AIR BALLOONS

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 those 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 flout ring through which passes the rod k: upon this rod the feet are to be formerly set; then the hands are to be applied to the two handles i 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 k. 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 unscrews 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 Z Z, which throws up the fear y a, and disengages it from the notch a, upon which the strong spring W W 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 s, 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 resistance 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 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 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 every body, 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 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 freshened, 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

who used all his interest for that purpose, was shamefully neglected.

A machine capable of answering the same purpose was invented by Mr Defagulier, 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 circumstanced, 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 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 Defagulier'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 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 Defagulier, the ventilators of Dr Hales, already mentioned, and those called wind-sails, are likewise used for the same purpose. The former of which is an improvement of the Hessian-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.