engines that would combine _efficiency_ with _light weight_ and _economy
of space and cost_. The trade demanded compressors at inaccessible
localities, and in many cases it was preferred to sacrifice isothermal
results to simplicity of construction and low cost.

It is evident that an air compressor which has the steam cylinder and
the air cylinder on a single straight rod will apply the power in the
most direct manner, and will involve the simplest mechanics in the
construction of its parts. It is evident, however, that this straight
line, or direct construction, results in an engine which has the
greatest power at a time when there is no work to perform. At the
beginning of the stroke steam at the boiler pressure is admitted behind
the piston, and, as the air piston at that time is also at the initial
point in the stroke, it has only free air against it. 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.]