AIR, in Geography. See AIR.
Air-Bladder, in fishes. See COMPARATIVE ANATOMY and ICHTHYOLOGY Index.
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, 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 be inserted in this hole, the air will rush through the tube into the fire, and of consequence there will be a continual 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 fore-mentioned 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 above mentioned 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 light 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 Air-Pipes place.
"It is therefore proposed, that, in order to clear the holds of ships of the bad air therein contained, the two holes above mentioned, 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 through those places where it would otherwise be most apt to stagnate and putrefy. Several other contrivances have been used for the same purpose; and Dr 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 Desaguliers, 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 through the trunk. By turning the wheel swiftly, a strong blast of air would be continually forced out through 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
Air Pump of the walls; by turning the wheel swiftly round, the
Air-Shaft 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 Desagulier'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; though 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 Desaguliers, 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.