[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 of a steam-engine. The pellet in the tube
has no connection with the outside, and we can get nothing from it. So
we give it a stem, thus: and when we do so we change it into a piston
and its rod. Where it passes through the stopper at the end of the tube
it must pass air- (or steam-) tight. Then as we push the piston back and
forth we have a movement that we can attach to machinery at the end of
the rod, and get a result from. We also move the cocks, or valves,
automatically by the movement of the rod.

[Illustration: Fig. 3]

Turning now to Fig. 3 again let us imagine a connection made between the
rod and the end of the lever in Fig. 2. Now put on the air (or steam)
pressure, and when the piston has reached the right-hand end of the tube
it automatically, by its connections, closes B. and opens A., and opens
D. and closes C. The pellet will be pushed back in the tube and go to
the other end of it, through the pressure coming against the piston
through the part of the air tube where the cock D. is open. It reaches
the left-hand end of the tube, and we must imagine that when it gets
there it, in the same manner and by the proper connections, closes D.,