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
plunger is used instead of a piston, and as it always moves in water the
result is more satisfactory. The piston, or plunger, moves horizontally
in the lower part of a U shaped cylinder. Water at all times surrounds
the piston, and fills alternately the upper chambers. The free air is
admitted through a valve on the side of each column and is discharged
through the top. The movement of the piston causes the water to rise on
one side and fall on the other. As the water falls the space is occupied
by free air, which is compressed when the motion of the piston is
reversed, and the water column raised. The discharge valve is so
proportioned that some of the water is carried out after the air has
been discharged. Hence there are no clearance losses.

This hydraulic compressor seems to have a certain charm about it, which
has resulted in its adoption in Germany, France and Belgium, and by one
of the largest mines in the United States. Its advantages are _purely
theoretical_, and without certain adjuncts which have been in some cases
applied to it, even the _theory_ is a very bad one.

The chief claim for this water piston compressor is that its piston is
also its cooling device, and that the heat of compression is absorbed by
the water. So much confidence seems to be placed in the isothermal
features of this machine that usually no water jacket or spray pump is
applied. Mr. Darlington, who is one of the stanch defenders of this
class of compressors, has found it necessary to introduce "spray jets of
water immediately under the outlet valves," the object of which is to
absorb a larger amount of heat than would otherwise be effected by the
simple co