"The second great store of heat is the internal heat of the earth," said Mr. Wilton. "The importance of this store of heat we can easily understand by considering that the earth is a mass of molten mineral matter cooled and hardened upon the surface. The crust upon which we live is warmed from beneath by an ocean, or rather a globe, a world, of glowing molten rock. Deep excavations have been made in mining operations, and artesian wells have been bored to still greater depths--as deep as two thousand, three thousand, or thirty-five hundred feet. The heat of the sun penetrates not more than seventy-five or a hundred feet; below that depth the temperature of the earth remains the same throughout the year. Below the point of constant temperature the heat of the earth is found to increase regularly and constantly. The rate of increase varies in different regions, but the average rate is about one degree of temperature for each fifty or sixty feet of descent. From this rate of increase it is easy to calculate the temperature at any given depth. At a depth of less than two miles water would boil. At twelve miles in depth the rock becomes incandescent. At twenty-two miles silver melts, at twenty-four miles gold melts, and at thirty-five miles cast iron becomes liquid. Volcanic eruptions also demonstrate the existence of immense masses of molten rock in the interior of the earth; and we can account for the existence of volcanoes only by supposing that they now communicate or once communicated with the deep interior heat of the earth. The thickness of the earth's crust is, however, a matter of conjecture. The melting point of different substances rises as the pressure upon them increases, and as the density of the rock increases its conducting power becomes greater. The crust of the earth, therefore, may be fifty miles in thickness, or it may be one hundred miles or two hundred or three hundred miles. The effect of this internal heat in maintaining the temperature of the earth must be very great." "I want to ask," said Peter, "how this internal heat came to exist, and how it is maintained?"