HYDRAULIC ENGINES, engines of which the motive power is water under pressure. In principle they do not dif fer essentially from steam engines. The water acts by difference of pres sure—i. e., it is admitted at a high pres sure at the beginning of the stroke, and exhausted at a low pressure at the end of the stroke, thus giving a reciprocat ing motion to the plunger. They may also be rotary. As water is much more dense than steam, the induction and eduction pipes have to be larger and have no abrupt angles. The eduction valve is required to open promptly at the end of the stroke, and the induction valve must not close till the stroke is com pleted. In most of the hydraulic en gines relief valves or other expedients are used to prevent these concussions. Since hydraulic engines work under much greater pressure they can be made much smaller than steam engines. A common form is the three-cylinder sin gle acting engine; in each cylinder works a plunger; water is admitted by valves behind the plungers and forces them out; at the conclusion of the out stroke the pressure water-supply is cut off, and the exhaust valve opened, al lowing the plunger to push the water out of the cylinder on the return stroke, and so on.
Hydraulic engines have been intro duced in some special industries in large foundries, in the working of cranes and other heavy machinery, and also for draining of mines.
Hydraulic machines in a small form are used for many industrial purposes in the operating of printing presses, cir cular saws, lathes, etc. The water head is obtained from a reservoir or accumu lator or, in cities where considerable pressure is constantly supplied, through the water mains.