the air at less than 100 pounds pressure, so that with a properly
designed air cylinder the clearance space is so small that the capacity
of the compressor is not materially affected.

Two systems are in use by which the heat of compression is absorbed, and
the difference between one and the other is so distinct that air
compressors are usually divided into two classes (1) wet compressors,
(2) dry compressors.

A _wet_ compressor is that which introduces water directly into the air
cylinder during compression.

A _dry_ compressor is that which introduces no water into the air during
compression.

_Wet_ compressors may be subdivided into two classes.

(1) Those which inject water in the form of a spray into the cylinder
during compression.

(2) Those which use a water piston for forcing the air into confinement.

The injection of water into the cylinder is usually known as the
Colladon idea. Compressors built on this system have shown the highest
isothermal results, that is, by means of a finely divided spray of cold
water the heat of compression has been absorbed to a point where the
compressed air has been discharged at a temperature nearly equal to that
at which it was admitted to the cylinder. The advantages of water
injection during compression are as follows: