pressure steam is
required to inject a given weight of water, as compared with that used
in a single-stage injector.

[1] "The Steam-Engine," p. 3.

[2] By "weight of steam" is meant the steam produced by boiling a
certain weight of water. A pound of steam, if condensed, would form a
pound of water.

Chapter II.

THE CONVERSION OF HEAT ENERGY INTO MECHANICAL MOTION.

Reciprocating engines--Double-cylinder engines--The function of the
fly-wheel--The cylinder--The slide-valve--The eccentric--"Lap" of
the valve: expansion of steam--How the cut-off is managed--Limit of
expansive working--Compound engines--Arrangement of expansion
engines--Compound locomotives--Reversing
gears--"Linking-up"--Piston-valves--Speed governors--Marine-speed
governors--The condenser.

Having treated at some length the apparatus used for converting water
into high-pressure steam, we may pass at once to a consideration of the
mechanisms which convert the energy of steam into mechanical motion, or
_work_.

Steam-engines are of two kinds:--(1) _reciprocating_, employing
cylinders and cranks; (2) _rotary_, called turbines.

RECIPROCATING ENGINES.

[Illustration: FIG. 17.--Sketch showing parts of a horizontal
steam-engine.]

Fig. 17 is a skeleton diagram of the simplest form of reciprocating
engine. C is a _cylinder_ to which steam is admitted through the
_steam-ways_[3] W W, first on one side of the piston P, then on the
other. The pressure on the piston pushes it along the cylinder, and the
force is transmitted through the piston rod P R to the _connecting rod_
C R, which causes the _crank_ K to revolve. At the point where the two
rods meet there is a "crosshead," H, running to and fro in a guide to
prevent the piston rod being broken or bent by the oblique thrusts and
pulls which it imparts through C R to the crank K. The latter is keyed
to a _shaft_ S carrying the fly-wheel, or, in the case of a locomotive,
the driving-wheels. The crank shaft revolves in bearings. The internal
diameter of a cylinder is called its _bore_. The travel of the piston is
called its _stroke_. The distance from the centre of the shaft to the
centre of the crank pin is called the crank's _throw_, which is half of
the piston's _stroke_. An engine of this type is called double-acting,
as the piston is pushed alternately backwards and forwards by the steam.
When piston rod, connecting rod, and crank lie in a straight