452. Double-acting pump. Cylinder closed at each end, and piston-rod passes through stuffing-box on one end, and the cylinder has four openings covered by valves, two for admitting water and like number for discharge. A is suction-pipe, and B discharge-pipe. When piston moves down, water rushes in at suction-valve, 1, on upper end of cylinder, and that below piston is forced through valve, 3, and discharge-pipe, B; on the piston ascending again, water is forced through discharge-valve, 4, on upper end of cylinder, and water enters lower suction-valve, 2.