DIVING BELL

An apparatus, by means of which persons are let down and enabled to remain under water, and execute various operations; such as levelling or clearing the bottoms of harbours, preparing a foundation for buildings, bringmg up sunken materials, . The principle of the diving bell depends on the impenetrability of atmospheric air, and may be illustrated by a very familiar experiment. Bring the edge of an inverted tumbler, or any close ve el, to the surface of water, and, keeping the mouth horizontal, pre it down in the water. It will be seen that, though some portion of water ascends into the tumbler, the greater part of the spacer emains 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 is compre ed into a smaller volume, in proportion to the depth to which it is made to descend. Now, if we conceive a ve el 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 operation that could be executed on land in the same confined space. The machine, however, as thus described, is liable to two great defects, which must be obviated by other contrivances before any great advantage can be derived from it. In the first place, as the air by its compre ibility allows the water to enter the lower part 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 support life; so that it is nece ary to elevate the apparatus after short intervals, to admit a fresh supply. [s. 368]