bjection. 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, 51/2 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
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 851/2 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