The hydraulic or water-pressure cranes were invented by Sir W. Armstrong, and applied by him to the quays of Newcastle about the year 1846, in the first instance; but subsequently, their use has become general in other towns, wherein water is to be obtained under considerable pressure. In these engines the water is admitted to act upon one side of a piston working iu a tight cylinder, and bearing a piston-rod, upon the end of which is fastened a chain passing over two fixed pulleys, under the pivot of the crane, and over a moveable one on the head of the piston itself, in order to increase the distance traversed by the load, at the expense of the power. The stroke of the cylinders is usually long, and by thus passing the chain over the three pulleys, the load is raised through a height equal to three times the stroke; but the load is correspondingly reduced in proportion to the effort exerted on the piston. There are valves placed at the bottom of the cylinder to close the access of the water to the piston, and to open the escape passages; and when the latter are opened the water escapes, and the weight of the piston, and of the machinery attached, brings the piston back to its original position. Relief-valves are placed near the slide
valves, which give access or to the water, to guard against any sudden shocks from changes of direction in the movement of the water which might be likely to produce a hydraulic jar. The water-pressure cranes used at Newcastle had cylinders 12 feet long by 1 foot in diameter, and they worked under a head of water equal to 210 feet.
The principle of the hydraulic press is, in fact, the same as the one involved in the water-pressure engine and the hydraulic crane. It consists iu the faculty by which water transmits In all directions a power exercised upon any portion of its surface ; the difference in the mechanical arrangement being simply that in the hydraulic press additional force is applied by means of levers and pumps, whereas in the pressure-engines the statical pressure of the source of supply is alone brought into operation. [HYDRAULIC PRESS; HYDRODYNAMICS.]