Steam Steel and Electricity | Page 9

against it, so to speak, but by producing an actual
air-pressure upon it which is confined by the sides of the tube and
cannot go elsewhere. The only pressure necessary is enough to move
the pellet.
Now, if you push this little pellet one way by the air-pressure from your
mouth, and then, instead of reversing the tube in the mouth and pushing
it back again in the same way, reverse the process and suck the 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]

We are now getting the air-pressure through the pipe in both ends of the
tube alike, and do not move the pellet either way. To make it move we
must do something more, and open one end of the tube, and close that
fork of the air-pipe, and thus get all the pressure on one side of the
pellet. Remove one finger from the end of the tube, and pinch the fork
of the air-tube that is on that side. The pellet will now move toward that
end of the tube which is open. Reverse the process, and it can be
pushed back again with air-pressure to the other end, and so on
indefinitely.
Let us improve the process. We will close each end of the tube
permanently, and insert four cocks in the tube and forked pipe.
We have here two tubes inserted at each end of the large tube, and in
each of these is a cock. We have each cock connected by a rod to the
lever set on a pin in the middle of the tube. We must have these cocks
so arranged that when the lever is moved (say) to the right, A. is
opened and B. is closed, and D. is opened and C. is closed. Now if the
air-pressure is constant through the forked air-tube, and the cock E. is
open, if the top of the lever is moved to the right, the pellet will be
pushed to the left in the large tube. If the lever is moved to the left, and
the two cocks that were open are closed, and the two that were closed
are opened again, the pellet will be sent back to the other end of the
tube. This movement of the pellet in the tube will occur as often as the
lever is moved and there is any air-pressure in the forked tube. There is
a supply-cock, opened and an escape-cock closed, and an escape-cock
opened and a supply-cock closed, at each end of the tube, every time
the lever is moved.
[Illustration: Fig. 2]
We are using air instead of steam, and the movement of these four
cocks all at the same time, and the result of moving them, is precisely
that of the slide-valve of a steam-engine. The diagrams of this
slide-valve would be difficult to understand. The action of the cocks
can be more readily understood, and the result, and even much of the
action, is precisely the same.

But to make the arrangement entirely efficient we must go a little
further into the construction