0 degrees initial temperature it is
800 degrees, and with 100 degrees initial it is 900 degrees. It will be
observed that the zero heat curve is flatter than the others, indicating
that when free air is admitted to a compressor cold, the relative
increase of temperature is less than when the air is hot. This points to
the importance of low initial temperature.

We have now seen that the economical production of compressed air
depends upon the following conditions:

(1) A low initial temperature.

(2) Thorough cooling during compression.

It has been demonstrated by experiments made in France that the power
required to compress moist air is less than that for dry air. A table
showing the power required to compress moist and dry air has been
prepared from the data of M. Mallard and shows that for five atmospheres
the work expended in compressing one pound of dry air is 58,500 foot
pounds, while that for moist air is 52,500 foot pounds. In expansion
also moisture in the air adds to the economy, but in both cases the
saving of power is not great enough to compensate for the many
disadvantages due to the presence of water. Mr. Norman Selfe, of the
Engineering Association of N.S.W., has compiled a table which shows some
important theoretical conditions involved in producing compressed air.

So much for the theory of compression. We now come to the practical
production of compressed air.

The first record that we have of the use of an air compressor is at
Ramsgate Harbor, Kent, in the year 1788. Smeaton invented this "pump"
for use in a diving apparatus. In 1851, William Cubitt, at Rochester
Bridge, and a little later an engineer, Brunel, at Saltash, used
compressed air for bridge work. But the first notable application of
compressed air is due to Professor Colladon, of Geneva, whose plans were
adopted at the Mont Cenis tunnel. M. Sommeiller developed the Colladon
idea and constructed the compressed air plant illustrated in Fig. 2.

[Illustration: FIG. 2.]

The Sommeiller compressor was operated as a ram, utilizing a natural
head of water to force air at 80 pounds pressure into a receiver. The
column of water contained in the long pipe on the side of the hill was
started and stopped automatically, by valves controlled by engines. The
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, t