DIVING BELL. Much ingenuity has been devoted from an early period to the contri vance of apparatus for enabling men to de scend beneath the surface of water, to a greater depth, for a longer space of time, and with less exertion and danger, than is possible by the unassisted powers of the body. Ma chines which in some degree included the principle of the Diving-Bell were suggested, contrived, and sometimes used for the purpose of obtaining property sunk in the sea or in rivers. At length, in the 16th century, the Diving Bell itself was invented and used ; and Halley, in No. 349 of the ' Philosophical Transactions,' describes its defects, and sug gests a remedy for them. In its simplest form the diving bell is a strong heavy vessel of wood or metal, made perfectly air and water tight at the top and sides, but open at the bottom. If such a vessel be gradually lowered into the water, in a perfectly horizontal position, the air which it contains cannot escape, and there fore the vessel cannot become full of water. Where the diving-bell is used for descending to a very small depth, as the pressure of the water is small, it will not rise in the bell to a sufficient height to be inconvenient; but at the depth of thirty-three feet the pressure is so great as to compress the air into one half its original volume, so that the bell will become half full of water ; and at a greater depth the air will be still more compressed, and the wa ter will rise proportionately higher in the bell. The unpleasant pressure of the condensed air on the ear, the difficulty of breathing, and the confinement of space within the bell, have all led to various improvements, by Dr. Halley and others.
The diving-bell used by Dr. Halley was of wood, in the form of a truncated cone, five feet in diameter at the bottom, and three feet at the top, and containing about sixty cubic feet. This was coated with lead, and so weighted about the lower part that it would sink while empty, and would always remain in its proper position ; that is, with the large open end downwards, having its rim parallel with the horizon. In the top of the bell was a very strong glass window, and a cock, by opening Which the foul air might be allowed to escape.
About a yard below the mouth of the bell was suspended a stage, so weighted that it might hang steadily. The apparatus for conveying air to the diving-bell consisted of two barrels, holding thirty-six gallons each, weighted with lead to make them sink readily. Each of these had an open bung-bole in the lower end, to allow water to enter during their descent, so as to condense the air ; there was also a hole in the upper end of each, to which was fitted an air-tight leathern hose, long enough to fall below the bottom of the barrel, and having its loose end so weighted that it would fall natu rally into that position. When the open end
of the hose was turned up by the attendant, so as to be above the level of the water in the barrels, the air rushed out with great force into the bell, the barrels becoming at the same time full of water. By sending down these air-barrels in rapid succession, the air in the diving-bell was kept in so pure a state that five persons remained in it, at a depth of nine or ten fathoms, for more thin an hour and a half at a time, without injurious consequences. When the sea was clear, and especially when the sun shone, sufficient light was transmitted to allow a person in the bell to write or read ; and when the sea was troubled and thick, which occasioned the bell to be as dark as night, a candle was burnt in it. Halley some times sent up orders with the empty barrels, writing them with an iron pen on plates of lead.
The credit of having been the first to apply the diving-bell in aid of civil engineering ope rations is usually attributed to Smeaton, who used it in 1779 in repairing the foundations of Hexham Bridge. The bell used on this occa sion was an oblong box of wood, four feet high, two wide, and three and a half long ; and it was supplied with air by a pump fixed on the top. The river being shallow, the bell was not covered with water. In 1788 the di ving bell was used in a much more important work, Ramsgate harbour, by the same engi neer. The depth here being considerable, an apparatus was employed for sending in a sup ply of air through a flexible pipe, by means of a forcing-pump. The bell used was of cast iron, similar in form to that at Hexham, but four feet and a half high, four and a half long, and three wide. Its weight was 50 cwts., and the thickness was so adjusted that it would, without the addition of any weights, sink in the proper position. In levelling foundations under water by this machine the surface of the water at the bottom of the bell formed a convenient and unerring level to work to; and in this, as well as in the subsequent operation of building, every necessary motion was given to the bell by the tackle by which it was sus pended ; signals being made from below by striking one, two, three, or more blows upon the side of the bell with a hammer.