This noble bridge was designed and executed by the cele brated Rennie. The whole of the iron castings were made at the extensive works of Messrs. Walker and Company. at Rotherham, and there put into arches before being shipped to London. The masonry was executed by Messrs. Jolliflc and Banks. The cost of the whole, including its connecting avenues, amounted to nearly £800.000.
Iron bridges had, about this time, also attracted the atten tion of the French engineers; and the fidlowing description of the Pont des Arts at Paris a ill be not only from its being the first iron bridge erected in France, but also from its presenting much novelty or design.
The Pont Iles Arts was desianed by M. de Cossa•t, the inspector-general of roads and bridges, but some variations were afterwards made from the original design by Dijon, to whom the execution of the work was intrusted.
The bridge is a nine-arched structure, the width between the piers measuring between 56 feet 91 inches, and the thick ness of the piers themselves, 6 feet inches above the foot ings. Each arch consists of five ribs, 7 feet 11:1- inches apart from centre to (wire, each rib a large arch, 6 inches in depth. and inches in thickness, formed of the two beams meeting at the coiwn of the arch. The extremi ties of the arch rest on skew-backs of cast-iron, embedded in masonry at the top of the piers. The chord of the are measures 60 feet 81 inches, and the versed sine 10 feet S inches. The smaller arches of iron are turned over the piers springing front the haunches of the adjacent principal arches, and supporting the roadway above the piers. l'pright sup porters are carried from the centre of the pie's to the crown oldie small arches, and are strengthened by struts supported on the larger arches. The ribs of each arch are bound together by tie-rods fixed to the top of the large arches. At first, the upright supports of the smaller arches were con nected by only one cross-piece, and had no braces in the upper part ; hut owing to the visible elfect produced upon the bridge by a large of persons crossing suddenly from one side to another on the OceINI in of some public spec tacle. it was deemed expedient to insert additional braces. The general appearance of the bridge is very good—especially for a tbot bridge—ffir which purpose it is admirably adapted. It is of exceedinly light and elegant appearance, as also of skilful and scientific construction, and would form an excellent model for a bridge not subject to any great strain.
iron arch-bridges of various dimensions have. since the
completion of the great works we have described, become so numerous, that it is impossible, as indeed it is unnecessary, to describe Or enumerate them. With the extension of rail way works, however, it became apparent that some modifica tion of the ordinary fi•m of those bridges was desirable. It is well known by persons conversant with railway matters, how much trouble engineers have to ene(mnter in arranging the crossings of the numerous roads, &c., by which a line of railway is intersec-ted in its course over an extent of country. Either in passing over or under the line, there is frequently great difficulty in obtaining sufficient height, or " headway," as it is termed, the the railway trains to pass under the road bridge, as in the former case, or for the road traffic to pass under the railway, as in the latter case. As the legislature imperatively requires a certain headway should be given, and will not permit any alterations in the levels of roads which shall increase their acclivities beyond a certain extent, immense expense has been entailed on railway companies in fulfilling all these required conditions.
The desideratum fur railway-bridges then was the means of adapting iron to the construction of bridges perfectly fiat, or in which might be preserved the minimum distance from the under side, or soffit of the girder, to the level of the roadway above. Bridges of this kind, it is true, had been built of timber, that is, simply of horizontal beams laid across the (penile* to be spanned ; but the application of cast-iron girders in the construction of had as yet been made only to those of very limited span. For, the maximum length of hearing to which single cast-iron girders, liable to be loaded with heavy weights. could be safely applied, having been commonly taken at 40 feet only, it followed that the use of these girder bridges was necessarily much restricted. The convenience of this ffirm of structure, however, was so obvious, and so desirable was it to extend its application to bridges of larger span, that attempts were continually made to combine in every variety of construction wrought iron with cast metal, in such a manner as should impart to the compound structure the power to resist the extension of wrought-iron itself. A paper, by Mr. John Storey, describing an ingenious mode of so combining malleable iron bars or rods with cast-iron girders, and thus forming as it were a kind of !octal trussing, was read at one of the meet ings of the Institution of Civil Engineers.