CHAP. VL SOUTHWARK BRIDGE. 191 



solid piece of masonry does not exist than Southwark 

 Bridge. 



The iron work consists of eight arched ribs, the main 

 strength of the arches being embodied in their lower 

 parts, which. are solid. The lower or main arch is 

 divided into thirteen pieces, with a rib 5i inches thick 

 at the top and bottom, and 2J inches in the centre. 

 The joints radiate outwards from the lower edge, and 

 form so many cast iron instead of stone voussoirs, from 

 6 to 8 feet deep and 13 feet long. At the junction of 

 each of these main rib pieces there are transverse 

 plates of the same depth, having flanges cast upon 

 them on both sides in a wedge form, so that the ends 

 of the main rib piers fit into them on one side, whilst 

 on the other there is a cast iron wedge, driven in 

 between the rib and flange piece, and enabling the whole 

 to be accurately adjusted and connected together. In 

 addition to this, each rib piece had a flange, cast at each 

 end with a certain number of holes three quarters of 

 an inch in diameter, into which wrought iron screw 

 bolts were introduced to connect the whole firmly toge- 

 ther in the direction of the arch. These rib pieces 

 were also of great importance during construction, the 

 chief dependence being placed upon their lateral thrust 

 in holding the arches " together. At each pier and 

 abutment there was a similar cast iron bed or abutting 

 plate, let accurately H inch into the stone ; but between 

 the end of each main rib which sprang from this plate 

 there was a groove cut out of the solid stone behind 

 the springing plates and main iron ribs of the arches, 

 18 inches wide, 3 inches thick at the top, and 2 inches 

 at bottom. This groove was accurately dressed and 

 polished. Three cast iron wedges, 9 feet long, 6 inches 

 wide, and 3i inches thick at top and 2 inches at bottom, 

 were then made and most accurately chipped and filed, 

 so as to fit exactly the groove above mentioned to within 

 12 inches of its bottom. When the whole of these wedges 



