Changes in the Position of the Axis of the Earth's Crust. 541 



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 per- 

 petually 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 exca- 

 vated 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 

 equilibrium, 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 ; 

 but 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 screws, 

 though of course what was the uppermost pole may become the 

 lower one ; and in some cases the wheel may be in cequilibrio with a 

 projecting screw either above or below the equator, in which case 

 there may be four readings on the circle at the index-point, according 

 as the one pole or the other is uppermost, and the projecting screw 

 is above or below the equator. 



With the screws on the wheel evenly balanced, a slight alteration 

 in the adjustment of any of them immediately tells upon the position 

 of what, for convenience sake, may be called the poles, except, in- 

 deed, in such cases as screwing outwards those already at the equator, 

 or making similar alterations in the adjustment of twoserewsat equal 

 distances on either side of one of the poles. If a screw be turned 

 outwards so as notably to project at any spot, no matter how near 

 to the pole, it will be found, after the machine has been a short time 



