[Illustration:  Fig. 16.]

[Illustration:  Fig. 17.]

[Illustration:  Fig. 18.]

Fig. 19 is a longitudinal section of a common wood-burning locomotive.

[Illustration:  Fig. 19.]

THE ENGINE.

93. Q.—­The steam passes from the boiler through, the steam pipe into the cylinder of the engine?

A.—­And presses up and down the piston alternately, being admitted alternately above and below the piston by suitable valves provided for that purpose.

94. Q.—­This reciprocating motion is all that is required in a pumping 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 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