ELECTRICAL CURRENTS OF THE EARTH. 225 



eartli iDcreases with tlie altitude of terrestrial ])oints, it can be con- 

 ceived that a metallic line, one extremity of which touches the bottom 

 of a valley and the other a summit, should be traversed by an ascendiug 

 electric current, with an inteusity ])roportional to the difference of the 

 electric potentialities of the two points, ^ince, then, the electric state 

 of the earth varies even in clear and cahn days at the various hours of 

 the day, the electric current of the earth also might derive, from that 

 law of atmospheric electricity, its origin in metallic lines, touching the 

 ground with their extremities, provided the lines be very long ones. 



But i)henomena so obscure and complex demand great reserve in anj" 

 attempts which are made to explain them, and it is only from those new 

 and persevering researches which we invoked at the beginning of this 

 memoir, that greater light can be hoped for in regard to a phenomenon of 

 so much importance in terrestrial physics. 



15 s 69 



