CHAP, ii.] LIGHTNING-CONDUCTORS. 533 



tion of lightning-conductors come from a commission of the French 

 Academy of Sciences, which reported, on the 14th of January, 1867, 

 through M. Pouillet. Our description will be founded on this 

 report. 



We commence by explaining the theory of storm-clouds and 

 that of the action of the lightning-conductors on the electricity 

 they contain. 



1. Storm-clouds which produce lightning are nothing else than 

 ordinary clouds charged with a great quantity of electricity. 



The lightning which cleaves the sky is an immense electric spark, 

 whose two poles are the two clouds, separated from each other and 

 charged with opposite electricities. 



The thunder is the noise of the spark. 



The lightning is the spark itself, or the recombination of the 

 opposite electricities. 



When one of the poles of the lightning is on the surface of the 

 ground we say that the thunder, or rather the lightning falls, and that 

 the terrestrial objects are struck by lightning. Then all the points of 

 the tongue of lightning are still the recombination or neutralization 

 of the two opposite electricities, one of which is furnished by the 

 cloud and the other by the ea,rth itself. 



How comes the earth, which -is generally in a natural state, and 

 without apparent electricity, to be thus charged with electricity, and 

 that contrary to the electricity of the cloud at the very moment it is 

 struck? 



This is the first question we have to examine. 



2. Before the lightning falls the storm-cloud that contains it, 

 notwithstanding it is several furlongs above the ground, acts by 

 induction to repel the electricity of like kind, and to attract the 

 electricity of the opposite kind. This induction tends to influence 

 all bodies, but it is really only effectual on good conductors ; such 

 are, in different degrees, the metals, water, moist earth, living 

 creatures, vegetables, &c. 



The same conductor experiences very different effects from the cloud, 

 according to its own form and dimensions, and above all according as 

 its communication with the ground is perfect or imperfect. 



A tree, for instance, when it stands in ground only moderately 

 moist is but little influenced by induction, because the electricity of 



