pposite sides of the machine. The
valve-stems extend upward through ordinary stuffing-boxes, and are
attached to the notched cross-heads by means of a threaded end which is
prevented from screwing in or out by a compression nut on the lower end
of the cross-head. Each cross-head is actuated by a pair of
reciprocating pawls, or dogs (shown more plainly in the enlarged view,
Fig. 18), one of which opens the valve and the other closes it. The
several pairs of pawls are hung on a common shaft which receives a
rocking motion from a crank driven from a worm and worm-wheel by the
turbine shaft. The cross-heads have notches milled in the side in
which the pawls engage to open or close the valve, this engagement
being determined by what are called shield-plates, A (Fig. 18), which
are controlled by the governor. These plates are set, one a little
ahead of the other, to obtain successive opening or closing of the
valves. When more steam is required the shield plate allows the proper
pawl to fall into its notch in the cross-head and lift the valve from
its seat. If less steam is wanted the shield-plate rises and allows the
lower pawl to close the valve on the down stroke.

[Illustration: FIG. 17]

[Illustration: FIG. 18]

The valves, as can easily be seen, are very simple affairs, the steam
pressure in the steam chest holding the valve either open or shut until
it is moved by the pawl on the rock-shaft. The amount of travel on the
rock-shaft is fixed by the design, but the proportionate travel above
and below the horizontal is controlled by the length of the
connecting-rods from the crank to the rock-shaft. There are besides the
mechanical valve-gear the electric and hydraulic, but these will be left
for a future article.

The Governor

The speed of the machine is controlled by the automatic opening and
closing of the admission valves under the control of a governor (Fig.
19), of the spring-weighted type attached directly to the top end of the
turbine shaft. The action of the governor depends on the balance of
force exerted by the spring, and the centrifugal effort of the
rectangular-shaped weights at the lower end; the moving weights acting
through the knife-edge suspension tend to pull down the lever against
the resistance of the heavy helical spring. The governor is provided
with an auxiliary spring on the outside of the governor dome for varying
the speed while synchronizing. The tension of the auxiliary spring is
reg