Diving is the art of descending under water to considerable depths, and of remaining there some time, so as to be able to collect valuable articles, such as pearls (see Pearl Fishing), sponges, coral, and other submarine productions, from the bottom of the sea or rivers, or property from the sunken wrecks of vessels.

This art is one of great utility, but is attended with peculiar difficulties, owing to the very limited powers which man naturally possesses within the liquid element. On the surface he may no doubt continue a long time floating or swimming, and hence arises the wonderful art of navigation. But the moment he plunges within the mass he is cut off from the vital air, and life is speedily extinguished. The necessity of a constant supply of air for the support of life is shown by simply attempting to withhold it by shutting the mouth and nostrils. No one can continue holding in the breath in this manner much longer than a minute or a minute and a half. If we begin to hold after having made an expiration, we cannot do it longer than a quarter of a minute; but if we take a large inspiration, and fill the lungs, this supply is found to last longer; so that we can readily hold breath a full minute, and, with practice and great exertion, some may even continue to do so two minutes. Now this is exactly what the diver must do to remain alive under water; and accordingly we find that, in general a person cannot remain longer than half a minute without the danger of suffocation, and the most practised divers not above two minutes; such is the necessity for fresh air continually present in the lungs.

The nature and cause of this necessity for air has been illustrated by the discoveries of modern chemistry. These have proved that it arises from a certain chemical action which the atmosphere exerts on the blood as it passes through the lungs, and which is continually going on, and cannot for a moment be intermitted. The nature of this action is not yet exactly understood, but the object of it undoubtedly is to purify the blood, as it becomes vitiated by circulating through the system. For this purpose, the air inspired into the lungs, and coming there into contact with the blood, imparts to it its oxygen, a small portion of which is supposed to combine with the blood, and to give it renewed vigour; but by far the greater portion combines with the carbonaceous matter of the blood, and carries off this impurity in the shape of carbonic acid at each expiration. This is proved by a very simple experiment. Let a person, for instance, respire by means of a pipe into a bag or bladder of air of the capacity of a gallon or more; he will breathe freely enough at first, but in a very short time with great difficulty, and at last will feel the sense of suffocation the same as in holding the breath in the ordinary way. If the air in the bladder be now examined, it will be found to have entirely changed its nature; it will no longer support the flame of a candle, but extinguish it, the moment it is immersed, thus showing the loss of oxygen. Hence arises that sense of closeness and oppression which is felt in crowded assemblies, where, as generally happens, the ventilation is imperfect. The same air being breathed again and again, becomes unfit for respiration, and produces those unpleasant sensations which are usually felt. A very curious and interesting set of experiments on respiration were made by Messrs Allen and Pépys, and narrated in the Philosophical Transactions for 1808. The following bears particularly on the present subject.

"Three hundred cubic inches of common air contained in one of the mercurial gasometers were respired. In less than a minute it became necessary to take deeper and deeper inspirations, and at last the efforts were so violent that the glass was in danger of being broken. A great sense of oppression and suffocation was now felt in the chest; vision became indistinct, and after the second minute the attention of the operator seemed to be withdrawn from surrounding objects, and fixed upon the experiment. A buzzing in the ears took place, as in breathing nitrous oxide; and after the third minute there was left only sufficient recollection to close the gasometer after an expiration; after which he became insensible, having made thirty-five inspirations. The expired air contained ten per cent. of carbonic acid, four of oxygen, and eighty-six of azote."

With 300 cubic inches of air, then, in the gasometer, the Quantity operator began to be insensible in the space of two minutes; and if we suppose that the lungs, which were in their natural state at the commencement of the operation, contained 100 cubic inches of atmospheric air, then it would follow that 200 inches each minute would be necessary to support life, so as to remain at the same time quite sensible. Hence, supposing the lungs to contain, with a full inspiration, 250 cubic inches, which is a pretty large allowance, it would follow that a man might hold breath, or remain under water, a minute and a quarter, which agrees very well with what occurs in ordinary cases. But a very curious fact has been mentioned to us by Professor Faraday of the Royal Institution, London, and was first noticed to him by a gentleman connected with the Asiatic Society, a fact which may often be of great importance, not only in diving, but in cases of fire, and of accidents in brewers' vats, &c. The lungs in their natural state are charged with a large quantity of impure air, being a portion of the carbonic acid gas which is formed during respiration, but after each expiration still remains lodging among the involved passages of the pulmonary vessels. In proof of this, it is only necessary to breathe by a small pipe, or roll of paper, into a common water bottle, throwing away the first portion of the expiration, and propelling the last into the lower parts of the vessel. Then insert this over a taper, and it will be instantly extinguished. Now, by breathing hard for a short time, as one does after taking any violent exercise, this impure air is expelled, and its place is filled up with atmospheric air. The consequence is, that if we then take a full inspiration, the breath can easily be held for two minutes. This experiment any one can make. On trying it in the ordinary way, we could hold breath for about three quarters of a minute, but this with great difficulty. We then made eight to ten forced respirations, and on closing the mouth and nostrils felt no inconvenience even on the first trial, till after a minute and a half, but continued, however, to the end of the second minute. The knowledge of this fact might be of essential use in diving, and, we have no doubt, might often be the means of saving life; for if in the ordinary way we can only remain a single minute under water, of what importance is it to be capable of doubling the time? A single minute in these cases must be invaluable. Whether the professed divers are aware of this circumstance or not, we do not know; but it is probable, at any rate, that in many cases the exertion induced by swimming may have the effect of clearing the lungs. Another curious fact illustrative of the same principles occurred to Mr Brunell in descending to examine the breach which the river had made in the Tunnel under the Thames. Having lower- Diving.

Diving, ed the diving-bell nearly thirty feet to the mouth of the opening; this was found too narrow to admit the bell, so that no further observation could be made on the state of the Shield and other works, which were perhaps eight or ten feet deeper. Brunell, therefore, laying hold of the end of a rope, left the bell, and dived himself down the opening; his companion in the bell being alarmed at the length of his stay; now about two minutes, gave the signal for pulling up; and the diver, unprepared for the signal, had hardly time to catch hold of the rope which he had let go, and was surprised on coming up to find that so much time had elapsed. On descending again, he found that he could with ease remain fully two minutes under water. The reason evidently was, that the atmosphere in the bell being condensed by a column of water nearly thirty feet in height, contained nearly double the quantity of air in the same bulk, and thus nearly a double supply in the lungs.

Besides the difficulty of holding the breath, another arises in diving, particularly at considerable depths, from the external pressure of the fluid on the chest, and on every cavity of the body. On the chest this tends to compress it together, and to expel the air out of it, and thus increases greatly the difficulty of holding the breath. At each foot of descent this pressure will increase upwards of sixty lbs. on every square foot of the body; and if we suppose the chest to expose half a square foot, we have, at the depth of fifteen feet, a force equal to the weight of 450 lbs. loading the chest, and tending to propel the included air. A very great muscular exertion, therefore, will evidently be required to resist this enormous strain; nor is it practicable, by any breastplate or other contrivance, to defend the chest from this pressure, as this, to do any good, would require to be so large, and of such strength, as greatly to obstruct the free motions of the diver. It is this pressure of the deep water, and the violent exertion necessary to overcome it, that causes, in divers who go down frequently, the eyes to become blood-shot, and brings on a spitting of blood.

The art of diving having always in it, and particularly during the infancy of science, something of the marvellous, the most extraordinary accounts have been given, by different authors, of the feats of some of the most noted divers. The most singular of these is that given by Kircher, of the Sicilian diver Nicola Pese, taken, as he states, from the archives of the kings of Sicily.

"In the times of Frederick king of Sicily," says Kircher, "there lived a celebrated diver, whose name was Nicholas, and who, from his amazing skill in swimming, and his perseverance under water, was surmamed the fish. This man had from his infancy been used to the sea, and earned his scanty subsistence by diving for corals and oysters, which he sold to the villagers on shore. His long acquaintance with the sea at last brought it to be almost his natural element. He was frequently known to spend five days in the midst of the waves, without any other provisions than the fish which he caught there, and ate raw. He often swam over from Sicily into Calabria, a tempestuous and dangerous passage, carrying letters from the king. He was frequently known to swim among the gulfs of the Lipari islands, noway apprehensive of danger.

"Some mariners out at sea one day observed something at some distance from them, which they regarded as a sea monster; but upon its approach it was known to be Nicholas, whom they took into their ship. When they asked him whither he was going, in so stormy and rough a sea, and at such a distance from land, he showed them a packet of letters which he was carrying to one of the towns of Italy, exactly done up in a leather bag, in such a manner as that they could not be wetted by the sea. He kept them thus company for some time in their voyage, conversing and asking questions; and after eating a hearty meal with them, he took his leave, and, jumping into the sea, pursued his voyage alone.

"In order to aid these powers of enduring in the deep, nature seemed to have assisted him in a very extraordinary manner; for the spaces between his fingers and toes were webbed, as in a goose; and his chest became so very capacious that he could take in, at one inspiration, as much breath as would serve him for a whole day.

"The account of so extraordinary a person did not fail to reach the king himself, who commanded Nicholas to be brought before him. It was no easy matter to find Nicholas, who generally spent his time in the solitudes of the deep; but at last, after much searching, he was found and brought before his majesty. The curiosity of this monarch had been long excited by the accounts he had heard of the bottom of the Gulf of Charybdis; he now therefore conceived that it would be a proper opportunity to have more certain information. He therefore commanded our poor diver to examine the bottom of this dreadful whirlpool, and, as an incitement to his obedience, he ordered a golden cup to be flung into it. Nicholas was not insensible of the danger to which he was exposed, dangers best known only to himself, and therefore he presumed to remonstrate; but the hopes of the reward, the desire of pleasing the king, and the pleasure of showing his skill, at last prevailed. He instantly jumped into the gulf, and was instantly swallowed up in its bosom. He continued for three quarters of an hour below, during which time the king and his attendants remained on shore anxious for his fate; but he at last appeared, holding the cup in triumph in one hand, and making his way good among the waves with the other. It may be supposed he was received with applause when he came on shore; the cup was made the reward of his adventure; the king ordered him to be taken proper care of; and, as he was somewhat fatigued and debilitated by his labour, after a hearty meal he was put to bed, and permitted to refresh himself by sleeping.

"The diver then, according to the account, gave a narrative of the wonders he had seen, which so excited the curiosity of the monarch, that he again tempted the diver to a second and fatal descent. After plunging into the whirlpool, he was never more heard of.

"But to return to more authentic statements, these authors agree very well with the views already stated. Among the pearl divers at Ceylon and other parts of the East, instances have been known of a diver remaining six minutes under water; but these are very rare; the ordinary time seldom exceeds a minute, and sometimes it is a minute and a half, or two minutes. There are generally ten divers in each of the boats belonging to the fishery; five descend into the sea at a time, and the other five remain above to recruit their strength. In order to hasten their descent, a large stone is used, with a rope attached to it, which the diver seizes with the toes of his right foot, while he grasps a bag of net-work with those of the left. He then seizes another rope with his right hand, and keeping his nostrils shut with his left, plunges into the water, and soon reaches the bottom. Then hanging the net round his neck, he speedily collects the oysters, and resuming his former position, he makes a signal to those in the boat, and is immediately hauled up, and the stone which assisted his descent is pulled up afterwards.

"The divers are all Indians, who are accustomed to this seemingly dangerous occupation from their infancy, and who fearlessly descend to the greatest depths. They will frequently make from forty to fifty plunges in a day; but the exertion is so extremely violent, that in coming up they Diving.

Discharge water, and sometimes blood, from their mouths, ears, and nostrils. Some of them rub their bodies with oil, and stuff their ears to prevent the water from entering; but the greater part use no precautions whatever. They take no food while in the boats, nor till they return on shore and have bathed themselves in fresh water. The only danger to which they are exposed is from meeting, while at the bottom, with the ground-shark, which is a common inhabitant of those seas, and of which the divers are under dreadful apprehensions; some of them indeed are so expert as to avoid this enemy, even when they remain under water for a considerable time; but the uncertainty of escaping is so great, that, in order to avert the danger, they consult, before they begin, their priests or conjurers, in whom they place implicit confidence.

Dr Hallet relates, as a remarkable circumstance, that he observed a Florida Indian diver at Bermuda, who could remain two minutes under water. He states, that the divers for sponges in the Archipelago are in the practice of taking down in their mouths a piece of sponge dipped in oil, and by this are enabled to dive longer than others who have none. It is not easy to conceive how this can assist the diver's breathing; for the introduction of any foreign substance into the mouth must necessarily diminish the quantity of air he can take down. But it has been lately said that the real object of taking oil in their mouth is to calm those small waves on the surface of the sea which prevent the light being so steadily transmitted to the bottom as is necessary to enable the divers to find the small objects they search for without delay. By ejecting a little oil from their mouths, it rises to the surface, and spreading upon it, calms the waves in a most remarkable manner, and gives a brilliant light at the bottom.

Many nations, and particularly the savages in the South Sea and other islands, are remarkable for the expertness they acquire by habit in diving and moving about in the water. Being accustomed to it from their infancy, the element becomes so natural to them that they seem to have the use of all their faculties in the water the same as on the dry land. According to the accounts of voyagers, they are such expert divers, that when a nail or other piece of iron was thrown overboard, they would instantly jump into the sea after it, and never fail to recover it. On one occasion a smith's anvil is said to have fallen overboard. Not being able to bring this up, the islanders notwithstanding contrived to bring it ashore, by descending a great many times to the bottom, and rolling it over and over till it reached the land.

Such is the length to which diving has been carried by the natural powers of the body alone. But from the curious and difficult nature of the object, and the many important purposes to which the art might be employed, ingenious men were led to the invention of various contrivances for the use of the diver, which have greatly extended his powers and the usefulness of the art. A multitude of these contrivances of different descriptions have been brought forward by mechanical projectors for the last two hundred years. They all resolve themselves into three different kinds.

1st. Water-tight armour or dresses for the body, so strong as to protect it from the external pressure of the fluid; and, along with this, the means of supplying the diver with fresh air, so as to enable him to remain any time under water.

2ndly. Water-tight vessels of metal for inclosing the diver, and of such capacity as to contain a supply of air for a limited period of perhaps half an hour or an hour or more, and giving him also the use of his hands and arms externally by a sort of flexible sleeves.

3rdly. The diving-bell, which, from its simplicity, safety, and perfect efficiency, has now almost entirely superseded every other, though there is no doubt that in many cases these may still be of considerable utility in subservience to the bell.

In regard to dresses or armour, a number of different Water-plans of this kind are detailed in Leopold's Theatrum machinarum Hydrolicarum. At depths of twelve or fifteen feet these may often be of essential use; but beyond this they become inapplicable, owing to the great pressure on the limbs of the diver, which must either be exposed, or covered only with a flexible material, not to impede his motions; and in that case the pressure, acting on all sides like a ligature, is liable to obstruct the circulation of the blood in the limbs, and to drive it from these into those parts of the body within the armour, causing extreme pain. In any great depth, also, the necessary strength of the armour renders it unwieldy; and it is extremely difficult, if not impracticable, to fit it tightly on every part; while the smallest opening, by admitting water, may endanger the life of the diver.

One of the best of these contrivances is perhaps that Klingert's proposed by M. Klingert, and described in a pamphlet published at Brestan in 1798. The harness or armour is made of strong tin-plate, in the form of a cylinder, with a round end to inclose the head and body, and, for the convenience of putting it on, is made in two parts, the head-piece or helmet, and the body. Besides this, there is a leather jacket, with short sleeves, and a pair of drawers of the same, which are made water-tight, buttoned on the metal part where they join, and made tight with brass hoops, going round the leather and the metal upon the outside. The chief peculiarity in this machine is the mode in which fresh air is supplied, and respiration effected. This is done by two distinct flexible pipes proceeding from the inside of the helmet to the surface of the water; the one is for inhaling the air, and terminates in an ivory mouth-piece, which the diver may embrace with his lips and inhale the air; the other enters the helmet at the same place, and opens merely into the inside of the machine, so as to allow the foul air to be discharged. The diver, therefore, draws in the fresh air by the mouth, and discharges it into the helmet by the nostrils; and from the interior of the machine it is propelled by the act of inspiration, the expansion of the chest contracting the space between it and the armour, and forcing out exactly as much air as is drawn in, keeping up always a due equilibrium. This is certainly a very ingenious arrangement; for, if there were no second pipe to discharge the air, the expansion of the chest would compress the air round the body of the diver, and, unless this were of large capacity, which would be inconvenient, would create a difficulty in the operation. The construction of the apparatus will be understood from the drawing, fig. 1, Plate CLXXXIX., which is a front view of the diver, and by the following description: A is the helmet-piece, fifteen inches in height, and the diameter adapted to the size of the body, of the diver; BB is the lower part of the cylinder, of the same diameter, and of such a height as to meet the other at the dotted line C; ddC is the jacket, and ffE the drawers; these are attached to the cylinder by buttons, as seen; and a, c, bb are the three brass hoops fitted over each joint to make it water-tight; the hoops are made of brass-plate, with their ends turned up, and fitted with screws, by means of which they can be drawn very tight upon the leather. The cylinder has holes for the arms, one half in the upper piece and one half in the lower; and when the jacket is fastened on, it binds the upper and lower parts of the cylinder together. It is fastened at the arms with brass screw hoops, dd, and the Divings drawers by similar ones at f; k k represent the breathing pipes, the first for drawing in the air, the second for discharging it; these are united to a little metal cylinder, which screws on the helmet at the aperture g; this is shown more particularly at fig. 2, where a partition will be observed in the cylinder dividing the fresh air compartment from the other, the one terminating in the ivory mouth-piece v, the other just entering the machine at t. W is a small reservoir at the lower part of the pipes, for condensing any air, or receiving what may penetrate through the pipes. To resist the external pressure of the water on the limbs, the leather drawers have a framing of iron within them, represented at fig. 3; this consists of a semicircular piece u, also seen at u, fig. 1, extending between the legs of the diver, and fastened to the lower extremity of the cylinder at the front and back; also two irons m outside the thighs, which are joined to the cylinder, and extend down to f, where they are attached to a hoop surrounding the thigh; there is another hoop for each thigh further up at q; these hoops are farther connected by irons, which at the upper end are fitted to slide upon the semicircular hoop, as at t; and by this means, though the frame-work is very strong, the diver is at liberty to walk. Now are weights hooked on the cylinder, to keep the diver down. P is a small pump for discharging any leakage water which may penetrate through the joints.

When the different parts of the machine have been fitted to the body of the diver, and the proper weights are attached, he enters the water till it rises as high as his eyes, while the end of the pipe is held by an assistant above the surface; and if he finds that he can breathe freely, and no water is forced into the pipe, he may venture to go deeper; and, stopping for some time, to ascertain whether respiration be not inconvenient from the want of fresh air, he may advance to still greater depths, while he makes the proper signals by means of the rope which is secured to one of his arms, or by speaking through the pipe. By this kind of exercise for some time, the diver acquires confidence and ease for conducting the necessary operations. When he is desirous of ascending he has only to unhook the weights attached to the apparatus, or to fix them to a rope let down for the purpose, that they may not be lost, and as he is then lighter than the same bulk of water, he rises to the surface.

By following these directions, any one may be able to use the apparatus, and dive to moderate depths, in a very short time. In one of the trials upon the Oder, near Breslau, the diver was a huntsman taught by the author; the water was of considerable depth, and the current strong, and there were a great number of spectators present. He sawed through the trunk of a tree which was lying at the bottom; he showed also that he could have fastened sunk bodies to a rope in order to be drawn up, and that in case any impediment should prevent the use of the saw, the trunks of trees might be hewed to pieces by an axe. On the whole, this apparatus, or one similar, might certainly be of great use in many cases, particularly in hydraulic works, where the diving-bell and the machinery connected with it might not be attainable. The water-proof cloth of Mackintosh might also be substituted with good effect for the leather.

Apparatus Another mode of supplying air to the diving apparatus by Tonkin has been adopted in some cases. This consists in forcing the fresh air into the machine by a bellows or pump, till its elastic force is equal to the pressure of the water. The foul air may in this case be suffered to escape into the water through a valve, or may be conducted to the surface by a pipe. Of this kind is the apparatus contrived by Mr Tonkin, and employed for some time in raising parts of the wreck of the Abergavenny East India ship, which was unfortunately lost off Weymouth in 1804. It consisted of a body of copper with iron boots, put together and jointed in the manner of coats of mail; the whole is then covered with leather, and afterwards with canvas to distinguish it under water. The arms are made of strong water-proof leather; and the place for sight is about eight inches diameter, glazed over with a plate of glass an inch thick. The diver is sunk in this machine by means of weights, fastened equatorially round the waist of it; and he is suspended by a rope, by means of which his situation is changed at pleasure. A flexible air-tube communicates with an air-vessel in the boat above. Through this tube the diver gives his instructions and obtains his supply of fresh air. This machine was used with very good effect in a depth of water of near seven fathoms, and enabled the diver to direct the operations of several curious machines, such as saws for clearing away the ship's decks, and making sufficient openings to give him access to the treasure below, as well as tongs, &c. for taking up the heavy goods by tackle in the vessel above.

In regard to the second kind of diving machines, that Borelli proposed by Borelli is only curious as showing the low diving state of physical knowledge in his time. He proposed to have a copper vessel, or vesica as he terms it, about two feet diameter, to contain the diver's head, and to be fixed to a habit of goat skin for the body. Within the vessel there were pipes contrived to produce a circulation of air, by which Borelli supposed that the objections to other diving machines from the want of air would be obviated; "the moisture," as he says, "by which it is clogged in respiration, and by which it is rendered unfit for the same use again, being taken from it by its circulation through the pipes, to the sides of which it would adhere, and leave the air as free as before." It also contained an air-pump, by means of which the diver could raise or lower the apparatus, by condensing or rarefying the air, on the principle of the air-bladder of fishes.

Mr Martin, in his Philosophia Britannica, mentions an apparatus contrived by an Englishman, consisting of strong divin leather, so prepared that no air could pass through. It fitted to his arms and legs, and had a glass window placed in the fore part of it. When dressed in this apparatus, which was large enough to contain half a hogshead of air, he could walk on the ground at the bottom of the sea, and enter the cabin of a sunk ship to take out the goods. The inventor is said to have himself used this machine very extensively in recovering wrecks, and with such success as to have acquired considerable property by it. We are not informed of the depths to which he descended.

Mr Klingert, the inventor of the water armour, also contrived a diving chest, of the form of a hollow cylinder, to be used along with it. This contained fifty-eight cubic feet of air, which, he estimated, would last two hours. It was suspended from a boat, but could be raised and depressed independently of this by a pump compressing or dilating the included air. Thus the ballast is so adapted to the size of the machine, as to make it sink so far that only a cubic foot of it remains above water. In this state an additional weight of a hundred pounds will depress it below the surface, or make it sink to the bottom. The effect of adding extra weights is produced by diminishing the volume of contained air, by condensing it into a smaller space. To accomplish this, a large cylinder is applied in the bottom of the vessel, and provided with a piston, which, by a rack and pinion, can be moved from one end of the cylinder to the other, when the diver turns a handle, coming through the side of the machine, and communicating motion by a worm and wheel to the pinion of the rack before mentioned. The lower end of the cylinder is open to the water, and the upper end opens within the machine; therefore, when the diver turns the handle in the direction to raise up the piston in its cylinder, it necessarily diminishes the bulk of the included air, and the machine will sink; but on depressing the piston in the cylinder, it will ascend again. The inventor proposed to furnish the machine with two small oars to move it in the water, and an anchor or grapnel to make it fast whilst the diver walks about on the bottom, within the limits of the length of the pipe, to examine sunk bodies, and discover the best mode of raising them. To prevent danger from any accident happening to the machine, the diver is to be provided with the means of quickly detaching the pipes from the machine, and retaining a sufficiency of air in the armour to carry him to the surface when he throws off the weight suspended from his girdle.

Another diving machine or chest was invented by Mr Rowe in 1753, and is represented in Plate CLXXXIX, fig. 4. It consists of a trunk or hollow copper vessel AB, soldered or riveted together with strength proportioned to the depth of water where it is to be fixed. It contains the diver's body, and also a sufficiency of air for the time he intends to dive. He enters with his feet first at the open end A, which is then closed by a lid or cover screwed on by a number of screw bolts passing through the flanges. The vessel is bent at F, for the bearing of the diver's knees, and has a sufficiency of leaden ballast at B to sink it in the right position. There are two hoops surrounding it, which, at the same time that they strengthen it, afford points of suspension by a bar, which is attached to them, and is pierced with several holes to admit a span upon the rope, which is so adjusted as to suspend the whole, with the diver in it, nearly in the position of the figure, when he will be in a convenient posture for working with his arms, which come through openings C in the vessel, to which sleeves E, of very strong leather, are attached by a hoop or ring, screwed to the vessel with the leather between them. The sleeves are lined with cloth, and the edges round the holes are defended by soft quitting, from hurting the diver's arms by the pressure, as well as to prevent the sleeves and his arms being thrust inwards. D is an aperture covered by a strong lens, for the diver to see through. At H and G are two other openings in the upper part of the vessel, covered by screw caps, which are removed when fresh air is to be introduced into the machine by the nose pipe of a pair of bellows being applied to force fresh air into one, and drive out the foul air at the other. The lower opening is also of use to pump out any water which may leak through at the joints, though this is as much as possible prevented by fitting leather into the joints of the cover and the caps before they are screwed tight. The mass of lead F is fastened to the lower side of the vessel in a line between the diver's arms, by means of hoops. On this the whole rests if it comes to the ground, and remains in a proper position for the diver to work, and fasten ropes to anything which is to be drawn up, as shown in fig. 5.

If the water be very deep, the diver must wear a kind of saddle on his back, which, having a ridge touching the top part of the vessel withinside, enables him to keep his arms properly out of the apertures, otherwise he would not have strength to resist the pressure acting upon the surface of the arms and sleeves, which forces them into it with a weight proportional to the quantity of surface exposed, and to the depth of water. The diver gives his instruction to those above by a small line, which is laid through a staple at the side of the machine, and has a handle always hanging in reach of the diver's hand. The upper part of this line is held by a person in the boat or ship above, to whom any signal is given, by the diver matching or twitching the line a certain number of times, as has before been agreed upon. This is immediately felt by the person above, who gives orders accordingly. The size of the vessel is such that he can continue at the bottom about half an hour, without any pipes or other supply, and will be enabled to do many things very readily, such as recovering moorings, chains lost in rivers or harbours, hooking ropes for weighing up lost anchors, or any other purpose where there is free access to the object sought; though in entering and searching the wrecks of ships, it would be less convenient than some others which we shall describe.

Besides the above, several other projects of a similar kind have been proposed, not only with means within itself of raising and lowering the vessel, but with contrivances in the shape of screw arms for moving it when under water in any direction; but none with much success. This is said to have been tried in the reign of King James I. by a famous English projector, Cornelius Drebbel, who, we are told by Mr Boyle, made a submarine vessel, which would carry twelve rowers besides the passengers; and that he had also discovered a liquid which had the singular property of restoring the air when it became impure by breathing. This last circumstance, with the number of persons inclosed in the machine, and the imperfect state of mechanics at the period alluded to, render the whole story extremely improbable, though it shows clearly that the idea had been entertained, and perhaps some attempt made. The celebrated Bishop Wilkins, in his Mathematical Magic, takes up the scheme of Drebbell, and, with all the sanguine facilities of a projector, describes the benefits of these submarine enterprises. The submarine vessel of Mr Bushnell of Connecticut, in America, constructed in 1787, though very complex, appears to have been a curious and ingenious machine, and to have promised success if persevered in, according to the accounts published of it. It was intended to act chiefly as an engine of war, by advancing under water towards an enemy's ship, and fixing in the bottom of it a magazine of powder, which, by peculiar contrivances, was intended to take fire after the machine had got to a sufficient distance to be out of danger. But if this be the only use of such a machine, its failure need not be regretted. Let us now turn, then, to the most important of all diving machines yet contrived, namely,

The Diving-Bell.

The principle of the diving-bell is extremely simple. General Let any one insert a wine glass in a tumbler of water; on principles, sinking it to the bottom, the inside of the glass will be observed to remain nearly full of air, so that any small object within the glass will remain perfectly dry, the included air being confined on all sides, and by its impermeability excluding the water from its place. If this experiment be made with a pretty large bell-glass, inverted over a taper floating on the surface of the water in a still larger vessel, the taper will be observed to descend with the glass to the bottom; and though surrounded on all sides with water, it will be found to remain perfectly dry, and to continue burning for some time. Conceive then a vessel of wood or metal, in the shape of a wine-glass or truncated cone, but so large as, when inverted, to admit several persons within it, sitting, for instance, on a board along one of the sides. Let the whole then be suspended by a rope or chain over the side of a vessel, with a jib pulley and crane, to lower or raise the machine at pleasure. Then, on the machine being lowered and loaded with sufficient weight to sink it, the persons may all descend to a great depth in the sea, without being wetted in the small- est degree; and there is nothing to prevent them remaining any time in this situation, and moving about and doing operations at great depths.

The above, then, was the original construction of the diving-bell; and the great advantage of it, and what distinguishes it above every other similar invention, and renders it vastly superior, is, that being perfectly open below, the divers can get out and in with the utmost facility. This invention, according to Professor Beckmann, is generally assigned to the sixteenth century; and "I am of opinion," says he, "that it was little known before that period. We read, however, that in the time of Aristotle divers used a kind of kettle, to enable them to continue longer under the water; but the manner in which it was employed is not clearly described. The oldest information which we have of the use of the diving-bell in Europe is that of John Taisnir, who was born in Hainault in 1509, and had a place at court under Charles V., whom he attended on his voyage to Africa. He relates in what manner he saw, at Toledo, in the presence of the emperor and several thousand spectators, two Greeks let themselves down under water, in a large inverted kettle, with a burning light, and rise up again without being wet. It appears that this art was then new to the emperor and the Spaniards, and that the Greeks were caused to make the experiment in order to prove the possibility of it."

"When the English in 1588 dispersed the Spanish fleet called the Invincible Armada, part of the ships went to the bottom, near the Isle of Mull, on the western coast of Scotland; and some of these, according to the account of the Spanish prisoners, contained great riches. This information excited, from time to time, the avarice of speculators, and gave rise to several attempts to procure part of the lost treasure. In the year 1665, a person was so fortunate as to bring up some cannon, which, however, were not sufficient to defray the expenses. Of these attempts, and the kind of diving-bell used in them, the reader will find an account in a work printed at Rotterdam in 1669, and entitled G. Sinclari Ars novæ et magna gravitatis et levitatis. In the year 1680, William Phipps, a native of America, formed a project for searching and unloading a rich Spanish ship sunk on the coast of Hispaniola; and represented his plan in such a plausible manner, that King Charles II. gave him a ship, and furnished him with everything necessary for the undertaking. He set sail in the year 1683; but being unsuccessful, returned again in great poverty, though with a firm conviction of the possibility of his scheme. By a subscription, promoted chiefly by the Duke of Albemarle, the son of the celebrated Monk, Phipps was enabled, in 1687, to try his fortune once more, having previously engaged to divide the profit according to the twenty shares of which the subscription consisted. At first all his labour proved fruitless; but at last, when his patience was almost entirely exhausted, he was so lucky as to bring up, from the depth of six or seven fathoms, so much treasure, that he returned to England with the value of L200,000. Of this sum he himself got about sixteen, others say twenty thousand, and the duke ninety thousand pounds. After he came back, some persons endeavoured to persuade the king to seize both the ship and the cargo, under a pretence that Phipps, when he solicited for his majesty's permission, had not given accurate information respecting the business. But the king answered, with much greatness of mind, that he knew Phipps to be an honest man, and that he and his friends should share the whole among them, had he returned with double the value. His majesty even conferred upon him the honour of knighthood, to show how much he was satisfied with his conduct. We know not the construction of Phipps's apparatus; but of the old figures of a diving-machine, that which approaches nearest to the diving-bell is in a book on fortification by Lorini; who describes a square box bound round with iron, which is furnished with windows, and has a stock affixed to it for the diver. This ingenious contrivance appears, however, to be older than that Italian; at least he does not pretend to be the inventor of it.

"In the year 1617, Francis Kessler gave a description of his water-armour, intended also for diving, but which cannot really be used for that purpose. In the year 1671, Witsen taught, in a better manner than any of his predecessors, the construction and use of the diving-bell; but he is much mistaken when he says that it was invented at Amsterdam. In 1679 appeared, for the first time, Borelli's well-known work De Motu Animalium; in which he not only described the diving-bell, but also proposed another, the impracticability of which was shown by James Bernoulli. When Sturm published his Collegium curiosum in 1678, he proposed some hints for the improvement of this machine, on which remarks were made in the Journal des Sciences." The diving-bell, as hitherto used in the above simple form, is liable to two great defects, viz:

1. The elasticity of the included air prevents it from resisting entirely the entrance of the water into the lower part of the bell. The water, by the universal law of fluids, presses the bell on all sides, in proportion to the depth of the immersion. This pressure therefore exerts upwards on the bottom of the bell, and against the included air; but the air being extremely compressible, yields to the pressure, and is contracted into a smaller volume, allowing the water to enter and occupy the lower portion of the bell. Such is the effect of this pressure, that at the depth of thirty-three feet the air becomes compressed into half its volume, and the bell fills half full of water; and the same proportion at every other depth.

But,

2. The air within the bell, by continued respiration, becomes speedily unfit to support life; and the whole apparatus therefore must be raised from time to time, to receive a fresh supply. Suppose that only two persons descend in the bell at a time, we have seen that a supply of two hundred cubic inches of air per minute is absolutely necessary for each person to keep in life and sensibility. But in order to breathe freely, at least double that quantity would be required; say for two persons half a cubic foot per minute. If then we have a bell six feet long, and four feet average diameter, this would contain about seventy cubic feet, and would last upwards of two hours. So that for at least one hour or more respiration might be carried on with all manner of freedom.

At great depths, such as twenty, thirty, forty, and sixty feet, where the usual pressure on the body from the atmosphere above is doubled and tripled, amounting in the latter case to nearly forty pounds in every square inch, one would imagine that respiration, and indeed the whole system of the body, would be deranged under so thick and confined an atmosphere. But experience proves that no great inconvenience arises from this circumstance; and the reason is, that the air pressing into every cavity within the body, as well as externally, the pressure is exactly balanced; so that the effect of the actual increase is rendered nearly insensible. The only particular sensation felt pain in descending in the bell is a severe pain in the ears, particularly at first. This increases a little as we descend, but, after resting at the bottom, goes entirely off. It arises from the effect of the condensed air acting externally on the tympanum of the ear, before the air within the tympanic cavity has acquired the same density to counterbalance it. The tympanum on the outside communicates directly with the atmosphere, the pressure of which therefore acts instantaneously. But on the inside the tympanum bounds the tympanic cavity; and this has no communication with the external air, excepting by the Eustachian tube, which leads from the cavity into the mouth. Through this tube, therefore, the condensed air must pass from the mouth, to supply what is necessary within the cavity for restoring the same equilibrium within and without. But the Eustachian tube is a long and narrow passage; at its commencement in the ear it has a bony structure, but towards its termina- tion in the mouth, behind the nostrils, it becomes soft and fleshy, so as readily to close the passage, particularly with any pressure acting externally. It admits therefore an easy passage from the ear to the mouth; but when any pressure arises in the opposite direction, it acts in some degree like a valve, shutting the passage, until the increas- ing pressure again forces it open. Some time then elapses before all this can be accomplished; and during this time the external air pressing with full force on the tympanum, produces the pain which is felt. When the Eustachian tube opens, it is generally all of a sudden, and with a slight explosion or pop, which is followed by instant relief from the pain. This relief may often be produced by fill- ing the mouth, or gulping the air and pressing it into the tube.

Different accounts have been given of this effect on the ears in the diving-bell; but the above seems the most ac- curate, and what really takes place. The effect, indeed, may be shown experimentally by shutting the mouth and nostrils, and exhausting the air from them by the action of the lungs. The air in the tympanic cavity immediately rushing through the Eustachian tube into the mouth, the external air acts on the tympanum, and produces a slight sensation of deafness, such as is felt in the bell. But, in- stead of exhausting the air, attempt to compress it, and force it through the tube into the internal ear; at first no effect is produced; but after exerting a considerable pres- sure, a slight pop is felt, and a little pain in the ear, which is just the sudden opening of the tube.

The great inconveniences of the diving-bell already mentioned were completely removed by the labours of the celebrated and ingenious philosopher Dr Halley, who about the year 1715 introduced the grand improvement of supplying it with fresh air for any length of time without raising the bell out of the water. This he effected by letting down from the vessel from which the bell was sus- pended, barrels of fresh air, which, by means of pipes, dis- charged their contents into the bell; while the foul air escaped by a small cock in the top of the bell. In this manner the air within the bell was kept perfectly fresh, and for any length of time. Another remarkable advan- tage arose from this plan. The force of the air in the barrels was made to discharge the whole of the water out of the bell, which the elasticity of the included air had hitherto allowed to enter and partially to fill the cavity. This was easily done by stopping the cock at the top, and letting down the barrels below the level of the bell, by which means the air included in them received a sufficient preponderating pressure to enter the bell and drive out the water. In this manner the whole cavity of the bell became available for working; and, what was of still more importance, the diver could with ease descend and walk on the bottom of the sea, the feet being only slightly im- mersed. The following is the interesting account which Dr Halley gives of his arrangements:

"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 let 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 one hundredweight 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 to this bell 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 hav- ing a bung-hole in its lowest parts 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 hose 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 labour 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 of 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 in- cluded in the upper parts of them was blown with great force into the bell, whilst the water entered at the bungs- 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 sort of 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 ne- cessary to be let down gradually at first, as about twelve feet at a time; and then to stop and drive out the air that entered, by receiving three or four barrels of fresh air be- fore 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 over shoes 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 re- turn 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. This I take to be an invention applicable to various uses, such as fishing for pearls, diving for coral or sponges, and the like, in far greater depths than has hitherto been thought possible. Also for the fitting and placing of the foundations of moles, bridges, &c., in rocky bottoms, and for the cleaning and scrubbing of ships' bottoms when foul, in calm weather, at sea. I shall only intimate, that 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 stream, by small flexible pipes; which pipes may serve as a clue to direct him back again when he would return to the bell."

Plate CLXXXIX., fig. 5, represents the construction and operations of Dr Halley's bell as thus described.

In 1721, shortly after the above experiments were made, Dr Halley contrived additional apparatus, to enable the diver to go out from the bell to a considerable distance, and stay a sufficient time in the sea, and walk about on the bottom, with full freedom to act as occasion required. Considering that the pressure being greater on the surface of the water in the bell than on any other surface which was higher than that in the bell, the air would pass by a pipe from the bell into any cavity for air; where the surface of the water was higher, he concluded that a man, by putting on his head a bell or cap of lead, made sufficiently heavy to sink empty, and in form resembling the bell itself, might keep his head dry, and might receive a constant stream of air from the great bell, so long as the surface of the water in the cap was above the level of that in the bell, by means of a flexible pipe which he would carry coiled on his arm.

In pursuance of this idea he procured pipes to be made, which answered all that was expected from them. They were secured against the pressure of the water by a spiral brass wire, which kept them open from end to end, the diameter of the cavity being about the sixth part of an inch. These wires being coated with thin glove leather, and neatly sewed, were dipped into a mixture of hot oil and bees-wax, which, filling up the pores of the leather, made it impenetrable to water; several thicknesses of sheep's entrails were then drawn over them, which, when dry, were covered with paint, and then the whole defended with another coat of leather to keep them from fretting. Several of the pipes were as much as forty feet long, the size of a half-inch rope. One end of a pipe being fixed in the bell at some height above the water, the other end was fastened to a cock which opened into the cap. The use of the cock was to stop the return of the air whenever there was occasion to stoop down or go below the surface of the air in the bell, which occurred as often as there was occasion to go out or return into the machine. The diver, therefore, when he has descended to the bottom in the great bell, puts on his cap with the pipe hanging on his arm like the coil of a rope. As soon as he leaves the bell, he opens the cock in the pipe, and walks on the bottom of the sea, giving out the coils of his pipe as it is required; and this serves as a clue to direct him back again to the great bell, from whence he derives his supply of air by means of the pipe.

The weight of a man being very little more than that of his bulk in water, he could not act with any strength, nor stand with any firmness, especially if there is any current, without a considerable addition of weight; the leaden caps were therefore made to weigh about half a hundredweight, to which was added a girdle for the waist, formed of large weights of lead nearly of as great weight in the whole; also two clogs of lead for the feet, of about twelve pounds each. With this accession of weight Dr Halley found a man could stand well in an ordinary stream, and even go against it. It is necessary for the diver to be provided against the cold of the water, which, though it could not be removed so that a man could endure it long, yet it was much eased by wearing a waistcoat and drawers made close to the body, of that thick woollen stuff of which blankets are made. This becoming full of water, would be a little warmed by the heat of the body, and keep off the chill of new cold water coming on.

When the water is not turbid, things are seen sufficiently distinct at the bottom of the sea; but a small degree of thickness makes perfect night in a moderate depth of water. To obtain an open view from the leaden caps, which, from their use, the doctor called caps of maintenance, he at first used a plain glass before the sight, but soon found that the vapour of the breath made such a dew on the surface of the glass that it lost its transparency. To remedy this, he found it necessary to prolong that side of the cap which was before the eyes, and thereby enlarge the prospect of what was beneath.

Another plan of the diving-bell was proposed by Mr Martin Triewald, F.R.S. and military architect to the king of Sweden, which, for a single person, is in some respects thought to be more eligible than Dr Halley's, and is constructed as follows. AB, fig. 6, is the bell, which is sunk by lead weights DD hung to its bottom. This bell is of copper, and tinned all over in the inside, which is illuminated by three strong convex lenses P, with copper lids to defend them. The iron ring or plate below the bell serves the diver to stand on when he is at work, and is suspended at such a distance from the bottom of the bell by the chains, that when the diver stands upright, his head is just above the water in the bell, where the air is much better than higher up, because it is colder, and consequently more fit for respiration. But as the diver must always be within the bell, and his head of course in the upper part, the inventor has contrived, that even there, when he has breathed the hot air as well as he can, he may, by means of a spiral copper tube be placed close to the inside of the bell, draw the cooler and fresher air from the lowermost parts; for which purpose a flexible leather tube, about two feet long, is fixed to the upper end of the copper tube; and to the other end of this tube is fixed an ivory mouth-piece, by which the diver draws in the air, at the same time expiring by the nostrils. This bell may be supplied with fresh air by barrels, the same as Dr Halley's.

The next improvements introduced in the construction of the diving-bell were those by Mr Spalding of Leith, Edinburgh, and for which the Society of Arts voted him a reward. These are certainly deserving of attention, although they do not appear to have afterwards been adopted in practice. Mr Spalding had, in the two preceding years, acquired considerable experience in the management of a bell on Dr Halley's plan, which he had constructed in the hopes of recovering some of a considerable property which had been lost in a ship wrecked on the Scares, or Fern Islands, in 1774, in the night, when all the crew perished. Some of the light goods were thrown on shore, and it was proposed to recover the rest by diving, the remainder of the owners giving up the management of the whole to Mr Spalding. His first experiments were made in depths of five, six, and eight fathoms, in Leith Roads; and having in these made his apparatus tolerably perfect, he sailed for Dunbar, thirty miles distance, in an open long boat, sloop-rigged, and of about six or eight tons burthen. By a mistaken account he had been informed the bottom of the Fox ship of war lay there; but upon his arrival, the oldest seaman in the place could give him no intelligence; and as that vessel had perished in the night with all on board, somewhere in Dunbar Bay, and by storms, so long before as thirty years, it was thought to be sunken up. In order to gratify the curiosity of some friends there, he still determined to descend where it might be thought probable her bottom lay; but in seven and eight fathoms water he found nothing but a hard sandy bottom, from which he was led to conjecture that the proprietors of the valuable effects which were on board that vessel might have found their account in sweeping for her. Being informed that a vessel, which was thrown up by accident in the river Tay, near Dundee, with a large quantity of iron, lay within two fathoms of the surface at low water, he determined to make trial there, and accordingly sailed across the frith to that place, about fifteen leagues distant from Dunbar. Here he went down three different times, changing the ground at each going down, and at last fell in with a stump of the wreck, sunk five fathoms deep at low water to a level with the soft bed of the river, which is composed of a light sand intermixed with shells. The principal parts of this wreck were supposed to have been carried away by an immense body of ice the year before. He found that the muddiness of the river occasions a darkness at only two fathoms from the surface that cannot be described; and from the smallness of his machine, which contained only forty-eight English gallons, it was impossible to have a candle burning in it, which would consume the air too quickly for any man to be able to work, and at the same time pay attention to receiving the necessary supplies of air.

These trials were only preparatory to his views at the Scares, hoping to acquire experience which would enable him to surmount the dangerous difficulty of the unequal rocky bottoms which he expected to meet with; but in the preceding trials, and different alterations of the machinery, so much time had been lost, that the weather became stormy, and he was obliged to wait at Bamborough Castle some time till the weather became more favourable. He then sailed to the Scares with his brother, three sailors, and two pilots. It was four in the afternoon, about high water, when he went down at a small distance from the place where he judged the wreck to lie. The depth was about ten fathoms. He fortunately alighted on a flat part of the rock, within a small space of a dreadful chasm, and had just gone two steps with his machine, when the terror of the two pilots was so great, that, in spite of his brother, they brought him up very precipitately, before he had in any degree examined around him. On coming into the boat, they remonstrated on the danger of the machine being overturned either on the wreck or the rocks, and also on the impossibility of raising any of the weighty goods with so small a purchase in an open boat, and in a place where, at this season, no large vessel would venture to lie, as the nights were then so long, and only two passages for a small vessel to run through, in case of a gale of easterly or southerly wind; one of the passages being extremely narrow, and both of them dangerous.

"Convinced from this," says Mr Spalding in his account, "that with an open boat nothing could be accomplished, and that, except in June and July, no man would risk himself with me in a sloop, to continue a few days and nights at anchor there, I was obliged to abandon my project; yet I determined to take a view of the guns of a Dutch ship of war lost in the year 1704; and as they lay two or three miles nearer the land, I could execute this design with less difficulty, especially as the weather continued still favourable. Having procured all the intelligence possible, we went to the place, where I went down four different times, but could find no marks of any wreck, notwithstanding my walking about in five and six fathoms water, as far as it was thought safe to allow the rope to the bell, continuing generally twenty minutes each time at the bottom. On this occasion I was obliged to carry a cutting hook and knife, and clear away the sea weeds, which at this place are very thick and strong; without this method I could not move about. At the fifth going down, each trial being in a different place, I was agreeably surprised to find a large grove of tall weeds, all of them from six to eight feet high, with large tufted tops, mostly in regular ranges, as far as the eye could reach, a variety of small lobsters and other shell-fish swimming about in the intervals." He then discovered the place where one of the cannons lay; but was too much exhausted, by having been down at intervals for near three hours, to attempt bringing it up.

In these descents Mr Spalding found out two very serious dangers attendant on the use of the bell on Dr Halley's plan. These are, 1. By Dr Halley's construction, the sinking or rising of the bell depends entirely upon the people who are at the surface of the water; and as the bell, even when in the water, has a very considerable weight, the raising of it not only requires a great deal of labour, but there is a possibility of the rope breaking by which it is raised, and thus every person in the bell would inevitably perish. 2. As there are, in many places of the sea, rocks which lie at a considerable depth, the figure of which cannot possibly be perceived from above, there is danger that some of their ragged prominences may catch hold of one of the edges of the bell in its descent, and thus overset it before any signal can be given to those above, which would infallibly be attended with the destruction of the people in the bell, especially as it must always be unknown, before trial, what kind of a bottom the sea has in any place.

To obviate these defects, Mr Spalding introduced a Spalding's balance-weight suspended below the bell, and which, improves when it reached any rocky or uneven ground, settlements down first, and then the bell being made too light to sink without the weight, remained suspended and free from danger; and for the purpose of raising or levelling the bell without aid from above, he divided with an air-tight partition the upper portion of the bell from the lower. The former was capable of being filled either with water or air at pleasure, and of thus increasing or diminishing the buoyant effect at pleasure, on the same principle as the air-bladder in fishes.

Plate CLXXXIX. fig. 7, represents these arrangements, which will be understood from the following description: ABCD represents a section of the bell, which is made of wood; ae are iron hooks, by means of which it is suspended by ropes QBFe, and QAERe, and QSs as expressed in the figure; ae are iron hooks, to which are appended lead weights, that keep the mouth of the bell always parallel to the surface of the water, whether the machine, taken altogether, is lighter or heavier than an equal bulk of water. By these weights alone, however, the bell would not sink; another is therefore added, represented at W, and which can be raised or lowered at pleasure by means of a rope passing over the pulley, and fastened to one of the sides of the bell at M. As the bell descends, this weight, called by Mr Spalding the balance-weight, hangs down a considerable way below the mouth of the bell: In case the edge of the bell is caught by any obstacle, the balance-weight is immediately lowered down, so that it may rest upon the bottom. By this means the bell is lightened, so that all danger of oversetting is removed; for, being lighter without the balance-weight than an equal bulk of water, it is evident that the bell will rise as well as the length of the rope affixed to the balance-weight will allow it. This weight, therefore, will serve as a kind of anchor, to keep the bell at any particular depth which the divers may think necessary; or, by pulling it quite up, the descent may be continued to the very bottom.

By another very ingenious contrivance, Mr Spalding rendered it possible for the divers to raise the bell, with all the weights appended to it, even to the surface, or to stop at any particular depth, as they might think proper; and thus they could still be safe, even though the rope designed for pulling up the bell was broken. For this purpose the bell is divided into two cavities, both of which are made as tight as possible. Just above the second bottom EF, are small slits in the sides of the bell, through which the water entering as the bell descends, displaces the air originally contained in this cavity, which flies out at the upper orifice of the cock GH. When this is done, the divers turn the handle G, which stops the cock; so that if any more air was to get into the cavity AEFD, it could no longer be discharged through the orifice H, as before. When this cavity is full of water, the bell sinks; but when a considerable quantity of air is admitted, it rises. If, therefore, the divers have a mind to raise themselves, they turn the small cock g, by which a communication is made between the upper and under cavities of the bell. The consequence of this is, that a quantity of air immediately enters the upper cavity, forces out a quantity of the water contained in it, and thus renders the bell lighter by the whole weight of the water which is displaced. Thus, if a certain quantity of air is admitted into the upper cavity, the bell will descend very slowly; if a greater quantity, it will neither ascend nor descend, but remain stationary; and if a larger quantity of air is still admitted, it will rise to the top. It is to be observed, however, that the air which is thus let out into the upper cavity must be immediately replaced from the air-barrel; and the air is to be let 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 bbb, and on these ropes the divers may sit without any inconvenience. I and K are two windows made of thick strong glass, for admitting light to the divers. N represents an air-cask with its tackle, and NP 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. F is a small cock by which the hot air is discharged as often as it becomes troublesome. Fig. 5 is a representation of the whole diving apparatus, which it is hoped will be readily understood without any further explanation. Two air-barrels are represented in this figure; but Mr Spalding was of opinion that one air barrel capable of containing thirty gallons is sufficient for an ordinary machine.

An improvement has been suggested on Mr Spalding's plan of raising or lowering the bell, by shutting up the upper bell entirely, and forming it into a magazine of condensed air, which being charged by two air-pumps within the bell, could be let off at pleasure, and filling the lower bell, would displace the water and increase the buoyancy.

The last great improvement on the diving-bell, and what stands next in importance to that of Halley, and has brought the machine to that perfect state in which it is now so successfully employed, was introduced by the celebrated engineer Mr Smeaton. This consisted in substituting for the air-barrels of Halley a forcing air-pump, by which a continued stream of air was poured into the bell without any farther trouble or apparatus than a man or two to work the pump. It was about the year 1779, in the repairs of the foundations of Hexham Bridge, that Mr Smeaton first tried the use of the diving-bell; and this was the first attempt indeed to introduce it into the operations of engineering, where it has since rendered such essential service. The piers of the bridge having been undermined by the violence of the current sweeping away the gravel from under the floor timbers of the caissons by which they were founded, it occurred to Smeaton that by means of the diving-bell the cavities under the foundations might be filled up with rough stones, rammed and wedged firmly together. His diving-bell consisted of a square box or chest of wood, three and a half feet long, two feet broad, and four feet high. The pump for supplying it with air was fixed on the top of the bell, and worked by a handle at one side. The depth of the river being small, it was not intended to go down so as to cover the whole of the bell, else the air-pump would have required to be removed; it was only necessary to sink the mouth of the bell down to the level of the caisson bottom. With the assistance of this machine Mr Smeaton succeeded in underpinning the foundations of some of the piers. The calamitous accident which followed in 1782, when the whole structure was carried away by a sudden and violent flood, only proved the great insufficiency of the natural bed of the river.

In 1788 Mr Smeaton constructed a second diving-bell, Open for the operations contemplated at Ramsgate harbour, on a much more substantial and improved plan; and this is the model on which all the succeeding diving machines have been formed. Instead of the usual form of a bell or conical inverted tub of wood, sunk by weights attached to the outside, this consisted of a square chest of cast iron, four and a half feet long, four and a half feet high, and three feet wide, affording sufficient room for two men at a time to work under it. Instead of the weights applied externally, the bell itself was cast of such thickness, particularly at the bottom, that its own weight, viz. fifty cwt., was more than sufficient to sink it when full of air. The pump also for supplying fresh air was placed in a boat by itself, on which several hands were stationed, to keep the pump continually in action. The air from the pump was conveyed to the machine by a flexible tube, which allowed the bell to be moved up or down, or in any direction, independent of the motion of the boat. From the above dimensions, the bell would always contain about fifty cubic feet of air, which, from what we have already shown, would be sufficient to support life for two persons for about an hour, independent of any supply from above; so that any idea of danger from this source is completely removed. It was in clearing the foundations for the advanced pier at Ramsgate that it occurred to Mr Smeaton the operation might be facilitated by the diving-bell. A large quantity of stones had been thrown in, to secure the old pier head; and it seemed doubtful whether they could be got up in nine and ten feet water by the usual method of tongs from the barges. The diving-bell was found to answer completely the object intended. In the course of two months the foundations were cleared; and it was computed that of 160 tons of stone raised out of the foun- The pier, which was afterwards built on the foundation thus cleared, was founded by caissons, but in the course of years was found to require renewal in some places, and in others to be protected by an apron or outside wall of regularly-built masonry; and here a new application of the diving-bell arose in the building of this wall under water. For this purpose the bell is suspended by powerful tackle to the extremity of a long wooden frame, which rests on the top of the pier, the one end projecting over the pier, and the other running back and turning on a centre pin, which is fixed in a heavy stone on the pier. The frame thus sweeping with a long radius, and the weight of the whole being borne by a roller running along near the edge of the pier on a cast-iron plate or rail in the segment of a circle, the bell is capable of having a considerable motion right or left along the wall, and the block of the tackle being moveable along the frame, the bell is by this means shifted out or in from the wall at pleasure; and by these two motions can be set in any required position within the sweep of the apparatus. The directions for moving it are given by the divers, and communicated to those who have charge of the apparatus above, by merely striking with a hammer on the inside of the bell. From the great facility with which water conducts sound, the strokes of the hammer are heard at a great distance, and have a peculiar character, which is not easily mistaken for any other. To convey various directions, the divers have established a sort of language from the number of blows of the hammer. One blow, for instance, denotes more air; two, stand fast; three, heave up; four, lower down; and so on. The first operation in the building is to clear and level the foundation. If this be loose materials, they are removed by dredging, in the usual manner; but wherever rock occurs, it is done by the bell, with two men in it, being let down to the bottom, which, at Ramsgate, is a hard chalk rock. When it stands thereon, it lays the chalk dry to the level of the bottom edge of the bell; but if the surface is uneven, the bell cannot descend so low but that it will leave six or eight inches of water on the bottom. The surface of this water is the level they work to, and by cutting away every eminence which rises above the water, they soon obtain a perfectly level surface. They work with a small pick, made something like a narrow adze, for this purpose; and the work proceeds rapidly, for the chalk is not very hard. When they have accumulated as much rubbish as becomes inconvenient, they give three knocks on the bell to order the people to draw it up, till they, standing on the bottom, find themselves knee deep; then two knocks to stand fast. They now take in a shallow basket which has been previously let down from above, and fill the rubbish into it, then snatch it to order it to be drawn up, and strike four times on the bell, that they may be lowered down to proceed with their work. Having in this manner hewed away the surface till the water, standing equally all over it, shows it to be a perfect level plane, they give orders to be removed to a new situation, yet at such a small distance that part of the surface they before levelled is still beneath the bell, in order that both may be brought to one plane. Thus continuing the work, they get all the rock prepared for the stone-work, without any other level than the water.

The foundation being thus levelled, the stones are in the mean time all prepared and jointed, either square or with dovetails. These are first hoisted from the pier by means of a crane, and let down to their places in the work, as nearly as can be done, by the crane. As each stone is thus laid, the divers direct themselves right or left, up or down, until they be exactly over the stone; then making fast a strong chain to the lewis of the stone, the other end of which is attached to a ring in the top of the bell, they give the signal to heave, and the bell, with the stone under it, are both suspended by the tackle, and being moved right or left until it cover exactly over its place in the wall, it is then let down, and the chain being detached, the operation proceeds with another stone in the same manner, until the wall be completed. No cement is generally used to unite the stones; their own weight, and the accuracy of the joints, being sufficient to hold them together.

Since the completion of Ramsgate harbour, the diving-bell has been applied with great success to various other bell operations of a similar kind in different parts of the kingdom, and particularly at Dublin, Donaghadee, and other places, harbours in Ireland, and at Holyhead and Portpatrick on this side the channel. Plate CLXXXVIII. contains drawings of the bell and machinery used for the harbour of Houth, near Dublin, under the direction of the late eminent Mr Rennie, and with which the foundations of the pier wall were laid with success at very considerable depths below water.

Fig. 1 is a section showing the machine and the bell viewed in the direction of the length of the wall which is to be erected, and fig. 2 is an elevation of the same as if seen when viewed from the sea. A is the bell, which is made of cast iron. It is suspended by strong chains passed through eyes rr, fig. 3, and through the ring m of a tackle B. FF, figs. 1 and 2, are strong beams supported in a horizontal position by cross beams G, resting at one end on the shore, and the other ends supported by a scaffolding L of piles firmly braced. On the beams F two iron railways are laid for the wheels of two carriages to run upon; one of these carriages contains the tackle which suspends the bell, and the other has a similar tackle to hoist the large stones, which are to be laid on the wall X. Each carriage runs with four wheels aa upon the railways F, and has a smaller or upper carriage running upon it in a transverse direction; and this upper carriage contains the windlass purchase tackle, by which the bell or the stone is raised. Thus F' is the timber frame of the principal carriage, on the top of which are railways for the wheels dd of the upper carriage, of which D is the frame; and C is the roller or barrel to wind up the rope or fall of the great purchase tackle B, which is suspended from the frame of the carriage, and bears the weight of the bell. On the end of the barrel is a large cog-wheel M, which is turned round by a pinion fixed on the axis N of a second wheel O, and this is turned by a pinion, to which the handles H are applied. By turning these, two men can raise or lower the bell with ease. In order to move the bell in either direction, the wheels aa of the lower carriage E are provided with cogs at one edge, and pinions b work in the teeth of these; both pinions b are fixed on the same axis, which extends across the frame; and wheels e are also fixed on each extremity of the axis. These wheels have holes or mortises in them to receive handspikes or levers, by which they can be turned round, and will then move the lower carriage and the bell along the railways FF, in the direction of the length of the wall, which is to be built as shown by X. In like manner the wheels dd of the upper carriage are provided with cogs and pinions e, on the end of which are the capstan head f to receive handspikes, when it is required to move the upper carriage and the bell in a transverse direction. By means of these two motions in transverse directions, the bell or the stone can be suspended over any required spot in the wall, and lowered down thereupon as the men in the bell direct. Fig. 5 is a section of the bell, and fig. 6 a