526 



BRIDGE. 



Practice, distant from the caisson. Between these piles, rows 

 s ^ ' of boards were let down into the grooves to the bed 

 of the river. 



The length of the caisson from point to point (the 

 shape corresponding with that of the pier,) was 80 

 feet, the breadth 30 feet, the height, including the 

 bottom, 18 feet. The sides were formed by fir beams 

 laid horizontally upon one another, and pinned with 

 oak trenails. They were framed, and had oaken knees 

 bolted and screwed at all the angles, except the sa- 

 lient angles, where they were secured by iron work, 

 which being unscrewed, permitted the caissons, if ne- 

 cessary, to part in two halves. The sides were plank- 

 ed across the timbers, both inside and outside, with 

 three inch planks ; the thickness of the sides was 18 

 inches at the bottom, and 15 inches at the top. The 

 bottom was also formed with beams, with three inch 

 planking across their under sides, and timbers nine 

 inches thick across their upper side, making the whole 

 two feet in thickness. Upon this bottom, around the 

 outer edge, a strong curbing was fixed, to prevent 

 the first course of stone from pressing against the 

 sides. All the joints, both of the sides and bottom, 

 were caulked and pitched. 



The sides were fastened to the bottom by 28 pieces 

 of timber, 8 by 9 inches, fixed on the outside, and 18 

 within, reaching to and lapping over the tops of the 

 sides. The lower end of those pieces had one side 

 cut dovetailed, to fit a mortice made in the outer curb- 

 ing 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 up- 

 right shaft or dado part of the pier. 



The caisson being launched and fixed exactly over 

 the place with cables fastened to the adjacent piles, 

 the masons laid the first course of stones within it. 

 This done, a sluice made in the side was opened near 

 low water, on which the caisson sunk 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 foundation was levelled. The masons cramped 

 the first course of stones, and laid a second course; 

 which being likewise cramped, a third course was 

 laid : then the sluice being opened again, proper 

 care was taken that the caisson should sink in its pro- 

 per place. The stone-work being thus raised to with- 

 in two feet of the common 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 small 

 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 make it unsafe to proceed any farther ; 

 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 aehore to be 

 fixed to another bottom for the next pier. See Plate 

 XCVI1I. 



A few years after Westminster bridge was com- 

 pleted, that over the river Thames at Blackfrjars 



xcvm. 



was begun, from the plans, and under the direction 1'racti 

 of Mr Mylne, who followed the example of Mr La- * v~ 

 belye, by laying the foundations by means of cais- 

 sons. The sides, ends, and bottom were constructed 

 in a similar manner to those at Westminster, but the 

 form of the caisson was rectangular, the length was 

 86 feet, the breadth 33 feet, and the height, in- 

 eluding 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 pro- 

 tect the workmen ; upon the highest 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 dis- 

 engaged, the barges lifted up the caisson, which, 

 when raised to clear the bed of the river, was floated 

 off. This mode of removing by barges became ne- 

 cessary, 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 p LATI 

 was required, Mr Smeaton, with his usual ingenui- XCVi: 

 ty, contrived one of corresponding simplicity : The *"'{> 2- 

 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 out- 

 side 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 ot 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 up- 

 per side of the frame. The spreading at the top tend- 

 ed to render their hold in the socket firm, and wheft 



