DIVING-BELL. From what has been stated in the preceding article DIVING, it will be at once understood that for all such purposes as subaqueous works upon the founda tions of piers, bridges, etc., or the exploration and raising of sunken vessels, the efforts of the unaided• diver would be almost valueless, and accordingly various contrivances for supplying air to the diver have been made. The embus aquaticus, or aquatic kettle, described by Taisnier as having been used by two Greeks in Spain, at Toledo, in 1538, in the presence of the emperor Charles V. and a Multitude of spectators, is one of the earliest reliable accounts of a diving-bell. From his description, this must have been similar in principle and construction to the modern diving-bell, but of clumsy dimen sions, and wanting in efficient means of renewing the supply of air. Dr. Halley's diving-bell, about 1720, was a wooden chamber of about 60 ft. internal capacity, open at the bottom, where it was loaded with lead to keep it perpendicular in.its descent. Strong pieces of glass were set in the upper part to admit light. Casks filled with air, and loaded with lead, were let down with the bung-hole downwards; and from these a supply of air was drawn by means of a hose. The form of diving-bell now in use was first constructed by Smeaton for the works at Ramsgate harbor, 1788. It was of cast iron, and weighed 50 cwt.; its height, 4-.} ft; length, the same; and width, 3 feet. It sunk by its own weight, and was lighted by stout pieces of bull's-eye glass firmly cemented by brass rings near the top. The principle of the diving-bell will be easily understood by floating a piece of lighted candle or a wax-match on a cork, and then covering it with an inverted tumbler, and pressing downwards; the candle will descend below the level of the surrounding water. and continue burning for a. short time, although the tura bier be entirely immersed. The reason is obvious enough• the air in the tumbler baying no vent, remains in it, and prevents the water from occupying its place, so that the cork and candle, though apparently under water, are still floating, and surrounded by the air in the tumbler; the candle continues burning until the oxygen of the air is exhausted, and then it goes out, as would the life of a man under similar circumstances. If vessels
full of air, like the barrels of Dr. Halley, were submerged, and their contents poured into the tumbler, the light might be maintained; but this could be better done if a tube passed through the tumbler, and air were pumped from above through the tube into the tumbler.
The modern diving-bell, which is made of cast-iron like Smeaton's, is supplied with air in this manner. It must be remembered that air is compressible, and diminishes in bulk in proportion to the pressure, so that at a depth of about 33 ft. in water, it would occupy half the space it filled at the surface; if the inverted tumbler were carried to this depth, it would be half filled with water. A considerable quantity of air has, therefore, to be pumped into the diving-bell, merely to keep it full as it descends; the air thus compressed exerts a corresponding pressure, and would rush up with great force if the tube were open and free. This is prevented by a valve opening downwards only. When the diving-bell has reached its full depth, the pumping is continued to supply air for respiration; and the redundant air overflows, or rather underflows, by the open mouth, and ascends to the surface in great bubbles. The diving-bell is pro vided with a platform or scat for the workmen, and suspended from a suitable crane or beams projecting from a barge or pier; men above are stationed to work the pumps, and attend to the signals of the bellman. These signals are simply made by striking the sides of the iron diving-bell with a hammer, and as sound is so freely communicated through water, they are easily heard above. One blow signifies "more air;" two blows, "stand fast;" three, "heave up;" four, "lower down ;" five, "to eastward;" six, "to westward," etc. These, of .course, may be modified as agreed upon. Messages are also sent up, written on a label attached to a cord. The sensations produced in descend ing are rather curious. Immediately on the mouth of the diving-bell striking the water, a feeling like a slight blow on the internal ear is produced; a dull ringing in the ears and a sense of deafness follows.