The two pistons
move simultaneously, and the resistance in the air cylinder rapidly
increases as the air is compressed. To get economical results it is, of
course, necessary to cut off in the steam cylinder, so that at the end
of the stroke, when the steam pressure is low, as indicated by the
dotted line (Fig. 5), the air pressure is high, as similarly indicated.
The early direct-acting compressor used steam at full pressure
throughout the stroke. The Westinghouse pump, applied to locomotives, is
built on this principle, and those who have observed it work have
perhaps noticed that its speed of stroke is not uniform, but that it
moves rapidly at the beginning, gradually reducing its speed, and seems
to labor, until the direction of stroke is reversed. This construction
is admitted to be wasteful, but in some cases, notably that of the
Westinghouse pump, economy in steam consumption is sacrificed to
lightness and economy of space.
[Illustration: FIG. 5.]
Many efforts were made to equalize the power and resistance by
constructing the air compressor on the crank shaft principle, putting
the cranks at various angles, and by angular positions of steam and air
cylinders.
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