Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/380

334 BRIDGES [EXAMPLES. Crai?- ellachie. London Bridge. Waterloo Bridge. of 390 feet, being the largest opening ever spanned by wood. Mr Smiles states that the first attempt to build a cast- iron bridge was made in 1755 at Lyons, and that one of FIQ. 120. Coalbrookdale Bridge. the arches was put together in a builder s yard, but that the project was abandoned as too costly. Mr Abraham Darby, the owner of iron works of Coalbrookdale, was Coalbroc the first person who actually erected a cast-iron arch, dale. This bridge (fig. 120) crosses the Severn by a span of 1 00 feet, near the town of Ironbridge, which has sprung up in the neighbourhood. Each of the ribs consisted of two pieces. The design is a bold and original one, and has been practically successful. Wearmouth Bridge, com pleted in 179G, is an arch built of open cast-iron panels, acting as voussoirs ; the span is 230 feet, with a rise oi 34 feet ; the springings begin 95 feet above the bed of the river ; and the width of the bridge was 32 feet. It contained 214 tons of cast-iron and 40 tons of wrought iron. The name of Thomas Paine, the well-known author, has been associated with the design of this bridge; but Mr L. D. B. Gordon (first Professor of Engineering in Glasgow) assures the writer that after careful investigation he finds that Rowland Burdon, mem ber for the county, was engineer, architect, and paymaster for this remarkable bridge. It was repaired and widened by Robert Stephcnson in 1858. The bridge erected by FlG. 121. Cast-Iron Bridge at Cruiffulladiio Telford at Oaigellachie (fig. 121), over the. Spcy, in the beginning of this century, shows a great advance in the conception of what was the safest form in which to apply cast-iron to an arch. 77. 1817 to 1845. London Bridge and Waterloo Bridge. London new bridge (fig. 40, supra, and Plate XVIII. fig. 2), is as fine an example of the modern stone arch bridge as can be found. The shape of the arches, the variation in their span, the slight curvature of the roadway, and the simple yet bold architectural details, are combined so as to produce a singularly beautiful structure. It is now insufficiently wide for the traffic it has to convey, but all who value beauty must earnestly desire that it may not be disfigured by having overhanging footpaths fitted to it as has been frequently proposed. London can well afford to pay for new bridges, but can by no means afford to part with a single object of real beauty. The design was made by Mr George Rennie, and the acting engineer was his brother, Sir John Rennie. The centre arch has a span of 152 feet, and rises 29 feet inches above Trinity high water mark; the arches on each side of the centre have a span of 140 feet, and the abutment arches 130 feet. The total length of the bridge is 1005 feet, its width from outside to outside 50 feet, and height above low water GO feet. The two centre piers are 24 feet thick, the exterior stones are granite, the interior, half Bramley Fall and half from Painshaw, Derbyshire. The voussoirs of the centre arch (all of granite) arc 4 feet 9 inches deep at the crown, and increase to not less than 9 feet at the springing. The general depth at which the foundations are laid is about 29 feet 6 inches below low water. Seven years and a quarter were spsnt in the con struction of London bridge, which was opened in 1831. The total cost was 1,458,311, but the contractor s tender for the bridge alone was 425,081. Waterloo Bridge, Plate XVIII. fig. 3, is another fine structure of the same character (1817). Introduction of Suspension Bridges. It will be ob served that from the earliest ages in which we have records of the construction of permanent bridges until very lately, the stone or brick arch has been the structure princi pally relied on. Timber bridges more or less permanent have also been employed for great spans, as in the noble bridges erected by the brothers Grubenmann (1757); and after the construction of the bridge at Coalbrook dale (1777) cast-iron was not unfreqnently employed in England. The theory of the metal arch was, however, very imperfectly understood, and the great metal arch of Southwark bridge (completed 1819), Plate XVIII. fig. 4 (largest span 240 feet), is little more than a heavy and wasteful imitation of a stone ring. By the use of timber or cast-iron instead of stone, the opening which a bridge could span was, however, somewhat increased. An immense stride in this direction was made when suspension bridges were introduced. A bridge of this kind over the Tees, 70 feet in length, was built in 1741 for the use of miners. Similar bridges are also said to have been used by Mr Finlcy in America, but the introduction of the modern suspension bridge practically dates from about 1820. (Gala- shiels bridge, 112 feet in length, was constructed in 1816, also a bridge of similar dimensions at Peebles over the Tweed). In 1819 Telford began the construction of the Menai suspension bridge (Plate XIX. fig. 2), in which the span of the catenary is 570 feet and the dip 43 feet. The success of this structure led to the construction of many other large suspension bridges, as at Fribourg (span 870 feet), Hammersmith (span 422 feet), Pesth (span COG feet). This form of bridge was not, however, found suitable for railway traffic ; and on the introduction of railways engineers were for many years dependent on stone, brick, or cast-iron arches. 78. Britannia Bridge, 1845. The design by Robert Stephenson of a bridge to carry the Chester and Holy- head Railway across the Menai Straits led to a complete revolution in engineering practice. Mr Stephcnson s first conception was that of a tube partly carried by chains. This would have practically been a suspension bridge stiffened by a girder. Under Mr Stephenson s direc- SoiltllW! Bridge. Tecs Bridge. Galasliit Peebles. Menai Bridge. Britanni

Bridge.