1866.] Position of the Axis of the Earth^s Crust. 



51 



screws, though of course what was the uppermost pole may become the 

 lower one ; and in some cases the wheel may he in equilibrio with a 

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

 may be four readings on tbe 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, indeed, in such 

 cases as screwing outwards those already at the equator, or making similar 

 alterations in the adjustment of two screws at 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 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 partially 

 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 tbe 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 homogeneous 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 supposing 

 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 consequence 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 the 

 slightest degree. 



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

 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 currents the shallower 

 parts of the universal ocean were deepened and the deeper parts filled up. 



