One further evidence, and that not the least important, is deducible from geology. From the known rate at which the temperature rises as we pierce deeper into the substance of the Earth, it has been inferred that its solid crust is some forty miles thick. And if this be its thickness, we have a feasible explanation of volcanic phenomena, as well as of elevations and subsidences. But proceeding on the current supposition that the Earth's interior is wholly filled with molten matter, Prof. Hopkins has calculated that to cause the observed amount of precession of the equinoxes, the Earth's crust must be at least eight hundred miles thick. Here is an immense discrepancy. However imperfect may be the data from which it is calculated that the Earth is molten at forty miles deep, it seems very unlikely that this conclusion differs from the truth so widely as forty miles does from eight hundred. It seems scarcely conceivable that if the crust is thus thick, it should by its contraction and corrugation, produce mountain chains, as it has done during quite modern geologic epochs. It is not easy on this supposition to explain elevations and subsidences of small area. Neither do the phenomena of volcanoes appear comprehensible. Indeed to account for these, Prof. Hopkins has been obliged to make the gratuitous and extremely improbable assumption, that there are isolated lakes of molten matter enclosed in this thick crust, and situated, as they must be, not far from its outer surface. But irreconcileable as appear the astronomical with the geological facts, if we take for granted that the Earth consists wholly of solid and liquid substances, they become at once reconcileable if we adopt the conclusion that the Earth has a gaseous nucleus. If there is an internal cavity of considerable diameter occupied only by aeriform matter--if the density of the surrounding shell is, as it must in that case be, greater than the current supposition implies; then there will be a larger quantity of matter contained in the equatorial protuberance, and an adequate cause for the precession. Manifestly there may be found some proportion between the central space and its envelope, which will satisfy the mechanical requirements, without involving a thicker crust than geological phenomena indicate.[R] [R] Since this was written, M. Poinsot has shown that the precession would be the same whether the Earth were solid or hollow.