CHAP, ii.] LIGHTNING-CONDUCTORS. 531 



CHAPTER II. 



LIGHTNING-CONDUCTORS. 



I. THE PRINCIPLES ON WHICH LIGHTNING-CONDUCTORS ARE 



CONSTRUCTED. 



ARAGO in his admirable Notice sur le Tonnerre passes in review the 

 various processes to which, from ancient times to that of Franklin, 

 or even of ourselves, popular prejudice and the prejudice of men of 

 science attributed the property of dissipating the clouds and escaping 

 the lightning. A great number of these processes were only practices 

 originating in superstitious credulity, and need not be mentioned. 

 Some were founded on hypotheses not justified by experience, or as to 

 which observation has hitherto furnished contradictory results. For 

 example, it has been thought that great fires kindled in the open air 

 took away from the clouds, at least in part, their fulminating properties. 

 It was the opinion of Voltaire, based no doubt on the experimental 

 fact that flames and hot gases are good conductors of electricity. But 

 in the case of fires kindled in the open air could the gaseous columns 

 rise to a sufficient height to reach the thunder clouds ? Anyhow, we 

 have heard of places where the peasants have been in the habit of 

 lighting, on the approach of storms, heaps of straw distributed here 

 and there on the fields, and these places have not in fact suffered 

 from lightning or hail. But on the other hand great conflagrations 

 have happened a little before or during great storms, without the 

 clouds which were nearest even to the scene of the accident having 

 been deprived, to all appearance, of the smallest part of their elec- 

 tricity. The efficacy of this method is therefore at least doubtful. 



Another means of dissipating clouds of every kind, and conse- 

 quently storm clouds, has been pretty frequently employed by sailors 

 and agriculturists. It is that of firing off pieces of artillery, cannons, 

 or other firearms. But the very precise examples cited by Arago for 



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