If the above rate of increase continues uniform the temperature of the crust would be sufficiently hot to boil water at a distance of about 8,000 feet below the surface, while at a depth of about thirty miles the temperature would be sufficiently high to melt all known substances at ordinary conditions of atmospheric pressure; that is, to melt all known substances if they were subjected to such a temperature at the level of the sea. In considering the above we must not lose sight of the fact that this increase in temperature with descent below the surface of the earth's crust occurs, not only in places where there are volcanoes, but over all parts of the earth, thus seeming to point out that there is something hot below the surface which fills the entire inside of the earth. It is true the greatest distance to which man has actually gone down through the earth's crust is but a few miles. We do not, therefore, know by actual experience that the interior is anywhere in a fused condition, yet the escape of lava or molten rocks in all latitudes, and in the enormous quantities referred to above, seems to show that the entire inside of the earth is at a temperature sufficiently high to melt all known substances under ordinary conditions. It may be interesting in this connection to examine some of the proofs of this increase in temperature with descent below the surface. The following figures are given by Dana: Borings to great depths have been made in various parts of the earth, both for artesian wells as well as for the shafts of mines. After passing the line of invariable temperature, the rate of increase for a total distance of 4,000 feet below the surface is in the neighborhood of from one degree for fifty-five to sixty feet, or an average of fifty-seven and a half feet for each degree of heat. In the case of the deep artesian well bored at Grenelle, Paris, where a temperature of eighty-five degrees Fahrenheit was reached at a distance of 2,000 feet, the rate of increase was somewhat more rapid, being one degree Fahrenheit for every sixty feet. In a deep well bored in a salt mine at Neusalzwerk, Prussia, a depth of 2,200 feet showed a temperature of ninety-one degrees Fahrenheit at the bottom. This was at the rate of one degree for every fifty feet of descent. At Schladenbach, in Prussia, a well has been dug to the depth of 5,735 feet with a temperature of 134° F. A boring at Wheeling, in West Virginia, reached a depth of 4,500 feet, 3,700 feet below the level of the sea. Here the rate of increase of temperature in the upper half was one degree Fahrenheit for every eighty feet, and in the lower half of one degree for every sixty feet.