435. Since, then, neither the hypothesis of the Neptunists or the Vulcanists, affords any good explanation of the figure of the earth, or such a one as can connect it with the other appearances in its natural history, it remains to inquire, whether the system that supposes a partial and successive fluidity, like Dr Hutton's, has any resource for explaining this great phenomenon.
Of this subject Dr Hutton has not treated; and when I was first made acquainted with his system, it appeared to me a very serious objection to it, that it did not profess to give an explanation of so important a fact as the oblate figure of the earth. On considering the matter more closely, however, I found that there were principles contained in it from which a very satisfactory solution (and, I think, the only satisfactory solution) of that difficulty might be deduced. This solution I shall endeavour to explain, in as far, at least, as is necessary for the purpose of general illustration.
It is laid down in Dr Hutton's theory, that the surface of the earth is perpetually changed by the detritus of the land; and that from the materials thus afforded, new horizontal strata are perpetually formed at the bottom of the sea. If this be true, and if the alternations of decay and renovation have been often repeated, it is certain, that the figure of the earth, whatever it may have originally been, must be brought at length to coincide with the spheroid of equilibrium.
436. Here it is necessary to remark, that the expressions, figure of the earth, and surface of the earth are each of them occasionally taken in two different senses.
The surface of the earth, in its most obvious sense, is that which bounds the whole earth, and includes all its inequalities; it is a surface extremely irregular, rising to the tops of the mountains, descending to the bottoms of the valleys, and having the continuity of its curvature often interrupted, or suddenly changed. This may be called the actual surface, and the figure bounded by it, the actual figure, of the earth.
The surface of the earth, in another sense, is one that is every where horizontal, and is the same which water assumes when at rest.
This superficies is determined by the circumstance of its being constantly perpendicular to the direction of gravity; it is the surface marked out by levelling, and may be supposed to be continued from the sea, through the interior of the land, till it meet the sea again. The figure bounded by this horizontal surface, may properly be called the statical figure of the earth.
When it is said that the figure of the earth is an oblate spheroid, it is the statical, not the actual figure which is meant; and the degrees of the meridian which astronomers measure, are also referred to the superficies of the former.
437. Suppose now a body like the earth, but with its actual figure infinitely more irregular, having a sea circumfused around it, the water will descend into the lowest situations, and will so arrange itself, that its surface shall be perpendicular every where to the plumb-line, or to the direction of gravity, in which state only it can remain at rest. The figure of the superficies which the sea must thus take will be of a continuous curvature, and will return into itself; though it may, if the actual figure is very irregular, be far either from a sphere or a spheroid. If, however, we suppose the solid parts of this mass subject to be dissolved or worn away, and carried down to the ocean, there will be a tendency to give to the whole body the same figure that it would have assumed, if it had been entirely fluid, and subject to the laws of hydrostatics. This tendency is the result of two principles.
438. Let us suppose the body just described to have no rotation, so that the particles of it are actuated only by the forces of cohesion and of attraction.