3i8

MODERN STEAM PRACTICE.

water ” as the vertical wheels are. There are several forms of suchwheels, depending upon the direction in which the water is allowedto impinge upon the vanes or blades. These vanes are curved, andit is of importance that the water should be directed upon them insuch a manner as to cause as little shock as possible; the propellingaction being due to the pressure and reaction on the vane due to thegliding of the water along its surface. Curved vanes are found inthis manner to be more efficient than flat surfaces, their surfacesbeing in the direction of the resultant of the lines of motion of thejet and vane. Turbines are now largely used on natural falls,notably so in America and in France and Switzerland .

HYDRAULIC CRANE.

Of the various applications of water pressure, the most com-mon is that of a hydraulic press with a set of sheaves used in theinverted order of blocks and pulleys, with the object of obtainingan extended motion in the chain from a comparatively short strokeof piston. The general arrangement of the machinery for workingsuch a crane may be described as follows:—The pressure cylinder A(Fig. 203) is fixed horizontally below the surface of the ground in acha.mber at the foot of the crane, and is fitted with the ram B, carry-ing the pulleys C at its outer extremity. The lifting chain is madefast at one end to the cylinder A, and passes alternately round themovable pulleys C and the pulleys D at the inner end of the cylinder;and is then led round the guide pulley E up to the crane post F,and along the'jib to the load. The motion of the lifting chain is con-trolled by means of the handle G, acting upon the inlet and outletvalves, which are kept closed by the weights H and I; by openingthe inlet valve H (Fig. 204) the water is let into the cylinder A fromthe pressure pipe J, and acting on the plunger raises the load; byopening the outlet valve I the water escapes from the cylinder intothe exhaust pipe K, allowing the load to descend. The travel ofthe ram B in the outward stroke is prevented from exceeding theproper limit by the pulley block C coming in contact with a stopconnected with the handle G, which closes the inlet valve H, andprevents the load from being lifted too high. The return stroke ofthe ram is efifected by the load suspended from the chain; and inthe absence of any load, a small supplementary ram L is employedto force the main ram B back, the slack chain being made to runout by the permanent weight M.