REGULATION OF STEAM.

IOI

or backward eccentric rod in a line with the valve-spindle, we havethe power of moving the crank IN or OUT, as while one eccentricis in gear the other is simply doing nothing; they have joinedhands, and are ready at a moment’s notice for either going forwardsor backwards, or by lifting the rods, and placing the link at mid-way between the centre of the eccentric rods, on a line with thevalve-spindle; thus no motion is imparted to the valve, or but atrifle, and in this position the steam is shut off from the cylinder.In fact, the motion of the one eccentric is identically opposed tothat of the other, and they can only move in contrary directionsto open the ports as required; and being linked together, it is im-possible that the one can do the duty of the other, or that bothcombined can ever fail in making the main crank of the enginetravel in the direction required. When the link is down, the topeccentric may be named the driver (as in the locomotive) for theforward motion; but when the link is raised, the bottom eccentricrod becomes the driver, while the top rod simply moves to and froalong with the link which oscillates on the pin and block on thecentre line of the valve-rod, reciprocating motion being imparted tothe valve by the forward or backward eccentric, as the case may be.The radius of the link is found by placing the valve and adjunctsat half stroke, and the distance from the centre of the pin, for takingthe valve-block, to the centre of the crank-shaft is the radius of thelink, all the other dimensions or lengths being calculated accordingly;the point of Suspension of the link is generally on this arc, describedby the radius of the link, placed midway between the eccentric-rodends, the distance between the eccentric-rod ends being usuallythree times the throw of the eccentric. When the link is drawnhalf up, the lifting arm being level, the suspending link should benearly vertical, so as to equalize the motion; there are variousmethods of holding the link in position, which will be treated indetail further on.

The slide-valves for compound engines are so arranged, that twovalves, fitted to one casing, is sufficient, one at the top and anotherat the bottom of the high-pressure cylinder, the port at the top andbottom admits the steam into the high-pressure cylinder, the nextport is in communication with the passages for the low-pressurecylinder, while the third, or middle ports, are in communicationwith the condenser; thus there are three ports at the top, and thesame number for the bottom, of the high-pressure cylinder.