gh. The manner in
which the boilers are made will be presently explained.

The steam generated in the boilers is conveyed to the engine, where it
is to do its work, by what is called the steam pipe. The steam pipe of
the larboard engine, that is, of the one nearest the spectator, is not
represented in the engraving, as it would have intercepted too much the
view of the other parts. That belonging to the starboard engine,
however, may be seen passing across from the boiler to the engine, on
the back side of the room. The destination of the steam is the
_cylinder_.

[Illustration: GENERAL VIEW OF A MARINE ENGINE.]

The cylinder, marked C, is seen on the extreme right, in the view. It
may be known, too, by its form, which corresponds with its name. The
cylinder is the heart and soul of the engine, being the seat and centre
of its power. The steam is generated in the boilers, but while it
remains there it remains quiescent and inert. The action in which its
mighty power is expended, and by means of which all subsequent effects
are produced, is the lifting and bringing down of the enormous piston
which plays within the cylinder. This piston is a massive metallic disc
or plate, fitting the interior of the cylinder by its edges, and rising
or falling by the expansive force of the steam, as it is admitted
alternatively above and below it.

The round beam which is seen issuing from the centre of the head of the
cylinder is called the piston rod. The piston itself is firmly secured
to the lower end of this rod within the cylinder. Of course, when the
piston is forced upward by the pressure of the steam admitted beneath
it, the piston rod rises, too, with all the force of the expansion. This
is, in the case of the largest marine engines, a force of about a
hundred tons. That is to say, if in the place of the cross head--the
beam marked H in the engraving which surmounts the piston--there were a
mass of rock weighing a hundred tons, which would be, in the case of
granite, a block four feet square and eighty feet high, the force of the
steam beneath the piston in the cylinder would be competent to lift it.

The piston rod, rising with this immense force carries up the cross
head, and with the cross head the two _side rods_, one of which is seen
in full, in the engraving, and is marked S. There is a side rod on each
side of the cylinders. The lower ends of these rods are firmly connected
with the back ends of what are called the