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 placed 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 place. Air-Pipes.
"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 Desagniers, 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 pass 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
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 Desaguliers'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-fails, 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.
Air-Pump, a machine by which the air contained in a proper vessel may be exhausted or drawn out. See PNEUMATICS.
Air-Sacs, in Birds. See COMPARATIVE ANATOMY.
Air-Shafts, among Miners, denotes holes or shafts let down from the open air to meet the adits and furnish fresh 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 expence 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 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 sideways 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 ore as well 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-Trunk, is also 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 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 swift 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 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-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 BOTANY INDEX.