DIVING BELL. An apparatus by means of which persons are let down and enabled to remain under water, and execute various operations ; such as le velling or clearing the bottoms of har bors, preparing a foundation for build ings, bringing up sunken materials, &c. The principle of the diving bell depends on the impenetrability of atmospheric air, and may be illustrated by a very fa miliar experiment. Bring the edge of an inverted tumbler, or any close vessel, to the surface of water, and, keeping the mouth horizontal, press it down in the water. It will be seen that, though some portion of water ascends into the tumbler, the greaterpart of the space remains empty, or only filled with air ; and any object placed in this space, though surrounded on all sides with water, would remain perfectly dry. In fact, the quantity of air remains the same, but it is compressed into a smaller volume, in proportion to the depth to which it is made to descend. Now, if we conceive a vessel of wood or iron, sufficiently capacious to hold several men, to be suspended by a chain, and lowered by means of weights attached to it, to any moderate depth under water, it is evident that they may remain there for a considerable time, and perform any ope ration that could be executed on the same confined space. The machine, however, as thus described, is liable to two great defects, which must be obvi ated by other contrivances before any great advantage can be derived from it. In the first place, as the air by its compressibility allows the water to enter the lowerpart of the bell, the dry space is not only diminished, but the bottom on which the bell rests, and where the operations are to be carried on, is also covered with water to a proportional depth. In the second place, the air within the bell, by repeated respiration, soon becomes mephitic, and unfit to sup port life ; so that it is necessary to ele vate the apparatus after short intervals, to admit a fresh supply.
It is not known at what period the diving bell was invented. Beckmann, in his Ilistory of Inventions, mentions that at Toledo, in the sixteenth century, two Greeks, in the presence of the emperor Charles V. and several thousand specta tors, let themselves down under water in a large inverted kettle with a burning light, and rose again without being wet. George Sinclair, the author of Satan's In visible World Displayed, in his work en titled Ars Nova et _Magna Gravitatis et Levitatis, mentions some attempts that were made about 1665 to raise, by means of a diving bell, the treasure from the ships of the Invincible Armada that went to the bottom near the Isle of Mull in the Hebrides, and describes the kind of bell that was employed. But, on account
of the defects to which we have alluded, the diving hell continued to be of very little use till the time of Dr. Halley, who contrived a means of introducing fresh air into the bell while under water, and of allowing the mephitic or breathed air to escape. The bell he made use of he describes as having been of wood, con taining about 60 cubic feet in its cavity, and of the form of a truncated cone, whose diameter at the top was three feet, and at the bottoni five. This was coated with lead, so heavy that it could sink empty,- and the weight so distri buted about its bottom that it could only descend in a perpendicular direction In the top a clear glass was fixed, to let in the light from above, and a cock to let out the air that had been breathed. To supply the air to the bell he caused a couple of barrels, of about 86 gallons each, to be eased with lead so as to sink empty, each of them having a bung-hole in its lowest part, to let in the water as the air in them condensed on their de scent, and to lot it out again when they were drawn up full from below. To a hole in the uppermost part of the barrels trunk or hose was fixed, long enough to fall below the bung-hole, and kept down by a weight, so that no air could escape by the hose till its end was raised np. The barrels thus prepared were let down by the side of the bell. A man stationed on a stage suspended from the bell was ready to take up the hose ; and, as soon as their ends were brought to the surface of the water in the barrels, all the air that was included in the upper parts of them was blown with great vio lence into the bell, while the great en tered at the bung-holes below and tilled the barrels. By means of this contriv ance the air was not only kept fresh, but another great advantage was gained ; namely, that by admitting a sufficient quantity of it the whole of the water was expelled from the inside of the bell, and the bottom of the sea laid dry.