GEOLOGY. 257 



" To begin with the case most favorable for the production of a 

 large effect, I will suppose the earth to be a perfectly rigid sphe- 

 roid, which has, in distant ages, and under proper circumstances, 

 assumed the form of equilibrium, and whose axis of rotation coincides 

 with its axis of figure, and is a principal axis. And I will suppose a 

 mountain mass to be elevated by something like a gaseous explosion 

 (the hollow being filled up by the influx of neighboring matter), in 

 latitude not very different from 45 ; it is evident that such elevation, 

 either at the pole or at the equator, will not disturb the parallelism 

 of the axis of rotation. 



" As the mountain mass partook of the rotatory movement before 

 its elevation, its elevation will not disturb, in any material degree, 

 the velocity of rotation. The principal effect of the elevation is, that 

 the axis of rotation is no longer a principal axis, that the principal 

 axis of largest moment is now a little way beyond the present axis of 

 rotation, and the two other principal axes, whose moments formerly 

 were equal, now have moments very slightly different. The effects 

 which follow are scarcely affected by this last modification. 



"It is well known that under these circumstances the axis of rota- 

 tion will wander in the solid earth. But it will not wander indefi- 

 nitely; its pole will describe on the earth's surface an ellipse not 

 sensibly different from a circle, whose centre is the pole of the new 

 principal axis, and after a certain time it will return to its former 

 position. The greatest change, therefore, in the terrestrial position 

 of the earth's pole will be double the distance of the new principal 

 axis from the former principal axis. 



" A very slight investigation suffices to show that the angular 

 change of the position of the principal axis will depend upon the pro- 

 portion which the moment of inertia of the mountain mass (or rather 

 the increased moment of inertia from all the changes in its neigh- 

 borhood, some being elevations and some depressions) bears to the 

 excess of the moment of inertia round the polar axis above the mo- 

 ment of inertia round an equatorial axis. This latter excess is about 

 half the moment of inertia of the equatorial protuberance, that is, 

 half the moment of inertia of a mass of matter twenty-five thousand 

 miles long, six thousand miles broad, and thirteen miles deep. And 

 what mountain mass can compare sensibly with this? *Even if a 

 mountain mass contained one-thousandth part of this matter (which 

 I apprehend is very far above the fact), the shift of the earth's pole 

 would be only two or three miles ; and this, though it would greatly 

 surprise astronomers, and might sensibly affect the depth of waters 

 in harbors, would produce no such changes of climate as those which 

 it is desired to explain. 



" Now, let us suppose that the earth is not absolutely rigid, but 

 that there is susceptibility to change of form, either from that degree 

 of yielding or fracture to which most solid substances are liable, or 

 from the hydrostatic pressure of internal fluid. This, as I conceive, 

 puts an end to all supposition of change of axis. The first day's 

 whirl would again make the axis of rotation to be a principal axis, 

 and the position of the axis is then permanent. 



" The density of the sea is so much less than that of the solid parts 

 of the earth, that it is not very important for us to consider it ; but, 

 22* 



