50 Mr. J. Evans on Geological Changes in the [Mar. 15, 



If, however, the equiUbrium of the shell or crust he destroyed, as, for 

 instance, by the addition of a mass of extraneous matter at H, midway 

 between the pole and the equator, not only would the position of the axis 

 of rotation be slightly affected by the alteration in the position of the 

 centre of gravity of the now irregular sphere, but the centrifugal force of 

 the excess of matter at H would gradually draw over the shell towards D 

 until, by sliding over the nucleus, it attained its greatest possible distance 

 from the centre of revolution by arriving at the equator. The resultant 

 effect would be that though the whole sphere continued to revolve around 

 an axis as nearly as possible in the line F G, yet the position of the pole 

 of the hollow shell would have been changed by 45°, as by the passage 

 of H to the equator the points I and K would have been brought to the 

 poles by spirals constantly decreasing in diameter, while A and B, by spirals 

 constantly increasing, would have at last come to describe circles midway 

 between the poles and the equator. 



The axis of rotation of the hollow sphere and that of its fluid contents 

 would now again coincide, and would continue to do so perpetually unless 

 some fresh disturbance in the equilibrium of the shell took place. 



If instead of the addition of fresh matter at H we had supposed an 

 excavation or removal of some portion of the shell, a movement in the axis 

 of rotation of the shell would also have ensued, since from the diminished 

 centrifugal force of that portion of the hollow sphere where the excavation 

 had taken place, it would no longer equipoise the corresponding portion on 

 the opposite side at I, and the excavated spot would eventually find its 

 way to the pole. 



In order more clearly to exhibit these effects, I have prepared a model 

 in accordance with a suggestion of Mr. Francis Galton, F.R.S., in which 

 a wheel representing a section of a hollow sphere has its axis, upon which 

 it can freely turn, fixed in a frame, which is itself made to revolve in such 

 a manner that the axis of its rotation passes through one of the diameters 

 of the wheel, and coincides with what would be the axis of the sphere of 

 which the wheel is a section. 



In the periphery of the wheel are a number of adjustable screws with 

 heavy heads, so that, by screwing any of them in or out, the addition of 

 matter or its abstraction at any part of the sphere may be represented. 



If by adjusting these screws the wheel could be brought into perfect equi- 

 librium, its position upon its own axis would remain unchanged in whatever 

 position it was originally placed, notwithstanding any amount of rotation 

 being given to the frame in which it is hung ; hut practically it is found 

 that with a certain given position of the screws a certain part of the wheel 

 coincides with the axis of the frame, or becomes the pole around which the 

 sphere revolves. The rim of the wheel is graduated so as to show the 

 position of the poles in all cases, and generally speaking the wheel always 

 settles down after rotation with the pole within three or four degrees of 

 the same spot, if no alteration has been made in the adjustment of the 



