e beam is usually loaded, so as to cause it to
preponderate when the engine is at rest; and the whole effort of the steam
is employed in overcoming this preponderance until a stroke is performed,
when, the steam being shut off, the heavy end of the beam again falls and
the operation is repeated.

95. _Q._--in the double-acting engine the piston is pushed by the steam
both ways, whereas in the single-acting engine it is only pushed one way?

_A._--The structure and action of a double-acting land engine of the kind
introduced by Mr. Watt, will be understood by a reference to the annexed
figure (fig. 20), where an engine of this kind is shown in section. A is
the cylinder in which a movable piston, T, is forced alternately up and
down by the alternate admission, to each side, of the steam from the
boiler. The piston, by means of a rod called the piston rod, gives motion
to the beam V W, which by means of a heavy bar, P, called the connecting
rod, moves the crank, Q, and with it the fly wheel, X, from which the
machinery to be driven derives its motion.

96. _Q._--Where does the steam enter from the boiler?

[Illustration: Fig. 20.]

_A._--At the steam pipe, B. The throttle valve in that pipe is an
elliptical plate of metal swivelling on a spindle passing through its edge
from side to side, and by turning which more or less the opening through
the pipe will be more or less closed. The extent to which this valve is
opened or closed is determined by the governor, D, the balls of which, as
they collapse or expand, move up or down a collar on the governor spindle,
which motion is communicated to the throttle valve by suitable rods and
bell-cranks. The governor, it will be seen, consists substantially of two
heavy balls attached to arms fixed upon an upright shaft, which is kept in
revolution by means of a cord driven by a pulley on the fly wheel shaft.
The velocity with which the balls of the governor revolve being
proportional to that of the fly wheel, it will follow, that if by reason of
too rapid a supply of steam, an undue speed be given to the fly wheel, and
therefore to the balls, a divergence of the balls will take place to an
extent corresponding to the excess of velocity, and this movement being
communicated to the throttle valve it will be partly closed (see fig. 1),
the supply of steam to the engine will be diminished, and the velocity of
its motion will be reduced. If, on the other hand, the motion of the engine