CHAP, ii.] LIGHTNING-CONDUCTORS. 535 



must be considered as simply media which it finds in the way and 

 strikes in passing. 



" At the same time we must not hence conclude that these media are 

 essentially passive, and never contribute in any way to modify or 

 even to determine the direction of the lightning-stroke. It is certain, 

 on the contrary, that they exercise in this respect an influence which 

 is all the greater, as they are of more considerable size and of better 

 conducting power. When a vessel, for example, is struck in the middle 

 of the sea, it is very probable that the lightning has not taken the 

 path which would be geometrically shortest to reach the water it is 

 seeking, and where it will be neutralized by the opposite electricity, 

 but that it has chosen the way that was electrically the shortest, on 

 account of the decomposition by induction which the cloud has 

 previously produced, on the masts, rigging, and other conductors of the 

 vessel, which are more or less elevated and good conductors. 



" This phenomenon is analogous to that of the spark drawn at a 

 great distance between the conductors of a powerful electrical machine; 

 it may be turned aside from its most direct path by the presence of 

 one or more insulated conductors placed near its line of traverse ; it 

 passes to strike the same point, but it reaches it by a path electrically 

 shorter, although it is longer in appearance. 



" Here the insulated conductors change the direction of the spark. 

 The media, of which we spoke just now, change the direction of the 

 lightning. We limit ourselves to the simple enunciation of this 

 fundamental principle which we cannot develop here: it contains the 

 explanation of all the movements, often so strange, of strokes of 

 lightning and of all the destructive effects they produce. We can never 

 account for them without having thoroughly recognised the two poles 

 or points of departure, and between these two points a series of media 

 which have been struck by the fork of the lightning, sometimes 

 single sometimes multiple." 



Here ends the theoretical part of the report, which in the opinion 

 of the members o.f the commission, serves as a base for those practical 

 instructions afterwards laid down for the construction and fixing of 

 lightning-conductors. We now return to these instructions as far as 

 they are essential. 



