A supplement to Ures Dictionary of Arts, Manufactures, and Mines, : containing a clear exposition of their principles and practice. . in the chain from a comparatively short stroke of thepiston. This construction, which characterizes nearly all the varieties of the hoistingand hauling machines to which I have applied hydraulic pressure, is exhibited fig. 329,which represents one of these presses with sheaves attached, to multiply the motion four-fold. In cases where the resistance to be overcome varies very considerably, I generallyemploy three such cylinders, with rams or pistons acting cithe
A supplement to Ures Dictionary of Arts, Manufactures, and Mines, : containing a clear exposition of their principles and practice. . in the chain from a comparatively short stroke of thepiston. This construction, which characterizes nearly all the varieties of the hoistingand hauling machines to which I have applied hydraulic pressure, is exhibited fig. 329,which represents one of these presses with sheaves attached, to multiply the motion four-fold. In cases where the resistance to be overcome varies very considerably, I generallyemploy three such cylinders, with rams or pistons acting cither separately or conjointlyupon the same set of multiplying sheaves, according to the amount of power required. In hydraulic cranes the power is applied, not only for Hfting the load, but also forswinging the jib, which latter object is effected by means of a rack or chain operatingon the base of the movable part of the crane, and connected either with a cyhnder and 602 HYDRAULIC CPwANES. piston having alternate motion, like that of a steam-engine, or with two presses appliedto produce the same effect by alternate action. 329. The absence of any sensible elasticity in water renders the motions resulting fromits pressure capable of the most perfect control, by means of the valves which regulatethe inlet and outlet passages of the machines; but this very property, which gives somuch certainty of action, tends to cause shocks and strains to the machinery, by resist-ing the momentum acquired by the moving parts. Take, for example, the case of a hy-draulic crane, swinging round with a load suspended on the jib, the motion being pro-duced by the water entering on one side of a piston and escaping from the other. Undersuch circumstances, if the water-passages be suddenly closed by the regulating valve, itis obvious that the piston, impelled forward by the momentum of the loaded jib, but metby an unyielding body of water deprived of outlet, would be brought to rest so abruptlyas to caus
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