does it add to the
economy of an engine except in so far as it enables it to cut off early
in the stroke, and to equalize the power and resistance. In other words,
a fly wheel is not a _source_ of power, and in many cases it is only a
means by which we accomplish rotative speed. It takes power to move
matter, and assuming that other conditions are equal, every engine that
carries a fly wheel that is larger than is necessary consumes a certain
number of foot pounds in turning so much metal around through space.
Were it possible to cut off at the same point and rotate as positively
without a fly wheel, it would be done away with entirely. Some straight
line air compressors are so constructed that the momentum of the piston
and other moving parts is nearly sufficient to equalize the strains
without a fly wheel; but the fly wheel is there because it insures a
definite length of stroke, and because it enables us to operate
eccentrics and to regulate the speed of the engine uniformly.

Objections to the duplex construction are: The strains are indirect,
angular and intermittent. It is necessary therefore to largely increase
the strength of parts; to add a crank shaft of large diameter with
enormous bearings, and to build expensive and very secure foundations.
Should the foundations settle at any point, excessive strains will be
brought upon the bearings, resulting in friction and liability to
breakage. A steam engine meets with a resistance on its crank shaft that
is uniform throughout the stroke; while an air compressor is subject to
a heavy maximum strain at the end of the stroke, hence the importance of
direct straight line connection between power and resistance.

[Illustration: FIG. 8.]

The friction loss on a duplex compressor seldom gets lower than 15 per
cent., while straight line compressors show as low a loss as 5 per cent.
Fig. 8 illustrates the Rand Duplex Air Compressor, a machine largely
used in America, especially in the Lake Superior iron mines. Fig. 9
illustrates a Duplex Compound Condensing Corliss Air Compressor built by
the Ingersoll-Sergeant Drill Company. This is a compressor made of the
best type of Corliss engine, with air cylinders connected to the tail
rods of the steam cylinders. One of these machines, of about 400 horse
power capacity, is now at work furnishing compressed air power for the
Brightwood Street Railway in Washington, D.C. Fig. 10 illustrates the
Norwalk direct-acting straight line a