Every day's experience confirms more and more the opinion that the central heat doctrine has less foundation than formerly it was supposed to possess. Its great supporters have gradually increased the necessary thickness of the solid crust in proportion to the internal supposed fluid core from forty to eight hundred miles at least: rather a wide difference in itself, but not perhaps so very great in respect to the absolute diameter of the earth, to which such a relationship would be about in proportion to the thickness of a sheet of cartridge-paper round a 12-inch globe. We know nothing, however, so perfectly a non-conductor that so thin would resist the heat of the internal molten mass. Moreover, upon the alleged increase of temperature with depth in coal and other mines, much doubt has been thrown by the subsequently ascertained facts that in many instances the higher temperatures have disappeared after the mines had ceased to be worked. The necessity, if the interior were fluid, for internal tides below the supposed solid crust, also militates against the existence of a fluid core, because we can detect no such tides at the surface of our earth; and if they existed, it is difficult to conceive the rigidity and strength of so thin a crust to be equal to restraining them entirely; and if the crust were in the least degree yielding or elastic, we must have evidence of such tides in the heavings of the surface.