STATIONARY ENGINES.

159

of the cylinder, the central ring being accurately bored out. Theseating consists of a bottom ring and a solid disc at the top, havinga guiding piece at the top of the disc, with a loose flange securedby bolts; the bottom ring and disc plate are connected by feathersor arms, cast all in one piece. The bottom ring and top plate arerecessed for the reception of a ring of wood or soft metal, termedthe beating surface. The amount of contraction at the top of thevalve is due to its weight, and the pressure brought to bear on itshould be slightly in excess of the total weight of the valve. Thusvery little force is lost in lifting it; and as the head of water in thedischarge pipe only acts on a small area, in comparison to thewater way through the bottom ring—and as the lift of the valve ismoderate, owing to the water being forced or drawn through twocircumferential openings—the beat onthe woodenor white metal rings, ifso fitted, is very gentleand but little feit, incomparison with thatof flap valves made ofleather, fitted with me-tallic facings to preventthe leather being forcedthrough the seatings.

For moderate lifts, how-ever, and more especiallyfor lift pumps, the flapvalve is still used.

The top and bottomvalves, or clacks, as theyare technically termed,have deep seatings ofcast iron, turned slightlyconical, fitting into cor-responding parts boredout in the pump Cast-ings, and are fixed inPosition with thin red lead and spun yarn laid into recessesturned out on the circumference of the seat; there is a singlefeather cast along with the seat, having an oblong hole in the centre.

Fig. 95.—Clack Valve with Leather Hinge.

1, Clack seat. b, Leather disc. C c, Top and bottom plates.d, Bow. e, Cross piece with cotter for securing the bars.f f, Recesses.