It is seen that the ideal air-compressing machine is one which will take all the heat from the air as rapidly as it is developed during compression. Such "isothermal compression" is never reached in practice, the best work yet done lacking io% of it. It follows that the most inefficient compressing machine is one which takes away no heat during compression—that is, works by "adiabatic compression," which in practice has been much more nearly approached than the ideal. It also follows that the ideal motor for using compressed air is one which will supply heat to the air as required when it is expanding. Such "isothermal" expan sion is often attained, and sometimes exceeded in practice by supplying heat artificially. Finally, the most inefficient motor for using compressed air is one which supplies no heat to the air during its expansion, or works by adiabatic expansion, which was long very closely approached by most air motors. In practice isothermal compression is approached by compressing the air slightly, then cooling it, compressing it slightly again and again cooling it until the desired compression is completed. This is called compression in stages or compound compression. Iso thermal expansion is approximately accomplished by allowing the air to do part of its work (as expanding slightly in a cylinder) and then warming it, then allowing it to do a little more and then warming it again, and so continuing until expansion is complete.
It will be seen that the air is carefully cooled during compression to prevent the heat it develops from working against compression, and even more carefully heated during expansion to prevent loss from cold developed during expansion. More stages of compres sion of course give a higher efficiency, but the cost of machinery and friction losses have to be considered. The reheating of air is often a disadvantage, especially in mining, where there are great objections to having any kind of combustion underground; but where reheating is possible, as W. C. Unwin says, "for the amount of heat supplied the economy realized in the weight of air used is surprising. The reason for this is, the heat supplied to the air is used nearly five times as efficiently as an equal amount of heat employed in generating steam." Practically there is a hot-air engine, using a medium much more effective than common air, in addition to a compressed-air engine, making the efficiency of the whole system extremely high. (See also PNEUMATIC TOOLS.) (A. DE W. F.) PNEUMATICS, the branch of physical science concerned with the properties of gases and vapours (Gr. rvEiµa, wind, air).