that admits steam to the engine cylinder; and
the other, D, being a cam to cut off the steam supply at the required
point in the engine stroke. The positions of these cams with relation to
the position of the crank-pin need not be commented upon here, more than
to remark that obviously the cam C must operate to open the steam inlet
valve in advance of cam D, which operates to close it and cause the
steam to act expansively in the cylinder, and that the angle of the
throw line of the cut-off valve D to the other cam or to the crank-pin
varies according as it is required to cut off the steam either earlier
or later in the stroke.

[Illustration: Fig. 267.]

The cam yoke is composed of two halves, Y and Y', bolted together by
bolts B, which have a collar at one end and two nuts at the other end,
the inner nuts N N enabling the letting together of the two halves
of the yoke to take up the wear. It is obvious that as the shaft
revolves and carries the cam with it, it will, by reason of its shape,
move the yoke back and forth; thus, in the position of the parts shown
in Figure 267, the direction of rotation being denoted by the arrow, cam
C will, as it rotates, move the yoke to the left, and this motion will
occur from the time corner _a_ of the cam meets the face of Y' until
corner _b_ has passed the centre line _d_. Now since that part of the
circumference lying between points _a_ and _b_ of the cam is an arc of a
circle, of which the axis of the shaft is the centre, the yoke will
remain at rest until such time as _b_ has passed line _d_ and corner _a_
meets the jaw Y of the yoke; hence the period of rest is determined by
the amount of circumference that is made concentric to the shaft; or, in
other words, is determined by the distance between _a_ and _b_.

The object of using a cam instead of an eccentric is to enable the
opening of the valves abruptly at the beginning of the piston stroke,
maintaining a uniform steam-port opening during nearly the entire length
of stroke, and as abruptly closing the valves at the termination of the
stroke.

Figure 268 is a top view of the mechanism in Figure 267; and Figure 269
shows an end view of the yoke. At B, in Figure 268, is shown a guide
through which the yoke-stem passes so as to be guided to move in a
straight line, there being a guide of this kind on each side of the
yoke.

[Illustration: Fig. 268.]

The two cams are bolted to a collar that is secured to the crank-shaft,