ATMOSPHERIC ELECTRICITY. 333 



tion to the hygrometric state of the air, gave the following explana- 

 tion : "Towards the end of the night the electricity appears very 

 feeble, either because the evaporation is nearly nothing, or because the 

 moisture of the preceding evening and that of the night which fol- 

 lowed it have transmitted to the earth nearly the whole electric fluid 

 accumulated in the air. But when the sun begins to warm the earth 

 again the electricity of the atmosphere increases, because the vapor 

 which the heat causes to rise from tiie earth conveys electricity into the 

 air, and transmit, in part, that which begins to accumulate. But 

 when the sun reaches a certain height the heat increases in a greater 

 degree than the evaporation ; the air is dry, and transmits with 

 difficulty the electric fluid accumulated in the height of the atmo- 

 sphere. It follows that our electrometers, placed near the surface of 

 the earth, indicate a decrease of electricity, although the electric fluid 

 continues to accumulate in the elevated regions. Again, as the sun 

 approaches the end of his career the air is cooled, becomes damp, and 

 begins to transmit more abundantly to the earth the electric fluid 

 accumulated in the higher regions. The electrical intensity must 

 then increase with the moisture until two or three hours after sunset. 

 Finally, when the air begins to be exhausted of its moisture the elec- 

 tricity decreases anew until the next day." De Saussure, according 

 to the same i)rinciple, explains why atmospheric electricity is much 

 less powerful in summer than in winter. The air in the first case 

 being warm and dry resists with more force the escape of the electric 

 fluid accumulated in the upper regions of the atmosphere ; while in 

 the winter the moist air must produce a contrary effect. He thought 

 that the accumulation of free electricity, in summer, in the elevated 

 regions of the atmosphere is probably one of the causes of the fre- 

 quency of thunder storms in this season. 



This manner of accounting for the daily variations of atmospheric 

 electricity, which M. Becquerel* and other philosophers have also 

 adopted, rests on the development of electricity during the evaporation 

 of water — a phenomenon which does not appear sufficiently proved in 

 the present state of science, as we have heretofore shown. It sup- 

 poses, also, that an electrometer is charged by the contact of the sur- 

 rounding air ; and we have seen, according to M. Peltier, that the 

 contrary is true. Besidts, it is difficult to conceive how vapor can 

 transmit tlie electricity Vv'ith which it is charged to the higher strata 

 of the atmosphere at the same time that it transfers towards the 

 earth that which is accumulated in these same strata. Finally, 

 among the lower moist and conducting strata there must be found 

 other strata sufficiently dry, and consequently so imperfect in their 

 conducting power as not to permit the transmissions of electricity. 



Schiibler's observations establish a direct relation between the 

 daily variations of the electricity of the air and those which occur in 

 its state of relative moisture. In fact, if we examine the table which 

 contains the hourly observations made by thisphilosopher on the 11th 

 of May, 1811, we see that atmospheric electricity is at its miimnum a 



« Traite de 1' Electricity et du Magn^tisme, torn. IV, p. 102. 



