llustration: FIG. 23.--The eccentric and its rod.]

THE ECCENTRIC

is used to move the slide-valve to and fro over the steam ports (Fig.
23). It consists of three main parts--the _sheave_, or circular plate S,
mounted on the crank shaft; and the two _straps_ which encircle it, and
in which it revolves. To one strap is bolted the "big end" of the
eccentric rod, which engages at its other end with the valve rod. The
straps are semicircular and held together by strong bolts, B B, passing
through lugs, or thickenings at the ends of the semicircles. The sheave
has a deep groove all round the edges, in which the straps ride. The
"eccentricity" or "throw" of an eccentric is the distance between C^2,
the centre of the shaft, and C^1, the centre of the sheave. The throw
must equal half of the distance which the slide-valve has to travel over
the steam ports. A tapering steel wedge or key, K, sunk half in the
eccentric and half in a slot in the shaft, holds the eccentric steady
and prevents it slipping. Some eccentric sheaves are made in two parts,
bolted together, so that they may be removed easily without dismounting
the shaft.

The eccentric is in principle nothing more than a crank pin so
exaggerated as to be larger than the shaft of the crank. Its convenience
lies in the fact that it may be mounted at any point on a shaft, whereas
a crank can be situated at an end only, if it is not actually a V-shaped
bend in the shaft itself--in which case its position is of course
permanent.

SETTING OF THE SLIDE-VALVE AND ECCENTRIC.

The subject of valve-setting is so extensive that a full exposition
might weary the reader, even if space permitted its inclusion. But
inasmuch as the effectiveness of a reciprocating engine depends largely
on the nature and arrangement of the valves, we will glance at some of
the more elementary principles.

[Illustration: FIG. 24.]

[Illustration: FIG. 25.]

In Fig. 24 we see in section the slide-valve, the ports of the cylinder,
and part of the piston. To the right are two lines at right angles--the
thicker, C, representing the position of the crank; the thinner, E, that
of the eccentric. (The position of an eccentric is denoted
diagrammatically by a line drawn from the centre of the crank shaft
through the centre of the sheave.) The edges of the valve are in this
case only broad enough to just cover the ports--that is, they have no
_lap_. The piston is about to commence its stroke towards the