Of or pertaining to hydraulics, or to fluids in motion; conveying, or acting by, water; as, an hydraulic clock, crane, or dock. Hydraulic accumulator, an accumulator for hydraulic machinery of any kind. See accumulator. Hydraulic brake, a cataract. See cataract. Hydraulic cement, a cement or mortar made of hydraulic lime, which will harden under water. Hydraulic elevator, a lift operated by the weight or pressure of water. Hydraulic jack. See jack. Hydraulic lime, quicklime obtained from hydraulic limestone, and used for cementing under water, etc. Hydraulic limestone, a limestone which contains some clay, and which yields a quicklime that will set, or form a firm, strong mass, under water. Hydraulic main An inverted cup with a partition dipping into water, for opening or closing communication between two gas mains, the open ends of which protrude about the water.