458 THE POPULAR SCIENCE MONTHLY. 



feet of the air, lie mentioned the fact that in Norway there were 

 two kinds of thunderstorms : one occurred in the summer, he be- 

 lieved, when the lightning clouds were high, and very little dam- 

 age was done ; on the contrary, in winter time the clouds were 

 very low, and the churches were frequently struck. 



Lord Rayleigh, Sir William Thomson, and Prof. Rowland dis- 

 cussed the questions whether or not the experiments actually rep- 

 resented the actual conditions. M. de Fonveille called attention 

 to the most extraordinary lightning conductor in existence, the 

 Eiffel Tower, and the fact that Paris was practically free from 

 calamities produced by lightning, because a sufficient number of 

 lightning rods had been erected according to the principles advo- 

 cated by the many official boards, substantially the same as the 

 conference. Prof. George Forbes thought a copper alternative 

 path better than an iron one. Sir James Douglas, speaking from 

 an experience of forty years with a large number of conductors, 

 was of the opinion that the rods, when properly constructed, were 

 entirely adequate. In the matter of copper versus iron he pointed 

 out the practical consideration that iron corroded rapidly com- 

 pared with copper. Mr. Walker further pointed out that this 

 corrosion of iron was not a question of weather alone, but, as in 

 the case of the top of the chimney of a factory, there would be 

 some chemical action. Mr. Symons brought oat with respect to 

 iron conductors that galvanizing would not entirely overcome the 

 difficulty. 



We now come to Lodge's book upon Lightning Conductors 

 and Lightning Guards, and shall get from it a more satisfactory 

 understanding of his experiments and deductions. He believes 

 that the current ideas on the character of the lightning dis- 

 charge were not altogether correct, because the momentum of an 

 electric current and the energy of an electrostatic charge had 

 both been more or less overlooked. The application of the 

 known fact of electrokinetic momentum revolutionized the treat- 

 ment of certain phenomena. The old drain-pipe idea of convey- 

 ing electricity gently from cloud to earth was thus proved falla- 

 cious, and the problem of protection became at the same time 

 more complex and more interesting. His position, therefore, is 

 not that lightning rods are useless, but that few or none of the 

 present types are absolute and complete safeguards ; and he be- 

 lieves it possible to so modify existing protective systems as to 

 afford more certain protection. The problem is one, he very 

 clearly shows, far removed from the old idea of conduction. 



To-day we know from the experiments of Hertz, Lodge, and 

 others that when an electric current flows steadily in one direction 

 in a conductor, its intensity is the same in all parts of the wire ; 

 but if it be of an oscillatory character — i. e., a current revers- 



