these valves are placed in each piston of a double-acting air cylinder,
the piston being hollow and the free air being admitted through a
tail-rod pipe, letter E, Fig. 15. JJ are water jacket passages for
cooling the air during compression. Owing to the absence of inlet
valves, large water jackets are provided, not only around the cylinder
itself, but through the heads. As the heat of compression is greater
near the end of the stroke, the advantage of a cool head is manifest. H
H are the discharge valves through which the compressed air is forced.

[Illustration: FIG. 16. PISTON INLET VALVE OPERATED BY THE NATURAL LAWS
OF MOMENTUM.]

The most interesting feature of this cylinder is the piston inlet valve.
It is evident that this valve being attached to the piston needs no
springs or other connections, but is opened and closed exactly at the
right time by its natural inertia. With only about ¼ of an inch throw of
valve a large area is opened, through which the free air is drawn. The
valve is made of a single piece of composition metal and is practically
indestructible. Its construction is such that it fills the clearance
spaces to a greater extent than is usual in air compressors. A singular
feature is that indicator cards taken on these cylinders show a free air
line in some cases a little above the atmospheric line. Poppet valve
compressors almost invariably show a slight vacuum, due to several
causes, mainly the duty performed in compressing the springs of the
valves, but the vacuum is also influenced by insufficiency of valve
area, hot air cylinders, etc. This cylinder gives its full volume of
air, and apparently a little more at times, because the air is admitted
by a concentrated inlet in which free _air is always moving in one
direction_. After it has been started, the speed of the compressor is
such that the air attains a momentum due to its velocity and density;