368 ATMOSPHERIC ELECTRICITY. 



of intensity in the report may also result, according to M. Kaemtz,* 

 from the fact that the compression of air must naturally be stronger 

 towards the apex of the angles where the inflexions of the flash of 

 lightning take place. There would then be, according to these phi- 

 losophers, a close connexion between the claps of thunder and the zig- 

 zags of the streaks of lightning, which it would be important to con- 

 firm by direct observation. 



CHAPTEE III 



OF THE PRINCIPAL EFFECTS PRODUCED BY LIGHTNING AT THE SURFACE OF 



THE EARTH. 



After having followed the lightning in its course through the air 

 and discussed the phenomena which it presents, it only remains for 

 us succinctly to state the principal effects which mark its passage to 

 the surface of the earth. 



Definite observations show us that lightning follows the best con- 

 ductors of electricity which it meets in its passage ; that it turns oif 

 from its original course to strike metallic masses behind large piles 

 of masonry or even inside of them , when a conductor is wanting or does 

 not ofier a passage for its sufficient escape. Though harmless as long 

 as it passes through a metallic rod of sufficient size, we observe it 

 issuing from the extremity of the metal by breaking, pulverizing, or 

 throwing off the substances which oppose its course. 



As in the case of the discharge of an electric battery, the lightning 

 in its passage considerably raises the temperature of the conducting 

 bodies, and frequently melts or volatilizes them. It is not unim- 

 portant to know what is the greatest thickness of different metals which 

 lightning has been known to fuse, and to assign to this phenomenon the 

 limits that have been observed. M. Arago,t in his notices on thunder, 

 directed his attention to this question, and from tlie sura of the obser- 

 vations he was able to collect, deduced the following conclusions: 

 that a flash of lightning could melt completely and through its 

 whole length a chain of iron 130 fetet long, of which the diameter of 

 the different links was not above a tourth of an inch; that it might effect 

 the fusion of a conical bar of copper 9 feet long and one-third of an 

 inch diameter at the base ; that it could also melt a leaden pipe of 3 

 inches in diameter and half an inch thick ; and, finally, that lightning 

 has never yet brought to red heat a bar of iron of half an inch in 

 'diameter. 



When lightning traverses conducting wires and the heat which it 

 sets free is not enough to fuse the metal it shortens them. This singular 

 phenomenon was brought to view by the observation of an English 



* Lebrbuch der Meteorologie, tome II, page 434. 

 f Annnaiie pour 1838, p. 305, &c. 



