XIV. ON TfiE PROBABLE CAUSES OF THE EARTH- 

 CURRENTS. 



Proceedings of the Royal Irish Academy, 1862. 



IN a former communication to the Academy, I endeavoured to 

 prove that the diurnal changes of the horizontal needle were the 

 results of electric currents traversing the earth's crust. The ex- 

 istence and continuous flow of such currents had been established, 

 as I believe, by the observations of Mr. Barlow made on two of 

 the telegraphic lines of England ; and it only remained to show 

 that their laws corresponded with those of the magnetic changes. 



In that communication I refrained from offering any conjec- 

 ture as to the origin of the currents themselves. Every speculation 

 of this kind must remain a pure hypothesis, until it can be con- 

 fronted and compared with facts; and the magnetic phenomena 

 presented at different points of the earth's surf ace are so diversified, 

 that a wide collection of the facts is necessary in order to form the 

 basis of any sound physical theory. For these reasons, I have 

 deemed it the more proper course to ascertain the laics of the 

 diurnal changes of the earth-currents at many places, so far as they 

 may be inferred from the magnetic phenomena, before proceeding 

 to the consideration of their causes. 



It has been shown, in the Paper referred to, that the earth- 

 currents, as inferred from the changes in the two horizontal com- 

 ponents of the magnetic force, observe certain general laws, which 

 are common to all the stations at which these changes have been 

 observed ; while, on the other hand, their departures from a com- 

 mon type are various and considerable. We thus learn that tin- 

 phenomena are produced by a common cause, the effects of which 

 are greatly modified by the physical peculiarities of the parts of the 



