AIR-COMPRESSORS. Machines for compressing air, which is afterward to be used in suitable engines as a motor, or through its expansion as a means of reducing the temperature of adjacent bodies, or as a blast for forges, etc. The machines performing the last-mentioned duty are known as blowing engines and blowers. The name "blower " is more commonly applied to rotary machines, either force, blast, or fan, and " blowing "engine" to piston apparatus. The former, having a wide range of uses, are separately treated under BLOWERS. For mechanical applications of compressed air, see BRAKES , CAISSONS, DIVING , FOUNDATIONS, HAMMER , LOCOMOTIVE, RAILROAD, REFRIGERATING MACHINERY , and TELEGRAPH. For theoretical considerations, see STEAM.

Apparatus for compressing air includes, first, a motor; second, a machine wherein the air is compressed. Compressors may be classified as follows:

1. With regard to air-pressure generated. Low-pressure compressors are those in which air is compressed to a 3 pressure not exceeding 2 absolute atmospheres-that is to say, to less than one effective atmosphere. Medium-pressure compressors are those in which the pressure attained is compressed between 2 and 4 absolute atmospheres, or between 1 and 3 effective atmospheres. High-pressure compressors are those in which the air is compressed to between 4 and 8 absolute atmospheres --that is to say, below 2 effective atmospheres. Very high-pressure compressors are those in which the air is compressed to pressures above 8 absolute atmospheres.

2. With regard to volume furnished at a given pressure, each one of the foregoing classes may be divided into low-duty and high-duty machines. Each of these subdivisions may be again divided into piston-compressors, the primitive type of which is the blast-machine of blast-furnaces, in which the air contained in a cylinder is brought to the desired pressure by means of a piston which gradually decreases the volume of the cylinder to that which corresponds to the pressure desired; and compressors without pistons, the primitive type of which is the trompe or water-bellows of Catalan forges, and which includes all other machines not coming under the piston-compressor class.

In piston-compressors the piston may act on the air either directly or by the intermediary of water, which serves as packing. Hydraulic piston-compressors, as the last-mentioned class may be termed, may be again divided in accordance with the means used for cooling the air and the cylinder. As each class above mentioned will be considered in turn, for the convenience of the reader the various groupings are recapitulated as follows in their proper connection: