, the art, or act of descending under water, to considerable depths, and remaining there a competent time.

The uses of Diving are very considerable, particularly in the fishing for pearls, corals, sponges, &c.

Various methods have been proposed, and engines contrived, to render the bufiness of diving more safs and easy. The great point in all these, is to furnish 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, help themselves by carrying 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 much that little will be contracted by the pressure of the incumbent water, such a supply cannot long subsist the diver. For it is found by experiment, that a gallon of air included in a bladder, and by a pipe reciprocally inspired and expired by the lungs, becomes unfit for respiration in little more than one minute of time. For though its elasticity be but little altered in passing the lungs, yet it loses its vivifying spirit, and is rendered effete. In effect, a naked diver, Dr. Halley assures us, without a sponge, cannot remain above two minutes inclosed in water; nor much longer with one, without suffocating; nor without long practice, near so long; ordinary persons beginning to be suffocated in about half a minute. Besides, if the depth be confiderable, the pressure of the water on the vessels makes the eyes blood shotten, and frequently occasions a spitting of blood. Hence, where there has been occasion to continue long at the bottom, some have contrived double flexible pipes, to circulate air down into a cavity inclosing the diver, as with armour, both to furnish air, and to bear off the pressure of the water, and give leave to his breast to dilate upon inspiration; the fresh air being forced down one of the pipes with bellows, and returning by the other, not unlike to an artery, and vein.

But this method is impracticable when the depth ex ceeds three fathoms; the water embracing the bare limbs so closely, as to obstruct the circulation of the blood in them; and withal pressing so strongly on all the junctures where the armour is made tight with leather; that if there be the least defect in any of them, the water rushes in, and instantly sills the whole engine, to the great danger of the diver's life. |

Diving-Bell, is a machine contrived to remedy all these inconveniencies. In this the diver is safely conveyed to any reasonable depth, and may stay more or less time under the water, as the bell is greater or less. It is most conveniently made in form of a truncated cone, the smallest base being closed, and the larger open. It is to be poised with lead, and so suspended, that it may sink full of air, with its open basis downward, and as near as may be in a situation parallel to the horizon, so as to close with the surface of the water all at once.

Under this covercle the diver sitting, sinks down with the included air to the depth desired; and if the cavity of the vessel can contain a ton of water, a single man may remain a full hour, without much inconvenience, at five or six fathoms deep. But the lower he goes, still the more the included air contracts itself, according to the weight of the water that 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 compressed air in the upper part of its capacity, is to the space filled with water, as thirty-three feet to the depth of the surface of the water in the bell below the common surface of it. And this condensed air, being taken in with the breath, soon insinuates itself into all the cavities of the body, and has no ill effect, provided the bell be permitted to descend so slowly as to allow time for that purpose.

One inconvenience that attends it, is found in the ears, within which there are cavities which open only outwards, and that by pores so small, as not to give admission even to the air itself, unless they be dilated and distended by a considerable force. Hence, on the first descent of the bell, a pressure begins to be felt on each ear, which, by degrees, grows painful, till the force overcoming the obstacle, what constringes these pores, yields to the pressure, and letting some condensed air slip in, presently ease ensues. The bell descending lower, the pain is renewed, and afterwards it is again eased in the same manner. 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 recruit it; besides the uncomfortable abiding of the diver, who is almost covered with water.

To obviate the difficulties of the diving-bell, Dr. Halley, to whom we owe the preceding account, contrived some further apparatus, by which not only to recruit and refresh the air from time to time, but also to keep the water wholly out of it at any depth; which he effected after the following manner:

His diving-bell (plate vii, fig. 6.) was of wood, three feet wide at top, five feet at bottom, and eight feet high, containing about fixty-three cubic feet in its concavity, coated externally with lead so heavy, that it would sink empty; a particular weight being distributed about its bottom R, to make it descend perpendicularly, and no otherwise. In the top was fixed a meniscus glass D, concave downwards, like a window, to let in light from above; with a cock, as at B, to let out the hot air; and a circular seat, as at LM, for the divers to sit on: and, below, about a yard under the bell, was a stage suspended from it by three ropes, each charged with a hundred weight, to keep it steady, and for the divers to stand upon to do their business. The machine was suspended from the mast of a ship by a sprit, which was secured by stays to the mast-head, and was directed by braces to carry it overboard clear of the side of the ship, and to bring it in again.

To supply air to this bell when under water, he had a couple of barrels, as C, holding thirty-six gallons each, cased with lead, so as to sink empty, each having a bung-hole at bottom, to let in the water as they descended, and let it out again as they were drawn up. In the top of the barrels was another hole, to which was fixed a leathern pipe, or hose, well prepared with bees wax and oil, long enough to hang below the bunghole; being kept down by a weight appended. So that the air driven to the upper part of the barrel by the encroachment of the water, in the defcent, could not escape up this pipe, unless the lower end were lifted up.

These air-barrels were fitted with tackle, to make them rise and fall alternately, like two buckets; being directed in their descent by lines fastened to the under edge of the bell: so that they came readily to the hand of a man placed on the stage, to receive them; and who taking up the ends of the pipes, as soon as they came above the surface of the water in the barrels, all the air included in the upper part of it was blown forcibly into the bell; the water taking its place.

One barrel thus received, and emptied; upon a signal given, it was drawn up, and at the same time the other let down; by which alternate succession, fresh air was furnished so plentifully, that the learned Doctor himself was one of five, who were all together in nine or ten fathoms deep of water for above an hour and a half, without the least inconvenience; the whole cavity of the bell being perfectly dry.

All the precaution he observed, was, to be let down gradually about twelve feet at a time, and then to stop, and drive out the water that had entered, by taking in three or four barrels of fresh air, before he descended farther. And, being arrived at the depth intended, he let out as much of the hot air that had been breathed, as each barrel would replace with cold, by means of the cock B, at the top of the bell, through whose aperture, though very small, the air would rush with so much violence, as to make the surface of the sea boil.

Thus, he found, any thing could be done that was required to be done underneath. And by taking off the stage, he could, for a space as wide as the circuit of the bell, lay the bottom of the sea so far dry as not to be over shoes in water. Besides, by the glass window so much light was transmitted, that, when the sea was clear, and especially when the sun shone, he could see perfectly well to write or read, much more to fasten, or lay hold of any thing under him that was to be taken up. And by the return of the air barrel he often sent up orders written with an iron pen on a plate of lead, directing how he would be moved from place to place.

At other times, when the water was troubled and thick, it would be as dark as night below; but in such cases he was able to keep a candle burning in the bell.

Dr. Halley observes, that they were subject to one inconvenience in this bell; they felt at first a small pain | in their ears, as if the end of a tobacco pipe were thrust into them; but after a little while there was a small puff of air, with a little noise, and they were easy.

This he supposes to be occasioned by the condensed air shutting up a valve leading from some cavity in the ear, full of common air; but when the condensed air pressed harder, it forced the valve to yield, and filled every cavity. One of the divers, in order to prevent this pressure, stopped his ear with a pledget of paper; which was pushed in so far, that a surgeon could not extract it without great difficulty.

The same author intimates, that by an additional contrivance he has found it practicable for a diver to go out of the bell to a good distance from it; the air being conveyed to him in a continued stream by small flexible pipes, which serve him as a clue to direct him back again to the bell. For this purpose, one end of these pipes, kept open against the pressure of the sea, by a small spiral wire, and made tight without by painted leather, and sheep's guts drawn over it, being open, was fastened in the bell, as at P, to receive air, and the other end was fixed to a leaden cap on the man's head, reaching down below his shoulders, open at bottom, to serve him as a little bell, full of air, for him to breathe at his work, which would keep out the water from him, when at the level of the great bell, because of the same density as the air in the great bell. But when he stooped down lower than the level of the great bell, he shut the cock F, to cut off the communication between the two bells. Phil. Trans. abr. vol. iv. part ii. p. 188, &c. vol. vi. p. 550, &c.

The air in this bell would serve him for a minute or two; and he might instantly change it, by raising himself above the great bell, and opening the cock F. The diver was furnished with a girdle of large leaden weights, and clogs of lead for the seet, which, with the weight of the leaden cap, kept him firm on the ground; he was also well clothed with thick flannels, which being first made wet, and then warmed in the bell by the heat of his body, kept off the chill of the cold water for a considerable time, when he was out of the bell.

Mr. Martin Triewald, F. R. S. and military architect to the king of Sweden, contrived to construct a diving-bell on a smaller scale, and less expence, than that of Dr. Halley, and yet capable of answering the same intents and purposes. This bell, AB (fig. 7.) sinks with leaden weights DD, suspended from the bottom of it. It is made of copper, and tinned all over on the inside; three strong convex lenses GGG, defended by the copper lids HHH, illuminate this bell. The iron plate E serves the diver to stand upon, when he is at work; this is suspended by chains FFF, at such a distance from the bottom of the bell, that when he stands upright, his head is just above the water in the bell, where he has the advantage of air fitter for respiration, than when he is much higher up; but as there is occasion for the diver to be wholly in the bell, and consequently his head in the upper part of it, Mr. Triewald has contrived, that, even there, after he has breathed the hot air as long as he well can, by means of a spiral copper tube placed close to the inside of the bell, he may draw the cooler and fresher air from the lowermost parts; for which purpose a flexible leather pipe, about two feet long, is fixed to the upper end of the tube at b; and to the other end of the pipe is fastened an ivory mouth-piece, for the diver to hold in his mouth, by which to respire the air from below. We shall only remark, that as air rendered effete by respiration is somewhat heavier than common air, it must naturally subside in the bell; but it may probably be restored by the agitation of the sea-water, and thus become fitter for respiration. See Fixed Air. Phil. Trans. abr. vol. viii. p. 634. Or Desaguliers's Exper. Phil. vol. ii. p. 220, &c.

The famous Corn. Drebell had an expedient in some respects superior even to the diving bell, if what is related of it be true. He contrived not only a vessel to be rowed under water, but also a liquor to be carried in the vessel, which supplied the place of fresh air.

The vessel was made for king James I. carrying twelve rowers, besides the passengers. It was tried in the river Thames; and one of the persons in that submarine navigation, then living, told it one, from whom Mr. Boyle had the relation.

As to the liquor, Mr. Boyle assures us, he discovered by a physician, who married Drebell's daughter, that it was used from time to time, when the air in that submarine boat was clogged by the breath of the company, and rendered unfit for respiration: at which time, by unstopping the vessel full of this liquor, he could speedily restore to the troubled air such a proportion of vital parts, as would make it serve again a good while. The secret of this liquor Drebell would never disclose to above one person, who himself assured Mr. Boyle what it was. Boyle's Exp. Phys. Mech. of the Spring of the Air.

We have had many projects of diving machines, and diving ships of various kinds, which have proved abortive.

Diving-Bladder, a term used by Borelli for a machine which he contrived for diving under the water to great depths, with great facility, which he prefers to the common diving-bell. The vesica, or bladder, as it is usually called, is to be of brass or copper, and about two feet in diameter. This is to contain the diver's head, and is to be fixed to a goat's skin habit, exactly fitted to the shape of the body of the person. Within this vesica there are pipes, by means of which a circulation of air is contrived; and the person carries an air pump by his side, by means of which he may make himself heavier or lighter, as the fishes do, by contracting or dilating their air bladder: by this means, the objections all other diving machines are liable to are obviated, and particularly that of the air; the moisture by which it is clogged in respiration, and by which it is rendered unfit for the same use again, being here taken from it by its circulation through the pipes, to the sides of which it adheres, and leaves the air as sree as before. Borelli Opera Posthuma.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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DITTON (Humphrey)