STATIONARY ENGINES.

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is controlled by means of a slide valve worked by a handle situatedalongside the handle G, so that while the water is admitted to onecylinder the other is open to the exhaust. The travel of the ramsis limited by means of a tappet rod connected with the handle ofthe slide valve, whereby the crane is prevented from being turnedround too far. Small hydraulic rotary engines have been intro-duced for working cranes, and in many cases they can be easilyattached to existing hand cranes.

The absence of any sensible elasticity in water renders the motionsresulting from its pressure capable of the most perfect control bymeans of the valves which regulate the inlet and outlet passages;but this property, which gives so much certainty of action, tends tocause shocks and strains to the machinery by suddenly resistingthe momentum acquired by the moving parts. Take, for example,the case of a hydraulic crane swinging round with a load suspendedfrom the jib: the motion being produced by the water enteringinto one cylinder and escaping from the other, it is obvious that ifthe water passages be suddenly closed, the ram, impelled forwardby the momentum of the loaded jib, but met by an unyielding bodyof water deprived of outlet, would be brought to rest so abruptlyas to cause in all probability some damage to the machine. Soalso, in lowering a heavy weight, if the escape passages were toosuddenly closed, a similar risk of injury would arise from the suddenstoppage of the weight. But these liabilities to injury are effectuallyremoved, in the case of a single-acting cylinder, by fitting a reliefvalve in connection with the water passages, consisting of a smallclack valve N opening upwards against the effective pressure, so asto permit the pent-up water in the cylinder to be forced back intothe pressure pipe, whenever it becomes subject to a compressiveforce exceeding the pressure given by the accumulator; and inthe case of a double-acting cylinder fitted with a piston and slidevalve, or where two single-acting cylinders with rams workingalternately are controlled by a slide valve—as in the instance of thecylinders N and O —for turning the crane, relief valves are fitted inconnection with the slide valve. These consist of four small leatherflap valves (Fig. 207), with metal pieces at the top and bottom. Thepassages PP communicate with the pressure pipe J, and the pas-sages E E with the exhaust K. When the slide valve is moved inthe direction of the arrow the pressure is first cut off from theport R by the top of the valve, the port S being still open to the