engine?

_A._--The prevailing form of the pumping engine consists of a great beam
vibrating on a centre like the beam of a pair of scales, and the cylinder
is in connection with one end of the beam and the pump stands at the other
end. The pump end of the 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