Men having discovered the power of falling water, which after all is comparatively slight, how eagerly do they seek out and improve these _privileges?_ Let a difference of but a few feet in level be discovered on some stream near a populous town, some slight occasion for gravity to act, and the whole economy of the neighborhood is changed at once. Men do indeed speculate about and with this power as if it were the only privilege. But meanwhile this aerial stream is falling from far greater heights with more constant flow, never shrunk by drought, offering mill-sites wherever the wind blows; a Niagara in the air, with no Canada side;—only the application is hard. There are the powers too of the Tide and Waves, constantly ebbing and flowing, lapsing and relapsing, but they serve man in but few ways. They turn a few tide mills, and perform a few other insignificant and accidental services only. We all perceive the effect of the tide; how imperceptibly it creeps up into our harbors and rivers, and raises the heaviest navies as easily as the lightest chip. Everything that floats must yield to it. But man, slow to take nature’s constant hint of assistance, makes slight and irregular use of this power, in careening ships and getting them afloat when aground. The following is Mr. Etzler’s calculation on this head: To form a conception of the power which the tide affords, let us imagine a surface of 100 miles square, or 10,000 square miles, where the tide rises and sinks, on an average, 10 feet; how many men would it require to empty a basin of 10,000 square miles area, and 10 feet deep, filled with sea-water, in 6¼ hours and fill it again in the same time? As one man can raise 8 cubic feet of sea-water per minute, and in 6¼ hours 3,000, it would take 1,200,000,000 men, or as they could work only half the time, 2,400,000,000, to raise 3,000,000,000,000 cubic feet, or the whole quantity required in the given time. This power may be applied in various ways. A large body, of the heaviest materials that will float, may first be raised by it, and being attached to the end of a balance reaching from the land, or from a stationary support, fastened to the bottom, when the tide falls, the whole weight will be brought to bear upon the end of the balance. Also when the tide rises it may be made to exert a nearly equal force in the opposite direction. It can be employed wherever a _point d’appui_ can be obtained.