We do not know what were M. Delaunay's proofs that the thickness of the crust has no bearing whatever on precession and nutation, but if they were complicated with the fluidity, or even viscosity, of a liquid interior beyond a depth of 800 to 1000 miles, they must be entirely changed under the notion of a hollow sphere where there could be no really liquid molten matter, except near the inner surface. One thing we may be certain of, and that is, there must be something to account for precession and nutation, and we believe that the hollow shell, with the greatest density where the mass is greatest, is a much more rational cause for these phenomena than the bulging out of the earth to the extent of 13 miles or so at the equator. It is very difficult to find out what geologists consider to be the nature of the interior of the earth in its details, but for our purpose no particular knowledge is required. However, it is necessary to allude to the principal features of their theories in order to note and remark how far they will agree to, or be facilitated, or the reverse, when applied to a hollow sphere. It would seem that almost all geologists are agreed that the central part is solid, and possibly extremely rigid owing to the enormous pressure of superincumbent matter; that it has a solid crust of several hundreds of miles in thickness; and that under this there is a sub-crust divided into two or more layers of different densities, partially liquid or at all events plastic, extending all over the solid interior matter; the chief purpose for which it is required being apparently to supply matter for volcanic action and surface movements.