A hydraulic ram is a water-powered cyclic pump. It harnesses the "water hammer" effect to pump water.

In operation, moving water presses on gas inside a pressure vessel. A check valve closes, and the compressed gas pushes the water uphill. The mass of the water moving in the pipe exerts a suction. The suction lowers the pressure in the gas chamber. The check valve opens and the cycle repeats.

See: hydraulics.