The doctrine of a central heat is as old as the days of Bishop Burnet, who imagined that the internal fire, pre-existent in the bowels of the earth, was the agent employed in breaking up the fountains of the deep for the production of the deluge. Leibnitz and Buffon regarded the earth as an extinguished sun or vitrified globe, which, according to the calculations of the latter, required seventy-five thousand years to cool down to its present temperature; and that, in ninety-eight thousand years more, the heat will be utterly exhausted, and productive nature extinguished. Whiston fancied that the earth was created from the atmosphere of one blazing comet, and deluged by the humid tail of another. And Whitehurst, one of the oldest of modern geologists, regarded all the strata of every formation, as concentrically arranged over the surface of the globe, and then employed the expansive agency of internal fire to account for their upheaved disrupted condition. These cosmogonies, it is needless to remark, all now rank with the speculations of the alchemists, and that Behmen and their authors are considered as of equal authority in the sciences of chemistry and geology. The searching tests of experiment, as already noticed, have been brought to illustrate the subject of internal heat, but the results have not been decisive, nor very satisfactory. It has been stated, as a general rule in the mines examined, that, in proportion to their depth, the heat increases as we descend; and the mean result of all the best observations, as given by Cordier, amounts to one degree of heat for every forty-five feet of depth. Different mines, however, it has been ascertained, vary in their degrees of downward temperature; as in the Durham and Newcastle coal-pits, the increase is estimated at one degree for every forty-four feet,—in the Cornwall iron-mines at one in seventy-five feet,—and in Saxony some of the mines give an increase of only one degree in one hundred and eighty or one hundred and ninety feet of perpendicular descent. Much of this difference may, indeed, be readily accounted for by the nature of the contents of the mines themselves, their position in the system, and the quality of the rocks among which they are situated. But a difficulty will remain, how to dispose of the increasing ratio of temperature, and the changes that must necessarily result from it in the bowels of the earth. Thus, assuming it to be a uniformly increasing ratio in proportion to the depth, it will follow from this law of increase that we reach a point, about twenty-four miles down, hot enough to melt iron: at double that distance, such a heat as will fuse every substance with which we are acquainted: and at a hundred miles, that a temperature will exist, of whose resolving powers we have no experience, and cannot even conjecture. The astronomical theory of a thousand miles of crust melts into airy nothing in its presence, and