102

MODERN STEAM PRACTICE.

The cylinders are arranged side by side on the centre line of the

engine; the steam,

after doing duty on the top of the small orhigh-pressure piston, expands to the bottom ofthe low-pressure piston, and then passes intothe condenser; steam is also admitted to theunder side of the small piston, from there to thetop of the large piston, and then into the con-denser; two long ports or passages are cast inthe low-pressure cylinder, and there is a beitcast around the high-pressure cylinder, fittedwith a pipe leading to the condenser. Such anarrangement, designed by the author,'was fittedto a pair of engines for driving the machineryat the Royal Gun Factory, Woolwich. Thevalve gear was simply a crank-pin fitted to acast-iron disc-plate; on the pin was secured athree-cornered cam, described from the centreof the disc-plate, one cross shaft driven bybevel wheels and shaft off the crankshaft, suited for both engines. Thisdass of engine requires no cover onthe valves, consequently the lengthfrom the centre of the disc-plate tothe pin or centre of the cam is ex-actly the width of the exhaust-portinto the low-pressure cylinder.This cam-motion opens the portsquickly, while the valve hangs, as itwere, at the top and bottom of thestroke. With coupled engines the cams are of course set at rightangles to each other, and as the weight of the valves is considerable,they are balanced with a weight, having a lever and links connectedto the valve-spindle. There was no hand-gear, as factory-enginesrarely require to be moved by hand, more especially when set atright angles to each other.

The slide-valve for the locomotive-engine differs very little fromland-engines; but certainly the locomotive type has arrived at ahigher state of perfection than what is usually seen in engines forordinary work. The various schemes for working the slide-valveof the locomotive,—and many arrangements have been tried,—have

Fig. 51.—Valves for High and Low PressureCombined Engines.

a b, Throw of the cam. 1h, High-pressure ports.

, Passage to condenser.l, Low-pressure ports.