322 ATMOSPHERIC ELECTRICITY. 



gence by its contact with the air, if it is the latter which becomes 

 electrified? We believe that the explanation of this fact may be 

 derived from what we have said (part 1, ch. II) in quoting the opinion 

 of M. Gay Lnssac on the distribution of the electricity of the atmos- 

 phere; besides, we ought to remember that the lower strata of the air 

 can possess, in a free state, but a feeble quantity of electricity. Many 

 causes may, in fact, be opposed to the accumulation of free electricity 

 in these strata. In the first place, there is the proximity of the ground, 

 which, charged by induction with an electricity contrary to that of the 

 air, reacts consequently, by its attraction, to neutralize it. Another 

 cause is found in the great conductibility which the air of the same 

 strata naturally possess in consequence of the elevated temperature to 

 which they are subjected, and the approach of the molecules of water to 

 the state of vapor, circumstances which permit an easy escape for all the 

 free electricity. In fine, even though it should be supposed that the 

 air of these strata could contain a greater quantity of free electricity, 

 it appears, according to the experiment of Mr. Faraday and that of 

 M, Peltier himself, that the electric action being the same in every 

 direction, no divergence could take place in the instrument. 



CHAPTER II. 



or THE VAEIATIOXS OF ATMOSPHERIC ELECTRICITY IN CLEAR WEATHER. 



§1.0/" the injluence of local causes and of the heigJU on atmospheric 

 electricity in clear loeather. 



By means of his electrometer, furnished with a metallic stem, De 

 Saussure * observed that local causes exercised a very decided influ- 

 ence on the electrical indications obtained under a clear sky. He 

 discovered that they did not become sensible in the open plain, except 

 at a height of from 4 to 5 feet above the ground; that they are stronger 

 in the most elevated and isolated places; there are none in houses, 

 under trees, in streets, in courts, and, in general, in localities wholly 

 inclosed; that, nevertheless, in cities they are sensible in the midst 

 of large, open places, on quays, and especially on bridges where they 

 have been found more powerful than in the open country. 



These results are confirmed by the observations which Schiiblerf 

 made during a journey in which he twice crossed the chain of the Alps 

 separating Germany from Italy. He always observed an increase in 

 the intensity of atmospheric electricity in proportion as the places 

 which he passed through were more elevated and isolated, and this 

 increase was so much the more remarkable the farther he was 

 removed from forests and habitations. Thus, on the 13th of April, 

 1813, at 4 o'clock in the evening, the electric tension at Mount St, 



t^- Voyages dans les Alpes, torn. II, § 792, page 205, and § 800, page 219. 

 I Journal de Schv.-eigger, torn. IX, pages 348 and 350, 1813. 



