After all, however, the crucial question is, whether the theory of central heat has any positive evidence to support it. Here we meet, in the first place, with the undisputed fact that heat increases as we descend from the surface of the earth. Sir Charles Lyell admits that the fact of augmentation is proved. Experiment and observation, no doubt, have not yet enabled us to determine the ratio in which the heat increases as we penetrate into the crust; but this does not neutralise the force of the fact itself. Sir Charles endeavours to parry its effect by remarking that if we take a certain ratio of increase, a ratio which seems to be countenanced by experiment, we shall, 'long before approaching the central nucleus,' arrive at a degree of heat so great 'that we cannot conceive the external crust to resist fusion.' It is surely a sufficient reply to this to say that our conceptions as to the consequences arising from an admitted fact can neither invalidate its evidence nor annul the obvious inferences from it. The reader of the 'Principles of Geology,' besides, who has been told by Sir Charles Lyell that the interposition of a few feet of scoriæ and pumice enables him to stand without inconvenience on molten lava, may be permitted to form a high estimate of the power of many miles of stratified and unstratified rock to resist fusion by the internal fires. Sooth to say, however, it will be time to consider an objection grounded on the ratio of the increase in heat from the surface of the earth downwards, when the ratio in question has been ascertained. The fact of increase is admitted; the ratio of increase is an unknown quantity: it is curious logic to impugn the direct bearing of the former, on the strength of consequences conceived to arise from the latter. Hugh Miller believed that the existence of the equatorial ring, in virtue of which the polar diameter of the earth is shorter than the equatorial, furnished explicit evidence that the planet once was molten. 'If our earth,' he wrote, 'was always the stiff, rigid, unyielding mass that it is now, a huge metallic ball, bearing, like the rusty ball of a cannon, its crust of oxide, how comes it that its form so entirely belies its history? Its form tells that it also, like the cannon-ball, was once in a viscid state, and that its diurnal motion on its axis, when in this state of viscidity, elongated it, through the operation of a well-known law, at the equator, and flattened it at the poles, and made it altogether the oblate spheroid which experience demonstrates it to be.'