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 groser part. The augurs also drew prelages from the clouds, thunder, lightning, &c.

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

music, is taken in different senses. In a vulgar acceptation, it signifies any particular manner of execution: thus, we say of a practical musician, that he performs with a good or bad, proper or improper, air. But this is certainly a solecism of speech. The animating graces, and moving touches, with which melody is adorned and heightened in execution, are refutable either into manner or expression.—Air is likewise 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 duett, 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 M. 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. See Music, Art. 252.

The airs of our operas are, if we may be permitted the expression, the canvass or subliratum upon which are painted all the pictures of imitative music; melody is the design, and harmony the colouring; every picturesque object selected from the most beautiful parts of nature, every reflected sentiment of the human heart, are the models which the artist imitates; whatever gains attention, whatever interests the soul, whatever charms the ear, or causes emotion in the heart, these are the objects of his imitation. An air which delights the ear, and discovers the learning of the composer; an air invented by genius, and composed with taste; is the noblest effort of music: it is this which explores the compass, and displays the delicacy, of a beautiful voice; it is in this where the charms of a well-conducted symphony shine; it is by this, that the passions, excited and inflamed by nice gradations, reach and agitate the soul through the avenues of external sense. After hearing a beautiful air, the mind is acquiescent and serene: the ear is satisfied, not disgusted: it remains impressed on the fancy, it becomes a part of our essence, we carry it with us, we are able to repeat it at pleasure: without the ability acquired by habit to breathe a single note of it, we execute it in our imagination in the same manner as we heard it upon the theatre: one sees the scene, the actor, the theatre; one hears the accompaniments and the applauds. The real enthusiasm in music never forgets the beautiful airs which he has heard; when he chokes, he causes the opera to recommence.

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, for ordinary, 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 but 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, which, in pathetic airs, frequently seem, though they are not always so, improperly placed; whilst the heart is affected with a sentiment exquisitely moving, it often expresses its emotions by inarticulate sounds, more strongly and feebly 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 rondos.

AIRS, in the menage, are the artificial motions of taught horses; as the demivolte, curvet, capriole, &c. *See Dictionary of Air-Bladders, in fifles. See Comparative Anatomy, &c.

**Air-Gun**, a pneumatic machine for exploding bullets, &c. with great violence.

The common air-gun is made of brass, and has two barrels; the inside barrel A, fig. 1, which is of a small bore, from whence the bullets are exploded; and a large barrel barrel ECDR on the outside of it. There is a syringe SMNP fixed in the flock of the gun, by which the air is injected into the cavity between the two barrels through the valve EP. The ball K is put down into its place in the small barrel, with the rammer, as in any other gun. At SL is another valve, which, being opened by the trigger O, permits the air to come behind the bullet, so as to drive it out with great force. If this valve be opened and shut suddenly, one charge of condensed air may be sufficient for several discharges of bullets; but if the whole air be discharged on one single bullet, it will drive it out with a great force. This discharge is effected by means of a lock, fig. 2, placed there as usual in other guns; for the trigger being pulled, the cock will go down and drive the lever O, fig. 1, which will open the valve, and let in the air upon the bullet K.

The Magazine Air-gun was invented by that ingenious artist L. Colbe. By this contrivance ten bullets are so lodged in a cavity, near the place of discharge, that they may be drawn into the shooting-barrel, and successively discharged so fast as to be nearly of the same use as so many different guns.

Fig. 3 represents the present form of this machine, where part of the flock is cut off, to the end of the injecting syringe. It has its valve opening into the cavity between the barrels, as before. KK is the small shooting-barrel, which receives the bullets from the magazine ED, which is of a serpentine form, and closed at the end D when the bullets are lodged in it. The circular part abc, is the key of a cock, having a cylindric hole through it, ik, which is equal to the bore of the same barrel, and makes a part of it in the present situation. When the lock is taken off, the several parts Q, R, T, W, &c., come into view, by which means the discharge is made by pushing up the pin Pp, which raises and opens a valve V, to let in the air against the bullet I, from the cavity FF; which valve is immediately shut down again by means of a long spring of brass, NN. This valve V being a conical piece of brass, ground very true in the part which receives it, will of itself be sufficient to confine the air.

To make a discharge, you will pull the trigger ZZ, which throws up the lever y a, and disengages it from the notch a, upon which the strong spring WW moves the tumbler T, to which the cock is fixed. This, by its end u, bears down the end v of the tumbling lever R, which, by the other end m, bears at the same time the flat end of the horizontal lever Q; and by this means, of course, the pin Pp, which stands upon it, is pushed up, and thus opens the valve V, and discharges the bullet. This is all evident from a bare view of the figure.

"To bring another bullet to succeed that marked I, instantaneously, turn the cylindric cavity of the key of the cock, which before made part of the barrel KK, into the situation ik, so that the part i may be at K; and hold the gun upon your shoulder, with the barrel downwards, and the magazine upwards, by which means that bullet next the cock will fall into it out of the magazine, but go no farther into this cylindric cavity than the two little springs ss, which detain it. The two circles represent the cock-barrel, wherein the key abovementioned turns upon an axis not represented here, but visible in fig. 4. This axis is a square piece of steel, on which comes the square hole of the hammer H, fig. 5.; by which the cylindric cavity mentioned is opened to the magazine. Then opening the hammer, as in that figure, the bullet is brought into its proper place near the discharge-valve, and the cylindric cavity of the key of the cock again makes part of the inward barrel KK.

It evidently appears how expeditious a method this is of charging and discharging a gun; and were the force of condensed air equal to that of gunpowder, such an air-gun would answer the end of several guns.

In the air-gun, and all other cases where the air is required to be condensed to a very great degree, it will be requisite to have the syringe of a small bore, viz. not exceeding half an inch in diameter; because the pressure against every square inch is about 15 pounds, and therefore against every circular inch about 12 pounds. If therefore the syringe be one inch in diameter, when one atmosphere is injected, there will be a resistance of 12 pounds against the piston; and when 10 are injected, there will be a force of 120 pounds to be overcome; whereas ten atmospheres act against the circular half-inch piston (whose area is only one-fourth part so big) with only a force equal to 30 pounds; or 40 atmospheres may be injected with such a syringe, as well as 10 with the other. In short, the facility of working will be inversely as the squares of the diameter of the syringe.

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, an invention for drawing foul air out of the ships, or any other close places, by means of fire. These pipes were first found out by one Mr Sutton, a brewer in London; and from him have got the name of Sutton's Air-pipes. The principle on which their operation depends is known to everybody, being indeed no other than that air is necessary for the support of fire; and, if it has not access from the places most adjacent, will not fail to come from those that are more remote. Thus, in a common furnace, the air enters through the ash-hole; but if this is closed up, and a hole made in the side of the furnace, the air will rush in with great violence through that hole. If a tube of any length whatever is inserted in this hole, the air will rush through the tube into the fire, and of consequence there will be a continued circulation of air in that place where the extremity of the tube is laid: Mr Sutton's contrivance then, as communicated to the Royal Society by Doctor Mead, amounts to no more than this.—"As, in every ship of any bulk, there is already provided a copper or boiling-place proportionable to the size of the vessel; it is proposed to clear the bad air, by means of the fire already used under the said coppers or boiling-places for the necessary uses of the ship." It is well known, that under every such copper or boiler, there are placed two holes, separated by a grate; the first of which is for the fire, and the other for the ashes falling from the same; and that there is also a flue from the fire-place upward, by which the smoke of the fire is discharged at some convenient place of the ship.

It is also well known, that the fire once lighted in these fire-places, is only preserved by the constant draught of air through the forementioned two holes and flue; and that if the said two holes are closely stopped up, the fire, though burning ever so briskly before, is immediately put out.

But if, after shutting up the abovementioned holes, another hole be opened, communicating with any other room or airy place, and with the fire; it is clear, the said fire must again be raised and burn as before, there being a like draught of air through the same as there was before the stopping up of the first holes; this case differing only from the former in this, that the air feeding the fire will now be supplied from another place.

It is therefore proposed, that, in order to clear the holds of ships of the bad air therein contained, the two holes abovementioned, the fire-place and ash-place, be both closed up with substantial and tight iron-doors; and that a copper or leaden pipe, of sufficient size, be laid from the hold into the ash-place, for the draught of air to come in that way to feed the fire. And thus it seems plain, from what has been already said, that there will be, from the hold, a constant discharge of the air therein contained; and consequently, that that air, so discharged, must be as constantly supplied by fresh air down the hatches or such other communications as are opened into the hold; whereby the same must be continually refreshed, and its air rendered more wholesome and fit for respiration.

And if into this principal pipe so laid into the hold, other pipes are let in, communicating respectively either with the well or lower decks; it must follow, that part of the air consumed in feeding the fire, must be respectively drawn out of all such places to which the communication shall be so made.

This account is so plain, that no doubt can remain concerning the efficacy of the contrivance; it is evident, that, by means of pipes of this kind, a constant circulation of fresh air would be occasioned thro' those places where it would otherwise be most apt to stagnate and putrefy. Several other contrivances have been used for the same purpose; and Doctor Hales's ventilators, by some unaccountable prejudice, have been reckoned superior in efficacy and even simplicity to Mr Sutton's machine, which at its first invention met with great opposition *, and even when introduced by Dr Mead, who used all his interest for that purpose, was shamefully neglected.

A machine capable of answering the same purpose was invented by Mr Defaguliers, which he called the ship's lungs. It consisted of a cylindrical box set up on its edge, and fixed to a wooden pedestal. From the upper edge of the box issued a square trunk open at the end, and communicating with the cavity of the box. Within this box was placed a cylindrical wheel turning on an axis. It was divided into 12 parts, by means of partitions placed like the radii of a circle. These partitions did not extend quite to the centre, but left an open space of about 18 inches diameter in the middle; towards the circumference, they extended as far as possible without interfering with the case, so that the wheel might always be allowed to turn freely.—Things being thus circumcised, it is plain, that if the wheel was turned towards that side of the box on which the trunk was, every division would push the air before it, and drive it out through the trunk, at the same time that fresh air would come in through the open space at the centre, to supply that which was thrown out thro' the trunk. By turning the wheel swiftly, a strong blast of air would be continually forced out thro' the square trunk, on the same principles on which a common fanner winnows corn. If the wheel is turned the opposite way, a draught of air may be produced from the trunk to the centre.—If this machine, then, is placed in a room where a circulation of air is wanted, and the trunk made to pass through one of the walls; by turning the wheel swiftly round, the air will be forced with great velocity out of that room, at the same time that fresh air will enter through any chinks by which it can have access to supply that which has been forced out.

It is evident, that the circulation which is promoted by this machine, is entirely of the same kind with that produced by Mr Sutton's; the turning of the wheel in Mr Defaguliers's machine being equivalent to the rarefaction of the air by fire in Mr Sutton's; but that the latter is vastly superior, as acting of itself, and without intermission, requires no arguments to prove. Mr Sutton's machine has yet another convenience, of which no other contrivance for the same purpose can boast; namely, that it not only draws out putrid air, but destroys it by causing it pass through fire; and experience has abundantly shewn, that though putrid air is thrown into a great quantity of fresh air, it is so far from losing its pernicious properties, that it often produces noxious diseases. We do not say, indeed, that putrid air becomes salutary by this means; but it is undoubtedly rendered less noxious than before; tho' whether it is equally innocent with the smoke of a fire fed in the common way, we cannot pretend to determine.

Besides this machine by Mr Defaguliers, the ventilators of Doctor Hales, already mentioned, and those called Wind-sails, are likewise used for the same purpose. The former of which is an improvement of the Hellian-bellows*: the other is a contrivance for throwing fresh air into those places where putrid air is apt to lodge; but this has the last-mentioned inconvenience in a much greater degree than any of the others, as the blast of fresh air throws out that which was rendered putrid by stagnation, in such a manner as to contaminate all around it. See Wind-sails.

Air-Trunk, is also a contrivance by Doctor 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, Air-trunks will continually go out through it. These effluvia arise in very considerable quantity, being calculated by the late Dr Keil at no less than 39 ounces from one man in 24 hours.

These trunks were first made trial of by Mr Yeoman, over the House of Commons, where they were nine inches wide within; and over the Court of King's-bench in Westminster-hall, where they were five inches wide. They are sometimes made wider, and sometimes narrower; but the wider they are, the longer they ought to be, more effectually to promote the ascent of the vapour. The reason why vapours of this kind ascend more swiftly through a long trunk than a short one, is, that the pressure of fluids is always according to their different depth, without regard to the diameter of their basins, or of the vessel which contains them; and, upon this principle, a gallon of water may be made to split a strong cask*. When the column of putrid effluvia is long and narrow, the difference between the column of atmosphere pressing on the upper end of the trunk, and that which presses on the lower end, is much greater than if the column of putrid effluvia was short and wide; and consequently the ascent is much swifter.—One pan of a single pair of scales, which was two inches in diameter, being held within one of these trunks, over the house of commons, the force of the ascending air made it rise so as to require four grains to restore the equilibrium, and this when there was no person in the house; but when it was full, no less than 12 grains were requisite to restore the equilibrium; which clearly shows that these trunks must be of real, and very great efficacy.

Air-Pump, a machine by which the air contained in a proper vessel may be exhausted, or drawn out†.

Air-Shafts, among miners, are holes made to meet the adits, and supply them with fresh air.

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. See Aranea.

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. See Plants, n° 35. and the figure there referred to.