AIR, in Physics. See Atmosphere, Meteorology, and Pneumatics. Air, in Mythology, was adored by the heathens under the names of Jupiter and Juno; the former representing the superior and finer part of the atmosphere, and the latter the inferior and grosser part. The augurs also drew presages from the clouds, thunder, &c.

Air, in Painting, &c. denotes the manner and very life of action; or it is that which expresses the disposition of die agent. It is sometimes also used in a synonymous sense with gesture or attitude.

Air, in Music, is taken in different senses. It is sometimes contrasted with harmony, and in this sense it is synonymous with melody in general.—Its proper meaning is, a tune, which is set to words, or to short pieces of poetry, that are called songs.

In operas we give the name of air to such pieces of music as are formed with measures and cadences to distinguish it from the recitative; and, in general, every piece of music is called an air which is formed for the voice, or even for instruments, and adapted to stanzas, whether it forms a whole in itself, or whether it can be detached from any whole of which it forms a part, and be executed alone. *

If the subject admits of harmony, and is set in parts, the air is, according to their number, denominated a duet, a trio, a quartetto, &c. We need not follow Rousseau and the other philologists in their endeavours to investigate the etymon of the word air. Its derivation, though found and ascertained, would contribute little to illustrate its meaning in that remote sense to which, through a long continuance of time, and the various vicissitudes of language, it has now passed. The curious may consult the same article in the Dictionnaire de Musique by Μ. Rousseau.

In modern music, there are several different kinds of airs, each of which agrees to a certain kind of dancing; and from these dances the airs themselves take their specific names.

The words to which airs are adapted are not always rehearsed in regular succession, nor spoken in the same manner with those of the recitative; and though in general they are very short, yet they are interrupted, repeated, transposed, at the pleasure of the artist. They do not constitute a narrative, which once told is over; they either delineate a picture which it is necessary to contemplate in different points of view, or inspire a sentiment in which the heart acquiesces with pleasure, and from which it is neither able nor willing to be disengaged; and the different phrases of the air are nothing else than different manners of beholding the same image. This is the reason why the subject of an air should be one. It is by these repetitions properly placed, it is by these redoubled efforts, that an impression, which at first was not able to move you, at length shakes your soul, agitates you, transports you out of yourself; and it is likewise upon the same principle that the runnings, as they are called, or those long, mazy, and inarticulated inflections of the voice, in pathetic airs, frequently seem, though they are not always so, improperly placed; for whilst the heart is affected with a sentiment exquisitely moving, it often expresses its emotions by inarticulate sounds, more strongly and sensibly than it could do by words themselves.

The form of airs is of two kinds. The small airs are often composed of two strains, which ought each of them to be sung twice; but the important airs in operas are frequently in the form of rondeaus.

Air- Gun, a pneumatic machine for exploding bullets, &c. with great violence. See Pneumatics. Air- Jacket, a sort of jacket made of leather, in which are several bags or bladders, composed of the same materials, communicating with each other. These are filled with air through a leather tube having a brass stop-cock accurately ground at the extremity, by which means the air blown in through the tube is confined in the bladders. The jacket must be wet before the air be blown into the bags, as otherwise it will immediately escape through the pores of the leather. By the help of these bladders, which are placed near the breast, the person is supported in the water, without making the efforts used in swimming.

Air- Pipes. See Ventilators. Air- Pump, a machine by which the air contained in a proper vessel may be exhausted or drawn out. See Pneumatics. Air- Shafts, among Miners, denote holes or shafts let down from the open air to meet the adits and furnish fresh [7:2:368] air. The damps, deficiency, and impurity of air which occur, when adits are wrought 30 or 40 fathoms long, make it necessary to let down air-shafts, in order to give the air liberty to play through the whole work, and thus discharge bad vapours, and furnish good air for respiration: the expense of which shafts, in regard of their vast depths, hardness of the rock, drawing of water, &c. sometimes equals, nay exceeds, the ordinary charge of the whole adit.

Air- Threads, in Natural History, a name given to the long filaments so frequently seen in autumn floating about in the air.

These threads are the work of spiders, especially of that species called the long-legged field spider, which, having mounted to the summit of a bush or tree, darts from its tail several of these threads, till one is produced capable of supporting the creature in the air. On this it mounts in quest of prey, and frequently rises to a very considerable height.

Air- Trunk, the name of a contrivance by Dr Hales, to prevent the stagnation of putrid effluvia in jails and other places, where a great number of people are crowded together in a small space. It consists only of a long square trunk open at both ends, one of which is inserted into the ceiling of the room, the air of which is required to be kept pure, and the other extends a good way beyond the roof. Through this trunk a continued circulation is carried on; and the reason is, that the putrid effluvia which do so much mischief when collected, being much lighter than the pure atmosphere, arise to the top of the room, and, if they there find a vent, will continually go out through it. These effluvia arise in very considerable quantity, being calculated by Dr Keil at no less than 39 ounces from one man in 24 hours.

Air- Vessels are spiral ducts in the leaves, &c. of plants, supposed to be analogous to the lungs of animals, in supplying the different parts of a plant with air.