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engines, and known as the equilibrium valve. This valve is represented in
fig. 34. It consists substantially of an annulus or bulging cylinder of
brass, with a steam-tight face both at its upper and lower edges, at which
points it fits accurately upon a stationary seat. This annulus may be
raised or lowered without being resisted by the pressure of the steam, and
in rotative engines it is usually worked by a cam on the shaft. The
expansion cam is put on the shaft in two pieces, which are fastened to each
other by means of four bolts passing through lugs, and is fixed to the
shaft by keys. A roller at one end of a bell-crank lever, which is
connected with the expansion valve, presses against the cam, so that the
motion of the lever will work the valve. The roller is kept against the cam
by a weight on a lever attached to the same shaft, but a spring is
necessary for high speeds. If the cam were concentric with the shaft, the
lever which presses upon it would remain stationary, and also the expansion
valve; but by the projection of the cam, the end of the lever receives a
reciprocating motion, which is communicated to the valve.

199. _Q._--The cam then works the valve?

_A._--Yes. The position of the projection of the cam determines the point
in relation to the stroke at which the valve is opened, and its
circumferential length determines the length of the time during which the
valve continues open. The time at which the valve should begin to open is
the same under all circumstances, but the duration of its opening varies
with the amount of expansion desired. In order to obtain this variable
extent of expansion, there are several projections made upon the cam, each
of which gives a different degree, or _grade_ as it is usually called, of
expansion. These grades all begin at the same point on the cam, but are of
different lengths, so that they begin to move the lever at the same time,
but differ in the time of returning it to its original position.

200. _Q._--How is the degree of expansion changed?

_A._--The change of expansion is effected by moving the roller on to the
desired grade; which is done by slipping the lever carrying the roller
endways on the shaft or pin sustaining it.

201. _Q._--Are such cams applicable in all cases?

_A._--In engines moving at a high rate of speed the roller will be thrown
back from the cam by its momentum, unless it be kept against it by means of
springs. In some cases I have emp