POTENTIAL AND EARTH CURRENTS. 177 



cooled with ice. Similarly also in the diurnal revolu- 

 tion of the earth, opposite sides are subjected daily to a 

 constantly changing temperature. And, in its annual 

 revolution, we have the same result in the changing sea- 

 sons; which also produce great changes in the conduct- 

 ing character of the surface ; from the frozen, snow- 

 clad surface of winter, to the verdure-clad surface of 

 summer. 



DLURNAL AND SEASONAL VARIATION. The change 

 of electric potential produced by these causes in the 

 earth, induces the opposite potential in the atmosphere; 

 which, by its lower strata, is insulated from it. Hence, 

 in observations made on the potential of the earth and 

 atmosphere, we find, as we should be led to expect, 

 daily maxima and minima potential, and also seasonal 

 maxima and minima. 



In several series of observations, made by different 

 observers in Europe, both on the continent and in the 

 British Isles, these maxima and minima were carefully 

 noted: and it was found, that, in winter, the daily 

 maxima occur at about 10 A. M. and 7 P. M.; in sum- 

 mer at about 8 A. M. and 10 P. M.; and in spring and 

 autumn, at about 9 A. M. and 9 P. M. The daily min- 

 ima occur, in summer, at about 3 P.M., and midnight; 

 but the daily winter minima are not given with suf- 

 ficient definiteness to be reliable. 



From this we see, that the daily maxima, occurring 

 soon after sunrise and sunset, correspond to the hours 

 of greatest change of temperature ; while the daily 

 minima occur at the hours when temperature is most 

 constant. 



The seasonal maximum occurs in winter, and the 

 seasonal minimum in summer: the maximum about 



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