ould take in no free air and would, of course, produce
no compressed air.

Loss in _capacity_ of air compressors by clearance is in direct
proportion to the pressure.

Owing to the loss of capacity by clearance space at high pressures, it
is important that compound air cylinders should be used for furnishing
air at high pressure. With compound air cylinders the air is compressed
to alternate stages of pressure in the different cylinders, and the
clearance loss is thus reduced because of the reduced density of the air
in the clearance spaces. In ordinary practice air compressors deliver
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:

(1) Low temperature of air during compression.

(2) Increased volume of air per stroke, due to filling of clearance
spaces with water and to a cold air cylinder.

(3) Low temperature of air immediately after compression, thus
condensing moisture in the air receiver.

(4) Low temperature of cylinder and valves, thus maintaining packing,
etc.

(5) Economical results, due to compression of moist air (see table 3).

TABLE 3.--SHOWING THE RELATIVE QUANTITY OF WORK REQUIRED TO COMPRESS A
GIVEN VOLUME AND WEIGHT OF AIR, BOTH DRY AND MOIST--ALSO RELATIVE
VOLUMES WIT