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_.