41 G On the Fluid Theory of the Figure of the Earth. 



much effect on the variation of gravity as the change of the'strata 

 in form, that is, relatively to the axis. This, indeed, is what might 

 have boon anticipated ; and it tells, I think, very greatly in favour 

 of the fluid theory. I can conceive it possible that a crude irre- 

 gular solid mass revolving round an axis might in the course 

 of indefinite ages assume a more or less external spheroidal form. 

 For the perpetual degradation of the solid prominences by the 

 process of weathering would set free particles which with the 

 fluid portions would find a position on the surface approaching 

 to conformity with the laws of the equilibrium of fluids. But 

 no process of this kind, or any other process, could make the 

 interior parts bulge out towards the equator in strata more or 

 less spheroidal with the axis of revolution as a common axis, 

 unless those parts were moveable like a fluid or semifluid mass. 



That the earth's mass consists of nearly spherical strata, is 

 not dependent on any theory. 



11. Before concluding, I would remark that it has, I think, 

 generally been assumed that the notion that the earth's mass 

 consists of strata more or less spherical about the earth's centre 

 is a deduction from the fluid theory, and that, apart from that 

 theory, we have no knowledge whatever regarding the distribu- 

 tion of the matter in the interior of the earth. This, as I have 

 intimated in paragraph 3, is hardly correct. 



There are two facts regarding the law of gravity at the surface 

 of the earth which observation has brought to light : (1) that 

 the plumb-line points in all parts of the world very nearly to the 

 geometrical centre of the globe; and (2) that the amount of 

 gravity is very nearly constant all over the globe, slightly in- 

 creasing from the equator towards the poles, but by such small 

 quantities that at Spitzbergen, in about latitude 80°, the increase 

 is only about yfu^ P ar ^ °f that at the equator. Now, if it 

 were true that at two or three places only the plumb-line pointed 

 nearly in the direction of the diameters of the earth at those 

 places, we could not infer anything more than that the matter 

 of the earth is distributed with reference to those particular 

 diameters in such a way as to produce no horizontal effect at 

 the surface. But since this is nearly the case with every dia- 

 meter, as observation teaches us, no other conclusion is possible 

 but that the matter of the earth is arranged nearly symmetri- 

 cally about every diameter, and therefore about the centre of the 

 earth. For, suppose there is a sensible preponderance of matter 

 on one side of the centre above that on the opposite side. The 

 plumb-line at the two places at the extremities of the diameter 

 passing through the midst of that preponderance may hang 

 towards the centre of the earth; but at places away from those 

 two particular spots, the plumb-line would be swayed away from 



