The power and resistance
being equalized through opposite cylinders, large fly wheels are not
necessary. Strange to say, the American practice seems to be to attach
enormous fly wheels to duplex air compressors. It is difficult to
justify this apparently useless expense in view of the facts shown in
Fig. 7. A fly wheel does not furnish power, nor 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.
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