The piston being raised by lowering the handle of the pump, a partial
vacuum is formed below the upper valve, which shuts down directly
the piston is raised by the pressure of the air; this vacuum causes
the external air to force the water some way up the tube _g_. On
the piston descending, the lower valve is forced down and the upper
one opened, this keeps the water where it is and allows the piston
to descend without forcing the water down again, and on its being
raised a second time the upper valve shuts and the lower one opens,
the water being drawn up still higher, and this takes place till the
box at the top is full to the spout, when it runs out. The air-pump
is on the same principle, and is generally made with two cylinders
worked by means of a “rack” and wheel (fig. 2); this is only to
save time, instead of pumping water it pumps out air from any vessel
called a “receiver,” because it receives any object to be placed in
a “vacuum,” that is to say a partial vacuum, for the air-pump cannot
produce a vacuum, as the air is only partly removed by each stroke
of the piston, leaving the air more rarefied inside; and although
each stroke of the piston increases the rarefaction, yet it cannot
get all, as it merely takes part, and always leaves part.