By far the most serious obstacle to water injection, and that which
condemns the wet compressor, is the influence of the injected water upon
the air cylinder and parts. Even when pure water is used, the cylinders
wear to such an extent as to produce leakage and to require reboring.
The limitation to the speed of a compressor is also an important
objection. The claim made by some that the injected water does not fill
the clearance spaces, but is aerated, does not hold good, except with an
inefficient injection system. The writer has increased the speed of an
air compressor (cylinders 12 in. and 12 in. by 18 in., injection air
cylinder) ten revolutions per minute by placing his fingers over the
orifice of the suction pipe of the water pump. The boiler pressure
remained the same, the cut-off was not changed and the air pressure was
uniform, hence this increase of speed arose from the fact that the water
was restricted and the clearance spaces were filled with compressed air,
which served as a cushion or spring. While the volume of compressed air
furnished by this compressor would be somewhat reduced by the
restriction of the water, yet the increase in speed which was obtained
without any increase of power fully compensated for the clearance loss.

Mr. John Darlington, of England, gives the following particulars of a
modern air compressor of European type:

"Engine, two vertical cylinders, steam jacketed, with Meyer's expansion
gear. Cylinders, 16.9 inches diameter, stroke 39.4 inches; compressor,
two cylinders, diameter of piston, 23.0 inches; stroke 39.4 inches;
revolutions per minute, 30 to 40; piston speed 39 to 52 inches per
second, capacity of cylinder per revolution, 20 cubic feet: diameter of
valves, viz., four inlet and four outlet, 5½ inches; weight of each
inlet valve, 8 lb.; outlet, 10 lb.; pressure of air, 4 to 5 atmospheres.
The diagrams taken of the engine and compressor show that the work