322 MODERN STEAM PRACTICE.
exhaust K; at the same instant the flap valve T opens upwardsand allows a small quantity of water to passfrom the exhaust K into the port R to followup the ram until brought to rest. Whenthe slide valve arrives at the central positionas shown, the port S is closed to the exhaust,and the pressure in it being increased bythe further motion of the ram before it iscompletely stopped, the second flap v israised, and a small quantity of water forcedback into the passage P communicating withthe pressure pipe J. When the slide valveis moved in the opposite direction, the twoF i s \ 2 7 ; 7 , Sect l 0n T 0 / ® llde , Va ' ve & remaining relief valves are brought into ac-
.Relief Valves for Hydraulic Crane. 0 0
e e, Exhaust passages. j, Pressure tion in the same manner - By these meanspipe. K , Exhaust, p p, Pressure a li r isk of concussion is avoided, and per-
passages. R and s, Ports. ...
fect control over the machine is combinedwith great softness of action.
DOCK GATES.
The method generally adopted for opening and closing dockgates by means of hydraulic pressure consists in applying to eachgate a pair of cylinders with rams and multiplying sheaves, similarto those used for the hoisting apparatus in hydraulic cranes. Oneof these cylinders opens the gate and the other closes it; and thewhole of the machinery is placed in chambers beneath the ground.The water is admitted from the pressure pipe J to the cylinder A(Fig. 208) through the inlet valve H by means of the handle G; thesame motion of the handle also opens the oütlet valve of the othercylinder B. The opposite motion of the handle G opens the outletvalve I, allowing the water to escape from the cylinder A into theexhaust pipe K, and at the same time admits the pressure to thecylinder B. A stop M connected with the handle G prevents theram from travelling too far in the out stroke, by closing the inletvalve; and the return stroke of the ram is effected by meansof the weight L. This arrangement has been applied to severalof the London and Liverpool Docks, as well as to some othersthroughout the country.
In Fig. 209 we give an engraving of the general plan of a