It is a very significant fact, that _the deeper we penetrate into the Crust of the Earth, the hotter it is_. At first, no doubt, for a short distance, the reverse is the case. When we begin to descend we find it cooler below than above, because the further we depart from the surface the more we are removed from the influence of the Sun. But at a certain point--in our climate at about fifty feet below the surface--the influence of the Sun’s heat ceases to be sensibly felt. When this limit is passed, the temperature begins to rise, and thenceforth the deeper we go the hotter the earth becomes. This broad and general fact has been tested by experiments in every part of the world, and has been found true in all countries, in all climates, in all latitudes, whether in coal-pits, or mines, or deep subterranean caves. “In one and the same mine,” says Sir John Herschel,[89] “each particular depth has its own particular degree of heat, which never varies: but the lower always the hotter; and that not by a trifling, but what may well be called an astonishingly rapid rate of increase,--about a degree of the thermometer additional warmth for every ninety feet of additional depth,[90] which is about 58° per mile!--so that, if we had a shaft sunk a mile deep, we should find in the rock a heat of 105°, which is much hotter than the hottest summer day ever experienced in England.” Now if the temperature continue to increase at this rate toward the centre of the Earth, it is quite certain that, at no very great distance from the surface, the heat would be sufficiently intense to reduce the hardest granite and the most refractory metals to a state of igneous fusion.