6 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 3, 



The solid nucleus of the earth is not a sphere, but a spheroid ; and 

 if we suppose the axis of rotation at any time to have occupied a 

 different place from that which it occupies at the present time, and 

 not identical with the axis of figure, if any resistance exists, it would 

 cause the pole of the axis of rotation to describe a spiral round the 

 pole of the axis of figure, and finally it would become, as it is at pre- 

 sent, identical with it. Moreover you would necessarily have a change 

 in the relative positions of sea and land. 



If the axis of rotation could suffer such a displacement by reason 

 of the causes which produce the precession of the equinoxes, you 

 would have another and a more natural way of accounting for the 

 existing phsenomena ; but this has been held to be impossible. 



If we admit the rotation of the earth to take place under such 

 conditions that the surface experiences any friction (i. e. resistance, 

 which Laplace did not consider), then the invariability of geographical 

 latitude which exists otherwise, noticed by Laplace (vol. v. Mec. Cel. 

 p. 268), is not a necessary consequence. 



Great obstacles interfere, so as to prevent a complete elucidation of 

 this question. Putting aside the great difficulty of an exact and 

 finished solution of the problem of the rotation of a body in a resisting 

 medium, we have no numerical data which would assist in deter- 

 mining the amount of such resistance. Still less do we know the 

 structure of the strata miderneath the earth's surface, or the history 

 of the changes which have taken place during the process of cooling, 

 which might enable us to trace the position of the axis of rotation in 

 remote times. So that I think the utmost that can be accomplished 

 by mathematics is to explain under what hypothesis a change of the 

 position of the axis of rotation is possible or not. 



In the Mec. Cel. vol. v. p. 14, Laplace lays it down, that it is im- 

 possible to account for the changes which have taken place on the 

 surface of the earth, and in the relative positions of land and water, by 

 a change in the position of the axis of rotation. But this dictum is 

 founded upon the absence of two considerations, both of which appear 

 to me to be essential : 



1 . The dislocation of strata by cooling ; 



2. The friction of the surface. 



If the earth were at any remote epoch a homogeneous spheroid, 

 formed for example of any one pure metal in a state of fusion, then, 

 in the process of cooling, I doubt not that the mass would always be 

 made up of concentric spheroids, so that it would always revolve 

 about the same principal axis of rotation, which would never deviate 

 from the axis of figure. But the condition of the earth which really 

 obtains is so very far, as we know, from being that of homogeneity, 

 that it seems to me quite within the limits of possibility, that in cool- 

 ing the position of the axis of rotation may have changed, apart from 

 the influence of friction at the surface. 



With regard to the second consideration, although not alluded to 

 in p. 14 of the Mec. Cel. vol. v., it is expressly referred to by Laplace 

 at p. 254 of the same volume. The inequalities to which Laplace 

 there alludes, and which he supposes and admits may have existed 



