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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 compressed 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 surface of it. One inconvenience that attends this condensed air 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 the ear; which, by degrees, grows painful, till, the force overcoming the obstacle, what constringes these pores yields to the pressure, and, letting condensed air slip in, ease presently ensues. The bell descending lower, the pain is renewed, and 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 situation of the diver almost covered with water.

To obviate the difficulties of the foregoing kind of diving bell, Dr. Halley contrived further apparatus, whereby 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. The manner in which this was effected, he relates in the following words:- The bell I made use of was of wood, containing about sixty cubic feet in its concavity, and was of the form of a truncated cone, whose diameter at the top was three feet and at the bottom five. This I coated with lead so heavy that it would sink empty; and I distributed the weight so about its bottom, that it would go down in a perpendicular direction, and no other. In the top I fixed a strong but clear glass, as a window, to let in the light from above, and likewise a cock to iet out the hot air that had been breathed; and below, about a yard under the bell, I placed a stage which hung by three ropes, each of which was charged with about 100 weight to keep it steady. This machine I suspended from the mast of a ship by a sprit, which was sufficiently secured by stays to the mast head, and was directed by braces to carry it overboard clear of the ship's side, and to bring it again within board as occasion required. To supply air when under water, I caused a couple of barrels, of about thirty-six gallons each, to be cased with lead, so as to sink empty, each of them having a bung-hole in its lowest part to let in the water, as the air in them condensed on their descent; and to let it out again when they were drawn up full from below. And to a hole in the uppermost part of these barrels I fixed a leathern trunk or nose, well liquored with bees' wax and oil, and long enough to fall below the bung-hole, being kept down by a weight appended so that the air in the upper part of the barrels could not escape unless the lower ends of these hose were first lifted up. The air-barrels

being thus prepared, I fitted them with tackle proper to make them rise and fall alternately, after the manner of two buckets in a well, which was done with so much ease, that two men, with less than half their strength, could perform all the labor required: and in their descent they were directed by lines fastened to the under edge of the bell, which passed through rings on both sides the leathern hose in each barrel; so that, sliding down by these lines, they came readily to the hand of a man who stood on the stage on purpose to receive them, and to take up the ends of the hose into the bell. Through these hose, as soon as their ends came above the surface of the water in the barrels, all the air that was included in the upper parts of them was blown with great force into the bell; whilst the water entered at the bung-holes below, and filled them and as soon as the air of one barrel had been thus received, upon a signal given, that was drawn up, and at the same time the other descended; and, by an alternate succession, furnished air so quick, and in so great plenty, that I myself have been one of five who have been together at the bottom in nine or ten fathom water, for above an hour and a half at a time, without any ill consequence: and I might have continued there as long as I pleased, for any thing that appeared to the contrary. Besides, the whole cavity of the bell was kept entirely free from water, so that I sat on a bench which was diametrically placed near the bottom, wholly dressed, with all my clothes on. I only observed that it was necessary to be let down gradually at first, as about twelve feet at a time, and then to stop and drive out the water that entered, by receiving three or four barrels of fresh air before I descended further. But being arrived at the depth designed, I then let out as much of the hot air that had been breathed, as each barrel would replenish with cool, by means of the cock 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, and to cover it with a white foam, notwithstanding the weight of the water over us. Thus I found that I could do any thing that required to be done just under us; and that, by taking off the stage, I 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 overshoes thereon. And, by the glass window, so much light was transmitted, that when the sea was clear, and especially when the sun shone, I could see perfectly well to write or read; much more to fasten or lay hold on any thing under us that was to be taken up. And, by the return of the air barrels, I often sent up orders, written with an iron pen on small plates of lead, directing how to move us from place to place, as occasion required. At other times, when the water was troubled and thick, it would be as dark as night below; but in such cases I have been able to keep a candle burning in the bell as long as I pleased, notwithstanding the great expense of air necessary to maintain flame. By an additional contrivance, I have found it not impracticable for a diver to go out of an engine to a good distance from it, the air being conveyed to him with a continued

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out very slowly, or the bell will rise to the top with so great velocity, that the divers will be in danger of being shaken out of their seats. But, by following these directions, every possible accident may be prevented, and people may descend to great depths without the least apprehension of danger. The bell also becomes so easily manageable in the water, that it may be conducted from one place to another by a small boat with the greatest ease, and with perfect safety to those who are in it. Instead of wooden seats used by Dr. Halley, Mr. Spalding made use of ropes suspended by hooks b, b, b; and, on these ropes, the divers may sit without any inconvenience. KK are two windows made of thick strong glass, for admitting light to the divers. N represents an air-cask with its tackle, and OCP the flexible pipe through which the air is admitted to the bell. In the ascent and descent of this cask, the pipe is kept down by a small weight appended, as in Dr. Halley's machine. R is a small cock by which the hot air is discharged as often as it becomes troublesome.

A considerable modern improvement is that of supplying air to a diving-bell, by means of a syringe or pump, which forces the air down in a continual stream into the bell, whence it escapes from beneath the lower edges of the bell, or from a waste pipe, as fast as it is supplied. In this way the air is kept very pure, and the people in the bell have no kind of trouble to obtain a supply. Mr. Smeaton was the first who put in practice the method to which we allude, though it had been frequently proposed by other inventors. His first attempt was in 1786, in shallow water, the bell being only intended to enable workmen to examine and repair the foundations of a bridge at Hexham, in Northumberland.

Mr. Smeaton, a few years afterwards, constructed another bell upon the same principle, for the works at Ramsgate harbour. It was used to raise up large stones, which had formerly been thrown into the sea around the base of the pier.

The bell was made of cast iron, of sufficient weight to sink without any extra ballast. In the top were lenses for the admission of light, and a strong shackle for the chain by which the bell was suspended. A strong leathern pipe was connected with the top of the bell, to convey air into it from an air-pump placed either in a boat or on the shore.

This kind of diving-bell has since been applied to the purposes of building foundations of masonry in deep water, under the direction of the late Mr. Rennie, who constructed machinery to move the bell under water in any direction, and which acts with such facility, that the masons in the bell make great despatch in laying the stones. It was used in Plymouth Sound to sweep the bottom for old anchors, &c. At Houth, in Dublin county, Ireland, the foundations for the pier were wholly laid by this machine. In many parts the rocky bottom was too uneven to work upon, and it was then necessary to blast it with powder. The divers bored the hole in the rock, and placed the powder in a tin cartridge, which was well secured in the hole, by running in small fragments of stone. A small tin pipe

was affixed to the canister, long enough to reach above the surface of the water. When all was prepared, the bell was drawn up out of the way, and a nail or other small piece of iron heated red hot, was dropped into the tin pipe, thereby to descend to the powder.

As the diving bell is, however, in any stage of improvement, necessarily very large and unwieldy, several attempts have been made to encase a man sufficiently to enable him to breathe and bear the pressure of the water. Among these the most successful is that of Klingert of Breslau, which is made of strong tin plate, in the form of a cylinder, which goes over the diver's head, and which consists of two parts, that he may conveniently thrust his arms through it and put it on; also a jacket with short sleeves, and drawers of strong leather. All these being watertight, and closely jointed round the body of the diver, secure every part of him, but his arms and legs, from the pressure of the water, which, at the depth of twenty feet, will occasion no inconvenience to these parts. Plate, DIVING BELLS, &c., fig. 4, represents the diver covered with the harness and drawers. Figs. 5 and 6 are representations of the cylinder, the diameter of which is equal to the breadth of a man at the top of the hipbone. It is fifteen inches in height, has a globular top, and is made of the strongest tin plate. In the inside of the cylinder, at a, is a strong broad iron hoop, to enable it to withstand better the pressure of the water; and in the inside of the top there are two pieces of a strong hoop of the same kind, placed over each other in the form of a cross at b; a strong ring of brass wire is soldered upon the outside at c, that the jacket may be fastened to it with an elastic bandage, to prevent it from slipping downwards; at dd are the upper halves of the apertures for the arms; and e, e, are holes to afford light, and into which the eye-glasses are screwed: fis the opening into which the mouth-piece of the breathingpipe is screwed, and g is an aperture for looking through, as well as for the purpose of breathing when out of the water, and which, by means of the cover h suspended from it, can be screwed up before the diver enters the water.

The lower part of the cylinder, which is also fifteen inches in height, is strengthened at i and k by iron hoops on the inside, in the same manner as the former. To the lower hoop k are soldered four small rings, to which are fastened strong leather straps, three inches in breadth, that can be buckled across over the shoulder, and support the whole machine; 1, 1, are the under halves of the apertures for the arms; m is also a ring of brass wire soldered to the cylinder, which serves to keep fast the jacket when buckled on, and to support the upper cylinder dd b, which slips over the under one; and on that account the under one is a little smaller, so as to fit into the upper one: there is also another such ring at n, in order to prevent the drawers from falling down.

At o is a strong semicircular piece of iron, the use of which is to prevent the drawers, when pressed by the water, from touching the under part of the body, otherwise the pressure, even at the depth of six feet, would be insupportable. As it is not possible to sew the leather so closely

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