72 Connexion between the Theory of the Earth, ^c. 



on the principles of the theory, subjected to a gradual cooling 

 process, it is quite manifest that the contracting influences have 

 had their principal seat of action on and near the surface, while 

 in the interior their operation has been comparatively feeble. 

 We seem therefore to be shut up to the conclusion, that the mo- 

 tion of the solid crust about the axis has been more accelerated 

 by the cooling and contracting processes than that of the internal 

 fused mass — that the angular velocity of the former is more rapid 

 than that of the latter — that the latter as a whole is gradually 

 falling back of the former. In other words, considering the solid 

 crust as fixed relatively to the observer, there is a gradual west- 

 erly revolution of the internal fused mass. 



Thus admitting the prevailing theory of the earth, we infer, as 

 a necessary physical consequence, a westerly revolution of the 

 internal mass ; and admitting the western revolution of the in- 

 ternal mass, the observed secular motions of the horizontal and 

 dipping needles would seem of necessity to follow. 



In looking back on the attempts which hat^e been made to as- 

 sign the physical causes of the variation of the declination of the 

 needle, it is curious to remark how completely the theory un- 

 der consideration embodies the hypothesis of Halley ; who 

 supposed the earth to have four magnetic poles, two fixed and two 

 movable. Regarding the resultant of the magnetic forces of the 

 solid crust as indicating the two fixed poles, we have the two 

 movable poles of Halley indicated by the resultant of the mag- 

 netic forces of the internal fluid mass. The hypothesis of Halley 

 may well be set down as an instance of that grasp of mind with 

 which " men before their time" seize upon truths, which it may 

 require centuries of investigation fully to develop and demon- 

 strate. 



It will be observed that in this paper I have confined my re- 

 marks to the connexion between the theory of the earth and the 

 sectdar motions of the magnetic line. I propose to make the 

 bearing of the theory on the subordinate oscillations of the needle 

 the subject of a future communication. 



Hamilton College, Sept. 1839. 



