2257. That the materials of the bank must be of a nature impervious to water, and also must adhere to the base or bottom, so as not to admit water to escape beneath it, are obvious conditions of the foregoing proposition. The practice of forming dams or heads is derived from this theory; but to guard against accident, the base of the triangle is always made three or more times greater than its height; the slope next the stream may form an angle with the horizon of from 40ﾦ to 20ﾦ, and that on the lower side is regulated by the uses of the dam. If for raising water so as to cover a hollow where there is little or no overflow expected, then the slope on the under side is generally of earth, 40ﾦ or 35ﾦ, turfed or planted; if for a cascade, the slope is regulated by the form or undulations on which the rocks to produce the breaking of the water are to be placed; and if for a waterfall, a perpendicular wall is substituted, over which the water projects itself in a sheet or lamina, in breadth proportioned to the quantity of the current. In all these cases, instead of forming the dam entirely of materials impervious to water, it is sufficient if a vertical stratum of wrought-clay be brought up its centre, and the surface of the bank rendered firm by a coating of gravel on the slope next the water.
