Since the time of Smeaton the diving-bell has been frequently, and with great advantage, employed in submarine works ; sometimeR. in situations in which it would have been impos sible to construct a coffer-dam, or to perform the required operations by any other means. The diving bells used in such works are usually formed on the model of that made for the works of Ramsgate Harbour ; but the mode of suspension differs according to circum stances. The bell may be suspended over the side or end of a vessel ; or through an opening in the centre of a barge; or from frame-work resting upon two barges, placed parallel with each other, but at such a distance apart as to allow the bell to descend between them ; or from a scaffolding supported by piles. Of the use of this important machine in recovering property from wrecks, the operations upon that of the Royal George afford a familiar ex ample. Smeaton's method of supplying air to the bell is that most commonly employed ; but that of Halley may, in some eases, have the advantage.
One of the largest diving.bells ever con structed is that at the Polytechnic Institution. This machine is made of cast-iron, and weighs three tons. It has twelve glazed openings, six at the sides and six at the top : the glass being half an inch thick. It is 5 feet high, 4 feet 8 inches diameter at the bottom, and from 11 to 21 inches thick. Air is supplied to the bell by two air-pumps, with 8 inch barrels. Many plans have been proposed for enab ling a man to walk beneath the surface of wa ter in such a manner as to assist in the reco very of property from wrecks, by means of waterproof coverings for the head and upper part of the body, or of strong vessels in which every part but the arms should be encased ; supply of air being either transmitted from above by a flexible pipe, or contained in the cavities of the protecting armour. Dr. Halley devised such an apparatus. The diver's head was covered by a heavy leaden cap, supplied with air by a flexible tube extending from the large bell. The diver was to coil this tube round his arm, and unwind it as he left the bell ; and to use it as a clue to direct him to the bell in returning. This pipe was formed of leather soaked in oil and hot wax, and was held open by a spiral coil of brass-wire ; its internal diameter being about one-sixth of an inch. The front of the helmet was glazed ;
and the diver, who was clothed in a thickwool len dress fitting close to the body, to diminish the effect of the coldness of the water, was enabled to walk by means of a weighted girdle and weighted clogs. The invention of water proof India-rubber cloth, which has been ap plied in various ways to diving-apparatus, affords great facilities for the manufacture of water-tight tubes for such a purpose. Many varieties of diving-dresses have been since em ployed. Mr. Thornthwaite introduced a few years ago a contrivance for facilitating the de scent and ascent of a diver. It consists of n hollow belt of India-rubber cloth, to which is attached a small but strong copper vessel. Into this vessel air is to be forced by a con densing-syringe until it has a pressure of thirty or forty atmospheres. The belt is then put on, in a collapsed state, so that it has no buoyancy, and does not impede the descent of the diver ; but when he desires to rise, he opens a valve, by which the condensed air es capes from the copper vessel into the belt. As it expands the belt, it affords sufficient buoy ancy to raise the diver immediately to the sur face.
A few years ago Dr. Payerne attracted a good deal of public attention by his attempts to dispense with an air-pump to supply air to diving-bells. He made his experiments in 1842, first at Paris, and then at the Polytech nic Institution in London. The ordinary air tube was removed from the diving bell; and he remained under water three or four hours without any connection with the external air. The experimenter took down with him a small box, containing some chemical agent which will absorb carbonic acid gas as fast as it is generated, and another agentwhich will evolve pure oxygen gas rapidly ; and as he vitiated the air in the bell by his breathing, the one chemical substance absorbed the carbonic acid from it, while the other furnished a fresh sup. ply of oxygen. Many such chemical agents are known; but we believe Dr. Payerne pro fessed to keep his method secret. He made one or two descents at Spithead, near the wreck of the Royal George.