The sides were lastened to the bottom by 28 pieces of timber, S by 9 inches, fixed on the outside, and 18 with in, reaching to, and lapping over the tops of the sides. The lower end of those pieces had elle side cut dove tailed, to fit a mortice made in the outer curbing of the grating of the bottom, and they were kept in their places by iron wedges. The bottom grating extended 5 feet beyond the face of the upright shaft or dado part of the Pier, The caisson being launched and fixed exactly over the place with cables lastehed to the adjacent piles, the ma sous laid the first course of stones within it. This done, a siniee made in the side was opened near low water, on which the caisson stink to the bottom ; if it did not set level, the sluice was shut and the water pumped out, so as to make it float, till such time as the founda tion was levelled. The masons cramped the first course of stones, and laid a second course; which being like wise cramped, a third course was laid : then the sluice being opened again, limier care was taken that the caisson should sink in its proper place. The stone work being thus raised to within two feet of the com mon Low-water mark, about two hours before low water the sluice was shut and the water pumped out by four pumps, eight inches square, each worked by eight men, and a smah pump three inches square, worked by two men, so that the masons could lay the next course of stones ; which they continued to do till the water was risen so high as to wake it unsafe to proceed any far ther; then they left the work and opened the sluice to let in the water. Thus they continued to work night and day at low-water till they had their work some feet higher than low-water mark ; after this the sides of the caisson were loosened from the bottom, which made them float, and they were carried ashore to be fixed to another bottom for the next pier. See Plate XCVIII.
A few years after Westminster bridge was completed, that over the river Thames at Blackfriars was begun, from the plans and under the directions of Mr Mylne, who followed the example of Mr Labclye, by laying the foundations by means of caissons. The sides, ends, and bottom were constructed in a similar manner to those at Westminster, but the form of the caisson was rectangu lar, the length was 86 feet, the breadth 33 feet, and the height, including the bottom, was 29 feet. The sides were fastened to the bottom by strong iron straps, six on each side, and three on each end ; each of them were about 20 feet in length : on one end a part about 10 feet in height above the bottom was fixed on hinges. At about 16 feet above the bottom, there was a floor to strengthen the sides and ends, and to receive a mill for working a chain-pump. There was another floor level with the top ; one end of this was sunk a little to receive the capstan for lifting stone ; over this capstan there was an awning to protect the workmen; upon the high est part of this floor was placed a triangle for raising stones, and a windlass for raising mortar. On each side
of the caisson were four upright pieces of timber, which formed part of triangles to be connected with barges for lifting the caissons. When the masonry was built up to the level of low water, a barge was laid along each side of the caisson at low water, and fixed to the upright pieces ; when the tide rose, the iron straps, and the moveable piece at one end, being disengaged, the barges lifted up the caisson, which, when raised to clear the bed of the river, was floated off. This mode of remov ing by barges became necessary, from the height of the caisson, and having floors and machinery constructed on them.
Mr Mylne caused piles to be driven into the bed of the river, and cut off level with the surface of the space which had been excavated for the foundations of the piers.
In a river where a caisson of 6 feet in depth only was required, Mr Smeaton, with his usual ingenuity, con trived one of corresponding simplicity : (Plate XCVII. Fig. 2.) The bottom consisted of two thicknesses of three-inch planks laid crossing each other, the sides and points calculated just to clear the masonry of the lowest course a few inches. They were composed of three inch planks, grooved and tongued, level with the top ; and all round the inside of the planking was a frame of timber, nine inches broad by six inches in thickness. At about two feet six inches below this was another frame six inches square ; around the outside of the planking, and rather above the level of the bottom, was a third frame nine inches by three. To all of those frames the planking was firmly bolted, the sides were fixed to the bottom by iron rods, which, passing through the inside frames, had each two of them their lower ends entered into an iron socket, which was let into the upper planking of the bottom, and secured by screws passing through both planks. The socket had one side perpendicular and the other dovetailed, and the ends of the rods were made to fit it, their upper ends passed through the upper curbing about nine inches from each other, and were secured by screws and washers on the upper side of the frame. The spreading at the top tended to render their hold in the socket firm, and when the sides were to be raised it was only necessary to unscrew the nuts. Across the top of the caisson there were seven ties of timber, each 12 inches by 6.
In some cases, in very deep water, where the bed of the river, though soft, is nearly level, or can be made so, a very strong grated frame of timber, much larger than the foundations of the masonry, may be let down by placing great weights around the edges of it, and having piles driven to keep it in its place, a caisson may be sunk upon it. This, when once settled by the weight of the pier, stands as well as if piles were driven under the bottom of the caisson. Sometimes pile planks should be driven round the frame.