OF THE EARTH-CURRENTS. 305 



in winter ; while with the electrical tension, and its changes, it is 

 the reverse. This objection, however, disappears when it is viewed 

 more closely. The physical quantity measured by our electro- 

 meters is not the absolute electric tension, but its variation with the 

 It eight ; while the electric changes which engender terrestrial cur- 

 rents are the variations as depending on horizontal distance. It is 

 easily conceivably that these should not correspond. In fact, it 

 is natural to suppose that in summer the zero-plane, which sepa- 

 rates the two electricities, should rise considerably ; and thus that 

 the variations for a given increase of altitude (which probably 

 diminish with the distance from that plane) should lessen, although 

 the absolute tensions, as well as the changes in horizontal distance, 

 may be greater. 



It would be of importance, in reference to this inquiry, to insti- 

 tute electrical observations of a totally different kind from any 

 which we now possess, and to measure the differences of tension 

 as depending on horizontal distance. There seems to be no diffi- 

 culty in the way of such observations, at least none greater than 

 those which present themselves in the ordinary observations of 

 atmospheric electricity ; and the results would probably do more 

 to clear up the physical aspect of these complex and interwoven 

 phenomena than any other observational means. 



