IRON BRIDGES.
In Britain, of late years, the application of iron having been greatly extended, and practical mechanics having been also brought to much perfection, that valuable metal has been used in the construction of large arches over rivers.
The first cast iron bridge was erected upon the river Severn, about two miles below Coalbrouk Dale, and between the villages of Madely- and Broseley, in the county of Salop. The form of the ribs or intrados is nearly semicircular, the span being 100 feet 6 inches, and the rise from the level of the springing plates to the soffit at the middle is 45 feet, the height from the ordinary low water to the springing plate is about 10 feet, making the whole height from the low water to the soffit 55 feet. This bridge was constructed by Abra ham Darby ; it was cast at the Coalbrook Dale foun clerics, and erected in 1777. The design was hold and well executed ; it formed a new xra in bridge build ing. The banks of the river adjacent to the bridge are exceedingly high and steep, and composed of alluvial matter which slips over the points of the coal strata. The effect of this operation not having been sufficiently provided against, some years ago, the top part of one of the stone abutments was pressed in a few inches, and of course raised up the iron work about the middle of the arch. Steps have been since taken to secure the western abutment ; but the other, by having valuable houses built close up to it, is more entangled, and it may in time suffer from that cause ; but the iron work has not been the least affected by the weather, or the intercourse over or under the bridge during 34 rears. See Plate XCI.
The next cast iron bridge in point of time, was like wise erected upon the river Severn, about three miles above the former, at the expence of the county of Salop. Here the banks being low, Mr Telford, as county sur veyor, under whose direction it was built, introduced the principle of suspending the roadway by two large ribs, one on each side of the bridge. The span is 130 feet, the versed sine of the ribs which bear the covering plates is 17 feet, the breadth across the soffit is 18 feet, and the height from ordinary low water to the soffit is 34 feet. The Coalbrook Dale Company performed both the masonry and iron work by contract, and it was finish ed in 1796. The eastern bank of the river is composed
of matter similar to that which injured the abutments of the former bridge ; but here, the foundations being placed upon rock, and the masonry made of a wedge form be hind, the whole has continued perfect during 15 years. See Plate XCI.
The third iron bridge, in regard to time and pro gressive increase of magnitude, is that erected upon the river Wear, at Sunderland, in the county of Dur ham. It is likewise the segment of a circle, the chord line being 236 feet, and the versed sine 34 feet : the height from the surface of ordinary low water to the soffit is 100 feet. The merit of having this bridge, instead of the ferry over the river, is chiefly due to Rowland Burdon, Esq. The iron work was cast at the founderies of Messrs Walkers, at Rotherham, in York shire, and was erected under the direction of Mr Thomas Wilson. It was opened for general use in August 1796, and forms a magnificent feature and a very con venient passage in that part of the country. See Plates The of this new species of bridge build ing was bold and rapid, setting out from a point, which arches constructed with stone seldom reached, that is to say. 100 feet span, extending next to 130, and from thence to 236 feet. Former experience was left far behind, and a principle introduced, to the practical operation of which We can at present assign no limit ; since a design still bolder than any of the foregoing was projected, when a proplsition was made by a committee of the House of Commons. for rebuilding London bridge. Mr Telford presented a plan of one arch of cast iron, the chord of which was 600 feet, and the versed sine 65 feet. This plan met with the approbation of the committee, and W..S by them submitted to the investiga tion of twenty persons, most eminent in Britain 'for scientific knowledge or practical skill. Their reports justified its being adopted ; but a rapid succession of political events. have hitherto prevented it from being carried into effect.
Two very neat cast iron bridges, each consisting of one arch of 100 feet, have been built over the river Avon, at Bristol, under the direction of :lir Jcssop, the engineer ; and one, 150 feet span, has been designed by, and is now building under the direction of, Mr Tel ford, over an arm of the sea, upon one of the IIigh