542 Royal Society : — Mr. J. Evans on a possible Cause of 



in revolution, in the region of the equator. Or again, if one or, 

 better still, two opposite screws at the equator be turned inwards, 

 they will be found after a short period of revolution at the poles. 



Now let us assume for a moment that, though the crust was par- 

 tially covered by water, the earth, instead of being a spheroid, was a 

 perfect sphere, consisting of a hardened crust of moderate thickness 

 supported on a fluid nucleus over which the crust could travel freely 

 in any direction, but both impressed with the same original rotatory 

 motion, so that without some disturbing cause they would continue 

 to revolve for ever upon the same axis, and as if they were one homo- 

 geneous body. Let us assume, moreover, that this crust, though in 

 perfect equilibrium on its centre of rotation, was not evenly spherical 

 externally, but had certain projecting portions, such as would be 

 represented in Nature by continents and islands rising above the 

 level of the sea. 



It is evident that so long as those continents and islands remained 

 unaltered in their condition and extent, the relative position of the crust 

 to the enclosed fluid nucleus would remain unaltered also. But sup- 

 posing those projecting masses were either further upheaved from 

 some internal cause, or worn down and ground away by the sea or 

 by subaerial agency and deposited elsewhere, it seems impossible but 

 that the same effects must ensue as we see resulting upon the model 

 from the elevation and depression of certain screws, and that the 

 axis of rotation of the crust of the sphere would be changed in con- 

 sequence of its having assumed a fresh position upon its fluid nucleus, 

 though the axis of the whole sphere might have retained its original 

 direction, or have altered from it only in Jhe slightest degree. 



An irregular accumulation of ice at one or both of the poles, such 

 as is supposed by M. Adhemar, would act in the same manner as an 

 elevation of the land ; and even assuming that the whole land had 

 disappeared from above the surface of the sea, yet if by marine cur- 

 rents the shallower parts of the universal ocean were deepened and 

 the deeper parts filled up, there would, owing to the different specific 

 gravity of the transported soil and the displaced water, be a disturb- 

 ance in the equilibrium of the crust, and a consequent change in the 

 position of its axis of rotation. 



Now if all this be true of a sphere, it will also, subject to 

 certain modifications, be true of a spheroid so slightly oblate as our 

 globe. 



The main difference in the two cases is, that in a sphere the crust 

 may assume any position upon the nucleus without any alteration 

 in its structure, while in the case of the movement of a spheroidal 

 crust over a similar spheroidal nucleus, every portion of its internal 

 structure must be more or less disturbed, as the curvature at each 

 point will be slightly altered. 



The extent of the resistance to an alteration of position arising from 

 this cause will depend upon the oblateness of the spheroid and the 

 thickness and rigidity of the crust ; while the thicker the latter is, 

 the less also will be the proportionate effect of such elevations, subsi- 

 dences, and denudations as those with which we are acquainted. The 

 question of friction upon the nucleus is also one that would have to 

 be considered, as the internal matter though fluid might be viscous. 



