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more obfervers at confiderable difhnces from each other, even as far as two miles.
In thefe laft experiments, and in many others of the like nature, Mr. Watfon obfervcs, that the eieSirical power, ac- cumulated in any non-eleSlric matter contained in a glafs phial, defcribes upon its explofion a circuit through any line of fubftances non- eieSirical in a confiderable degree ; if one end thereof is in contact with the external furface of this phial, and the other end upon the explofion touches ei- ther the eleSirified gun-barrel, to which the phial in charg- ing is ufually connected, or the iron hook always fitted therein. Thus if a perfon ftands upon a dry wooden floor with a coated phial ever ib highly charged in one of his hands, and if another perfon, without touching the firft, Itands but fix inches from him, and touches the iron hook of the phial, neither of them are fhocked ; becaufe the floor between them, though the distance is fo fliort, will not con- duit the eleSiricity fufficiently quick. But if thefe two per- fons tread upon a piece of wire laid between them, they each of them feel the eieSirical commotion in that arm which touches the phial and hook, and in that foot which treads upon the wire; the wire here conducting the eleSiricity quick enough, which the dry floor would not. The circuit is here formed by ihe coated phial, its hook, fo much of the bodies of thefe two perfons as form a curve line between the wire, the phial, and hook, and the wire between thefe perfons. If thefe perfons fland upon, or touch with any pat of their bodies any non-ekSlrics, which readily conduct eleSiricity, the circuit is completed, and the effect is the fame. Hence Mr. Watfon infers, that when the obfervers iiave been fhocked at the end of two miles of wire, the eieSirical circuit is four miles ; viz. two miles of wire, and the fpacc of two miles of the non-eleSlric matter between the obfervers, whether it be water, earth, or both. So in the experiments made near Weftminftcr bridge, where the river is above 400 yards over, the circuit mutt have been above 800 yards, viz. above 400 yards of conducting wire laid upon the Portland ftones of the bridge, and fomething more than 400 yards of water. That the water made part of the circuit, and that the fhock was not communicated from the Surry ihore to the other fide menely by the conducting wire, is evident from this, that if the obferver on the Weft- minfter fhorc did not communicate with the river, either by dipping his hand, or an iron rod held therein into the river, the fhock anting from the difcharge of the phial was not at all felt by him, although the obferver on the Surry fide felt it as before. This circuit of the eieSirical matter has been farther confirmed by feveral other ingenious expe- riments by the fame gentleman.
It is to be obferved, that the commotion is equally ftrong, whether the eleSiricity is conducted by water, or dry ground. And that if the wires, between the deSirifying machine and the obfervers, are conducted upon dry flicks, or other fub- ftances non-eleSlric, in a flight degree only, the effects of the eieSirical power are much greater, than when the wires in their progrefs touch the ground, rnoift vegetables, or other fubftances in a great degree non-eleSlric,
31. It is remarkable, that if the eieSirical machine, and the man who turns the wheel thereof, be mounted upon eieSiri- cal cakes, the eieSirical power is fo far from being increafed hereby, as fome might expect, that it is, on the contrary, much diminifhed, and fometimes is not at all perceptible. It was this experiment which induced Mr. Watfon to con- ceive, that the eieSirical power was not inherent in the glafs, but came from the floor of the room. And, he fays, he was confirmed in this opinion when he found, that if he touched the frame of the machine with one hand, while he ftood upon the floor, and with a finger of the other hand touched the gun-barrel, the fire would iflue, and the mapping would continue as long as he held his hand upon the machine, but would ceafe upon his taking it off. In like manner, if the man, who turned the wheel, 'put one of his feet upon the floor, the other remaining upon the wax, the eleSiricity at the gun-barrel would fnap, and ceafe upon his replacing his foot. But of this hypothefis more hereafter.
32. The accumulation of eleSiricity may be meafured to great exa£tnefs by the following method, difcovercd by Mr. Canton. When the phial is fufficiently eleSirified, by ap- plying the wire thereof to the glafs globe, which is known by the appearance of a brulh of flame at the end of the wire, hang a flender piece of wire to the fufpended gun- barrel, detached for this purpofe from the globes. Upon ■your applying the wire of the eleSirified phial to that hang- ing to the gun-barrel, you perceive a fmall fnap j this you difcharge by touching the gun-barrel with your finger, which likewife fnaps : and thus alternately eleSirifying and difcharging, you proceed until the whole eleSiricity of the water is diffipated, which is, fometimes, not done under an hundred difcharges. If you do not difcharge the eleSiricity every time, the fnaps from the wire of the eleSirified phial to the gun-barrel, are fcarcely perceptible. The acquired eleSiricity of the water is eftimated m proportion to the num- ber of itrokes.
This accumulation of deSirkity may perhaps be thought to Append, Vol. II.
deferve a farther explanation, and may be. thus iiluftratedi As we take it for granted, that there is always a determi- nate quantity of atmofphere furrounding the terraqueous globe, we conceive, when we fee the mercury in the ba- rometer very low, that there then is a 3efs accumulated co- lumn of this atmofphere impending over us, than when we fee the mercury high. In like manner, When we obferve that the eleSirified gun-barrel attracts or repels only very light fubftances, at a very fmall diftance, or that the fnap and fire therefrom are fcarce perceptible, we conceive then. a much iefs quantity of eieSirical atmofphere furrounding the gun-barrel. This power being more or lefs, is called the greater or lefs degree of the accumulation of eleSiricity. And this is only attainable to a certain point, if you elec- trify ever fo long ; after which, unlefs otherwife directed, the diffipation thereof is general. In the phial filled with water, in Mufchenbroek's experiment, or filled with any other non-eleSlric, as filings of iron, if the wire be held to the globe in motion, when the accumulation is complete, the furcharge runs off from the point of the wire, as a brufh of blue flame That by flopping the current of eleSiricity^ iion~clcSlrics might be excited ; and that by accumulating their power, they might be made to exert more force than deSirics per fe would at any point of time, is that capital difcovery of the late Mr. Gray ; which is to be regarded as the bafis upon which all the prefent improvements of our knowledge in eleSiricity are founded.
33. By comparing the refpective velocities of eleSiricity and found, that of eleSiricity, in any diftance yet experienced, ap- pears inftantaneous.
For the detail of thefe curious experiments, and of feveral others, we muft refer to Mr. Watfon's experiments and ob- fervations, tending to illuftrate the nature and properties of eleSiricity, Lond. 1745; and the fequel to the experi- ments, &c. Lond. 1746; as alfo to his account of the ex- periments made by fome gentlemen of the Royal Society^ Lond. 1748.
34. Mr. Franklin of Philadelphia, In his experiments and ob- fervations on eleSiricity, has obferved, that granulated lead is better than water for filling the phial or glafs jar to be eleSirified; and he has been able to excite the eieSirical force to fuch a degree, as not only to fire gunpowder a , and to kill a turkey of ien pound weight b , but to produce a fufion of metals c .— [« Phil. Tranf. Vol. XLVIL p. 291. "Ibid, p. 209. c Ibid. p. 290.]
This ingenious author's method of firing gunpowder by the eleSirk flame has fomething particular, as it does not require any inflammable vapour to be previoufly raifed. The powder may be fired thus : a fmall cartridge is filled with dry powder, hard rammed, fo as to bruife fome of the grains. Two pointed wires are then thruft in, one at each end, the points approaching each other in the middle of the cartridge, till within the diftance of half an inch : then the cartridge being placed in the circle, when four eleSirified glafs jars are difcharged, the eleSlric flame leaping from the point of one wire to the point of the other, within the cartridge among the powder, fires it, and the explofion of the powder is at the fame inftant with the crack of the eieSirical difcharge.
35. As to metals, this gentleman obferves, that he has been able by eleSiricity frequently to give polarity to needles, and to reverfe it. A fhock from four large glafs jars fent thro* a fine fewing needle, gives it polarity ; and it will traverfe, when laid on water. The polarity is given ftrongeft, when the needle is ftruck lying north and fouth ; and weakeft, when lying eaft and weft. In this cafe, the end entred by the eleSlric blaft points north : but when the needle lies north and fouth, the end that lies towards the north will continue to point north, whether the fire entred at that end^ or the contrary end.
36. In thefe experiments, the ends of the needles are fome- times finely blued, like a watch-fpring, by the ^nV flame. This colour, given by the flam from two glafs jars only, will wipe off; but four will fix it, and frequently melt the' needles. Sometimes the furface on the body of the needles is alfo run, and appears bliftered, when examined by a mag- nifying glafs. The jars Mr. Franklin made ufe of hold feven or eight gallons, and are coated and lined with tin- foil. Each of them takes a thoufand turns of a globe of nine inches diameter to charge it. Tin-foil has been melted between glafs by the force of two jars only. Phil. Tranf. Ioc. cit.
37. The effects of lightning and thofe of eleSiricity appear very fimilar. Lightning has been known to ftrike people blind, the eieSirical fhock has had the fame effect on ani- mals* Animals have been killed by both. The mariners compafs has fometimes loft its virtue by lightning ; and by Mr. Franklin's experiments it appears, that polarity may be given and rcverfed by eleSiricity. The late Mr. Stephen Gray obferved feveral years ago d , that the eleSlric fire feem- ed to be of the fame nature with that of thunder and light- ning. Others have fince fallen into the fame opinion. In- deed many experiments fhew, that the flame of eleSiricity has been juftly called by Mr. Franklin a mimic lightning % fince. it blinds and kills animals, and melts metals., &c
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