s 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. Several types are shown in Fig. 6.

[Illustration: FIG. 6.]

Angular positions of the cylinder involve expensive construction and
unsteadiness. Experience has conclusively proved that it does not pay to
build air compressors with vertical cylinders, and moreover we have
found out that there is nothing in the apparent difficulty in equalizing
the strains in a direct-acting engine. It is simply necessary to add
enough weight to the moving parts, that is, to the piston, piston rod,
fly wheel, etc., to cut off early in the stroke and secure rotative
speed with the most economical results and with the cheapest
construction. It is obvious that the theoretically perfect air
compressor is a direct-acting one with a conical air cylinder, the base
of the cone being nearest the steam cylinder. This, from a practical
point of view, is impossible. Mr. Hill, in referring to the fallacious
tendencies of pneumatic engineers to equalize power and resistance in
air compressors, says: "The ingenuity of mechanics has been taxed and a
great variety of devices have been employed. It is usual to build on the
pattern of presses which do their work in a few inches of the end of the
stroke and employ heavy fly wheels, extra strong connections, and
prodigious bed plates. Counterpoise weights are also attached to such
machines; the steam is allowed to follow full stroke, steam cylinders
are placed at awkward angles to the air-compressing cylinders and the
motion conveyed through yokes, toggles, levers; and many joints and
other devices are u