DIVING, the art of descending under water to considerable depths, and abiding there a competent time. The uses 'of diving are considerable, particularly in fishing for pearls, corals, sponges, wrecks of ships, &c. See PEARL, &c.
There have been various engines con. trived to render the business of diving safe and easy ; the great point is to fur-: nish the diver with fresh air, without which he must either make but a short stay, or perish. Those who dive for sponges in the Mediterranean, carry down sponges dipt in oil in their mouths ; but considering the small quantity of air that can be contained in the pores of a sponge, and how muck that little will contracted by the pressure of the incum bent air, such a supply cannot subsist -a diver long, since a gallon of air is not fit for respiration above a minute.
Diviwu bell. A diving bell is most con-, veniently made in form of a truncated cone, the smaller end being closed, and the ;urger opened. It, is to be poised j with lead, and so suspended, that the ves sel may sink full of air, and with its open base downwards, and as nearly as may be in a situation parallel to the horizon, as to close with the surface of the water all at once. The diver, sitting under this, sinks down with the included air to the . depth desired ; and if the cavity of the I vessel can contain a tun of water, a single man may remain a full hour without much inconvenience, at five or six fathoms deep ; but the lower he goes the includ ed air contracts itself, according to the weight of the water which compresses it, so that at thirty-three feet deep the bell becomes half full of water, the pressure of the incumbent water being then equal to that of the atmosphere : and at all other depths, the space occupied by the corn- ') pressed air in the upper part of the bell ;: will be to the under part of its capacity'' filled with water, as thirty-three feet to the surface of the water in the bell below. the common surfice, and this condensed air being taken in with the breath, soon accommodates itself to the existing cir cumstances, so as to have no ill effect, provided the bell is admitted to descend slowly ; but the greatest inconvenience of this engine is, that the water, entering it, contracts the bulk of air into so small a compass, that it soon heats, and becomes unfit for respiration, so that there' is a necessity for its being drawn up to re cruit it, besides the uncomfortable situa tion of the diver, who must be almost co vered with water.
To obviate these difficulties of the div ing bell, Dr. Halley, to whom we owe the preceding account, contrived a farther apparatus, whereby, not only to recruit the air from time -to time, but also to keep the water wholly out of the ma. chine at any depth. This bell was made of wood, containing about sixty cubic feet in its concavity, and was the form of a truncated cone, whose diameter at the top was three feet, and at the bottom , five. It was so loaded with lead, that it [ would go down in a perpendicular direc t 'ion and no other. In the top was a win dow to let in light, and likewise a cock to let out the hot air that had been breath ed ; and below, about a yard under the bell, was a stage suspended by three ropes, each of whiejt was charged with about one hundred 'weight, to keep it steady. To supply air, the bell had a cou ple of barrels so cased with lead as to sink when empty, each having a bung hole in its lowest part, to let in the wa ter as the air in them condensed on their descent, and to let it out again when they were drawn up full from below. To a hole in the uppermost part of these was fixed a leathern trunk or hose, long enough to fall below the bung-hole, and kept down by a weight, in such a way that the air in the upper part of the bar rels could not escape, unless the lower ends of these hose were first lifted up. These air barrels were made to rise and fall like two buckets in a well ; by means of these barrels fresh air was continually supplied from above, and it was done with so much ease, that two men, with less than half their strength, could per form all the labour required. By an ad ditional contrivance it was found practica ble for a diver to go out of the engine, to some distance from it, the air being conveyed to him in a continuable stream by small flexible pipes.