Chap. XXI. STEAM CONSIDERED AS A MOVING POWER.

1259

Fig. 1998.

Fig. 1997.

directly on the segments of the metallic rings, as in tig. 1998.

Wolf's Piston is so contrived as to enable the engine-man to tighten it without taking thecover off the cylinder ; the piston-rod forms part of a screw to which a toothed-wheel isattached; this is turned by means of a pinion provided with a square head rising in a recessin the cylinder surmounted by a cap.

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Fig. 2001. jessop’s PISTON.

Fig. 1999. wolf’s piston.

Fig. 2000. Cartwright’s piston.

Cartwright's Piston is very similar to the wedge metallic piston, but has not been foundquite successful in practice, when the cylinders are not truly bored; the pieces formingthe piston having a determined curvature, and being too strong to be sensibly flexible,cannot accommodate themselves to any irregularity, as is done bv the elastic stuffing of hemp.

Jessop's Piston consists of an expanding coil of metal which binds round the piston bodyin a spiral form, fig. 2001. : a bed of hemp packing is first prepared, which answers the doublepurpose of preventing steam passing at the joints, and of supplying a means of pressing thesprings against the surface of the cylinder; their pressure and wear is more equable thanin other metallic pistons, and they have been successful in practice.

V hen a piston-rod is drawn as well as pushed, the proportion of its diameter to that ofthe piston should be attended to, as the slightest inequality in the centering or in the fric-tion will render it liable to stick and not move properly : in high-pressure engines the frictionand loss are estimated at two-tenths of the power. The piston-rod, collar, or stuffing-box, is that which admits the smooth rod or plunger to pass into the cylinder or air-tight vessel; the stuffing-box with the hemp packing surrounds the piston-rod; round itis a collar, with a hole sufficient to allow the rod to pass, which is stuffed and screwed downwith similar stuffing to that already described.

The common Clack-valve is a piece of leather rather larger than the opening it is to close,and is attached by a joint or hinge; it is strengthened by a metal plate on each side,the lower one less and the upper larger than the aperture: an angle of 30° for it to openallows a free passage.

A double Clack-valve is used for pump-buckets; they should also rise to an angle of30°, and are decidedly preferable for large pistons.

Conical Steam-Valve is a metallic plate with bevelled edges, made exactly to fit intoa conical box, and is sometimes called a T valve; the valves of Watt’s engine wereof this kind. The diameter of the box should be to that of the greater diameter of thevalve as 3 to 2, and should rise at least one-fourth of its greatest diameter when open ;they are usually of brass, and are turned so as to exactly fit, after which they arefurther ground into each other with emery powder ; the angle is 45°. When this valveis to be self-acting, its weight must be equal to the square of the diameter multiplied