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, 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 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.