There are regions where the ground is red-hot at a depth of a hundred feet. While there is no water present to furnish steam, it is a simple matter to sink a water pipe down to the heated earth and then, around this pipe, to drive a ring of smaller pipes through which steam may find its way up to the top and be fed either into water heaters or directly into steam engines. In fact it has been suggested that such a scheme might be used almost anywhere. If we bore into the earth, we find that the temperature grows higher the deeper we go. The rate of increase of temperature varies with different localities, but it is very evident that anywhere on earth temperatures that will give a steam pressure of ten or more pounds per square inch can be obtained if we dig down far enough, and when we find it worth while to do so we shall probably riddle the earth’s crust with perforations through which water will be sent down to the subterranean furnaces and it will return to us laden with heat energy. After all, we shall not be plunged into dire want when our stores of coal are exhausted. There will be other sources of power to draw upon, most of which will be inexhaustible. Furthermore, we have recently discovered in atoms of matter stores of energy incomparably greater than any that have heretofore been used to work the will of man. How to utilize this energy we have not yet learned, but the energy is there, and no doubt, some day, probably long before coal takes its place in museum collections, we shall be possessed of a new slave, far more powerful than that which has served us so far. CHAPTER XII INVASION OF THE SEA THE POSSIBILITY that the wind was the first inanimate power utilized by man, has already been referred to. There are records of the use of sailing vessels in Egypt that date as far back as 6,000 years before Christ. Navigators of that early date, however, could hardly claim to have mastered the wind. They merely used wind power when the wind was disposed to help them. If the winds were adverse, they had no recourse other than to furl their sail, step the mast and depend upon oars to propel them to the desired port. It was not until thousands of years later that primitive mariners learned how to tack and pursue a zigzag course against the wind. When this knowledge was acquired we do not know, but it is certain that the Phœnicians, who rounded the continent of Africa 1,200 years before Christ, knew how to make use of the power of opposing winds. Of course they could not explain how it was that a breeze could be made to drive a vessel in a direction across and even opposed to that in which it was blowing. In order to understand this apparent paradox ourselves, we must go back to the very elements of mechanics. ELEMENTARY MECHANICS