POTENTIAL AND EARTH CURRENTS. 189 



It will be observed that while the observer's position 

 reaches the point of highest temperature at noon, the 

 point of lowest temperature is reached at sunrise. For 

 the heating of any given area begins at sunrise, 

 increases till noon, as the sun's rays become more 

 vertical; and declines from that hour till sunset, as 

 the rays become less vertical ; while the cooling is 

 constant from sunset to sunrise. So that the morning 

 difference of temperature, between east and west 

 regions, is greater than the evening difference ; and we 

 should expect to find a corresponding increase of elec- 

 tric potential, at the morning maximum. 



But the series of telegraphic observations given 

 shows the reverse ; which may result from the fact that 

 the line on which the observations were made, has the 

 Atlantic ocean at its eastern terminus, and the interior 

 of the continent at its western. And, as change of 

 temperature is much slower on a water surface than on 

 a land surface, the difference of temperature between the 

 Atlantic on-the east, receiving the sun's rays first, and the 

 interior on the west, would be less in the morning than in 

 the evening, when these relative positions are reversed. 



As the distance between heated and cooled regions 

 alternately increases or diminishes during the earth's 

 diurnal revolution, electric resistance increases or di- 

 minishes in the same ratio, and increase or decrease of 

 current intensity is a corresponding result : and electric 

 maxima and minima, and also reversal of current, must 

 follow from this cause, as well as from difference or 

 equality of temperature. But as increase or decrease 

 of distance is coincident with increase or decrease of 

 difference of temperature, the two causes intensify each 

 other's effects. 



