he water checked; a portion
of it was allowed to discharge and the space was filled with air, which
was in turn forced into the tank. The efficiency of this compressor was
about 50 per cent.

At the St. Gothard tunnel, begun in 1872, Prof. Colladon first
introduced the injection of water in the form of spray into the
compressor cylinder to absorb the heat of compression.

[Illustration: FIG. 3.]

Fig. 3 illustrates the air cylinder of the Dubois-Francois type of
compressor, which was the best in use about the year 1876. This
compressor was exhibited at the Centennial Exposition and was adopted by
Mr. Sutro in the construction of the Sutro tunnel. A characteristic
feature seems to be to get as much water into the cylinder as possible.
The water which flooded the bottom of the cylinder arose from the
voluminous injection; this water was pushed into the end of the cylinder
and some of it escaped with the air through the discharge valve.

An improved pattern of this compressor is shown in Fig. 4.

[Illustration: FIG. 4.]

These illustrations are interesting from an historical point of view, as
indicating the line of thought which early designers of air-compressing
machinery followed. As the necessity for compressed air power grew,
inventors turned their attention to the construction of air-compressing
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 i