LIGHTNING. 



ofe, and propofcJ a method, whicli he oon accomplilhcd, of 

 fecuring buildings from being damaged by liglitniii^, by 

 means of conduftors. The Enghfli philofophers had no: 

 been lefs attentive to this fiibjcft than their neighbours on 

 the continent ; but for want of proper opportunities for 

 trying the nccefTary experiments, and from fome incidental 

 circumdances that were unfavourable, they had failed of 

 fuccefs. However, in July, 1752, Mr. Canton fucceeded ; 

 and in the following month, Mr. Wilfon and Dr. Bevis ob- 

 ferved nearly the fame appearances which Mr. Canton had 

 obferved before. Mr. Canton alfo foon after obfervcd, in a 

 number of experiments, that fome clouds were in a pofitive 

 and fome in a negative ftate of eleClricity ; and that the elec- 

 tricity of his conduftor would fometimes change f.'om one 

 ftate to the other, five or fix times in lefs than half an hour. 

 This variable llnte of thunder clouds was difcovered by S. 

 Beccaria, before he heard of its having been obfervcd by 

 Dr. Franklin, or any other perfon : and he has given a very 

 exaft and circumftantial account of the external apj)earances 

 of thefe clouds. From his obfervations of the lightning 

 abroad, and of his apparatus within doors, he inferred, that 

 the quantity of cleflric matter, in an ufual ftorm of thunder, 

 is almofl: inconceivably great, confidering how many pointed 

 bodies, as trees, fpires, &c. are perpetually drawuig it olf, 

 and what a prodigious quantity is repeatedly difcharged to 

 or from the earth. This quantity is fo great, that he thinks 

 it impofiible for any cloud or number of clouds to contain it 

 all, fo as either to difcharge or receive it. Befides, he ob- 

 ferves, that, during the progrefs and increafe of the (lorm, 

 though the lightning frequently ftruck to the earth, the fame 

 clouds were the next moment ready to make a ftill greater 

 difcharge, and his apparatus continued to be as much affccled 

 as ever ; and, therefore, the clouds mull ha%'e received at 

 one place, in the fame moment when a difcharge was made 

 from them in another t and, upon the whole, he infers, that 

 the clouds ferve as co- duClors to convey the eleftric fluid 

 from thofe places of the earth that are overloaded with it, 

 to thofe which are exhaufted of it. This eleclric matter, 

 the rife of which, from the earth into the higher regions of 

 the atmofphere, is afcertained by the great quantities of fand, 

 aflies, and other light fubftances, carried up with it, and 

 fcattered uniformly over a large tra& of country, wherever 

 it iffues, attracts to it, and bears up with it the wat(*ry par- 

 ticles that are difperfed in the atmofphere. It afcends into 

 the higher regions of the atmofphere, being folicited by the 

 lefs refinance it finds there than in- the common mafs of the 

 earth, which, at thefe times, is generally very dry, and 

 confequently highly eledlric. The fame caufe which firll 

 raifed a cloud, from vapours difperfed in the atmofphere, 

 drawe to it thofe that are already formed, and continues to 

 form new ones, till the whole coUefted mafs extends fo far 

 as to reach a part of the earth where tliere is a deficiency of 

 the qlectric fluid. Thither, too, will thofe clouds, replete 

 with elctlricity, be llronjjly attraAed, and there will the 

 elediric matter difcharge itfclf upon the earth : a channel of 

 communication being m this manner formed, a frefli fupply 

 of electric matter will be raifed from the overloaded part, 

 and will continue to be conveyed by the medium of the 

 clouds, till the equilibrium of the fluid between the two 

 places of the earth be rellored. When the clouds are at- 

 tracted in their paflage by thofe parts of the earth, where 

 there is a deficiency of the fluid, thofe detached fragments 

 are formed, and a'fo thofe uniform defcending protuberances, 

 which are, in fome cafes, the caufe of Water-fpouts and Hur- 

 rkanes ; which fee. 



That the eletlric matter, which forms and animates the 

 thunder-clouds, ilTues from places far beiow the furface of 



2 



the earth, and that it buries itfclf there, is probable fronr 

 the deep holes that liave, in many places, been made by 

 lightning ; and from the flaflies that have been feen to arife 

 from fubterraneous cavitias and from wells ; as well as from 

 the inundations accompanying thunder-llorms, and occa- 

 fioned by water buriling out of the bowels of the earth. 

 The greatell difficulty attending this theory of the origin of 

 thunder-ltorms relates to the collection and infulation of 

 eletlric matter within the body of the earth. With refpeft 

 to the former, this ingenious philofopher has nothing to fay : 

 fome -operations in nature are certainly attended with a iofs 

 of the equilibrium in the_ elcflric fluid, but no perfon has 

 yet afiigned a ir.ore probiible caufe of the r(>dundancy of the 

 eleftric matter, which, in faft, often abounds in the c'otids, 

 than what we may fuppofe poflible to take place in the 

 bowels of the earth : and fuppofing the lufs of tiie eqi'.i- 

 librium poflible, the fame caufe that produced the cft'ecl 

 would prevent' the relloriiig of it ; fo that r.ot being able to 

 force a way, at lead one fufTiciently ready, through ths 

 body of the earth, it would ifl^ue at the fame convenient 

 vent into the higher regions of the air, as the better paflage. 

 3. Beccaria oblerves, that a wind always blows from tlia 

 place from which the thunder-cloud proceeds ; and it is cer- 

 tain, that the fuddeii congregation of fuch a prodigious 

 quantity of vapours muft dilplace the air, and repel it on all 

 fides. A great number cf obfervations rclati.ig to the de- 

 fcent of lightning, confirm his theory of the manner of its 

 afcent : for, in many cafes, it throws before it the parts of 

 condufting bodies, a::d diftribntcs them along the refilling 

 medium, through which it muil force its paifage. Upon 

 this principle, the longed flaflies of lightning feem to be 

 made, by its forcing into its v.-ay part of the vapours in the 

 air. One of the principal reafons wliy thofe flalhe.'i make fo 

 long a rumbling, is their being occafioned by the vail length 

 of a vacuum, made by the paffage of the eleftric matter. 

 For although the air coUapfes the moment after it has pafled^ 

 and the vitjra'ion, on which the found depends, commences 

 at the fame moment ; yet, if the flafh was diretfted towards 

 the perfon who hears the report, the vibrations excited at 

 the nearer end of the track will reach his car much fooner 

 than thofe excited at the more remote end ; and the found 

 will, without any rcpercuflion or echo, continue till all tlie 

 vibrations have fucceffively reached him. Mr. Lullin, in 

 order to account for the prqduflion of eledricity in the 

 clouds, made a long infulated pole to projedt from one fide 

 of the Alps, and obferved, that when fmad clouds of va- 

 pour, raifed by the heat of the fun, rofe near the foot of 

 the mountain, and afcended along the fide of it : if they 

 touched the extremity of the pole only, it was eleftrilied j 

 but if the whole pole, and confequently part of the hill on 

 which it flood, was likewife involved, it was not elcftrified. 

 Whence he concludes, that the elcflricity of the clouds is 

 produced by their paiTfng through the air while the fun 

 ihines upo.i them. But to whicii of thefe two circumltances, 

 namely, the motion through the air cr the action of the 

 fun's rays, this was ov.ing, he could not dctern.ine, though 

 he made leveral experiments for this purpofe. 



Upon liie whole, it is e, fy 'o conceive, that when pjrti- 

 cula- clouds or different parts of the ear;h pofllfs oppofite 

 eledlricities, fome being electrified pofi ively, and others 

 negatively, a dilcharga .vill take place within a certain dif- 

 tance ; or the one wi'l itrike into the other, and in the dif- 

 charge a flafli of lightning will be obfervcd. But how the 

 clouds or earth acquire this Hate, is lliil a qiicdion not ab- 

 folntely determined. Mr Canton queries, whether the 

 clouds become pofleffed of electricity b)' the gradual heating 

 and cooling of the air ; and whether air fuddenly rarcf ed, 



SSMf 



