Hydraulic Engine is a motor driven by water under pressure. The pressure-energy of the fluid is converted therein into the kinetic energy of the moving parts of the engine. In the case of waterwheels and turbines the water that enters the engine is possessed of a definite velocity; and impact occurring, there is a direct transfer of kinetic energy to the moving parts. Such cases do not, therefore, come exactly under the above definition of hydraulic engines, though the term is frequently employed inclusively. The ordinary engine worked by water pressure is similar in many respects to the steam-engine. A piston fits closely in a cylinder of cast-iron or cast-steel, and is forced backwards and forwards by water that is passed into the cylinder on one side or the other alternately. Since water is but slightly compressible, its expansion during portion of the stroke is not available, and it is necessary, therefore, to pass full-pressure water into the cylinder throughout the stroke, all of which is relieved of pressure during the return-stroke, deprived of its pressure energy, and passed into the exhaust. This inability of the working fluid to expand constitutes the important difference between such an engine and the ordinary steam-engine. Two, or preferably three, cylinders are used to drive the same crankshaft, for hydraulic engines are not adapted for great speeds, and it is necessary to carry any one piston past its dead-points by means of another. When three are used they may be arranged radially at 120° to each other, the three piston-rods pointing to the centre of the crank-shaft, and moving in the same vertical plane.