T H U 



T H U 



terval of time between tlie flafli and the noife. Dr. Wallis 

 obfervcs, that, ordinarily, the diflcrence between tlie two is 

 about feven fcconds, which, at the rate of 1 142 feet in a fe- 

 cond of time, gives the diftance about a mile and a half ; but 

 fometimes it comes in a fecond or two, which argues the ex- 

 ploiion very near us, and even among iis. And in iucli 

 cafes, the doftor afTures us, he lias more than once foretold 

 the mifchiefs that happened. 



Upon the whole, that there is in lightning a fulphurous 

 vapour has been argued from the fmell of Inlphur which at- 

 tends it, and from the fultrv heat in the air which ulually 

 precedes it ; and that there is a nitrous vapour along with it, 

 the fame writer concludes hence, that we know of no other 

 l)ody fo liable to a fudden and violent cxplofion. And as to 

 the kindling of thefe materials, we know that a mixture ot 

 fulphur and Iteel-fdings, with a little water, will of itfelf 

 break forth into attual flame. Nothing, therefore, is want- 

 ing to the explofioii but fome chalybeate or vitriohc vapour ; 

 and, among the various effluvia from tlie earth, the dottor 

 does not doubt but there mull be fome of that kind ; but, in 

 proof of what he leaves as a probability, the following fafts 

 have been alleged. 



In hiilory, we meet with inftances of its raining iron in 

 Italy, and iron-ftoncs in Germany. Jul. Scahger tells us, 

 !ie liad by him a piece of iron rained in Savoy. Cardan re- 

 ports 1200 Itones to have fallen from heaven, fome of them 

 weighing 30, fome 40, and one 120 pounds, all very hard, 

 and of the colour of iron. 



The matter of fadl is fo well attefted, that Dr. Liller, 

 in the Philolophical Tranfaftions, builds a whole theory 

 of thunder and lightning on it ; maintaining that they 

 both owe their matter to the vapour or exhalation of the 

 pyrites. 



The noife of thunder, and the flame of lightning, are 

 eafily made by art. If a mixture of oil or fpirit of vitriol 

 be made with water, and fome filings of fteel added to it, 

 there will immediately arife a thick fmoke or vapour out of 

 the mouth of the vefTel ; and if a lighted candle be applied 

 to this, it will take fire, and the flame immediately defcend 

 into the veffel, and this will be burft to pieces with a noife 

 like that of a cannon. 



This is fo far analogous to thunder and lightning, that a 

 great explofion and fire are occafioned by it ; but in this 

 they difter, that this matter when once fired is dellroyed, 

 and can give no more explofions ; whereas, in the heavens, 

 one clap of thunder ufually follows another, and there is 

 a continued fucceffion of them for a long time. M. Hom- 

 berg explained this by the lightnefs of the air above us in 

 comparifon of that here, which therefore would not fuffer 

 all the matter fo kindled to be difllpated at once, but kept it 

 for feveral returns. 



Ever fince the year 1752, in which the identity of the 

 matter of lightning, and of the electric fluid, has been afcer- 

 tained, philofophers have generally agreed in confidering 

 thunder as a concuflion produced in the air by an elettrical 

 explofion. For the illuibation and proof of this theory, fee 

 Lightning. See alfo Elkctricity. 



We (hall here obferve, that Mr. Henry Eeles, in a letter 

 written in 1751, and read before tlie Royal Society in 1752, 

 confiders tlie eleftrical fire as the caufe of thunder, and en- 

 deavours to account for it on this hypothefis ; and he tells us, 

 that he did not know of any pcrfon's having made the fame 

 conjefture. Phil. Tranf. vol. xlvii. p. 524, &c. 



That rattling in the noife of thunder, which makes it feem 

 as if it pafled through arches, or were broken varioufly, is 

 probably owing to the found being excited among clouds 



hanging over one another, and the agitated air pafling irre- 

 gularly between them. 



See this phenomenon particularly accounted for under 



LiGIlTNINi;. 



TiiUNDEn-5o//. If what we call lightning afts with ex- 

 traordinary violence, .and breaks or (hatters any thing, it is 

 called a tliunderbolt, which the vulgar, to fit it for fucK 

 eifefts, fuppofe to be a hard body, and even a (lone. 



But that we need not to have recourfe to a hard folid body 

 to account for the eflFects commonly attributed to the thunder- 

 bolt, will be evident to any one, who confiders thofe of the 

 pulvis fulminans, and of gunpowder : but more efpecially 

 the ailonifhing powers of eleftricity, even when collected 

 and employed by human art, and much more when diredled 

 and exerciled in the courfe of nature. 



When we confider the known effefts of eleftrical explo- 

 fions, and thofe produced by lightning, we (hall be at no 

 lofs to account for the extraordinary operation vulgarly 

 afcribed to thunderbolts. As Hones and bricks (buck by 

 lightning are often found in a vitrified Hate, we may reafon- 

 ably fuppofe, with fignior Beccaria, that fome ftones in the 

 earth, having been ftruck in this manner, firil gave occa- 

 fion to the vulgar opinion of the thunderbolt. 



Places ilruck with thunderbolts were held facred among 

 the ancients. Nigidius has a curious treatife on the thunder- 

 boh. 



The ancient painters and poets have armed Jupiter with a 

 fort of flaming dart, called a thunderbolt. Thus, it is faid, 

 he became mailer both of gods and men. 



Thefe thunderbolts are forged for Jupiter, according to 

 the poets, by the Cyclopes. 



The thunderbolt, in antiquity, reprefented fovereignty, 

 and a power equal to the gods ; on this account, Apelles 

 painted Alexander, in the temple of Diana of Ephefus, hold- 

 ing a thunderbolt m his hand : and on medals, the thunder- 

 bolt is fometimes found to accompany the emperor's heads, as 

 that of Auguflus. 



Appian informs us, that the thunderbolt was the principal 

 divinity of Seleucia ; adding, that it was adored, even in his 

 time, with various hymns and ceremonies. 



THUNDER-C/(7!;(/t, in Phyjlology, are thofe clouds which 

 are in a (late fit for producing lightning and thunder. From 

 fignior Beccaria's exaft and circumftantial account of the ex- 

 ternal appearances of thunder-clouds, we fliall extraft the 

 following particulars. 



The lirll appearance of a thunder-llorm, (which generally 

 happens when there is little or no wind), is one denfe cloud, 

 or more, increafing very fall in fize, and riling into the higher 

 regions of the air. The lower furface is black, and nearly 

 level ; but the upper finely arched, and well defined. Many 

 of thefe clouds often feem piled upon one another, all arched 

 in the fame manner ; but they keep continually uniting, fwell- 

 ing, and extending their arches. 



At the time of the rifing of this cloud, the atmofphere is 

 generally full of a great number of feparate clouds, motion- 

 lefs, and of odd and whimfical fliapes. All thefe, upon the 

 appearance of the thunder-cloud, draw towards it, and be- 

 come more uniform in their (hapes as they approach, till, 

 coming very near the thunder-cloud, their limbs mutually 

 (Iretch towards one another ; they immediately coalefce, and 

 together make one uniform mafs. Thefe he calls adfcititiout 

 clouds, from their coming in, to enlarge the ii/.e of the 

 thunder-cloud. 



But fometimes the thunder-cloud will fwell, and increafe 



very fall, without the conjunftion of any adfcititious clouds ; 



the vapours in the atmofphere forming themfelves into clouds 



4 F 2 wherever 



