16. Experience demonstrates that the temperature has been raised one degree by every thirty yards of depth: whence it follows that at a depth of three hundred yards the augmentation is ten degrees, at three thousand yards, one hundred degrees, a temperature of boiling water; at thirty thousands yards, seven to eight leagues (from twenty-one to twenty-four miles), one thousand degrees; at twenty-five leagues (seventy-five miles), more than thirty-three hundred degrees, a temperature at which no known material can resist fusion. From there to the center there is still a space of more than fourteen hundred leagues (forty-two hundred miles), may be twenty-eight hundred leagues (eighty-four hundred miles), in diameter, which must be occupied by molten substances.
Although this is only a conjecture judging cause by effect, it has all the elements of probability; and one arrives at this conclusion, that the Earth is still an incandescent mass covered with a solid crust of twenty-five or more leagues (seventy-five miles) in thickness, which is scarcely the one hundred and twentieth part of its diameter. Proportionally speaking, it must be much thinner than the thinnest rind of an orange.
For the rest, the thickness of the terrestrial crust is very variable in many places; for there are some countries, especially volcanic territories, where the heat and flexibility of the soil indicate that it is much thinner. The high temperature of hot springs is also an indication of close vicinity to the central fire.