HYDRAULIC PRESSURE ENGINE. A motor in which water is made to do work by means of its pressure only acting on a piston or plunger reciprocating in a cylinder. or, in some cases, on a revolving piston similar to those employed in a rotary steam-engine. In the case of a turbine the pressure of a column of water is, in the first instance, employed in giv ing motion to the fluid itself, and after this mo tion has been produced the energy due to it is utilized in doing work. In the ease of the pres sure engine the only pressure expended in giving motion to the fluid is that needed to make the water follow the piston and escape from the mo tor, and to secure efficiency this piston should be as small as practicable. This end is most nearly secured when the pressure is great and the speed of the engine is low. The first pressure engine was designed by Sir William Armstrong (q.v.),
and was of the rotary type. This engine was never actually used, and was replaced by a re ciprocating engine designed by the same engineer. Since Sir William Armstrong's invention a great variety of hydraulic pressure engines have been invented, the majority of which are of the re ciprocating type. Hydraulic pressure engines are particularly adapted to the use of high-pressure water in comparatively small quantities, and where these conditions prevail they have a con siderable field of usefulness. particularly as sec ondary motors for operating mechanical con trivances, such as the opening machinery for swing bridges, cranes, hoists, etc. For further information consult Iiodmer. Hydraulic Motors, Turbines, and Pressure Engines (New York, 1SS9). See TURBINE.