CHAP. XVIII. DETAILS OF HIGH LEVEL BRIDGE. 413 



within the coffer-dam, until it set, and the bottom was 

 then found to be secure. A bed of concrete was laid 

 up to the level of the heads of the piles, the foun- 

 dation course of stone blocks being commenced about 

 two feet below low water, and the building proceeded 

 without further difficulty. It may serve to give an 

 idea of the magnitude of the work, when we state that 

 400,000 cubic feet of ashlar, rubble, and concrete were 

 worked up in the piers, and 450,000 cubic feet in the 

 land-arches and approaches. 



The most novel feature of the structure is the use of 

 cast and wrought iron in forming the double bridge, 

 which admirably combines the two principles of the 

 arch and suspension ; the railway being carried over the 

 back of the ribbed arches in the usual manner, while 

 the carriage-road and footpaths, forming a long gallery 

 or aisle, are suspended from these arches by wrought- 

 iron vertical rods, with horizontal tie-bars to resist the 

 thrust. The suspension - bolts are enclosed within 

 spandril pillars of cast iron, which give great stiffness 

 to the superstructure. This system of longitudinal and 

 vertical bracing has been much admired, for it not only 

 accomplishes the primary object of securing rigidity in the 

 roadway, but at the same time, by its graceful arrange- 

 ment, heightens the beauty of the structure. The 

 arches consist of four main ribs, disposed in pairs, with 

 a clear distance between the two inner arches of 20 feet 

 4 inches, forming the carriage-road, while between each 

 of the inner and outer ribs there is a space of 6 feet 2 

 inches, constituting the footpaths. Each arch is cast in 

 five separate lengths or segments, strongly bolted 

 together. The ribs spring from horizontal plates of 

 cast iron, bedded and secured on the stone piers. All 

 the abutting joints were carefully executed by machinery, 

 the fitting being of the most perfect kind. In order to 

 provide for the expansion and contraction of the iron 

 arching, and to preserve the equilibrium of the piers 



