AIRE, a town of south-west France, in the department of Landes, on the Adour, 2 2m. S.E. of Mont-de-Marsan on the railway between Morcenx and Tarbes. Pop. , 1 ,949 Aire (Atura, Vices Julii) was the residence of the kings of the Visigoths, and there Alaric II. (q.v.), drew up his famous code. It is the seat of a bishopric (5th century) and has a cathedral (I 2th century) and an episcopal palace (I 1 th, I 7th and i 8th cen turies). Both have been restored. The church of St. Quitterie in Mas d'Aire, a suburb, is a building of the 13th and 14th centuries, with a choir in the Romanesque style, and a fine western portal now much disfigured. The crypt contains several Gallo-Roman tombs and the sarcophagus (5th century) of St. Quitterie.
The practical drawbacks to employing air as the working substance of a heat-engine are so great that its use has been very limited. Such attempts as have been made to design air-engines on a large scale have been practical failures, and are now interesting only as steps in the historical development of applied thermodynamics. In the form of domestic motors, air-en gines had some application, prior to the development of the inter nal combustion engine and the extended use of electricity. The type survives in a few motors of very small power. One of the chief practical objections to air engines is the great bulk of the working substance in relation to the amount of heat that is util ized in the working of the engine.
Another is the difficulty of get ting heat into the working air through the walls of the contain ing vessel. Air takes up heat from such a surface with much less readiness than does water.
The waste of heat in the chimney gases is accordingly greater; and further, the metallic shell is liable to be quickly burned away as a result of its contact at a high temperature with free oxygen. The temperature of the shell is much higher than that of a steam boiler, for in order to secure that the working air will take up a fair amount of heat, the upper limit to which its temperature is raised greatly exceeds that of even high-pressure steam.
A form of air-engine invented in 1827 by the Rev. R. Stirling is of special interest as embodying the earliest application of what is known as the "regenerative" principle, namely, the principle that heat may be deposited by a substance at one stage of its action and taken up again at another stage with but little loss, and with a great resulting change in the temperature of the sub stance at each of the two stages in the operation. The principle has since found wide application in metallurgical and other opera tions. In any heat-engine it is essential that the working sub stance should be at a high temperature while it is taking in heat, and at a relatively low temperature while it is rejecting heat.
An attempt to develop a powerful air-engine was made in America about 1833 by John Ericsson, who applied it to marine propulsion in the ship "Caloric," but without permanent success. Like Stirling, Ericsson used a regenerator, but with this difference, that the pressure instead of the volume of the air remained con stant while it passed in each direction through the regenerator.