of an iron rod is heated red-hot, the other extremity, though the barnbsp;is feveral feet long, will become fo warm in anbsp;little time that the hand cannot hold it; be-caufe the iron condudts heat readily ; though anbsp;tube of glafs, only a few inches long, may benbsp;held in the hand, even while the other end isnbsp;melting. The eleflric fluid, in the fame maii-Ynbsp;nbsp;nbsp;nbsp;ner.

322 nerj paffes with great velocity fr�m one endnbsp;of a rod of iron to the other; but it is anbsp;confiderable time before a tube of glafs, at onenbsp;end of which an excited eledtric is held, willnbsp;give eledric figns at the other.

Thefe obfervations prove, that feveral bodies that receive and lofe with difficulty their ac--tual degree of heat, receive and lofe alfonbsp;with difficulty their eledricity. To determinenbsp;if thisslaw is general, and what are the excep-*nbsp;tions'to it, will require a variety of experi*nbsp;mentis� gt;n:-

If� w� fuppofe two fubftances, one of which is elednfied, but the other not, that the firdnbsp;has a known degree of eledrlcity, and that thenbsp;laft in touching it, deprives it of a given degree of eledricity; this lofs of a part of itsnbsp;eledricity, determines the facility with whichnbsp;the body that touches it receives the eledricnbsp;fluid. Befides the figure and volume of thisnbsp;fubftance, the time the two bodies remain innbsp;contad, will alter the quantity taken fromnbsp;the eledrified fubftance ; fo that all othernbsp;circumftances being the fame, the property ofnbsp;bodies to deprive other bodies of their electricity, or, in other words, to condud the electric fluid, is, in the inverfe ratio of the time,nbsp;neceffary to make them lofe an equal degreenbsp;of eledricity.

Ttc

�LECTRlClTY.

Yhe inftrument which is reprefetited fig. 95, is conftru�ted �n thefe principles, and with itnbsp;the quantity of eledtricity that one body lofesnbsp;in a given time, when touched by another, maynbsp;be accurately afcertained. A B is a very fen-*nbsp;fible balance, at the extremity of each arm twonbsp;very light balls �f copper are affixedj GFDnbsp;a divided femicircle, which is faftened to thenbsp;cock which fupports the axis of the balance;nbsp;the degrees may be pointed out by a needle, �rnbsp;by the arms of the balance j the cock is fixednbsp;to a brafs cap, which is cemented on the glafsnbsp;pillar GG, which is fixed to the board ST;nbsp;this pillar fhould be at leaft 18 inches high;nbsp;U is a Leyden bottle ; to the wife Z Z, whichnbsp;communicates with its infide coating, threenbsp;horizontal wires, Vz, XZ, and ZY, are fixed;nbsp;the ends of thefe wires are furnilhed with hollow brafs balls; the bottle U is fo fixed to thenbsp;board, that when the beam is horizontal, thenbsp;ball B touches exaftly the ball V, as is repre-fented in the figuren

KN is a metal lever, which turns upon an axis at I, fo as to move freely in a verticalnbsp;plane, which fhduld coincide with the bar VX jnbsp;the lever K N is fupported by a wooden pillarnbsp;1H, which is fixed to the board QR S T ; atnbsp;the end K of the lever is a ferew, to hold thenbsp;lubfiance on which the experiment is to be made;

AN ESSAY

the upper end of this fubftance Ihould be turtl-cd into a convex form. A thread N O is tied to the end N of the lever J at O is a fmallnbsp;hook, on which a ball P is to be fufpended.nbsp;The diftarice of the pillar IH, from the bottlenbsp;Is to be fo adjufted, that wheii the end N isnbsp;lowered, the body L may touch in one pointnbsp;the ball X, the proportion between the weightnbsp;of the arms of the lever, the weight P and thenbsp;body Lj and the length �f the pillar IH tonbsp;the .thread 'N O, is to be fuch, that when thenbsp;fubftance L touches the ball X, at the famenbsp;moment the ball P will touch the boardnbsp;Qji S T, and be difengaged from the threadnbsp;NO; the fubftance L will alfo at the famenbsp;inftant quit the ball X.

To life this inftrumeht, connect the bottle U with the prime conductor by the ball Y,nbsp;and form a cOhirirunicatlori by a wire fromnbsp;Y to the cap G; charge 'the bottl�, and thenbsp;ball V will repell the ball B, the aflgle of rc-pulfton will be marked by the needle EF.nbsp;Suppofe this to be 20 degrees, and let L benbsp;brought, as before deferibed, to touch X, it willnbsp;abforb a quantity of electricity proportionablenbsp;to its conduding pow'er, and the ball B willnbsp;fall in proportion to the quantity abforbed, andnbsp;the difference will be feen on the fcmicircle,nbsp;J.et the difference be five degrees; repeat the

ON ELECTRICITY.

experiment, only fubftituting fome other fub-r �ance in the place of the body E ; fuppofe thatnbsp;with this fubftance the diminution of the anglenbsp;is 8 degrees, then is the condufting power ofnbsp;thefe two fubftances in the proportion of 5 to 8.

Fig, 98 reprefents an apparatus, to fet a wire on fire by the eleftric explofion in dephlogi-fticated air. I am obliged to defer the defcrip-tion and ufe of it to fome future opportunity,nbsp;as I have not had any time to try its fuc-cefs.

MAGNETISM.

Though the phccnomena of the magnet have, for many ages, engaged the attention of natural philofophers, not only by their fingularity and importance, but alfo bynbsp;the obfcurity in which they are involved; yetnbsp;very few additions have been made to the dif-coveries of the fir ft enquirers upon the fubjedl.nbsp;The powers of genius which have been hitherto employed in profecuting this fubjeft, havenbsp;pot been able to frame an hypothefis, that willnbsp;account, in an eafy and fatisfadlory manner, fornbsp;all the yarious properties of the magnet, or

AN ESSAY

point out tlie links of the chain which conneft J� with the other phoenamena of the unlverfe.

It is known by the works of Plato and Ari-ftotle, that thc^antients were acquainted with, the attradive and repulhve powers of the mag-,nbsp;net; but it does nor appear, that they knew ofnbsp;its pointing to the pole, or the yfe of the com-pafs. As they w'ere not acquainted with thenbsp;true method of philofophifing, and contentednbsp;themfelves wdth obfervation alone, their knowledge of nature was confined within very narrorvnbsp;limits, and did not afEsrd any confiderable advantage to fociety. Modern philofophers, bynbsp;combining experiment with obfervation, foonnbsp;extended the boundaries of fcience, and dif-covered the polarity of the loadftone, a property which in a manner conftitutes the balls of na^nbsp;vigation, and gives being to commerce.

The loadftone, or natural magnet, is an iron �Te or ferruginous ftone, found in the bowelsnbsp;�f the earth, generally in iron mines ; of a�nbsp;forms and fizes, and of various colours,

Loadftones are in general very hard and brif-* tie, and for the moft part more vigorous innbsp;proportion to their degree of hardnefs. Confiderable portions of, iron may be extra�lednbsp;from them. Newman fays, that they are al-tnoft totally foluble in fpirit of nitre, and par^nbsp;tially in the vitriolic and marine acids.

Artificial magnets, which are made of fteel, are now generally ufed in preference to the natural

fural magnet; not only as they may be procured with greater eafe, but becaufe they are far fuperior to the natural magnet in llrength,nbsp;affid communicate the magnetic virtue more,nbsp;powerfully, and may be varied in their formnbsp;more eafily.

The power poliefled by the loadftone, which is alfo communicable to iron and fteel, is callednbsp;Magnetlfm.

A rod, or bar, of iron or fteel, to which a permanent polarity has been communicated, isnbsp;called a Magnet,

The points in a magnet which feem to poffefs the greateft power, or in which the virtue feemsnbsp;to be concentrated, are termed the Poles of anbsp;magnet.

The Magnetical Meridian is a vertical circle in the heavens, which interfelt;Els the horizon innbsp;the points to which the magnetical needle, whennbsp;at reft, is dire�ted.

The Equator of a magnet isgt; a line perpen-, dicular to the axis of the magnet, and exaftlynbsp;between the two poles.

Secondly, The force by which it places it-felf, when fufj^ended freely, in a certain direc-mn towards the poles of the earth.

AN ESSAY

Fourthly, The property which it poflefTes of communicating the foregoing powers to iron prnbsp;Heel.

An HYPOTHESIS.

Mr, Euler fuppofes, that the two principal caufes vrhich concur in producing the wonderful properties of a magnet are, firft, a particu-;nbsp;far ftrudfure of the internal pores of the magnet,nbsp;and of magnetical bodies; and, fecondly, an externa} agent pr fluid, which adfs upon and paflesnbsp;through thefe pores. This fluid he fuppofes tonbsp;be the folar atmofphere, or that fubtil matternbsp;called ether, which fills our fyffem.

Indeed, mofl writers on the fubjeft agree in fuppofing, that there are corpufcles of a peculiar form and energy, wfoich continually circulate around and through a magnet; and that anbsp;vortex of the fame kind circulates around andnbsp;through the earth.

A magnet, befides the pores which it has in common with other bodies, has alfo othernbsp;pores confiderably Imaller, deflined only fornbsp;the paffage of the magnetic fluid. Thefe poresnbsp;are fo difpofed as to communicate one with thenbsp;ether, forming tubes or channels, by whichnbsp;the magnetic fluid paffes from one end to the

other. The pores are fo formed that this fluid �can only pafs through them in one diredion�nbsp;but cannot return back the fame way ; fimilarnbsp;to the veins and lymphatic veffcls of the animalnbsp;bodyj which are furnbhed with valves for thisnbsp;purpofe. So that the })ores of the magnet maynbsp;be conceived to be formed into feveral narrownbsp;contiguous tubes, parallel to each other, as atnbsp;A B, fig. 99, through which the finer parts ofnbsp;the ether pafl�s freely from A to B, but cafinotnbsp;return back on account of the refiftance k meetsnbsp;tvith at a, b, b, nor overcome the refiftance ofnbsp;the grofler ether, which occafions and continuesnbsp;the motion. For fuppofing the pole A of anbsp;magnet, filled with' feveral mouths or �pennbsp;'ends of fimilar tubes, the magnetic fluid,nbsp;prefled by the grofler parts of the ether, willnbsp;pafs towards B with an inconceivable rapidity,nbsp;which is proportionable to the elafticity of thenbsp;eth�r itfclf '; this matter which, till it arrives atnbsp;B, is feparated by the' tubes from the morenbsp;grofs parts, then meets with it again, and hasnbsp;its velocity retarded,' and its diredion changed ;nbsp;the ftream, re'fleded by the ether, with whichnbsp;it cannot immediately mix, is bent on bothnbsp;fides towards C and D, and deferibes, but withnbsp;kefs velocity, the curves D E and C F e, andnbsp;approaching by the curves and c, foils Jnnbsp;with the affluent matter m m, and again entersnbsp;the magnet; and thus forms that remarkable

AN ESSAY

atmofpher� which is vifible in the arrangement of fteel filings OH a piece of paper that is placednbsp;over a magnet.

is a tendency iti iron atid a magnet to approach each other, and attach therafehes together, and that with fuch force, as often to requirenbsp;a conjiderable weight to feparate them.

Thefe curious phcenomena may be illufiratec! by either of the magnets contained in the apparatus, as they will lift greater or fmallernbsp;weights in proportion to their ftrength.

Place a piece �f iron on a cork,- and put the cork into water^ the piece of iron wdll be at-^nbsp;trailed by, and follow, a magnet, in a pleafin^nbsp;manner.

On this principle many ingenious and entertaining' pieces of mechanifm have been contrived. Small fwans fwimming in the water have been made to point our the time of thenbsp;day, amp;c.

Place a magnet upon one of the brafs ftands, and prefent one end of a fmall needle towardsnbsp;it, holding the other end by a piece of threadynbsp;to prevent the needle fixing itfelf to the bar,,nbsp;and the needle wdll be pleafingly fufp�nded ininbsp;the air,

Sufpend a magnet under the fcale of a ba-lance, and counterpoife it by weights in the other fcale, then prefent a piece of iron tow�ards

ON MAGNETISM. 555

the magnet, it will immediately defcend, aatd, if the iron is not placed at too great a diftance,nbsp;will adhere to it; now fufpend the iron undernbsp;the fcale inftead of the magnet, then bring thenbsp;latter towards it, and the iron will defcend andnbsp;adhere to the magnet.

To give a detail of the Various proce�Tes which have been fuggelled, for the touchingnbsp;or communicating the properties of the magnet to iron or fteel, would alone fill a volume.nbsp;I lhall therefore only give an account of twonbsp;general and good methods Vvhich I prefumenbsp;avill be found adequate to every common pur-pofe.

I. Place two magnetic bars AB fig. too-. In a line, with the north or marked end of one,oppofed ronbsp;the fouth or unmarked end of the other, but atnbsp;fuch a diftance from each other, that the mag*nbsp;net to be touched may reft with its marked endnbsp;on the unmarked end of A, and its unmarkednbsp;end on the marked end of B, then apply thenbsp;north end of the magnet D and the fouth endnbsp;of E to the middle of the bar C, the oppofitenbsp;ends being elevated as in the figure ; draw Dnbsp;and E afunder along the bar C, one towards A,nbsp;the other towards B, preferving the fame elc--

33^ vation, rertiove D and C a foot or two from th#nbsp;Bar when they are off the ends, then bring th��nbsp;north and fouth poles of thefe magnets togethernbsp;and apply them again to the middle of the barnbsp;C as before'; repeat the fame procefs five or fixnbsp;times, then turn the bar, and touch the oppo-fite furface in the fame manner, and afterwardsnbsp;the two remaining furfaces, and by this meansnbsp;the bar will acquire a ftrong fixed mag-netifm.

Place the two bars which are to be touched parallel to each other, and then unite the endsnbsp;by tw'o pieces of foft iron called fupporters, innbsp;order to preferve, during the operation, the circulation of the magnetic matter ; the bars arenbsp;to be placed fo that the marked end B, fig. loi,nbsp;may be oppofite the unmarked end D, then placenbsp;the two attrafting poles G and I on the middle ofnbsp;one of the bars, to be touched, railing the endsnbsp;fo that the bars may form aft obtufe angle of i,oonbsp;or 120 degrees '; the ends G and I of the barsnbsp;are to be feparated two or three tenths of annbsp;inch from each other. Keepingthe bars in thisnbsp;pofition, move them flowly over the bar A B¹nbsp;from one end to the other, going from end tonbsp;end about fifteen times. Having done this,nbsp;change the poles of the bars, ¹ and repeat thenbsp;fame operation on the bar C D, and then on the

That is the marked end of one Is always to be againfi the onmarked end of the other.

ON MAGNETISM.

oppofite faces of the bars; the touch, thus communicated, may be farther increafed, bynbsp;rubbing the different faces of the bars withnbsp;fets of magnetic, bars difpofed as in fig.nbsp;102.

It feems, that in order to render fteel magne-tical, we rnuft fo difpofe the pores that they may form contiguous tubes parallel to eachnbsp;Other, capable of receiving the magnetic fluid,-and then propagating and perpetuating its motion, fo that the magnetic ftream may enternbsp;with eafe, and be made to circulate throughnbsp;It with the greateft force r to this end, it is ne-ceffary to be particularly attentive in the choicenbsp;of the fteel which is to be touched; the grainnbsp;fhould be equal, fmall, homogenous, and without knots, that it may prefent a number ofnbsp;�qual and uninterrupted� channels to the fluid,nbsp;from one end to the other ; this is more immediately important in the choice of the fteel fornbsp;the needles of fea conlpaffes, for, if the fteelnbsp;is impure, or the mode of touching improper,;nbsp;the needle may have different poles communicated to It, which will more or lefs impede thenbsp;adtion of the principal needle according to theirnbsp;ftrength and fituation.

The fteel fhould be well tempered, that the pores may preferve for a long time the djfjjofi-�nbsp;tion they have received, and better refift thofenbsp;changes in their diredtion, to which iron andnbsp;foft fleel are liable. The difference in the na-Xnbsp;nbsp;nbsp;nbsp;turh

AN ESSAY

ture of fteel is exceeding great, as is eafiiy proved by touching in the fame manner, andnbsp;with the fame bars, two pieces of Heel ofnbsp;�qual fize, but of different kind*

Steel that is hardened, receives a more permanent magnetifnt than foft f�eel, thcf it does not appear that they differ frbm- each other in anynbsp;thing but the arrangement of the parts; perhaps the foft fteel contairrs phlogift�n in itsnbsp;largefl p�res^ while hardened fteel contains itnbsp;in the fmaller* Iron, or fteel, have very littlenbsp;jiir incorporated in their potes; when they arenbsp;feparated from the ore, they are expofed to anbsp;moft intenfe degree of heat, and moft of thenbsp;changes t� which they are afterwards fubmit-ted, are effected in a red hot ftate* A piece ofnbsp;fpring-tempered fteel will not retain as muchnbsp;magnetilfn as hard fteel, foft fteel ftill lefs, andnbsp;iron fcarce retains any. From fome experiments of Mr. Muffchenbroefc, it appears, thatnbsp;when iron is united with an acid,, it will notnbsp;become magnetical; but if the acid is feparated,nbsp;and the phlogifton reftored, it will become asnbsp;magnetical as ever.

The dimcnfions and lhape of a� magnet will make a difference in its force, therefore, thenbsp;bars to be touched, ftiould neither be too longnbsp;nor too ftiort, but in proportion to their thick-nefs; if they are too long, the paffage of thenbsp;magnetic matter coming out of one pole, and

ON MAGNETISM.

proceeding round the magnet to enter the other, will be impeded, and its velocity leffened. Ifnbsp;they are too fhort, the fluid which comes outnbsp;from One pole, will be repelled and thrown'nbsp;back by the other afling parts of the magnet,nbsp;and thus be carried too far from the pole intonbsp;which it ought to enters and prevent the continued circulation of the magnetic rfiatter. Ifnbsp;they are too thin^ then the number of pores arenbsp;too few to receive a flream fufSciently ftrong tdnbsp;rcfift the Obflacles in the external fpace j w'hile,nbsp;if they are too thick, the ftrait and regular di-reflion of the channel is injured by the difficulty which takes place in th� arrangement ofnbsp;the interior channels, as the rriagnetic matter hasnbsp;hot fufficient force to penetrate the flccl to anynbsp;confiderable depth, and thus injufes the circulation of the fluid; ,

All the pieces ffiould be well poliffied; it is of the greatefl importance that the ends fhOuIdnbsp;be fiat and true, fo as to touchy in as manynbsp;points as is poffible, the ends of foft iron whichnbsp;keep up the circulation. Inequalities on thenbsp;faces, but principally near the poles, are to benbsp;Avoided, as thefe occafion irregularities in thenbsp;circulation, and thus diminifh its velocity,'nbsp;which is one of the principal fources of magnetic power.

While the bars are touching, the ends of foft iron fhould be kept in conftant contadf withnbsp;the bars, for a momentary reparation is fufficient

to deftroy the cfFeftof the operation, as the fliiid will be inftantly difperfed in the air.

The operator ought not to flop longer on the firft bar than is neceffary to open the pores, andnbsp;to arrange them magnetically, paffing immediately to the other, to form an opening fornbsp;the fluid which ilTues from the firftj

It is moft advantageous to turn the bar that is quitted, while the touching magnets are placednbsp;on the other; by this means, the ftream that isnbsp;to be excited will difpofe the channels of the firft,nbsp;and thus render the operation more efficacious;nbsp;befides, by only turning one bar at a time, thenbsp;touching bars need never be totally removednbsp;during thewholeoperation, acircumftance whichnbsp;will contribute to the ftrength of the magnet.

The touching bars ffiould never be feparated but at the equator of the magnet; and their motion over the others, ffiould be flow and regular.

The magnetic power of touched needles has been encreafed by leaving them for fome time innbsp;iinfeed oil.

It may contribute to the effedts of the operation if the bars A and B, fig. loo, are placed in the diredtion of the magnetic meridian, andnbsp;are inclined to the horizon in an angle equalnbsp;to the dip of the needle.

The fixed pow'er, thus communicated to a luagnet, is impaired if it is laid amongft iron,nbsp;or by ruft; it may be injured alfo by fire, asnbsp;each of thefe circumftances will change, ornbsp;confufe the direction of the magnetic flream.

Place

ON MAGNETISM.

Place a fmall n^agnetic needle on the pivot of one of the fmall Hands, and put it between twonbsp;magnetic bars, fo that the north end of the barnbsp;may be near the fouth end of the needle j thenbsp;fmall needle will, without any apparent caufe,nbsp;be thrown into a violent vibratory motion, andnbsp;feem as it were animated, till it is faturated withnbsp;magnetifm, when it will become quiefcent.nbsp;The vibratory motion is probably occafioned bynbsp;the irregularity of the impreffions it receivesnbsp;from the magnetic fluid, and the difficulty thatnbsp;fluid finds in entering the needle.

All caufes, that are capable of making the magnetic fluid move in a ftream, will producenbsp;magnetifm in thofe bodies which are properlynbsp;qualified to receive it.

have no fixed polarity ; but if upon cooling a bar of iron in w'ater, the under end is confider-ably hotter than the upper, and the upper endnbsp;is cooled firft, it will fometimes become thenbsp;north pole, but not always. If iron, or fteel,nbsp;jundergo a violent attrition in any one particular parr, they will acquire a polarity ; if thenbsp;iron is foft, the magnetifm remains very littlenbsp;Jqnger than while the heat continues. Light-

Z 3 nbsp;nbsp;nbsp;piing

If bars of iron are heated, and then cooled .equally, in various diredbions, as parallel, perrnbsp;-pendic.ular, or inclined to the dipping needle,nbsp;the polarity will be fixed according to their po-fition, ftrongeft when they are parallel tp thenbsp;dipping needle, and fo lefs by degrees, till theynbsp;perpendicular to it, when they will

AN ESSAY

ening is the ftrongeft power yet known in prg-lt;lucing a ftream of magnetifm; it will, in an inftant, render hardened fteel ftrongly magne-tical, and invert the poles of a magnetic needle^nbsp;To make a magnetical bar with feverainbsp;poles, place magnets at thofe parts where thenbsp;poles are intended to be, the poles to be of anbsp;contrary name to thofe required, and if a fouthnbsp;pole is fixed on one part, the two next placesnbsp;muft have north poles fet againft them; con-fider each piece between the fupporters as anbsp;feparate magnet, and touch it accordingly,

J'here are certain points in a magnet in which its �virtues jeera as it were concentrated.

Let a magnet be placed on one of the brafs {lands contained in the apparatus, and thennbsp;try what number of iron balls it will fuftain atnbsp;different parts; it will be found to fupport moldnbsp;near the ends, evincing that the magneticnbsp;powder is exerted there with the gmateft force.

Place the fmall brafs weight, which is in the box, on the north end of the fmall dippingnbsp;needle, and then prefent the fouth end of anbsp;magnet to the end of the arch, this will repelnbsp;the end of the needle to a certain degree; thennbsp;move the magnet progrefiively forwards, andnbsp;the needle will fall down gradually till it comesnbsp;to zero. If the magnet is moved farther, thenbsp;index will be attradled towards it.

Jiff

To find the Foks of .a Magnet.

� Let a magnet be placed under one of thofc .panes of glafs which are contained in the bottom of the box ; lift fome lie,el filings on thisnbsp;glafs, and theo Itrike it gently with a key^ innbsp;order to throw the glafs into a vibratory mo-^:ion ; this will difcngage the filings, and theynbsp;will foon be arranged in a pleafing manner:nbsp;thofe parts of the magnet from which thenbsp;curves feem to take their rife, and over whichnbsp;the filings feem to % almoft eredt, are thenbsp;poles of the magnet.

In this, as well as njany other magnetical experiments, a mecliaaiicai force is evidently exerted., detaching the particles .qf iron from one fitua-tion, removing them to another, and then retain-ino� them there with confiderab'le force.

The poles of a magnet may b.e afcertalned with greater accuracy by means pf the fmallnbsp;dipping needier place this on a magnet, andnbsp;move it backwards and forwards till the needlenbsp;is perpendicular to the magnet, it will thennbsp;point direftly to one of the poles ^ when it Isnbsp;betw^een the north and fouth poles, fo that theirnbsp;�mutual aftions balance each other, the centernbsp;pf the needle will ftand over what is called thenbsp;nquator of the magnet, and the needle will benbsp;pxadlly parallel to the bar, If it is then removednbsp;Z 4nbsp;nbsp;nbsp;nbsp;tQr

AN ESSAY

towards either pole, it will be dilferently inclined according to its diftance from the poles.

Hold a common fmall fewing needle (with fome thread in its eye) near a magnet for a fewnbsp;feconds, then bring it gradually towards thenbsp;middle of a magnetic bar, and the powers ofnbsp;the magnet will fo far counteradf the force ofnbsp;gravity as to keep it fufpended in the air, in anbsp;pohtion which is nearly parallel to that of thenbsp;magnet.

There is no magnetical attracfllon without polarity; it is confequently abfurd to fuppofe,nbsp;that a magnet may have a ftrong attradfivenbsp;power, but a weak polarity, or diredlivcnbsp;power.

JLei' an iron rod he exa�ily balanced and fuf-fended on a faint ^ fo as to revolve in a flam parallel to the horizon ; communicate the magnetic virtue to this rod, and one extremity will be always directed towards the north.

Place any of the untouched needles in the apparatus on a point, and it may be fixed, ornbsp;will remain in any required fituation; communicate the magnetic virtue to it, and it will nonbsp;longer be indilferent as to its fituation ; but willnbsp;fix upon one, in preference to any other, one endnbsp;pointing towards the north, the other towardsnbsp;the fouth.

ON MAGNETISM. 345

it is not improbable, that in fome future period, it may be difcovered, that moft bodies are pofleffed of a polarity, and will afliime directions relative to the various affinities of thenbsp;elements of which they are compounded.

The directive power of a touched needle is of the greatefl: importance to mankind, as it enables the mariner to traverfe the ocean, and thusnbsp;nnite the arts, manufadtures, and knowledge ofnbsp;diftant countries, together* The furveyor, thenbsp;miner, and the aftronomer, derive many advantages from this wonderful property.

The mariner�s compafs confifts of three parts, the box, the card or fly, and the needle.

The card is a circle of ftifr paper reprefent-ing the horizon, vdth the points of the compafs marked on It; the magiietical needle is fixed tonbsp;the under fide of this card ; the center of thenbsp;needle is perforated, and a cap,with a conical agatenbsp;at its top, is fixed in this perforation; this cap isnbsp;hung on a fteel pin, which is fixed to the bottom of the box; the-box has a cover of glafs,nbsp;and is mounted on jimbals. At fome futurenbsp;period, I mean to treat of the various modesnbsp;that have been adopted in conftruifiing fea com-pafies, and of their various faults and excel-Jcncies-

It is by no means clearly decided who was fhe original inventor of the mariner�s compafs;nbsp;by fome it is attributed to Flavio Gioa, of A-palfi, in Campania, who lived about the beginning

AN ESSAY

ginning of the 14th century ; feme fay it came from the Eaft ; pthers that it was known evennbsp;earlier in Europe.

The north poles of two magnetSjwhen brought contiguous, repel each other. The fouth polesnbsp;alfo, w'hen brought near, repel each other.

Sufpend on a point a touched needle, then prei'eiit towards its north pole the fouth pole ofnbsp;a magnet, and it will be attradled by, and flynbsp;towards it; prefent the other pole of the magnet, and the needle will fly from it.

Fix two needles horizontaler in two pieces of cork, and put them in water; if the poles of thenbsp;fame name are placed together, they will mutually repel each other. If the poles of a contrary denomination are turned towards eachnbsp;ether, they will be attradled and join.

Dip the north or fouth ends of two magnets in fteel filings, which will hang in clufters fromnbsp;the end of the bars ; bring the ends of the barsnbsp;towards each other, and the fleel filings on onenbsp;bar, will recede from thofe on the other. Dipnbsp;the fouth pole of one magnet, and the northnbsp;pole of the other, into fteel filings, then let thenbsp;ends be brought near to each other, and thenbsp;tufts of filings will unite, forming frnall circular arches*

ON MAGNETISM. 34^

Place the cylindric magnet, which forms part of the apparatus, on a Ihiooth horizontalnbsp;plane, and bring the fteel filh near and parallelnbsp;to it, with its head towards the marked end ofnbsp;the magnet, and the round bar will roll from it jnbsp;turn the fifh fo that the tail may be towards thenbsp;marked end, and the magnet will follow it.

This curious property of the magnet was the foundation of the experiments that were ihewn innbsp;JLondon Ibme years fince by Comus, a great variety of them are deferibed in � Hooper�s Rational Recreation.� To explain the nature ofnbsp;thefe, a piece of brafs, filed into the lhape ofnbsp;an heart, is included in the apparatus^ a magnetnbsp;is inferred in this piece of metal; put the heartnbsp;into its box, ajid place a compafs over the boxnbsp;with the north pqint towards the middle ofnbsp;that part of the box where the cover flide^nbsp;out; obferve the direftion of the needle; thennbsp;take out the metal^ invert it, put it in its placenbsp;again, and obferve the direftion qf the needle;nbsp;by keeping thefe obfervations in mind, younbsp;may readily difeover which fide of the heart isnbsp;uppermoft, though put in yinknowii to you.

�The magnetic matter moves in a ftream fro^ �ne pole to the other, internally., and is then carriednbsp;lack in curved lines, externally, till it arrivesnbsp;again at the pole, where itfirji entered, to he againnbsp;admitted.

Put one of the glafs panes over a magnetical |)ar, fift fteel filings on the glafs, then ftrike

AN ESSAY

the glafs gently^ and the filings will difpofe themfelves in fuch manner as to reprefent,nbsp;with great exadnefs, the courfe of the magnetic matter. The curves by which it returnsnbsp;back to the pole, where it firfl: entered, are alfonbsp;accurately exprefled by the arrangement of thenbsp;filings. The largefl: curves rife from one polarnbsp;furface, and extend to the other i they are largernbsp;in proportion as they rife nearer the axis ornbsp;center of the polar furface ; the curves whichnbsp;arife from the fidcs of a magnetical body, arenbsp;interior to thofe which arife from the polarnbsp;furfaces, and are fmaller and fmaller in proportion to their diliance from the ends. Thatnbsp;the magnetic matter does move back, in a direction contrary to that with which it paffesnbsp;through the magnetical body, is confirmed bynbsp;its action on a fmall compafs needle, when pre-fented to it at different places. See fig. 103.

The greater the diftance is between the poles of a magnet, the larger are the curves whichnbsp;arife from the polar furface.

�The immediate caufe why two or more magnetical bodies., at trail each other, is the pajfage �gt;f one and the fame magnetical Jlream throughnbsp;them.

Let two magnets be placed at fome diftance from each other, the fouth pole of one oppofednbsp;fo the north pole of the other, lay a pane ot

glal5

ON MAGNETISM.

glafs over them, and fprinkle it with flee! filings, then ftrike the pane gently with a key,nbsp;and the filings will arrange themfelves in thenbsp;dire�lion of the magnetic virtue. The filingsnbsp;which lay between the two polar furfaces, andnbsp;near the common axis, are difpofed in ftraitnbsp;lines going from the north pole of one, to thenbsp;fouth pole of the other : the pores being now innbsp;the fame diredtion, fo that the fl'uid which pafi�snbsp;thro� A B, fig. 104, finds the pores at the pole ^nbsp;open to receive them, it will therefore pafis^nbsp;through this, and coming out at h will turnnbsp;towards A, to continue its ftream through thenbsp;magnet, and thus form one atmofphere or vortex, which prelied, on all fides, by the elaflienbsp;force of the other, carries the magnets towanl-snbsp;each other. At different diftances from thenbsp;axis the filings deferibe regular curve lines,nbsp;which run from one pole fo the other, and diverge from each other in moving from thenbsp;fouth pole, till they come half way, they thennbsp;converge more and more, till they arrive at thenbsp;north pole. If the oppofed poles are diftantnbsp;from each other, fome arches will pafs fromnbsp;one pole to the other of the fame magnet; fewernbsp;will be formed in this manner if they arenbsp;brought nearer together, and more will proceednbsp;from one magnet to the other; the ftream ofnbsp;the magnetic matter will feem more concentrated and abundant.

Whik

AN ESSAY

While the magnets remain In the foregolfrg jpofition, place a Imall untouched bar or needlenbsp;in the ftream of the niagnetlc virtue; thisnbsp;will pa:fs through itj and give it a polarity innbsp;the direction of the ftream.

On the fame principle,, a large key^ or other Untouched piete Of iron,, will attra�l and fup-port a fraall piece of iron, while it is within thenbsp;fphere of aflion of the pole off a magnet, butnbsp;tviH let them fall when it is out of the magnetic ftream.

A ball of fbft iron in cohtaff with a rnagner,i will attraft a fecond ball, and that a third, tillnbsp;the ftream becomes too weak to fuppOrt st-greater weight:

Put into motion one of the fmall whirligigs with an iron axis, and then take it up by a!nbsp;magnet; it will preferve its rotatory motionnbsp;much longer than if it were left to whirl onnbsp;the table ; a feeond and a third whirligig maynbsp;be fufpended one under another,� according tonbsp;the ftrength Of th� magnet, and yet continue irtnbsp;motion.

Place a magnet upon each of the brafs ftands, with their poles of contrary names oppofed tonbsp;each other, and a pleaftng chain of iron balltnbsp;may be fufpended between them. Prefent-either pole of another magnet toivards them,nbsp;end they will fall down.

If a large piece of iron is held at one pole of a magnet, it will encreafe the attraction of the

Magnetic Repulfion arifes from the accumuta-� tion of the magnetic fluid, and the refifluncsnbsp;formed to its e?itrance in the magnet.

If the two poles Cff the fame name of two magnets ate brought near to each other, and placed under a pane of glafs,�n which iron filings havenbsp;been ftrewed, the filings will be difpofed intonbsp;curves, which feem to turn back from each othernbsp;^towards theoppofite polev The fluid which proceeds from B, fig. 103, meeting with refiftanccnbsp;from the pores at D, is forced to turn back, andnbsp;circulate round its own magnet, and thus form twonbsp;atmofpheres,which ad againfleachother,in proportion to the force and quantity of the ftreamnbsp;which paffes through the magnets.

Take a fteel needle, with a very fine pointy and rub it from the eye to the point five or fixnbsp;times with the north pole �f a magnetic bar i thenbsp;Cye will be the north, and the point the fouthnbsp;pole of the needle.

The attraction and repulfion of magnets is not hindered or encrealed by the interpolitionnbsp;�f any b�dy whatever.

Dip the point of the needle in fteel filings, and it will take up a confiderable quantity-�nbsp;Take the magnetic bar in one hand, and thenbsp;negdle with the filings in the other, hold thenf

AN ESSAY

parallel to the horizon, with the point of ihd needle near the fouth pole of the magnet, andnbsp;the fteel filings will fall from the point of thenbsp;needle ,� as foon aS the filings dfop off from thenbsp;point, withdraw it from the fphere of adlioii ofnbsp;the magnet, and the point will be fo far deprived of its attradtive quality, that it willnbsp;hot again attract the fteel filings. If the needlenbsp;is not taken away, but continues for a few' minutes about half an inch' from th� bar, the polarity of the needle will be changed

Hang a ftumber of balls to each other, by applying the fil'd to the north pole of a magnet, prcfcnt the fouth pole of another magnet to onenbsp;�f the middle balls, and all thofe below it willnbsp;thereby b'c deprived of the magnetic ftream,nbsp;and fall afunder the ball to w'hich thdnbsp;inagnet w'as applied w'ill be attradled by it, andnbsp;all the others will remain fufpended. If thenbsp;north end of the magnet be prefented, then thenbsp;ball, to which it is applied, will alfa dr�p.

A fingular fadt is related by fome ancient tvriters on magnetifm. That if twm loadftones,nbsp;a ftronger and a w'eaker, have their repellentnbsp;poles brought together, the weaker will havenbsp;its power confuted, and will not come to itfelfnbsp;for fome days; the polarity of the part, in con-taft, becomes iriverted by the ftronger pow'er;nbsp;but as that power reaches but a little way beyond

ybnd the polar furface, the unaltered power, in. � the remaining part of the ftone, is able, by its,nbsp;contrary force, td reftore the ednfufed part ofnbsp;the done in a few days.

It does not appear that there is any certain Idw of attraflion peculiar to maghetifin ; for innbsp;different jtuirs of magnets, the force will varynbsp;at different diftances. The magnetic attractionnbsp;is not to be computed from the center ofnbsp;the magnetsj but from the center of thenbsp;pole.

Tho� rhahy experiments have been made to dif-G�ver,whether the force by which two magnetS are repelled or attra�ted, afts only to a certainnbsp;diftance; whether the degrees of its aftidn within, and at this diftance, is uniform or variable,nbsp;and in what pr�p�rt�nn, to the difta'ncCS it en-creafes or diminifhes; yet w� can only infernbsp;from, them, that the magnetic power extends further at fome times, than it does acnbsp;others, and that the fphere of its aftion isnbsp;variable.

The fmaller the loadftone or the magnet is,� the greater is its force, cceteris �paribus, innbsp;proportion to its iize. Though when thenbsp;axis of a magnet is Ihort, and of cdurfe itsnbsp;ploles very near, their adtidn on each othernbsp;vfoakens the magnetic force. A variety of othernbsp;caufes will alfo oedafion great irregularity in the^nbsp;attraflion of magnetifm. If one end of a magnetnbsp;{5 dipped in fteel flings, we ftuU find that they

are very feldom diftributed with uniformity, buf difpofed in little tufts, fome places more thicknbsp;than others. The force of magnetic attraction between the fame magnets, and at the famenbsp;dihance, may be varied by turning the magnetsnbsp;on their axis, and making different parts of thenbsp;polar furfaces regard each other. If a flrongnbsp;magnet be applied to a weaker, a kind of re-�pulfiOn feems to take place even between poles'nbsp;of the fame name, but its force is overpowerednbsp;by the attfadlion of the ftronger.

If a touched needle is placed near a magnet,, �its diredtion to the magnetic meridian is fuf-pended, and it affumes a diredlion relative to itsnbsp;Situation and diftance from the poles of thenbsp;magnet. Place a fmall needle on the pointednbsp;end of one of the brafs hands, and then bringnbsp;it near the magnet, the needle will diredt itfelfnbsp;differently, according to its diftance fromnbsp;the poles of the magnet. Thefe relativenbsp;lituations and tendencies are more pleafinglynbsp;pbferved by placing feveral touched needlesnbsp;round the bar at the fame time. The motion ofnbsp;the fmall dipping needle further illuftrates thisnbsp;propofition. From the three laft experimentsnbsp;various others of confiderable importance maynbsp;be derived for accurately inveftigating thenbsp;ourves, according to which, the magnets adt,nbsp;and iiluftrating further fome of the intricatenbsp;branches of magnetifm.

The

OK magnetism. 3SS

The northern magnetifni is deftroyedby the feommunication of the fouthern, and vice verfa.nbsp;Hence it is clear,- that the two magnetic powersnbsp;counteract each other, and that if both be communicated to the fame arm of a magnet, thenbsp;magnetifra acquired by the arm will be thatnbsp;of the ftrongeft, and as the difference betweennbsp;the two powers.

Two ftrait magnets will not be weakened, if they are laid parallel to one another, withnbsp;poles of the oppofite denomination corref-ponding to each othetj the ends being connected together by pieces of iron, whichnbsp;will keep up and facilitate the circulationnbsp;Of the magnetic fluid through them ; but theynbsp;fltould never be fulfered to touch each other^nbsp;except when they lie in the fame direction, andnbsp;�with poles of contrary hamesi

A Angle ftrait magnet fltould be always kept with its Ibuth pole towards the north. Or downcnbsp;'wards in the northern magnetic heraifphere, andnbsp;vice verfa, in the fouthern hemifphere. Ironnbsp;fltould never be lifted but by the fouth pole ofnbsp;a ftrait magnet in this hemifphere of the world.-Every kind of violent percuflion weakensnbsp;the power of a magnet ,� a ftrong magnet hasnbsp;been entirely deprived of its virtue by receiving feveral flnart ftrokes of a hammer ,� indeed^nbsp;whate\'cr deranges, or difturbs the internal poresnbsp;of a magnet, will injure its magnetic force, asnbsp;the bending of touched iron, wires, amp;c.

A a � nbsp;nbsp;nbsp;Fill

Fill a fmall dry glafs tube with iron filings^' prefs them in rather clofe, and then touchnbsp;the tube as if it was a fteel bar, and the tubenbsp;will attraft a light needle, StCi ihake the tubenbsp;fo that the fituation of the filings may be dif-turbed, and the magnetic virtue will vanifh.

But though a violent percuflion will deftroy a fixed hiagnetifm, yet it will give polarity to annbsp;iron bar which had none before; for a few fmartnbsp;ftrokes of an hammer, on an iron bar, will givenbsp;it a polarity, ^nd by hitting, firft one end ofnbsp;the bar, and then the other, while it is held in anbsp;vertical fituation the poles may be changed.nbsp;Twill a long piece of iron wire backwardsnbsp;and forwards feveral times, then break it off atnbsp;the twilled part, and the broken end will benbsp;magneticab

If a magnet be cut through the axis, the feg-ments, which were joined before^ will avoid and fly from each other.

If a magnet is divided by a fcdlion perpendicular to the axis, the parts' which were joined before will have acquired contrary poles, onenbsp;jiorth, the other fouth, thus generating a newfnbsp;magnet at every fedtipn

From thefe, and fimilar experiments, Mr. Ecles infers, that magnetifm confilts of twonbsp;different diftindt pow'ers, which in their naturalnbsp;flate are conj,oined, and exert but little fen-flble cdlion, and llrongly attraft each other atnbsp;all times; but when they are feparatedby force,-they ad like thofe of eledricity j for if mag-

ON MAGNETISM.

iietifm is excited in two different pieces of fteel by the fouth pole of a magnet, the ends repelnbsp;each other; but if one piece be excited by thenbsp;north pole, and another by the fouth, they willnbsp;attra�l each other. He further liippofes, thatnbsp;a magnet attracts, and is attrafled, not entirelynbsp;according to its own ftrength, but according tonbsp;the quantity of iron to be attradled ; and thatnbsp;magnetifm is a quality inherent in all iron, andnbsp;of which it cannot be di veiled; for fire, whichnbsp;W�ill deftroy a fixed magnetifm, does not deprive it of its natural quantity ; on the contrary,nbsp;it will give it a polarity, or fixed magnetifm,nbsp;according to the manner of heating or coolingnbsp;the iron.

is carried Lack, on ali kdc3, in curved lines to the contrary poles ; but when armour, or platesnbsp;of iron, are applied to each pole, the diredionnbsp;of the magnetic fluid is changed, and it is conduced, united, and condenfed, at the feet ofnbsp;the armour; fo that if the feet are conncded bynbsp;another piece of iron, which is called a lifter,nbsp;the ftream proceeding from one pole is carriednbsp;by the lifter to the other, which caufes it tonbsp;adhere with confiderable force, A chain ofnbsp;balls may be formed between the two feet in-ftead of a lifter.

Place the armed magnet under a glafs plane, ftrewed over with fteel filings, and thefe will benbsp;A a 3nbsp;nbsp;nbsp;nbsp;arranged

AN ESSAY

The armour flrould be formed of foft homogenous iron, well fitted to the ends of the magnets ; it flrould alfo be thicker, in proportion as the diftance of the poles from each other encreafes.

Mr. Savery has adduced feveral inftances to fliev/ the force and adlion of the earth�s mag-netifm; among others, that 'it will fupport fmallnbsp;pieces of iron. He hung up a bar of iron, aboutnbsp;five feet long, by a loop of fmall cord, at thenbsp;upper end, and then carefully wdped the lowernbsp;end, and the point of a nail, that there mightnbsp;be no dufi; or moifture to prevent a good con-tadb ; then holding the nail under the bar, withnbsp;its point upward, he kept it clofe to the bar,nbsp;holding only one finger under its head for thenbsp;fpace of thirty or more feconds, then withdrawing his finger gently downwards that the nailnbsp;might not vibrate; if it fell off, he wiped thenbsp;point as before, and tried ibme other part ofnbsp;the plane at the bottom of the bar. If the endsnbsp;are fimilar, and the bat has no permanent virtue, it is indifferent which end is downwards jnbsp;if it has an imperfed: degree of polarityj onenbsp;end will anfwer better than the other.

The upper end A of a long iron rod, which has no fixed polarity, will attrad the north endnbsp;of a magnetic needle; the under end B repels thenbsp;north end of the needle ; invert the iron bar,nbsp;and tire end B, which is now the upper one,

ON MAGNETISM*.

-tvIII attradl the north pole of the needle that it repelled before ; the cafe is the fame, if the barnbsp;is placed horizontally in the magnetic meridian, the end towards the fouth will be a northnbsp;pole.

Iron bars of windows, which have remained long in a vertical pofition, acquire a fixed por-larity. Mr. Lewenhoek mentions an iron crofs,nbsp;which was fuppofed to have flood on the fleeplcnbsp;of a church, at Velft, about 200 years, whichnbsp;had acquired a flrong fixed magnetifm.

The needle of the mariner's compafs, does net point exadily to the norths hut is obferved to chait^enbsp;its azimuth, pointing fometimes towards the eajt,nbsp;and fometimes to the wejl of the meridian.

This deviation from the meridian is called the variation of the needle, and is difterent atnbsp;.diflferent parts of the world, being well at fomenbsp;places, eaft at others, and in parts where th^nbsp;variation is of the fame name, its quantity isnbsp;very different.

Though the diredlive power of the compafs was applied to the purpofes of navigation in thenbsp;fourteenth and fifteenth century, it does notnbsp;appear that there were any apprehenfions duringnbsp;that time of its pointing otherways-than duenbsp;north and fouth.

The variation of the compafs is faid tp have |jeen firft difeovered by Columbus, in hi$ voy-

age,

was the fame to all needles in the fame place, is generally allowed to be Sebaftian Cabot.nbsp;This was about the year 1497.

After the variation was difcovered by Cabot, it was thought, for a long time, to be invariablynbsp;the fame, at the fame places, in all ages ; butnbsp;IMr. GelUbrand, about the year 1625, difco-vcrcd that it was different, at different times,nbsp;ill the fame place.

If a needle, which is accurately balanced, apd fufpcnded, fo as to turn freely in a verticalnbsp;plane, be rendered magnetical, the north polenbsp;unll be deprefed, and the fouth pole elevated abovenbsp;the horizon; this property, which is called thenbsp;dip of the needle, was difcovered by Robertnbsp;Norman, about the year 1576.

It is clear, that the magnetic ppwer exerts iifflf in two manners on a compafs needle; bynbsp;qne force it is diredled towards the magneticnbsp;pieridian ; by the other, it forms an angle withnbsp;the horizon.nbsp;nbsp;nbsp;nbsp;*

The pofition of a dipping needle, when at reft in the magnetical meridian, is called thenbsp;magnetical line.

Various kinds of round mapnets, termed Te-rellas, have been conftrudled with a view to iriveftigate the phoenomena of the variation,nbsp;* � ' 'nbsp;nbsp;nbsp;nbsp;and

ON MAGNETISM. 361

apd the dip of the needle, by obferving the po-Ijtion of a compafs at different parts of the Te-rella, and comparing thefe pofitions with the obferved date of the magnetic needle on thenbsp;earth. Little progrefs has been made with thefenbsp;on account of the imperfedtion of their conftruc-tion; one has, however, been invented by-Mr, Magellan, which bids fair to be of real ufe,nbsp;in difeovering the laws by which thefe myfte-rious properties are regulated, It will be foundnbsp;that molt of the pheenomena attending j:he di-redlion of the needle, correfpond to what happens to a needle placed on the Terella.

About the year 1722 and 1725, Mr. George Graham made a great number of obfervationsnbsp;on the diurnal variations of the magnetic needle.nbsp;I4 the year 1750, Mr. Wargentin took noticenbsp;of the regular diurnal variation of the needle ;nbsp;and alfo of its being difturbed at the time ofnbsp;an aurora borealis. Ahput the latter end of thenbsp;year 1756, Mr. Canton began to make obfervations on the variation, and 1759 communicated the following valuable experiments to thenbsp;Royal Society.

The obfervations were made by him for 603 �days, on 574, out of thefe, the diurnal variation was regular. Th ahfolute variation of thenbsp;needle weftward^ was encreafmg, from about 8 ornbsp;9 o�clock in the morning, till about i or 2 innbsp;m afternoon, when the needle became fta-^nbsp;nbsp;nbsp;nbsp;tionarr

AN ESSAY

tionary for fome time ; after that, the variation weftward was decreafing, and the needle camenbsp;back again to its former fituation in the night,nbsp;or by the next morning.

needle moves Jlowly eaftward, in the latter part of the mornings or weftward in the latter part ofnbsp;the afternoon ; alfo when it moves much either waynbsp;after nighty or fuddenly both wap in a jhort time.

^ Thefe irregularities feldom happen more^than on ce or twice in a month, and arc always acrnbsp;oorapanied with an aurora borealis.

The attradtive power of a magnet will dc-creafe while it is heating, and encreafe while it is cooling ; the greater the force of the lame'nbsp;magnet, the more it will lofe in a given degree of heat.

About ENE from a compafs, a little more than three inches in diameter, Mr. Cantonnbsp;�placed a fmall magnet two inches long, halfnbsp;an inch broad, and three-fwentieths of an inchnbsp;thick, parallel to the magnetic meridian ; and atnbsp;fuch a diftance, that the power of the fouth endnbsp;of the magnet was but juft fufficient to keep thenbsp;north end of the needle to the NE point, ornbsp;to 45 degrees.

ON MAGNETISM.

The magnet being covered by a brafs weight ^of fixteen ounces, about two ounces of boilingnbsp;, M'ater was poured into it, by whigh means thenbsp;magnet was gradually heating for feven or eightnbsp;minutes; and during that time, the needlenbsp;moved about three quarters of a degree weft?nbsp;ward, and became ftationary at 44� ^; in ninenbsp;minutes more, it came back a quarter of a dernbsp;gree, or to 44�nbsp;nbsp;nbsp;nbsp;; but � was fome hours before

On each fide of the compafs, and parallel to the magnetic meridian, he placed a ftrong magnet, of the fize above-mentioned; fo that thenbsp;fouth ends of both the magnets afted equallynbsp;on the north end of the needle, and kept it innbsp;the magnetic meridian; but if either of thenbsp;magnets was removed, the needle was attra�lednbsp;by the other, fo as to ftand at 45 degrees. Thenbsp;magnets were both covered with brafs weightsnbsp;of fixteen ounces each. Into the eafiern weightnbsp;about two ounces of boiling water was poured ;nbsp;and the needle in one minute moved half a degree, and continued moving weftward tor aboutnbsp;feven minutes, when it arrived at 2� It wasnbsp;then ftationary for fome time; but, in twenty-four minutes from the beginning, it came backnbsp;to 2� I, and in fifty minutes to 2^ He then

3^4 filled the weftern weight with boiling water, andnbsp;in one minute the needle came back to i � ^; innbsp;fix minutes more it flood half a degree eafl-ward ; and after that, in about forty minutes,,nbsp;it returned to the magnetic north, or its firftnbsp;fituation.

It is evident, that the magnetic parts of the earth in the north on the eaft fide, and the mag-iretic parts of the earth in the north on the weftnbsp;fide of the magnetic meridian, equally attradtnbsp;the north end of the needle. If then the eafteninbsp;magnetic parts are heated fafler by the fun innbsp;the morning, than the weftern, tfie needle willnbsp;move weftward, and the abfolute variation willnbsp;encreafe; when the attradling parts of the e^rthnbsp;on each fide of the magnetic meridian have theirnbsp;heat encreafing equally, the needle will be fta-tionary, and the abfolute variation will then be,nbsp;greateft ; but, when the weftern magnetic partsnbsp;are either heating fafter, or cooling flower thannbsp;the eaftern, the needle will move eaftward, ornbsp;the abfolute variation wdll decreafe ; and when,nbsp;the eaftern and weftern magnetic parts are cooling equally faft, the needle will again be fta-tionary, and the abfolute variation w'lll then benbsp;leaft. This may be ftill further illuftrated, bynbsp;placing the compafs and two magnets, as in thenbsp;laft experiment, behind a fereen near the middlenbsp;of the day in fummer; then, if the fereen benbsp;lb moved, that the fun may ftiine only on thenbsp;eafteyn magnet, the needle will fenfibly vary in

ON MAGNETISM. 365

its direftion, and move towards the weft ; and if the eaftern magnet be lhaded, while the funnbsp;ihines on the weftern, the needle will move th�nbsp;contrary way; By this theory^ the diurnal variation in the fummer ought to exceed that innbsp;th� winter; and we accordingly ftnd by obfcf-vation, that the diurnal variation in the monthsnbsp;of June and July, is almoft double that of December and January.

The irregular diurnal variation muft arif� from fame other caufe than that of heat com*-'nbsp;municated by the fun ; and here we muft hav�nbsp;rccourfe to fubtcrranean heat, which is generated without any regularity as to' timcj andnbsp;which will, when it happens in the north, af-'nbsp;fcdt the attractive power of the magnetic partsnbsp;of the earth on the north end of the needle.-The Reverend Dr. Hales has a good obferva-don on this heat, in the Appendix to the fecondnbsp;volume of his Statical Eflays, which I lhallnbsp;here tranferibe. That the warmth df thenbsp;� earth, at fome depth under ground, has annbsp;� influence in promoting a thaw, as well asnbsp;the change of the weather from a freezing tonbsp;a thawing ftate, is manifeft from this obfer-vation; viz. Nov. 27, 1731gt; a little fnownbsp;^ having fallen in the night, it was, by elevennbsp;the next morning, moftly melted away onnbsp;the furface of the earth, except in feveralnbsp;places in Bufhy-Park, w'here there w'erenbsp;drains dug, and covered with earth, where

AN E S S A

the fnow continued to lie, whether thofc drains were full of water, or dry; as alfdnbsp;where elm-pipes lay under-ground; a plainnbsp;proof that thefe drains intercepted thenbsp;warmth of the earth from afcending frontnbsp;� greater depths below them ; for the fnow laynbsp;where the drain had more than four feetnbsp;� depth of earth over it. It continued alfo tonbsp;lie on thatch, tiles, and th� tops of walls.�nbsp;That the air neareft the earth will be moffinbsp;warmed by the heat of it, is obvious; and thisnbsp;has frequently been taken notice of in the morning, before day, by means of thermometersnbsp;at different diftances from the ground, by thenbsp;Reverend Dr; Miles, at Tooting, in Surrey ;nbsp;and is mentioned in p. 526, of the 48th volumenbsp;of the Ehilofophicai Tranfadlions;

The aurora borealis, which happens at the time the needle is diflurbed by the heat of thenbsp;earth, is fuppofed to be the eledricity of the'nbsp;heated air above it ; and this will appear chieflynbsp;in the northern regions, as the alteration in thenbsp;heat of thofe parts will be greateft. This hy-pothefis will not feem improbable, if it be con-lidered, that eledricity is now known to be thenbsp;Caufe of thunder and lightning, that it has beennbsp;extraded from the air at the time of an auroranbsp;borealis; that the inhabitants of the northernnbsp;countries obferve the aurora to be remarkablynbsp;ftrong, when a fudden thaw happens after fe-vere cold weather ; and that the curious in thefe

ON MAGNETISM.

matters are now acquainted with a fubdance,, that will, without friftion, both emit and ab-forb the electrical fluid, only by the encreafenbsp;or diminution of its heat: for if the Tourmalinnbsp;be placed on a plane piece of heated glafs, ornbsp;metal, fo that each fide of it, by being perpendicular to the furface of the heating body, maynbsp;be equally heated, it will, while heating, havenbsp;the electricity of one of its fidcs pofitive, andnbsp;that of the other negative ; this will likcwife benbsp;the cafe w^hen it is taken out of boiling waterynbsp;and fuflered to cool; but the fide that rvas pofitive while It was heating, will be negative whilenbsp;it is cooling, and the fide that was negatircy.nbsp;�'ll! be pofitive.

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CATALOGUE

GEORGE ADAMS,

Optical Inftruments.

I	I	0
X	16	0
0	15	0
i	10	0
0	10	6
0	7	6
0	5	0
0	3	6
0	7	0
0	4	0
0	I	0

Ditto mounted in tortoifeftiell and filver, pearl and filver, in various manners, and at different prices

Reading glaffes, from 2s. 6d, to Opera glaffes, from lOs, 6d. tonbsp;Ditto of an approved conftruCtionnbsp;nbsp;nbsp;nbsp;*

Acromatic profpe�ls, from 15s. to nbsp;nbsp;nbsp;�

Acromiatic telefcopes, with brnfs drawers, which may be drawn out at once, and thatnbsp;ihut up, conveniently for the pocket, fromnbsp;2I. 12s. 6d, to �nbsp;nbsp;nbsp;nbsp;�.

An optical vade mecum, or portable acromatic telefcope and microfeope, for tranf-parent and opake objects, amp;c. from 31.13s. 6d tonbsp;nbsp;nbsp;nbsp;�nbsp;nbsp;nbsp;nbsp;�

A thirty inch acromatic telefcope, different eve pieces for terreitrial and celeftial objects,nbsp;from 81. 8s. tonbsp;nbsp;nbsp;nbsp;�^nbsp;nbsp;nbsp;nbsp;.�

Ditto with rack, work

An acromatic telefcope, about three feet and an half long, witn different eye piecesnbsp;JRefle�ting telefcopes, of all the various ftzesnbsp;A three foot reflecting telefcope with four mag�-nifying powers

A ditto with rack work nbsp;nbsp;nbsp;-

�A two loot erieCting telefcope, with rack work and four magnifying powersnbsp;nbsp;nbsp;nbsp;�

A two foot reflecting telefcope with two mag-nifying powers nbsp;nbsp;nbsp;��nbsp;nbsp;nbsp;nbsp;,nbsp;nbsp;nbsp;nbsp;_

An eighteen inch ditto nbsp;nbsp;nbsp;�- ��

A twelve inch ditto nbsp;nbsp;nbsp;�nbsp;nbsp;nbsp;nbsp;�

pouble reflecting inicrofeopes, from 31.13s. 6d,

Ellis�s aquatic microfeope nbsp;nbsp;nbsp;�

Wilfon�s microfeope, improved nbsp;nbsp;nbsp;��

Adams�s lucernal microfeope for opake and tranfparent objeCts, being the moft perfeChnbsp;inftrument of the kind hitherto contrived,nbsp;affording more entertainment and inftruc-tion than any other microfeopenbsp;nbsp;nbsp;nbsp;-r-

Solar microfeopes nbsp;nbsp;nbsp;�

Ditto for opake objeCts pittonbsp;nbsp;nbsp;nbsp;�

Camera obfeuras, from, 12s. to Glafs prifms, from ys, hd. tonbsp;nbsp;nbsp;nbsp;�^

Magic laiiAorns

6	16	6
3	3	0
40	0	0
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31	10	0
1000 le	0	0
31	10	0
12	12	0
3	3	0
800	0	0

Adams�s new eighteen inch globes mounted in the moft approved form, from 9I. gs. tonbsp;Ditto in the Ptolemaic or common mannernbsp;Ditto, twelve inches in diameter, mounted innbsp;the beft manner, from 5I. 5s. tonbsp;Ditto in the common mannernbsp;nbsp;nbsp;nbsp;�

Adams�s improved equatoreal dial, or po obfervatory, from 7!. rys. 6d. tonbsp;Horizontal fun dials, from 5s. tonbsp;Univerfal ring dials, from ys, 6d. tonbsp;Tranfit intirumentsnbsp;Aftronomical quadrants, from 50I. to

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Adams�s fedtoral, elliptical, and callipper compafles in one inltrumentjfrom 4I. 14s.6d. to Adams�s protradling parallel rules, and univerfal plotting fcale

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b	0	0
14	14	0
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8	18	6
31	ro	0
10	10	0
5^	15 nbsp;nbsp;nbsp;6 Phi-

s. i.

to - - 40 o o

4	14	6
6	16	6
10	10	0
31	10	0
30	0	0
2	2	0
2	12	6
to 5	5	0
i I	It	6
0	18	0
2	12	6
3	13	6

Small fingle barrelled air pump Small double dittonbsp;A larger dittonbsp;A table air pump

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14	AN E S	s A y
White filk	7 Pofitive	Black filk, metals, black cloth.
White filk	^ Negative ^	Paper, hahd, hairs, Weafel�S Ikin.
	1 Pofitive 1	Sealing wax.
Black filk	gt; nbsp;nbsp;nbsp;r	Hate�s, weazePsj and fer
	(Negative J	ret�s fkins, loadllonej brals, filver, iron, hand.
Sealing wax	^ Pofitive 1 gt; nbsp;nbsp;nbsp;f	Metals Hare�s* weafel�s, and fer
Sealing wax	(Negative J	ret�s ikins, hand, leather, Woollen cloth, paper.
	(Negative J	ret�s ikins, hand, leather, Woollen cloth, paper.
	') Pofitive \	Silk
Baked wood.	\ 1 V Negative lt;	Flannel*

	- '-■- nbsp;nbsp;nbsp;: -l'-nbsp;nbsp;nbsp;nbsp;. . '■ -■	Vv'-v	' ' nbsp;nbsp;nbsp;—-nbsp;nbsp;nbsp;nbsp;'nbsp;nbsp;nbsp;nbsp;.tj

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PREFACE.

PREFACE.

much

preface. V

PREFACE.

the-

CONTENTS.

CHAP. I.

CHAP. ir.

CHAP. III.

CONTENTS.

NTS.

CHAP. VII.

99

C H A P.

CHAP. X.

CHAP.

CHAP. XI.

CHAP. XU.

CHAP. XIII.

C PI A P. XIV.

0 ...

ELECTRICITY.

C.-H A P. I.

hands

AN ESSAY

ON ELECTRICITY.

AN ESSAY

ON ELECTRICITY. 7

Catalogue of conduding fubftances.

AN ESSAY

4. Metals

AN ESSAY

ON ELECTRICITY. if

was

AN ESSAY

White

ON ELECTRICiTY. i$

i6 nbsp;nbsp;nbsp;ANESSAY

CHAP.

AN ESSAY

Ca nbsp;nbsp;nbsp;If

AN ESSAY

D and

ON ELECTRICITY. it

C 3 nbsp;nbsp;nbsp;lurs

AN ESSAY

any

ON ELECTRICITY.

C 4 nbsp;nbsp;nbsp;The

AN ESSAY

AN ESSAY

ON ELECTRI Cl T Y. 29

AN ESSAY

ON ELECTRICITY.

AN ESSAY'

which

AN ESSAY

e H A p. III.

Properties of E�eSiric AttraBion and Repulfon, illujlr(ited by Experiments onnbsp;light Bodies.

3^ nbsp;nbsp;nbsp;� AN � f �� A 't

3. nbsp;nbsp;nbsp;Bodies eledlrified negatively repel eachnbsp;other.

ON'ELECTRICITY. 39

Ex-

AN essay

AN ESSAY

owh

AN ESSAY

ON ELECTRICITY. 49

AN ESSAY

CHAP. IV.

AttraSlion and Keputfion corijidered^ relative to the two Jiates of EleSlricity.