ATMOSPHERIC ELECTRICITY. 



155 



ways at 2 P. M. The hour of the second maximum also gradually approached 

 nearer to noon from summer to winter, and receded from it again from winter 

 to summer. 



The series of observations on the diurnal changes of atmospheric electricity, 

 which Schubler made, in 1811-12, were repeated by M. Arago at Paris, in 

 1830, who obtained similar results. Thus, in the month of March, 1811, 

 Schubler found that the mean time of the morning maximum, was 8 hs. 30 m., 

 and M. Arago found the mean time for the same month, 8 hs. 48 m. 



LOCAL VARIATIONS OF THE ELECTRICITY. 



i In all the preceding observations, the sources which supply positive electri- 

 city to the air, are supposed to be uniformly distributed on the surface of the 

 earth. A great variety of local causes, however, interrupt this uniformity. 

 Saussure's observations show that the positive electricity of the air has greatest 

 intensity in the most elevated places, and in those which are best insulated. 

 In the interior of buildings,, under trees, in the streets, courts, and other en- 

 closed and sheltered parts of towns, no free electricity is found in the air. In 

 the midst of squares, and other open places in cities, on the quays, but more 

 specially on bridges, it is even more intense than in an open, flat country. In 

 particular localities, such as Geneva, where fogs prevail, which lie low, and 

 are not converted into rain, the positive electricity of the air is most intense. 

 Although the general correspondence between the diurnal and annual variations 

 of the normal electric state of the air indicates, unequivocally, its dependance 

 on the variation of the sun's declination, and the diurnal motion of that body, 

 and the local variations accord with the hypothesis, that evaporation is the 

 chief source of the electricity of the air ; still, no complete and satisfactory 

 explanation has yet been proposed for the diurnal and annual electric periods. 



Schubler observed that some correspondence may be perceived between the 

 diurnal variation of the magnetic needle, and the diurnal variation of the elec- 

 tricity of the air, and that, if such correspondence be admitted, it would follow 

 that both these phenomena must be ascribed to the same cause. But this cor- 

 respondence is far from being so exact as to justify even a probable conjecture 

 as to their identity of cause. The maximum variation of the needle east takes 

 place at half past eight in the forenoon, from which time till a quarter past one 

 in the afternoon, it turns gradually round toward the west, attaining its maxi- 

 mum western variation at the latter hour. From that time till half-past eight 

 the following morning, it returns gradually eastward. The times of greatest 

 eastward and westward variation correspond nearly to the times of the morn- 

 ing maximum, and evening minimum, but there are no effects exhibited by the 

 needle corresponding to the other maximum and minimum. 



Becquerel proposes the following explanation of the diurnal variations of the 

 electricity of the air. Toward the morning the electricity ought to have a 

 feeble intensity, because the humidity of the evening and night has restored to 

 the earth a part of the electricity which had been accumulated in the air. 

 When the sun, at its rising, begins to warm the earth, evaporation is promoted 

 and positive electricity supplied to the air. Hence, after sunrise, for some 

 hours, the intensity of the electricity of the air will be augmented. When the 

 sun has attained a certain elevation, and the heat has increased, the air is dried, 

 and transmits with less facility the electric fluid, accumulated in the higher re- ( 

 gions of the air ; electrometric instruments, therefore, placed near the surface ' 

 of the earth, will indicate a diminution of electricity, even though the electric < 

 fluid should continue to be augmented in the higher parts of the air. As sun- j 

 set approaches, the air is cooled, becomes humid, and begins to transmit the < 



