expended in compressing one cubic meter of air to 4.21 effective
atmospheres was 38,128 lb. According to Boyle and Mariotte's law it
would be 37,534 lb., the difference being 594 lb., or a loss of 1.6 per
cent. Or if compressed without abstraction of heat, the work expended
would in that case have been 48,158. The volume of air compressed per
revolution was 0.5654 cubic meter. For obtaining this measure of
compressed air, the work expended was 21,557 pounds. The work done in
the steam cylinders, from indicator diagrams, is shown to have been
25,205 pounds, the useful effect being 85½ per cent. of the power
expended. The temperature of air on entering the cylinder was 50 degrees
Fah., on leaving 62 degrees Fah., or an increase of 12 degrees Fah.
Without the water jacket and water injection for cooling the temperature
it would have been 302 degrees Fah. The water injected into the
cylinders per revolution was 0.81 gallon."

We have in the foregoing a remarkable isothermal result. The heat of
compression is so thoroughly absorbed that the thermal loss is only 1.6
per cent.; but the loss _by friction of the engine_ is 14.5 per cent.,
and the net economy of the whole system is no greater than that of the
best American dry compressor, which loses about one-half the theoretical
loss due to heat of compression, but which makes up the difference by a
low friction loss.

The wet compressor of the second class is the water piston compressor,
Fig. 18.

[Illustration: FIG. 18.--HYDRAULIC AIR COMPRESSOR.]

The illustration shows the general type of this compressor, though it
has been subject to much modification in different places. In America, a