272 Professor de la Rive on Electricity. 



to enable us to judge of the theories by which it has been at^ 

 tempted to explain this remarkable phenomenon of our globe. 

 The anomalies which the direction and the intensity of magne- 

 tic forces present in some places, and the changes which, in all, 

 they invariably undergo along with time, cannot be reconciled 

 with the theory of magnetic poles ; unless we consent to multi- 

 ply their number, and to attribute to them a movement which 

 would make them to be always changing their site ; hypotheses 

 too improbable, and too much opposed to the known phenomena, 

 to be admitted. It may easily be perceived that the supposi- 

 tion of electrical currents, distributed over all the surface of the 

 globe, at the same time that it satisfactorily explains the gene- 

 ral direction of the needle, can much better than any other ac- 

 count for the anomalies and irregularities to which in some places 

 this direction is liable : all indeed that it is necessary to admit is, 

 that it i^ not improbable that some particular circumstance may 

 slightly modify the direction and intensity of the electrical cur- 

 rents in the places where they occur. 



But whence can these terrestrial currents originate ? And 

 how can their existence agree with the constitution of the globe, 

 and the different phenomena which it presents ? These are the 

 questions to which we would now give an answer, by supplying 

 a very abridged exposition of the ideas that have been elicited 

 on the point. Before, however, commencing this subject, which 

 will close this communication, we take leave now to oppose 

 an hypothesis by which it has been proposed to explain ter- 

 restrial currents, as conceiving them owing to the simple rota- 

 tory motion which the earth experiences. This theory, which 

 is based on the action which Arago has discovered to be exer- 

 cised upon the needle by a revolving disc, cannot be applied to 

 the case of the terrestrial globe, because the needle participates 

 in its movement, and by consequence is, to it, in a state of re- 

 lative rest : On the other hand, the necessary condition for the 

 development of magnetism by rotation, is the relative move^ 

 ment of the needle, and the body which is to act upon it. 



It is necessary then to seek elsewhere than in the needle it- 

 self, for the origin of terrestrial currents. Starting with the 

 idea first thrown out by Davy, that the globe is formed of a 

 metaUic nucleus, covered over with an oxidized crust, M. Am- 

 pere thought that he found the source of these currents in the 



