Hydraulic Mining.— This term is applied to that system of mining in which heavy streams of water are played on a bank of earth or gravel to wear it away. The streams are thrown by Igiants,)) which are simply huge nozzles by which the water is delivered from some high head, giving great pressure. Where plenty of water is to be had there is no such cheap way of opening up a hillside for the miner as the hydraulic system. The giant streams bring down the gravel rapidly and in just the right condition for easy panning of the gold or other valuable metal content. But this system ruins the locality for agricultural purposes, because all the soil is buried beneath deep beds of gravel, and so hydraulic mining is in disfavor with all other interests, and in some States is prohibited by law. The °miner's inch b originally a measure of water for the placer miner, and now also a unit of measure for water used in irrigation in the arid regions of the West and Southwest, has interest as a hydraulic flow through an orifice and subject to the laws governing such flows. It is ad judged to be that amount of water which will flow through an inch-square opening in an inch board, all the edges being sharp, under a head of six inches (sometimes seven inches) above the upper edge of the opening. The amount of water which actually passes under such conditions is 1.52 cubic feet per minute. It varies, however, in different localities. In Arizona, Idaho, Nevada and Utah the custom ary miner's inch is a deliverance of 1.2 cubic feet per minute. In California and Montana it is fixed by law at 1.5 cubic feet per minute.
Dams and The hydraulic engi neer is called upon to design and construct dams under ever varying conditions. Most modern river dams are made of concrete. or part concrete and part stone; the concrete is of ten reinforced. It is common to curve the dam up stream so as to form an arch, thus greatly increasing the strength of the dam, as its re sistance may be considerably more than its weight. The up-stream side of a dam is usu ally sloped, and if the dam is of rubble or rip rap, is faced with timber or concrete; the down stream side of a dam may approach the perpen dicular. Every dam requires a waste weir with gates to let out the surplus water in time of flood or emergency. Insufficient dams have oc casionally burst, flooding the valley below, as at Johnstown, Pa., but these calamities have served to inform engineers more fully as to conditions, and there is every reason to place confidence in modern dams. See IRRIGATION; WATER SUPPLY.
Canal engineering is a part of hydraulics, and has reached its highest development in the building of the Panama Canal (q.v.). The principle of a canal lock is simple. Where there is a change of level of the water in a canal a lock is formed by building two dams having great gates and located a few hundred feet apart. The space between these dams is termed the lock. When a vessel is to pass, say from the lower to the higher level, the water within the lock is let out means of small gates until the water of the interior of the lock is at the lowest level, the upper gates of course being kept closed. The large lower gates are then opened wide and the vessel is floated in to the lock; then the lower gates are shut, and small gates in the upper end of the lock are opened, so that water flows in until the lock is filled to the upper level, when the upper large gates are opened, and the vessel floated up and out of the lock. To pass a vessel down through a canal lock the operation described is reversed. Water is lost from the upper level at every such passage, but in designing a canal the engineers find streams to supply water to keep the upper levels full.
Wave One of the most interest ing fields of hydraulic study is connected with wave motors. In theory the waves on the ocean constitute a tremendous power, which it ought to be easy to harness for man's use; in prac tice the wave motor has never been able to compete with the steam-engine. The wave motor has been constructed in hundreds of ways, and the files of the Patent Office are flooded with designs, most of them ingenious, and very many of which have been built and tried out; but though the machines develop immense power at times, they accomplish little at low tide and in very calm weather, while in storms they are apt to be broken to pieces by the tremendous exercise of surplus power. It remains for some inventor of the future to make the wave motor commercially practicable.
See CANAL; ENGINEERING, HYDRAULIC; HY DRAULIC RAM; HYDRODYNAMICS; HYDRO-ECO NOMICS; HYDRO-ELECTRIC DEVELOPMENT; HYDRO STATICS ; HYDROGRAPHY; HYDROLOGY; IRRIGA TION; WATER-WORKS, etc.
Bibliography.— Fanning, J. T.. 'Practical Treatise on Hydraulics and Water Supply En gineering' (New York 1902) ; Gibson, A. H., 'Hydraulics and Its Applications' (New York 1915) ; Hughes, H. J., and Safford, A. T., Treatise on Hydraulics) (New York 1911); Merriman, M. 'Treatise on Hydraulics) (New York 1914) ; Parker, P. A. M., The Control of Water' (London 1913); Smith, H., Jr., (New York).