ve, or
circular valve, working up and down in a tubular seating. It may best be
described as a rod carrying two pistons which correspond to the faces of
a D-valve. Instead of rectangular ports there are openings in the tube
in which the piston valve moves, communicating with the steam-ways into
the cylinder and with the exhaust pipe. In the case of the D-valve the
pressure above it is much greater than that below, and considerable
friction arises if the rubbing faces are not kept well lubricated. The
piston valve gets over this difficulty, since such steam as may leak
past it presses on its circumference at all points equally.

SPEED GOVERNORS.

[Illustration: FIG. 33.--A speed governor.]

Practically all engines except locomotives and those known as
"donkey-engines"--used on cranes--are fitted with some device for
keeping the rotatory speed of the crank constant within very narrow
limits. Perhaps you have seen a pair of balls moving round on a seating
over the boiler of a threshing-engine. They form part of the "governor,"
or speed-controller, shown in principle in Fig. 33. A belt driven by a
pulley on the crank shaft turns a small pulley, P, at the foot of the
governor. This transmits motion through two bevel-wheels, G, to a
vertical shaft, from the top of which hang two heavy balls on links, K
K. Two more links, L L, connect the balls with a weight, W, which has a
deep groove cut round it at the bottom. When the shaft revolves, the
balls fly outwards by centrifugal force, and as their velocity increases
the quadrilateral figure contained by the four links expands laterally
and shortens vertically. The angles between K K and L L become less and
less obtuse, and the weight W is drawn upwards, bringing with it the
fork C of the rod A, which has ends engaging with the groove. As C
rises, the other end of the rod is depressed, and the rod B depresses
rod O, which is attached to the spindle operating a sort of shutter in
the steam-pipe. Consequently the supply of steam is throttled more and
more as the speed increases, until it has been so reduced that the
engine slows, and the balls fall, opening the valve again. Fig. 34 shows
the valve fully closed. This form of governor was invented by James
Watt. A spring is often used instead of a weight, and the governor is
arranged horizontally so that it may be driven direct from the crank
shaft without the intervention of bevel gearing.

[Illustration: FIG. 34.]

The Hartw