op
of it.

"That is the cylinder," said Forester. "It is the _heart_ of the steam
engine, as I may say--the seat of its power. All the other machinery is
only to aid the cylinder, and to convey the power to the point where it
is wanted to do the work. Thus, the place where the steam exerts its
power, and on which the whole movement of the machinery depends, is the
cylinder."

Marco observed that a long iron rod, large and solid, and very bright,
kept ascending and descending through the top of the cylinder, as if
pushed up and drawn down again by some force within. Forester told him
that that was the piston rod.

"The piston rod," said Forester, "is fastened, at its lower end, to the
_piston_, which is a flat plate of iron, made to fit the inside of the
cylinder exactly."

"First," said Forester, "the steam comes in below the piston, and drives
it up; and then it is stopped from coming in below, and is forced in
above, and so drives it down."

"And how does the other steam get out?" asked Marco.

"There are two ways of getting rid of the steam that is below the piston
when the piston is coming down," said Forester. "One way is, to open a
passage to let it out _into the air_. On this plan, when the piston has
been driven up, the steam is cut off from coming in below the piston,
and is admitted above. At the same instant, the passage is opened to let
the lower steam out. Of course, the steam that comes in _above_, drives
the piston down, and forces the steam that is _below_, out into the air.
They generally have a pipe to convey it away, and as the piston goes up
and down, the steam comes out in puffs, as you saw it in that mill."

"Yes," said Marco, "I understand that; and now what is the other way?"

"That kind of engine is called a _high pressure_ engine," said Forester,
"because the piston, in coming down, has to drive out the steam from
below, against the pressure of the atmosphere; for the atmosphere above
passes into the pipe, and resists the movement of the steam in coming
out. It requires a greater force of steam to work the piston on this
plan than it does upon the other."

"What is the plan of the other?" asked Marco.

"On the other plan," said Forester, "the steam under the piston is
condensed, that is, turned suddenly into water; and this leaves a
vacancy or void below the piston, so that the piston can be forced down
much more easily than if it had to drive the steam out before it,
against the p