206 THE OCEAN. [Book IX. 



For tlie sake of illustration, let us suppose the 

 eartli in the condition described in the foregoing 

 chapter — namely, with an outer covering of air ; 

 beneath that air an unbroken expanse of water ; 

 beneath the water a hardenmg, but still more or less 

 pliant, surface of land ; and beneath the land a fluid, 

 incandescent, and gradually contracting mass, homo- 

 geneous with the materials whose solidification has 

 formed the outer crust or land. 



Under the sole action of its own force of gravita- 

 tion that globe would naturally tend to preserve its 

 form as a perfect sphere. But by the motion of 

 rotation round its axis a centrifugal force is created, 

 acting from the axis towards those parts of the 

 surface which, being most remote from the axis, 

 rotate with the greatest velocity. On the surface of 

 the globe this force acts from the poles of the axis 

 towards the equator. And, supposing the land or 

 outer crust of the globe to be sufficiently pliant, then 

 the liquid mass within it would bulge it out all round 

 the equator and draw it inwards at each of the poles ; 

 thereby causing its equatorial to be greater than its 

 axial diameter. The action of this force would not 

 tend to cause any difference between the hemispheres 

 lying on either side of the equator ; but, as far as its 

 action is concerned, those hemispheres would be 

 equal and their configuration similar. 



The tendency to contraction, as described in the 

 foregoing chapter, induces lateral pressure through- 



