312 ATMOSPHEEIG ELECTEICITT. 



PART II. 



OF THE STATE OF OUR KNOWLEDGE OF THE ELECTRICITY OF THE AIR. 



CHAPTER I. 



OF THE ORIGIN OF ATMOSPHERIC ELECTRICTTY, 



In the month of July, 1752, Cassini de Thnry * observed that a rod 

 placed on the observatory of Paris became electrified when there was no 

 appearance of a thunder cloud above the horizon ; but at this period it 

 was believed that such clouds were necessary to the production of this 

 penomenon, and this led him to suspect that there was a cloud near the 

 horizon, which, without being perceived, communicated to the air elec- 

 tricity enough to charge the insulated conductor. In the meantime the 

 observations made by Le Monniert soon left no doubt that the elec- 

 tro-atmospheric apparatus does become electric under a clear sky. 

 During a drought of six weeks, from the middle of September to the 

 end of October, 1752, this scientist found that the apparatus which he 

 had established in his garden of St. Germain en Laye, became sensibly 

 electrified, tlumgh the clearness of the sky was not disturbed by clouds 

 during all this time. The experiments of Beccaria, Cavallo, and 

 especially those made with the most delicate instruments by De Saus- 

 sure, Volta, and Schiibler, confirm this result. These philosophers 

 took pains to determine the character of atmospheric electricity, and 

 i'rom the sum of their observations we may conclude that in clear 

 weather the atmosphere is always positively electrified. 



Many hypotheses have been framed to account for atmospheric elec- 

 tricity. We shall present a brief exposition of some of these and show 

 how far they correspond with the present state of the science. 



When friction was the only known agent capable of producing the 

 decomposition of the electric fluids, it was natural to seek the source 

 of atmospheric electricity in the friction of the air against the clouds 

 and the earth and against itself. This opinion was entertained by 

 Nollet| and adopted by many philosophers; but it was afterwards 

 entirely abandoned, when observation taught that there was no rela- 

 tion between the intensity of electricity and the force of the wind. 

 Friction is no doubt a most active means of developing electricity ; 

 we know likewise from the experiments of Wilson that the air be- 

 comes electrified by its friction against glass. It is then probable 

 enough, as M. Koemtz § observes, that the friction of the air against 

 the clouds and the earth develo2)S electricity ; but it may not pro- 

 duce any sensible effect, either on account of its want of intensity or 

 because it is immediately dissipated by the ways of escape offered to it. 

 [Faraday found no electrical excitement from the friction of dry air.] 



* Uistoire de 1' Academic des Sciences de Paris, pour 1752, page 10. 

 J Mcmoire de I'Academie des Sciences de Paris, pour 1752, page 241. 

 j Memoires de I'Academie des Sciences de Paris, pour 17G4. page 409. 

 § Lehrbuch dcr Mcteorologie, torn. II, page 408. 



