A great improvement in the diving bell was made by the late Mr. Spalding, of Edinburgh. This construction seems designed to remedy some inconveniences of Dr. Halley, which are very evident, and of a very dangerous tendency; these are, 1. by Dr. Halley's constructions, the sinking er rising of the bell depends on the people who are at the surface of the water : as the bell when in the water Was a very considerable weight, the raising it not only requires a great deal of labour, hut there is a possibility of the rope i.ireaking, by which it is raised, and thus every person in the bell would inevitably perish : 2. As there are in many parts of the sea, rocks which lay at a considera ble depth, the figure of which cannot pos sibly be perceived from above, there is danger that some of their ragged promi nences may catch hold of one of the edges of the bell in its descent, and thus overset it, before any signal can be given to those above, which would infallibly be attended with the destruction of the peo ple in the bell ; and as it must always be unknown before trial what kind of a bot tom the sea has in any place, it is plain, that, without some contrivance to obviate this last danger, the descent in Dr. Hal ley's diving bell is not at all eligible.
How these inconveniences are remedi ed by Mr. Spalding's new contrivance will be easily understood from the follow ing descriptions : ABCD, fig. 3, represents a section of the bell, which is made of wood, ee are iron hooks, by means of which it is suspended by ropes QBF e and QAE e and QS, as expressed in the figure; cc are iron hooks, to which are appended leaden weights, that keep the mouth of the bell always parallel to the surface of the water, whether the machine taken altogether is lighter or heavieethan an equal bulk of water. By these weights alone, however, the bell would not sink ; another is therefore added, represented at L, and which can be raised or lower ed at pleasure, by means of a rope pass ing over the pulley a, and fastened to the sides of the bell M. As the bell de scends, this weight, called by Mr. Spald ing the balance weight, hangs down a considerable way below the mouth of the bell. In case the edges of the bell is caught by any obstacle, the balance weight is immediately lowered down, so that it may rest upon the bottom ; by this means the bell is lightened, so that all danger of oversetting is removed; for, be ing lighter °without the balance weight than an equal bulk of water, it is evident that the bell will rise as far as the length of the rope affixed to the balance weight will allow it. This weight therefore will serve as a kind of anchor, to keep the bell at any particular depth which the divers may think necessary, or by pulling it quite up, the descent may be continued to the very bottom.
By another very ingenious contrivance, Mr. Spalding rendered it possible for
the divers to raise the bell, with all the weights appended to it, even to the sur face, or to stop at any particular depth, as they think proper ; and thus they could still be safe, even though the rope de signed for pulling tip the bell was bro ken ; for this purpose the bell is divided into two cavities, both of which are made as tight as possible ; just above the se cond bottom, EF, are small slits on the sides of the bell, through which the wa ter entering, as the bell descends, dis places the air originally contained in ite cavity, which flies out at the upper ori fice of the cock H. When this is done, the divers turn the.handle, which stops the cock, so that if any mere air was to get into the cavity AEFB, it could no long er be discharged through the orifice H as before When this cavity is full of water, the bell sinks, but when a consi derable quantity of air is admitted, it ri ses. If therefore the divers have a mind to raise themselves, they turn the small cock by which a communication is made between the upper and under cavities of the hell ; the consequence of this is, that a quantity of air immediately enters the upper cavity, forces out a quantity of wa ter contained in it, and thus renders the bell lighter by the whole weight of the water which is displaced thus. If a cer tain quantity of air is admitted into the upper cavity, the bell will descend very slowly ; if a greater quantity, it will nei ther ascend nor descend, but remain sta tionary; and if a larger quantity of air be still admitted, it will rise to the top. It is to be observed, however, that the air which is thus let out into the upper' cavi ty must be immediately replaced from the air barrel, and the air is to be let out very slowly, or the bell will rise to the top with so great velocity, that the di vers will be in danger of being shaken out of their seats but by following these directions, every possible accident may -be prevented, and people may de scend to very great depth without the least apprehension of danger ; the bell al so becomes so easily manageable in the water, that it may be conducted from one place to another by a small boat with the greatest ease, and with perfect safety to those who are in it.
Instead of wooden seats, used by Dr Halley, Mr. Spalding made use of ropes suspended by hooks b, b, b, and on these ropes the divers may sit without any in convenience ; there are two windows made of thick strong glass for admitting light to the divers; N represents an air cask with its tackle, and C P the flexible pipe through which the air is admitted to the bell ; in the ascent and descent of this cask, the pipe is kept down by a small appended weight, as in Dr. Hal ley's machine ; R is a small cock, by which the hot air is discharged as often as it becomes troublesome.