line--that
is, when the piston is fully out, or fully in--the crank is said to be
at a "dead point;" for, were the crank turned to such a position, the
admission of steam would not produce motion, since the thrust or pull
would be entirely absorbed by the bearings.

[Illustration: FIG. 18.--Sectional plan of a horizontal engine.]

DOUBLE-CYLINDER ENGINES.

[Illustration: FIG. 19.]

[Illustration: FIG. 20.]

Locomotive, marine, and all other engines which must be started in any
position have at least _two_ cylinders, and as many cranks set at an
angle to one another. Fig. 19 demonstrates that when one crank, C_1,
of a double-cylinder engine is at a "dead point," the other, C_2, has
reached a position at which the piston exerts the maximum of turning
power. In Fig. 20 each crank is at 45 deg. with the horizontal, and both
pistons are able to do work. The power of one piston is constantly
increasing while that of the other is decreasing. If _single_-action
cylinders are used, at least _three_ of these are needed to produce a
perpetual turning movement, independently of a fly-wheel.

THE FUNCTION OF THE FLY-WHEEL.

A fly-wheel acts as a _reservoir of energy_, to carry the crank of a
single-cylinder engine past the "dead points." It is useful in all
reciprocating engines to produce steady running, as a heavy wheel acts
as a drag on the effects of a sudden increase or decrease of steam
pressure. In a pump, mangold-slicer, cake-crusher, or chaff-cutter, the
fly-wheel helps the operator to pass _his_ dead points--that is, those
parts of the circle described by the handle in which he can do little
work.

THE CYLINDER.

[Illustration: FIG. 21.--Diagrammatic section of a cylinder and its
slide-valve.]

The cylinders of an engine take the place of the muscular system of the
human body. In Fig. 21 we have a cylinder and its slide-valve shown in
section. First of all, look at P, the piston. Round it are white
grooves, R R, in which rings are fitted to prevent the passage of steam
past the piston. The rings are cut through at one point in their
circumference, and slightly opened, so that when in position they press
all round against the walls of the cylinder. After a little use they
"settle down to their work"--that is, wear to a true fit in the
cylinder. Each end of the cylinder is closed by a cover, one of which
has a boss cast on it, pierced by a hole for the piston rod to work
through. To prevent the escape of