The fall of an iron railway bridge over the Dee, at Chester, belonging to the Chester and Holyhead Railway Company, in 1847, has led to many enquiries on the nature of such bridges. Mr. Locke, who has constructed so many bridges of stone, brick, and timber, I stated in the evidence which he gave in rela tion to that catastrophe, that he thinks cast iron to he wholly unfitted for the construction of bridges for railways, unless under special circumstances.
In 1839 a bridge had to be constructed at Grisoles, over the branch canal of Garonne, in France ; and as building stone is scarce in that district, M. Lebrun determined to con struct it of beton or concrete. This lie accom plished, and the bridge was finished in 1841. Both the French beton and the English con are formed of lime, sand, and pebbles ; but there are slight differences in the mode of preparation. The beton was laid on course after course; and only as much was made each day as could be laid on in that day. The entire mass of the abutments, and the greater part of the arch, was made of beton : bricks being used only in a few places, to increase the strength.
Mr. Remington's &rid bridge, familiar to the visitors at the Surrey Zoological Gardens, has obtained its name-from its extreme light ness. It is entirely formed of thin pieces of wood : so thin and light as to excite general surprise that it should be able to bear the weight of many persons at one time. In 1848
Earl Talbot caused a bridge to be built on this system by Mr. Remington, over the river Trent, as part of an accommodation road on the earl's estate near Ingestre. The bridge is 150 feet span ; and yet, remarkable to ob serve, the beams or stringers, six in number, which extend from end to end, are only five inches square at each end, and diminish to 2i inches square at the centre of the bridge ; they are formed of pieces of timber, 20 to 25 feet long, scarped together at the ends. They rest, at either shore, on a light abutment formed of posts of oak, 6 inches square, framed together with iron clasps. The stringers hang into a slight curve, depressed about two feet in the centre; and the extreme lightness at the centre prevents the structure from being borne down by its own weight. The planks which form the bridge are placed crosswise upon the stringers. The bridge, although strong enough to bear a carriage and horses, cost only 200/. ; and the timber of which it is formed was growing on the earl's estate six weeks before the completion of the bridge. A similar bridge has been since thrown over the lake in Birkenhead Park, and others have been built in Birmingham and elsewhere.