THE CONQUEST OF NATURE 



practise this principle is utilized, where great pressure 

 is desired, by making a receptacle with an enormous 

 piston connecting with the force pump just described. 



An indefinite power may thus be developed, the 

 apparatus constituting virtually a gigantic lever. But 

 the principle of the equivalence of weight and distance 

 still holds, precisely as in an actual lever, and while the 

 pressure that may be exerted with slight expenditure 

 of energy is enormous, the distance through which this 

 pressure acts is correspondingly small. If, for example, 

 the piston of the force pump has an area of one square 

 inch, while the piston of the press has an area of several 

 square feet, the pressure exerted will be measured in tons, 

 but the distance through which it is exerted will be almost 

 infinitesimal. The range of utility of the hydrostatic 

 press is, therefore, limited, but within its sphere, it is 

 an incomparable transmitter of energy. 



Moreover, it is possible to reverse the action of the 

 hydraulic apparatus so as to gain motion at the expense 

 of power. A familiar type of elevator is a case in point. 

 The essential feature of the hydraulic elevator consists 

 of a ram attached to the bottom of the elevator and 

 extending down into a cylinder, slightly longer than the 

 height to which the elevator is to rise. The ram is 

 fitting into a cylinder with water-tight packing, or a 

 cut leather valve. Water under high pressure is ad- 

 mitted to the cylinder through the valve at the bottom, 

 and the pressure thus supplied pushes up the ram, 

 carrying the elevator with it, of course. Another valve 

 allows the water to escape, so that ram and elevator 

 may descend, too rapid descent being prevented by 



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