in natural philosophy, a thin, fluid, elastic, transparent, ponderous, compressible, and dilatable body, surrounding the terraqueous globe to a considerable height. See Aerology, Atmosphere, and Pneumatics.

Impregnation of Water with Fixed Air, and with Sulphurous Air. See Mineral Waters.

Medicine, &c. makes one of the six non-naturals.—From observations on bleeding in rheumatism, and after taking cold, it is evident, the air can enter with all its qualities, and vitiate the whole texture of the blood, and other juices.—From the palsies, vertigoes, and other nervous affections caused by damps, mines, &c. it is evident, that air thus qualified can relax and obstruct the whole nervous system. And from the colics, fluxes, coughs, and consumptions produced by damp, moist, and nitrous air, it is evident it can corrupt and spoil the noble organs, &c.

Circulation of Air in Rooms. To render the circulation of air sensible, let the air of a room be heated by a strong fire, whilst the air of a contiguous room is cold; then let the door between these two rooms be opened, in which case the hot air of one room being lighter, will pass through the upper part of the opening of the door into the cold room; and, on the contrary, the cold cold air of the other room being heavier, will pass into the former room through the lower part of the opening; accordingly, it will be found, that applying a lighted candle at the top, in the middle, and at the lower part of the opening between the two rooms, a strong current of air will appear to pass from the hot into the cold room near the top; a contrary current of air will appear to pass from the latter into the former room near the lower part of the said opening; whilst in the middle there is little or no motion at all, as may be clearly perceived by the direction of the flame of the candle.

It is for the same reason that when the fire is lighted in a chimney, a strong current of air is occasioned to enter the room, which may be felt by applying the hand near the key-hole, or other such small openings, if the doors and windows are shut; for the air over the fire being heated, becomes lighter, and ascends into the chimney, consequently other colder air must supply its place, which forces its way through all the small openings it can find. Were a room with a fire in it to be perfectly closed, excepting the chimney, the air in it would soon become unwholesome for respiration, and the fire would be soon extinguished, besides other inconveniences. Hence it appears, that those persons mistake who expect to keep the air of a room sweet and wholesome, especially for convalescents, by accurately stopping all the smallest openings that admit fresh air. When the current of air that enters into a room is on some side of it where it falls immediately upon the persons who sit in the room, then it may be offensive, especially to delicate constitutions. In that case, such opening should be closed: but at the same time another opening should be made for admitting fresh air, in another more convenient part; for a circulation of air, especially in rooms where a fire is kept, is not only salutary and useful, but is absolutely necessary.

In an ingenious publication, intitled, A Practical Treatise on Chimneys, there are the following remarks relating to the properest method of admitting air into a room, and of expelling the contaminated air. The author, directing to make a vent-hole near the top of the room, in order to expel the heated and contaminated air, "this," says he "might be done by means of a small tube opening into the room, either in or near the ceiling; which might either be carried to the top of the building, or be made to communicate with the external air by a small perforation through the wall at the roof of the room; by means of either of which, a proper circulation would be established, and the foul air be carried off.

N° 9.

(A) Such readers as have been little accustomed to speculations of this sort, will be at a loss to comprehend in what manner two holes, both of them in the roof of the room, and communicating with the air, without any valve, or other contrivance, for opening or closing of themselves, should yet answer the two very opposite purposes; one, of constantly bringing cool air into the room without emitting any warm air—and the other, of a constantly emitting warm and admitting no cool air. They will please to advert, that the one of these tubes communicates with the atmosphere at the bottom of the house, and the other towards the top; the opening of the one is beneath the level of the room, that of the other above it. Now, as the air is more dense at the surface of the ground than at any height above it, the warm rarefying air will naturally issue at that opening where it meets with least resistance, which must invariably be through that which opens to the external air at the greatest height; and as the cool air will naturally be pressed into the room by that opening where the air is most weighty, this must invariably be by that which is nearest the surface of the earth. equal degrees of heat and cold, as are unavoidable where our apartments are so open as to admit a ready passage to the external air during the winter-season.

"The reader will easily perceive, that all that has been here said has a reference only to those apartments in cold climates, and rigorous weather, where fire to warm them becomes necessary. In warmer regions, or during the summer-season, there can be no objection to the wheel-ventilator in the window.—It is a simple contrivance, and a safe and effectual mean of preserving the air in our apartments sweet and wholesome at that season."

It is a vulgar error among many people, to believe that fire purifies the contaminated air, by destroying the noxious particles mixed with it; and for this reason they think, that the fire kept in a room where the air is tainted, purifies the room, by rendering the air in it again fit for respiration. Indeed, a fire kept in a room or apartment where the air is tainted, as is the case with hospitals, goals, and the like, does certainly purify the apartment, and the practice is very useful; but this effect is only because the fire promotes the circulation of the air, and dries the dampness of rooms, furniture, &c.: so that it is not the infected air that is purified, but is new, fresh, and wholesome air, that by the action of the fire has taken the place of the infected air; which infected air, being rarefied by the heat, has been expelled from the apartment. Fire and combustion in general is so far from purifying contaminated air, that it actually contaminates a prodigious quantity of it in a short time; so that not only a common fire, but even a lighted candle, when kept in a well-closed room, wherein the external air has not a free access, instead of purifying, renders the air of that room noxious.

Instrument for ascertaining the Purity or Wholesomeness of respirable Air. See Eudiometer.

Air Balloons, a general name given to bags of any light substance filled with inflammable air, or other permanently elastic fluid, whose specific gravity is considerably less than that of common atmospheric air. The consequence of their being filled in this manner is, that if they are of any considerable magnitude, they ascend in the air to an amazing height; and will not only ascend in this manner by themselves, but carry up along with them great weights, and continue to rise till they attain an height in which the circumambient air is of the same specific gravity with themselves. In this situation they will either float or be driven in the direction of the wind or current of air in which they are exposed, remaining in these elevated regions till the fluid escapes by the bursting of the bags from the superior elasticity of the fluid, or by its gradual evaporation through the pores of the envelope. The history, principles, &c. of these machines are detailed under the article Aerostation.

Air-Bladder, in fishes. See Comparative Anatomy, chap. iii. and Ichthyology.

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. 8. which is of a small bore, from whence the bullets are exploded; and a large barrel ECDR on the outside of it. There is a syringe SMNP fixed in the stock 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. 9. placed here as usual in other guns; for the trigger being pulled, the cock will go down and drive the lever O, fig. 8. which will open the valve, and let in the air upon the bullet K.

Air-guns of late years have received very great improvements in their construction. Fig. 10. is a representation of one made by the late Mr B. Martin of London, and now by several of the mathematical instrument and gun makers of the metropolis. For simplicity and perfection it exceeds any other heretofore contrived. A is the gun-barrel, with the lock, stock, ram-rod, and of the size and weight of a common fowling-piece. Under the lock, at b, is a round steel tube, having a small moveable pin in the inside, which is pushed out when the trigger a is pulled, by the spring-work within the lock; to this tube b, a hollow copper-ball c screws, perfectly air-tight. This copper ball is fully charged with condensed air by the syringe B (fig. 7.) previous to its being applied to the tube b of fig. 10. It is then evident, that if a bullet be rammed down in the barrel, the copper ball screwed fast at b, and the trigger a be pulled, that the pin in b will, by the action of the spring-work within the lock, forcibly strike out into the copper ball; and thereby pushing in suddenly a valve within the copper ball, let out a portion of the condensed air; which air will rush up thro' the aperture of the lock, and forcibly act against the bullet, driving it to the distance of 60 or 70 yards or further. If the air is strongly condensed at every discharge, only a portion of the air escapes from the ball; therefore, by re-cocking the piece, another discharge may be made; and this repeated to the amount of 15 or 16 times. An additional barrel is sometimes made, and applied for the discharge of shot, instead of the one above described.

The air in the copper ball is condensed by means of the syringe B (fig. 7.), in the following manner: The ball c is screwed quite close on the top of the syringe at b, at the end of the steel pointed rod: a is a stout ring through which passes the rod k: upon this rod the feet use to be formerly set; then the hands are to be applied to the two handles i, fixed on the side of the barrel of the syringe. Now by moving the barrel B steadily up and down on the rod a, the ball c will become charged with condensed air; and it may be easily known when the ball is as full as possible, by the irresistible action that the air makes against the piston when you are working the syringe. At the end of the rod k is usually a four-square hole, which with the rod serves as a key to fasten the ball c fast on the screw b of the gun and syringe close to the orifice in the ball c. In the inside is fixed a valve and spring, which gives way for the admission of air; but upon its emission comes close up to the orifice, shutting up the internal Air-gun. air. The piston-rod works air-tight, by a collar of leather on it, in the barrel B; it is therefore plain, when the barrel is drawn up, the air will rush in at the hole h. When the barrel is pushed down, the air therein contained will have no other way to pass from the pressure of the piston but into the ball c at top. The barrel being drawn up, the operation is repeated, until the condensation is so strong as to resist the action of the piston.

Sometimes the syringe is applied to the end of the barrel C (see fig. 11.), the lock and trigger shut up in a brass case d; and the trigger pulled, or discharge made, by pulling the chain b. In this contrivance there is a round chamber for the condensed air at the end of the syringe at e, and it has a valve acting in a similar manner to that of the copper ball. When this instrument is not in use, the brass case d is made to slide off, and the instrument then becomes a walking-stick; from which circumstance, and the barrel being made of cane, brass, &c., it has received the appellation of the Air-cane. The head of the cane uncrews and takes off at a, where the extremity of the piston-rod in the barrel is shown; an iron rod is placed in a ring at the end of this, and the air condensed in the barrel in a similar manner to that of the gun as above; but its force of action is not near so strong and permanent as that of the latter.

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. 12. represents the present form of this machine, where part of the stock is cut off, to the end of the injecting syringe. It has its valve opening into the cavity between the barrels, as before. K K is the small shooting-barrel, which receives the bullets from the magazine E D, which is of a serpentine form, and closed at the end D when the bullets are lodged in it. The circular part a b c, is the key of a cock, having a cylindric hole through it, i k, 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 P p, which raises and opens a valve V, to let in the air against the bullet I, from the cavity F F; which valve is immediately shut down again by means of a long spring of brass N N. 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, raises at the same time the flat end of the horizontal lever Q; and by this means, of course, the pin P p, 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 K K, into the situation i k, 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 s t, 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. 13. This axis is a square piece of steel, on which comes the square hole of the hammer H, fig. 14.; 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 K K.

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 gun-powder, 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 retentive of 12 pounds against the piston; and when 10 are injected, there will be a force of 120 pounds to be overcome; whereas 10 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 flop-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 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 Air-pipes, 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 aforementioned 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 repiration.

"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 to plan, 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 lung. 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 farmer 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 shown, 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 dilettes. 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 Dr Hales, already mentioned, and those called wind-falls, are likewise used for the same purpose. The former of which is an improvement of the Heffian-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.

*See Sut-opposition*, and even when introduced by Dr Mead, 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, 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 six 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 basis, or of the vessel which contains them; and, upon this principle, a gallon of water may be made to split a strong cask. See Hydrostatics. When the column of putrid effluvia is long and narrow, the difference between the column of atmosphere preëxisting on the upper end of the trunk, and that which preëxists 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. See the article Pneumatics.

Air-Sacs, in birds. See Comparative Anatomy, chap. ii.

Air-Shafts, among miners, denote holes or shafts let down from the open air to meet the adits and furnish fresh air. The damps, want, 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.

Sir Robert Murray describes a method, used in the coal-mines at Liege, of working mines without air-shafts.

When the miners at Mendip have sunk a groove, they will not be at the charge of an air-shaft till they come at ore; and for the supply of air have Air-threads boxes of elm exactly closed, of about six inches in the clear, by which they carry it down about twenty fathoms. They cut a trench at a little distance from the top of the groove, covering it with turf and rods disposed to receive the pipe, which they contrive to come in sideway to their groove, four feet from the top; which carries down the air to a great depth. When they come at ore, and need an air-shaft, they sink it four or five fathoms distant, according to the convenience of the breadth, and of the same fashion with the groove, to draw as well ore as 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 the article Plants.

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 prefigurations from the clouds, thunder, lightning, &c.

painting, &c. denotes the manner and very life of action; 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. 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 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.

The airs of our operas, are, if we may be permitted the expression, the canvass or substratum 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. See 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 applause. The real enthusiasm in music never forgets the beautiful airs which he has heard; when he chooses, 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 inarticulate 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 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 rondellas.

Airs, or Ayr, in geography, a town of Scotland, capital of an extensive county of the same name. It stands on the river Ayr, and was formerly a place of good trade, and seat of fisheries; all of which have vanished, and the people now live by one another. Air appears, from history and other documents, to have been a considerable place at the time of the Norman conquest. The vouchers of its antiquity are corroborated by an elegant building, called the Croft, which hath escaped the destructive rage of the last and preceding century. The date on this fragment of antiquity is 1555, consequently it hath stood in its place above 730 years; and it is to be wished, that the majority of the inhabitants may unite in preserving it from being destroyed by persons who have expressed a strong desire to that purpose. In 1557, the tax levied upon Air was L.236 Scots; upon Glasgow only L.202. In 1771, Air was assessed at 15 s. Sterling and Glasgow at L.18, 10s. In 1751, the pickled herrings exported from Air were 6624 barrels; since the year 1777, none. These revolutions appear the more extraordinary, when we consider the very advantageous situation of Air both by land and water; the fertility of the country; the riches of the sea; its contiguity to the western fisheries on one side, and to Glasgow on the other; the large returns for cattle, grain, and coal; the ample revenues of the town; and particularly the conveniency of its harbour for fishing-vessels of every construction.—About a mile north from the town there is a lazaret-house, commonly called The King's Chapel, which King Robert de Bruce set apart for the maintenance of lepers.