satellites. For, as the moon is the cause of the tides, and the sun the cause of the wind, which, in turn, is the cause of the waves, all the work of this planet would be performed by these far influences.
"We may store up water in some eminent pond, and take out of this store, at any time, as much water through the outlet as we want to employ, by which means the original power may react for many days after it has ceased. . . . Such reservoirs of moderate elevation or size need not be made artificially, but will be found made by nature very frequently, requiring but little aid for their completion. They require no regularity of form. Any valley, with lower grounds in its vicinity, would answer the purpose. Small crevices may be filled up. Such places may be eligible for the beginning of enterprises of this kind."
The greater the height, of course, the less water required. But suppose a level and dry country; then hill and valley, and "eminent pond," are to be constructed by main force; or, if the springs are unusually low, then dirt and stones may be used, and the disadvantage arising from friction will be counterbalanced by their greater gravity. Nor shall a single rood of dry land be sunk in such artificial ponds as may be wanted, but their surfaces "may be cov-
