By means of this contrivance for the admission of fresh air, it was now pos sible to remain for any length of time under water; but the use of the appara tus was still found to be attended with some inconveniences, and even consider able danger. The divers within the bell having no power over it, its rising or sinking depends entirely upon the peo ple at the surface of the water; and as the bell, even when in the water, has a considerable weight, there is always a possibility of the chain by which it is raised breaking, which would inevitably be attended with the destruction of the divers. Another danger, still more to he apprehended, is, that if the mouth of the bell in its descent should come upon a sunken ship, or a rock projecting ab rup:ly from the bottom, it might he overset before any signal could be given to those above. These defects were ob viated by the very ingenious contrivances of dr. Spalding of Edinburgh. In order to avoid the risk of being upset when the bell descends on a rocky or uneven bottom, he suspended a considerable weight, which is called a balance weipbt, below the bell, by a rope passing over a pulley fixed in the inside ; and the other weights attached to the bell being so adjusted that they could not sink it without the balance weight, as soon as the latter rested on the ground the bell remained suspended in the water. In case of the month of the bell being caught by any obstacle, the balance weight is immediately lowered till it rests on %lie bottom ; and as the bell, when thus relieved, is buoyant, the di vers, having disengaged it from the rock, have it in their power either to descend by pulling in the rope, or by allowing it to run to ascend to the surface. Another contrivance of Mr. Spalding deserves mention. He divided the bell into two compartments, the one above the other, and communicating by means of a stop cock. The divers are stationed in the lower one, and the weights are so ad justed that when the cavity above is empty the bell is buoyant; when it is filled with water, the bell sinks. Imme diately above the partition are some slits in the sides of the bells ; and at the top is an orifice, which can be opened or shut at pleasure. Suppose now, this orifice being open, the bell is required to be lowered ; as it descends, the water enters at the slits, and the air escapes by the orifice. When the apparatus is en tirely under water, and the cavity conse quently completely filled, let the orifice he shut. The bell will now continue to descend ; but if the stopcock communi cating with the upper compartment be opened, the air will rnsh from the under to the upper, and displace a quantity of the water, and the apparatus will be lightened by the whole of the water so displaced. The divers have it thus in
their power to regulate the descent or rise as they please. By admitting a cer tain quantity of air into the upper cavity, the descent of the bell is arrested ; by admitting a greater quantity it becomes buoyant, and rises to the top. This me thod of constructing the diving bell has not, however, been adopted.
The greatest improvement on the div bell, since that of Halley, was made by the celebrated Mr. Sineaton, and con sists in forcing down a continued stream of air by means of an air-pump through a flexible tube ; and this plan is now al ways adopted. In the year 1788, Smea ton constructed a diving bell to be used in the operations then contemplated at Ramsgate harbor on a new and improved plan. Instead of a bell-shaped vessel stink by weights, his apparatus consisted of a square chest of east iron, four and a half feet long, four and a half feet high, and three feet wide, affording sufficient room for two men under it. It was cast of such a thickness that its own weight was sufficient to sink it ; end its thick ness was greatest near the mouth or lower part, to prevent it from being easily overset. This construction of the diving bell gave the men ithin it no power of raising or sinkin it ; but as the apparatus was made to be used at a place where the nature of the bottom was known, this advantage was not con sidered of great consequence ; and, in fact, it is found by experience that it is better to leave the bell to be entirely guided from above. On account of the facility with which water conveys sound, the strokes of a hammer on the inside of the bell can be heard at a great distance ; and the sound coining through the water has a peculiar character, which cannot be mistaken. By previous ar rangements any directions can be given in this manner. For instance, one blow may denote more air ; two, stand fast ; three, heave up ; four, lower down, and so on. With these successive improve ments, the diving bell is found to be a most important machine in all the great operations to be performed under water. It was used with great advantage by Mr. Rennie in the construction of the various harbors he projected ; and it has recently been successfully employed in deepening the Clyde between Glasgow and Green ock, and improving the condition of the river.