weight and momentum of the water forced a volume of air with such shock
against a discharge valve that it was opened and the air was discharged
into the tank; the valve was then closed, the 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