Air En'gine, an engine in which compressed air, or air heated and so expanded, is used as the motive power. A great many engines of the former kind have been invented, some of which have been found to work quite well where no great power is required. They may be said to be essentially similar in construction to the steam engine, though of course the expansibility of air by heat is small compared with the expansion that takes place when water is converted into steam. For this reason the cylinders of air engines are much larger than those of steam engines. Engines working by compressed air have been found very useful in mining and tunneling, and the compressed air may be conveyed to its destination by means of pipes. In such cases the waste air serves for ventilation and for reducing the oppressive heat. See AIR; AIR COMPRESSOR.