io8 Traces of Unity in the Various 



seems to me, no escape from this conclusion if the 

 doctrine of the correlation of the physical forces be 

 universally applicable : and, therefore, the only question 

 is as to the value of the evidence in support of the 

 soundness of the doctrine. And certainly I see no 

 reason so far to underrate the value of this evidence. 



About terrestrial magnetism it is less easy to come 

 to any conclusion beyond this that electricity and 

 magnetism must stand or fall together. Electricity in 

 motion must generate magnetism, and, vice versa, mag- 

 netism in motion must generate electricity. This rule, 

 however, would seem to fail when it is applied to the 

 explanation of terrestrial magnetism. Here the mag- 

 netism is an obvious phenomenon, but where are the 

 constant electric currents which ought to pass at right 

 angles to the magnetic plane ? There are vague and 

 partial and inconstant currents, and that is all. Indeed, 

 there seems to be no escape from this dilemma except 

 it be that the currents in question are masked, partly by 

 passing in a closed circuit through a very ample and 

 excellent conductor, and partly by being transformed 

 into motion or some other mode of force, and that, after 

 all, the magnetism itself may be the only possible mani- 

 festation of the currents under the circumstances. 



More than once in the course of these remarks it has 

 been hinted that the force of gravity may be obedient to 

 the law of correlation about which so much has been 

 said, but here again the subject is beset with difficulties 

 which are not easily disposed of. By the force of gravity 

 all material bodies, quite irrespective of their molecular 

 condition, are supposed to attract each other propor- 

 tionately to their mass, and inversely as the square of 

 their distance. The law is that of any other attractive 

 force, but the force itself is held to be independent. 



