The water engine is a positive-displacement engine, often closely resembling a steam engine with similar pistons and valves, that is driven by water pressure. The supply of water was derived from a natural head of water, the water mains, or a specialised high-pressure water supply such as that provided by the London Hydraulic Power Company. Water mains in the 19th century often operated at pressures of 30 to 40 psi, while hydraulic power companies supplied higher pressure water at anything up to 800 psi.
In the nineteenth century, the terms hydraulic motor and hydraulic engine often implied reference to any motor driven by liquid pressure, including water motors and water engines used in hydropower, but today mentions of hydraulic motors, unless otherwise specified, usually refer more specifically to those that run on hydraulic fluid in the closed hydraulic circuits of hydraulic machinery.
Because water is virtually incompressible, the valve gear of water engines is more complicated than that used in steam engines, and some water engines even had a small secondary engine solely to power the operation of their valves. Closing a valve too quickly can cause very large pressures to result, and pipework to explode (a phenomenon similar to water hammer), and in addition to valves designed to close slowly, many water engines used air chambers to provide some absorption of force by compressing the air in them.
The largest possible design of a water engine is the directly acting water-column engine or water column machine Such devices had been in use for pumping purposes in different mining areas since the middle of the eighteenth century and one was used, for example, by Georg Friedrich von Reichenbach in 1810 to pump brine from Berchtesgaden to Reichenhall