THE WORK OF AIR AND WATER 



the partial balancing of ram and elevator with weights 

 acting over pulleys. The ram, to the end of which 

 pressure is thus applied, need be but a few inches in 

 diameter. Water pressure is secured by bringing water 

 from an elevation. Such an elevator acts slowly, but 

 is a very safe and in many ways satisfactory mechanism. 

 Such elevators are still used extensively in Europe, 

 but have been almost altogether displaced in America 

 by the electric elevator. 



The hydraulic elevator just described is virtually a 

 water engine, the ram acting as piston. A veritable en- 

 gine, of small size, to perform any species of mechanical 

 work, may be constructed on precisely the same prin- 

 ciple, the piston in this case acting in a cylinder similar 

 to that of the ordinary steam engine. Such an engine 

 operates slowly but with great power. It has special 

 utility where it is desirable to apply power intermit- 

 tently, as in various parts of a dockyard, or in handling 

 guns and ammunition on shipboard. In the former 

 case in particular, it is often inconvenient to use steam 

 power, as steam sent from a central boiler condenses 

 in a way to interfere with its operation. In such a case 

 any number of small water-pressure engines may be 

 operated from a single tank where water is at a high 

 elevation, or where the requisite pressure is secured 

 artificially. In the latter case, the water is kept under 

 pressure by a large piston or ram heavily weighted, 

 the entire receptacle being, of course, of water-tight 

 construction and adapted to withstand pressure. The 

 pump that supplies the tank is ordinarily made to work 

 automatically, ceasing operation as soon as the ram 



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