Air Compres'sor, an air pump for forcing air into a closed vessel. The simplest form is the common bicycle pump. This has a valve in the piston opening downward, and another in the bottom of the cylinder opening outward. When the piston is raised, the cylinder below it is filled with air. When the piston is forced down, the valve in it is closed, the valve in the cylinder is forced open and the air is driven into the vessel. Whatever the size of an air compressor, it operates on this principle. Very large air compressors, operated by water power, steam engines or electric motors, are often used in mines and tunnels for forcing a circulation of air and for supplying air to operate machine tools. In this case the air acts the same as steam in a steam engine. Some of these compressors are so powerful that they will condense the air until it exerts a pressure of three thousand pounds to the square inch. The compressed air is stored in a reservoir, from which it is drawn as required. See AIR; COMPRESSED AIR.