air out from behind it, it comes back by the pressure of the outside
atmosphere. This was the way the first steam engines worked. Their only
purpose was to get the piston lifted, and air-pressure did all the
actual work.

If you turn the tube, and put an air-pressure first at one end and then
at the other, and pay no attention to vacuum or atmospheric pressure,
you will have the principle of the later modern, almost universal,
high-pressure, double-acting steam-engine.

But now you must imagine that the tube is fixed immovably, and that the
air-pressure is constant in a pipe leading to the tube, and yet must be
admitted first to one end of the tube and then to the other alternately,
in order to push the pellet back and forth in it. It seems simple.
Perhaps the young reader can find a way to do it, but it required about
a hundred years for ingenious men to find out how to do precisely the
same thing automatically. It involves the steam-chest and the
slide-valve, and all other kinds of steam valves that have been
invented, including the Corliss cut-off, and all others that are akin to
it in object and action.

But now imagine the tube closed at each end to begin with, and the
little moving pellet, or plunger, on the inside. To get the air into
both ends of the tube alternately, and to use its pressure on each side
of the pellet, we will suppose that the air-pipe is forked, and that one
end of each fork is inserted into the side of the tube near the end,
like the figure below, and imagine also that you have put a finger over
each end of the tube.

[Illustration: Fig. 1]