358 Lightning Conductors in Ships. 



21. It will be found, however, that the action of pointed metallic 

 bodies is purely passive ; that they only afford by the aptness of their 

 parts an easy transmission to the electric matter ; so that they can no 

 more be said to attract the matter of lightning, than a dike can be said 

 to attract the water which necessarily flows through it at the time of 

 heavy rain; and, as in the one case, the water is drawn down by a 

 force not peculiarly appertaining to the dike, so, in the other case, 

 the electric matter Is determined to a given point, in a somewhat simi- 

 lar way, by a force not appertaining to the metal. Moreover, it may 

 still further be reasoned by analogy, that, as the quantity of water 

 transmitted will depend on the capacity of the dike, and the final 

 protection it gives in conveying the fluid on the length to which it is 

 continued ; so, on the other hand, the protection afforded by a light- 

 ning-rod will also depend on its capacity, and the distance to which 

 It runs. If, In both cases, the length be extended until the force in 

 action be satisfied, the protection received will be as the capacity for 

 transmitting the current : if both be perfect, the protection will be 

 complete ; if the dike be not present, the water must be supposed 

 to run loose and undirected ; or, If its continuity be frequently inter- 

 rupted or narrowed to a small compass, the damage must then be 

 supposed to happen in the intermediate spaces. Such is, in fact, the 

 way In which all bodies of the conducting class already mentioned 

 (4) operate in conveying electrical discharges; and It must never be 

 forgotten as an Important feature in this discussion, that, whenever 

 we erect an artificial elevation on the earth's surface In the ordinary 

 way, we do, in fact, set up a conductor of electricity, upon which 

 the electricity of the atmosphere will fall, and no human power can 

 prevent It. Hence, if metallic bodies be present, those will be first 

 assailed ; if not, then the electric matter will fall on the bodies next 

 in conducting power, and so on. 



22. A curious Illustration of this principle will be found In an ex- 

 tract from the Memoirs of the Count de Forbin, which is given In 

 the forty-eighth volume of the Philosophical Transactions. '^In the 

 night," says the author of these memoirs, "it became extremely dark, 

 and it thundered and lightened dreadfully. As we were threatened 

 with the ship being torn to pieces, I ordered the sails to be taken In. 

 We saw upon different parts of the ship above thirty of St. Elmo's 

 fires ; amongst the rest there was one upon the top of the vane of 

 the main-mast more than a foot and a half In height ; I ordered one 

 of the sailors to take it down. When this man was on the top, he 



