ELECTRICITY

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contrary electricity on the opposite side. How thick a glass plateshould be to become incapable of being charged, hath not yetbeen determined.”

Or Atmospherical Electricity, and its Effects ; parti-cularly Lightning.

Electricity is not only to be observed by rubbing an electricor wanning a tourmalin, but has been found to be widely diffusedthrough the atmosphere. It has been discovered that the air, theclouds, and even the rain, contain it; and that thunder and light-ning, the aurora borealis, meteors, &c. are among its various andastonishing effects. The great similarity between lightning andthe electric fluid had been for some time remarked by philoso-phers, particularly by the Abbfi Nollet; but it was not suspectedthat they were the effects of the same cause, much less that thephxuomena of lightning could be imitated by electricity, andthose of electricity by lightning, till the celebrated Dr. Franklinhazarded the bold assertion, and advised the experiment tobe made, and till he and the French philosophers proved the factin 1752. Indeed, all the wonderful phxnoinena exhibited byelectricians are only imitations on a small scale of those greateffects which astonish and alarm us; and they depend upon thesame mechanism. The saint properties, and zig-zag sparks, theirsimilar action on conducting substances, the power of rending,inflaming, and dispersing in every direction the substances onwhich lightning acts with power, the giving polarity to ferruginousmatter, &c. all concur to shew their identity, But independentof these similarities, the fact is proved by the plainest and clearestevidence; when the atmosphere is charged with thunderclouds,we can by an electrical kite draw from it the matter of lightning,and with this matter perform every electrical experiment withwhich we are acquainted. There is not indeed a single pheno-menon of the one, but may be imitated by the other. Lightningdestroys ediiices, trees, and animals; it goes through the best con-ductors that it meets within its course, and if its passage be ob-structed by electrics, or less perfect conductors, it rends them,tears them to pieces, and disperses them in all directions. Light-,ning melts metals, and burns inflammable substances, &c. all ofwhich effects can be produced by electricity. The air at somedistance from houses, trees, masts of ships, Sec. is commonly elec-trified positively, especially in frosty and clear weather. It isalso electrified in loggy weather. “ Clouds as well as rain, snow,and hail, sa'ys Mr. Cavallo, are almost constantly electrified, butoftener negatively than positively. I low they come to be so, isnot yet clearly ascertained, unless upon the principle of electricityproduced by evaporation and condensation of lightning is theelfcet of their electricity, which darting from a cloud, or a num-ber of clouds, highly electrified, strikes into another cloud, orupon the earth; preferring the most lofty and pointed places, andthus producing all those dreadful effects which are known to beoccasioned by it,” and of which it may not be improper to give adescription, with an instance or two. When lightning is accom-panied with thunder, it is well defined, and has generally a zig-zag form; sometimes it only makes one angle like the letter V,sometimes it appears like the arch of a circle. But the most for-midable and destructive form which lightning is ever known toassume, is that of balls of tire. The motion of these is very oftenperceptible to the eye; but wherever they fall, much mischief isthe result of their explosion. The next to this, in its destructiveeffects, is the zig-zag kind; for that species, whose flashes are in-distinct, and whose form cannot easily be observed, is seldomknown to do much hurt. The colour of lightning is also an indi-cation of its power to do mischief, the palest and brightest flashesbeing most destructive in their effects. The zig-zag Kind of light-ning, when near, is remarkable for a kind of omnipresent property.If two persons be standing in a room, looking different ways,and a loud clap of thunder happens, accompanied with the zig-zaglightning, they w ill both distinctly see the flash, not only by (hatindistinct kind of illumination of the atmosphere, which is occa-sioned by tire of any kind, but the very form of the lightning it-self, and every angle it makes in its course will be as distinctlyperceptible, as though they had looked directly at the cloud fromwhence they proceeded. If a person happened at that time to bereading, he would distinctly see the form of the lightning betweenhimself and the hook. Lightning usually confines its effects to asmall space; seldom exhibiting pha'nomena like those which ac-

vol. it.—so. 72.

company explosions of gunpow der, or of inflammable air in mines.Instances, however, of terrible explosions have occurred. Thefollow ing is perhaps one of the most remarkable: “ August 2,1763, about six in the evening, there arose at Anderlighl, about aleague from Brussels, a conflict ot several w inds born upon a thickfog. This conflict lasted four or five minutes, and was attendedwith a frightful hissing noise, which could be compared to nothinghut the yellings of an infinite number of wild beasts. The cloudthen opening, discovered a kind of very bright lightning, and iban instant the roofs of one side of the houses w ere carried olf anddispersed at a distance; above 1000 large trees were broke oli,some near the ground, others near the top, some torn up J>v theroots; and many of the branches and tops carried to the distanceof 60, 100, or 120 paces. Whole coppices were laid down, as con*is by ordinary winds; and the glass of the windows, situated nearthe spot, was shivered to atoms.” It is not unusual for thunder-storms to produce most violent whirlwinds, such as are by somephilosophers attributed to electricity; nay, even to occasion anagitation in the waters of the ocean itself; and all this too after thethunder and lightning has ceased. Of this the following instancehappened at Great Malvern , October 26, 1761: at a quarter pastfour in the afternoon, the people were surprised with a mostshocking and dismal noise; 100 forges, all at work at once, couldscarce equal it. Upon the side of the hill, about 400 yards to theS. W. there appeared a prodigious smoke, attended with the sameviolent noise, as if a volcano had burst out of the hill; it soon de-scended, and passed on within-about 100 yards of the south endof the house ; it seemed to rise again in the meadow just below it,and continued its progress to the east, rising in the same mannertor four different times, attended with the same dismal noise as atfirst; the air being filled with a nauseous and sulphureous smell.It gradually decreased till it was quite extinguished in a turnipfield, about a quarter of a mile below the house; the turnip leaves,with leaves of trees, dirt, sticks, &c. filled the air, and flew higherthan any of the hills. The thunder ceased before this happened,and the air soon afterwards became calm and serene.” After theidentity of lightning and the electric fluid was discovered, electri-cians began to describe other phenomena, formerly unaccountedfor, to atmospherical electricity. Among these the aurora borea-lis was naturally, and with reason, ascribed to this cause. SeeAurora Borealis . This conclusion was drawn not only fromthe imitation of its light by the electrical spark, but also from theobservation, that the aurora borealis, when very strong, has beenknown to disturb the polarity of the magnetic needle. (See Phil.Trans, vol. lix. p. 88.) The late Mr. Canton made several expe-riments in continuation of this idea, by collecting considerablequantities of the electric fluid, during the time of an aurora borea-lis. IIis apparatus was very simple; it consisted of an insulatedfishing-rod, with a wire twisted round it, erected on the top of hishouse. From his experiments performed with the atmospherical-electrometer, and (lie electrometer for the rain, Mr. Cavailo-deduces the following conclusions, respecting the electricity oftile atmosphere:

“ I. That there is in the atmosphere, at all times, a quantity ofelectricity; for whenever I use the above-mentioned instrument,it always acquires some electricity.

“ 11’. That the electricity of the atmosphere, or fogs, is alwaysof the same kind, namely, positive; for the electrometer is alwaysnegative, except when it is evidently influenced by heavy clouds,near the zenith.

” III. That, in general, the strongest electricity is observable inthick togs, and also in frosty weathe r; and tile weakest, when it iscloudy, warm, and very near raining: but it does not seem to beless by night than in the day.

“ IV. That in a more elevated place the electricity- is strongerthan in a lower one; for having tried the atmospherical electro-meter both in the stone and iron gallery on the cupola of St.Paul’s cathedral , I found that, the balls diverged much more in(he latter, titan in the former less elevated place, lienee it ap-pears, that if this rule takes place at any distance from the earthrthe electric ity in the upper regions of the atmosphere must be ex-ceedingly strong.”

Of the Advantage’s dfrived from Electricity .

Under this head, to which M. Cavailo devotes a whole chapter,of his excellent treatise, he ob.vrves, that “ borides the field that..

3 U electricity