542 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



lightning-conductors which has been adopted in the British navy, and 

 which has the advantage over rods or ropes of metal of adapting 

 itself to all the movements and all the varying positions of the masts. 

 This system consists in placing sheets of copper round the mast and in 

 communication with the sheathing of the ship. The result of this is, 

 that in bad weather when the masts are broken by the violence of the 

 wind, the lightning always finds a system of conductors sufficient for 

 the discharge of the stroke and rendering it inoffensive. Arago states 

 that the English frigate Dryad was often exposed, off the coast of 

 Africa, to violent storms, called by sailors tornados (the ship was pro- 

 vided with Harris's new lightning-conductors). The electricity came 

 down along these continuous pipes of copper in such quantity as to 

 give rise to a sort of luminous atmosphere, and a noise like water 

 boiling very fast. The ship was nevertheless preserved throughout. 



Professor Clerk Maxwell, who has recently investigated this subject, 

 has come to somewhat different conclusions. Taking the extreme case 

 of a powder magazine, he states that, " It is quite sufficient to inclose 

 the building with a network of a good conducting substance. For 

 instance, if a copper wire, say No. 4, B.W.G. (0*238 inches diameter), 

 were carried round the foundation of the house, up each of the corners 

 and gables and along the ridges, this would probably be a sufficient 

 protection for an ordinary building against any thunderstorm in this 

 climate. The copper wire may be built into the wall to prevent theft, 

 but should be connected to any outside metal, such as lead or zinc on 

 the roof and to metal rain-water pipes. Tn the case of a powder-mill 

 it might be advisable to make the network closer by carrying one 

 or two additional wires over the roof and down the walls to the wire 

 at the foundation. 



"If there are water or gas-pipes which enter the building from 

 without, these must be connected with the system of conducting- 

 wires, but if there are no such metallic connections with distant 

 points, it is not necessary to take any pains to facilitate the escape 

 of the electricity into the earth. Still less is it advisable to erect 

 a tall conductor with a sharp point in order to relieve the thunder- 

 clouds of their charge." 



It is hardly necessary to add that it is not advisable during a 

 thunderstorm to stand on the roof of a house so protected, or to 

 stand on the ground outside and lean against the wall. 



