another, and with those lately measured in England, we find that they are best represented by a

1

150

of its semi

pheroid that has its compression. axis. * There is reason to think, therefore, that the meridians are not elliptical; and other observations seem to show, that they are not even similar to one another; or that the earth is not, strictly speaking, a solid of revolution; so, also, the comparison of the degree measured at the Cape of Good Hope, with those measured on the opposite side of the equator, creates a suspicion, that the northern and southern hemispheres are not perfectly alike, and that the earth is not equally compressed at the Arctic and the Antarctic poles. These irregularities, though they do not affect the general fact of the earth's compression at the poles, show that the true statical figure is but imperfectly attained; and though this may be accounted for, without having recourse to the principles involved in our theory, it is in a manner very unsatisfactory, and, by help of suppositions, not at all consistent with the original fluidity ascribed to the whole mass, or to the exterior crust of the earth.

442. As the principles here laid down explain how a solid body may attain very nearly the figure

* Exposition du Systême du Monde, par Laplace, p. 61, 2d edit.

which a fluid would acquire in order to preserve its parts in equilibrio; and since the oblate figure belongs to other of the planets as well as the earth, and the globular to all the great bodies of the universe, this suggests an analogy that goes deep into the economy of nature, and extends far beyond the limits within which the mineralogist is wont to confine his speculations.

443. That no very irregular figure is found among the planetary bodies, may therefore be considered as a proof of the universality of that system of waste and reconsolidation that we have been endeavouring to trace in the natural history of the earth. A farther proof of the same arises from considering, that for every given mass of matter, having a given period of rotation, there are two different spheroids that answer the conditions of establishing an equilibrium among its parts, the one near to the sphere, and the other very distant from it, and so oblate, as to have a lenticular form. Thus the earth, supposing it homogeneous, might either be in equilibrio, by means of the figure which it actually has, or of one in which the polar was to the equatorial diameter as 1 to 768. same is true of the other planets; and yet we no where find that this highly compressed spheroid is actually employed by nature. The reason, no doubt, is, that in so oblate a spheroid, the equilibrium between the gravitating and the centrifugal

The

force is of the kind that does not re-establish itself when disturbed; so that the parts let loose, and not kept in their place by firm cohesion, would fly off altogether. In such a body, the waste at the surface would lead to an entire change of form, and therefore the constitution here supposed could not be permanent.

444. In the system of Saturn, we have a great deviation from the general order, which, nevertheless, has led to a very unexpected verification of some of the conclusions deduced above. A principle extremely like that which is the basis of all the foregoing reasonings, led one of the greatest philosophers of the present age to discover the revolution of Saturn's ring on its axis, and even to determine the velocity of that revolution, such as it has been since found by observation. Laplace, laying it down as a maxim, that nothing in nature can exist, where there are causes of change, not balanced or compensated by other causes, cluded, that the parts of the ring must be held from falling down to the body of the planet by some other force than their mere cohesion to one another. Were it otherwise, every particle detached from the ring, by any means, must descend in a straight line, almost perpendicular to the surface of Saturn; and the final destruction of the ring must be in

Laplace, ubi supra, p. 242.

*

con

evitable. The only force that could balance this effect of gravitation, seemed to be a centrifugal force, arising from the rotation of the ring on an axis passing through its centre, and perpendicular to its plane. Laplace proceeded to inquire what celerity of rotation was adequate to this effect, and found that one of ten hours and a quarter would be required, which is almost precisely the time afterwards determined by Dr Herschel from actual observation. If, with this rotation, the ring is a solid annulus generated by the rotation of a very flat ellipsis about a given point in its greater axis, coinciding with the centre of Saturn, it may be so constituted, that the attraction of Saturn, combined with the centrifugal force, may produce a force perpendicular to its surface, and may enable detached parts to remain at rest, animals, for instance, to walk on its surface, and fluids to be in equilibrio. The system of Saturn is thus fortified against the lapse of time, as effectually as that of the earth itself; and the means by which this is accomplished, seem to prove, that the weapons which time employs, are in both cases the same, viz. the slow wearing and decomposition of the solid parts. This slow wearing may have produced the figure by which its action is most effectually resisted.

445. Thus Dr Hutton's theory of the earth comes at last to connect itself with the researches of physical astronomy. The conclusion to be

drawn from this coincidence is to the credit of both sciences. When two travellers, who set out from points so distant as the mineralogist and the astronomer, and who follow routes so different, meet at the end of their journey, and agree in their in their report of the countries through which they have passed, it affords no slight presumption, that they have kept the right way, and that they relate what they have actually seen.

NOTE XXVI. § 133.

Prejudices relating to the Theory of the Earth.

446. Among the prejudices which a new theory of the earth has to overcome, is an opinion, held, or affected to be held, by many, that geological science is not yet ripe for such elevated and difficult speculations. They would, therefore, get rid of these speculations, by moving the previous question, and declaring that at present we ought to have no theory at all. We are not yet, they allege, sufficiently acquainted with the phenomena of geology; the subject is so various and extensive, that our knowledge of it must for a long time, perhaps for ever, remain extremely imperfect. And hence it is, that the theories hitherto proposed have succeeded one another with so great rapidity, hardly any