The action of the common pump is as follows: When the handle is
raised, the piston-rod descends, and brings the piston-valve--called
the sucker, or bucket--to another valve, which is fixed, and opens
inward towards the piston. When the handle is drawn down, the piston
is raised, and, as it is air-tight, a vacuum is produced between the
two valves; the air in the barrel of the pump, betwixt the lower valve
and the water, then forces open the lower valve, and rushes through to
fill up this vacuum; and the air in the pump being less dense than the
external atmosphere, the water is forced a short way up the barrel.
When the piston again descends to the lower valve, the air between
them is again forced out by forcing open the upper valve; and when the
piston is raised, a vacuum is again produced, and the air below the
lower valve rushes up, and the water in consequence is again raised a
little further. This operation continues until the water rises above
the lower valve; at every stroke afterwards, the water passes through
the valve of the descending piston, and is raised by it, on its ascent,
until it issues out of the spout.