FORKED LIGHTNING 311 



" Diespiter 



Igni corusco nubila dividens 

 Plerumque, per purum tonantes 

 Egit equos volucremque currum." 



But Virgil's remark is not so commonly known. He is speaking of prodigies of 

 various kinds, and goes on : 



" Non alias coelo ceciderunt plura sereno 

 Fulgura; nee diri toties arsere cometae." 



It is very singular that he should thus have associated comets and meteorites 

 which quite recent astronomical discovery has shown to have a common origin. 



Another remarkable peculiarity, long ago observed, is the characteristic smell 

 produced when lightning strikes a building or a ship. In old times it was supposed 

 to be sulphurous ; now-a-days we know it to be mainly due to ozone. In fact, all 

 the ready modes of forming ozone which are as yet at the disposal of the chemist 

 depend upon applications of electricity. But besides ozone, which is formed from 

 the oxygen of the air, there are often produced nitric acid, ammonia, and other 

 compounds derived from the constituents of air and of aqueous vapour. All these 

 results can be produced on a small scale in the laboratory. 



Hitherto I have been speaking of lightning discharges similar in kind to the 

 ordinary electric spark, what is commonly called forked or zig-zag lightning. Our 

 nomenclature is very defective in this matter, and the same may be said of the 

 chief modern European languages. For, as Arago remarks, by far the most common 

 form of lightning flash observed in thunderstorms is what we have to particularise, 

 for want of a better term, as sheet-lightning. He asserts that it occurs thousand- 

 fold as often as forked lightning ; and that many people have never observed the 

 latter form at all ! It is not at all easy to conceive what can be the nature of 

 sheet-lightning, if it be not merely the lighting up of the clouds by a flash of forked 

 lightning not directly visible to the spectator. That this is, at least in many cases, 

 its origin is evident from the fact that its place of maximum brightness often takes 

 the form of the edge of a cloud, and that the same cloud-edge is occasionally lit up 

 several times in quick succession. You will remember that we are at present dealing 

 with the appearances observed in a thunderstorm, so that I do not refer to that 

 form of sheet-lightning which commonly goes by the name of summer-lightning, and 

 which is not, audibly, at least, followed by thunder. 



The next remarkable feature of the storm is the thunder, corresponding, of 

 course, on the large scale to the snap of an electric spark. Here we are on com- 

 paratively sure ground, for sound is very much more thoroughly understood than is 

 electricity. We speak habitually and without exaggeration of the crash of thunder, 

 the rolling of thunder, and of a peal of thunder ; and various other terms will 

 suggest themselves to you as being aptly employed in different cases. All of these 

 are easily explained by known properties of sound. The origin of the sound is in 

 all cases to be looked for in the instantaneous and violent dilatation of the air along 

 the track of the lightning flash ; partly, no doubt, due to the disruptive effects of 

 electricity of which I have already spoken, but mainly due to the excessive rise of 



