In the Encyclopaedia we have given an account of several machines for diving, and a description of the diving-bells of Halley and Spalding. In the present article we have to notice an improved method of supplying air to a diving-bell, by means of a syringe or pump, which forces the air down in a continual stream into the bell, whence it escapes from beneath the lower edges of the bell, or from a waste pipe, as fast as it is supplied. In this way, the air is kept very pure, and the people in the bell have no kind of trouble to obtain a supply. Dr Halley's original method of lowering the air in casks required the attention of a person within the bell, to let out a portion of the heated air from the bell, and then to admit the fresh air contained in the cask.

We believe that Mr Smeaton was the first who put in practice the method to which we allude; though it had been frequently proposed by other inventors. His first attempt was in shallow water, the bell being only intended to enable workmen to examine and repair the foundations of a bridge, which had been laid by the caisson method. This was at Hexham in Northumberland, in 1786.

A few years afterwards, Mr Smeaton constructed another bell upon the same principle, for the works at Ramsgate harbour. It was used to raise up large stones, which had formerly been thrown into the sea around the base of the pier. When it was propo- sed to build an advanced pier, by means of caissons, these stones required to be cleared away, and the ordinary means were not found sufficiently expeditious.

The bell was made of cast-iron, in one piece, and of a sufficient weight to sink in water without any extra ballast. In the top were proper apertures with lenses, for the admission of light, and also a strong shackle for the chain by which the bell was suspended. A strong leathern hose, or pipe, was connected with the top of the bell, to convey air into it from an air-pump placed either in a boat or on the shore.

This kind of diving-bell has since been applied to the purposes of building foundations of masonry in deep water, under the direction of Mr Rennie, who has constructed machinery to move the bell under water in any direction. This acts with such facility, that the masons in the bell can make great dispatch in laying the stones. Plate LXX. contains drawings of the diving-bell and machine, planned by this eminent engineer, for the harbour of Houth near Dublin, and which was used with success in building the foundation walls for the pier at a considerable depth below water.

Fig. 1. is a section showing the machine, and the bell viewed in the direction of the length of the wall which is to be erected; and fig. 2. is an elevation of the same as it appears when viewed from the sea. A is the bell, which is made of cast iron. It is suspended by strong chains passed through eyes rr and through the ring m of a tackle B. FF figs. 1 and 2. are strong beams supported in an horizontal position by cross-beams G, resting at one end on the shore, and the other ends supported by a scaffolding L of piles firmly braced. On the beams F two iron railways are laid for the wheels of two carriages to run upon; one of these carriages contains the tackle which suspends the bell, and the other has a similar tackle to hoist the large stones which are to be laid on the wall X. Each carriage runs with four wheels a a upon the railways F, and has a smaller or upper carriage running upon it in a transverse direction; and this upper carriage contains the windlass purchase tackle, by which the bell or the stone is raised. Thus E is the timber frame of the principal carriage, on the top of which are railways for the wheels dd of the upper carriage, of which D is the frame; and C is the roller or barrel to wind up the rope or fall of the great purchase tackle B, which is suspended from the frame of the carriage, and bears the weight of the bell. On the end of the barrel is a large cog-wheel M which is turned round by a pinion fixed on the axis N, of a second wheel O, and this is turned by a pinion, to which the handles H are applied. By turning these, two men can raise or lower the bell with ease. In order to move the bell in either direction, the wheels aa of the lower carriage E are provided with cogs at one edge, and pinions b work in the teeth of these; both pinions b are fixed on the same axis, which extends across the frame; and wheels c are also fixed on each extremity of the axis. These wheels have holes or mortices in them to receive handspeaks or levers, by which they can be turned round, and will then move the lower carriage, and the bell along the railways FF, in the direction of the length of the wall, which is to be built as shown by X. In like manner, the wheels dd of the upper carriage are provided with cogs and pinions c, on the end of which are the capstan head f to receive handspeaks, when it is required to move the upper carriage and the bell in a transverse direction. By means of these two motions in transverse directions, the bell or the stone can be suspended over any required spot in the wall, and lowered down thereupon as the men in the bell direct. Fig. 5. is a section of the bell, and fig. 6. a plan to show the apertures nn for the lenses which give light. Two men descend together, a seat s being fixed across on each side of the bell. The air pipe is screwed on at k, and proceeds to the airpump as shown in fig. 1. The pump is placed on the top of the scaffold G; it has two barrels II, which are worked by a lever K, by one or two men; they act as forcing pumps, and the air which is thrown down escapes from the lower edge of the bell, and rises up through the water in bubbles. By this means the air in the bell is at all times quite fresh and pure.

The stones which are to be used in building the wall are prepared on shore, and fitted to each other. When all is prepared, these stones are lowered down the bank by a capstan to the position w. The rope of the machine is then attached, and by the aid of both ropes the stone is lowered down upon the wall. The divers then descend in the bell, and the two carriages are brought close together, by which means the bell will hang partly over the stone W, fig. 2, so that the men can guide it into its place on the wall X, and make signals to those above to direct them which way to move the stone, and where to lower it. The bell was also employed, in the first instance, to clear the foundation for the walls. It was then lowered quite down on the bottom, and the men worked the rock to a level surface. In many parts it was requisite to blast it with gunpowder. The divers bored the hole in the rock, and placed the powder in a tin cartridge, which was well secured in the hole, by running in small fragments of stone. A small tin pipe was affixed to the canister, long enough to reach up above the surface of the water. When all was prepared, the bell was drawn up out of the way, and a nail or other small piece of iron heated red hot, was dropped into the tin pipe, thereby to descend to the powder.

Figures 3. and 4. represent a vessel which was fitted up under the direction of Mr Rennie, to carry a diving bell of cast-iron. This was used in Plymouth Sound, and was swept over the bottom to discover and take up old anchors, &c. The bell A is suspended over the bow of the vessel, by a strong tackle q, from the extremity of a pair of shears; that is, two masts DB DB, fig. 4. The fall or rope of the tackle q is drawn up by a windlass at C. There is also another strong tackle GH, extended between the head of the mast I and the top of the shears D. This is drawn by the windlass F. The use of this is to raise the shears upright, and bring the bell on board. A platform S is fixed on the deck to lower it upon, when out of use.