A. As the piston in the low pressure cylinder "D" moves up (see Fig.
19), a partial vacuum is formed below it, and air flowing through the
strainer passes downward through the air passage, then past the lower
receiving valve "W" into the lower end of the cylinder, filling it with
air at about atmospheric pressure. In the meantime the air that is being
compressed above the piston holds the receiving valve "U" to its seat,
and lifts the upper intermediate discharge valve "K" from its seat,
allowing the air to pass from the low to the high pressure cylinder "F".
The high pressure piston now moving up causes a partial vacuum to be
formed below it, and air from the atmosphere flows past the lower
receiving valve "N", filling this end of the cylinder with air at about
atmospheric pressure. The air above the piston being compressed, holds
the upper intermediate valve "K" and receiving valve "J" to their seats
and lifts the upper final discharge valve "M", allowing the air to pass
to the main reservoir. The action is the same on the down stroke, only
air is compressed in the opposite end of the cylinders and the opposite
air valves are used.