Upon the roadway, a layer of bitumen was laid about half an inch thick, in order to prevent the water from injtiring the timber of the bridge; however, it must be admitted, that after very heavy rain, the moisture has been found to have per colated through this pavement, either owing to its having been badly laid, or perhaps in consequence of crack's made in this coating by the contraction and expansion of the timber road way. It might lie better, therefore, in future, t i'race a layer of fine sand between the flooring-boards and time bitumen. Six drain-pipes in each Lay convey the water off the surface.
The pavement of the roadway consists of a flagging, 6 inches thick, laid upon sand in the ordinary manner. Each bay is loaded to the extent of about 40 tons.
There is another bridge over the Seine, at Tory, which was put up in the year 1828, and which greatly resembles the bridge of Grenelle in construction.
Besides these, which are given by way of example, there are almost an endless number of wooden-bridges erected throughout the world ; among which may be mentioned the bridge at Trenton, in America, of ISO feet span ; a bridge over the Tees, 150 feet span ; the bridge of Neucetringen, in Bavaria, 102 feet ; the bridge across the Necker, 210 feet span ; the bridge of Bamberg., with an opening of `201i feet, erected by M. NViebeking, an engineer, who has constructed an immense number of' thnber-bridges ; the bridge of Feld rich, with a span of 65 feet ; time bridge at Zete, built by M. Cuffinet, with a span of 125 feet, besides several put up by the celebrated c and a number of others reinark able fbr their ingenious construction and variety of fe•t].
Timber bridges are either supported upon piers and abut ments of masonry built on the solid foundation of the ground, or on it platform constructed ed upon piles driven into the earth, or they are supported upon piers f hared of one or more rows of piles driven in a line with the road or river passing under the bridge. There are an almost infinite variety of ways in which such props or piers may be made. It is, how ever, usual to drive the piles about a yard apart from centre to centre, and to capping-pieces or malings to the top of such piles, and either filling tip the spaces between with large stones laid dry, or else grunted with mortar; on this, the inasoury for the supports should be placed, or a timber framing. if desired, or else the piles may be carried up to the
height of the roadway. being kept in their places by walings and diagonal pieces, bolted on each side of them ; these piles should be about a foot square, and when they are driven in salt water, or in tidal rivers, their surfaces, up to mark, should be sheathed with copper, or protected by scup per-nails, from the ravages of the worth. At each end of the piers in the water, in cases where several rows of piles are driven, a sort of entwater should be limited, in order to ward i)tt heavy bodies, such as trees, ice, &e., and prevent them from injuring the superstructure. This is usually (lone by driving one pile by itself in advance of the rest, or by forming what is called a dolphin at each end of the pier. The piers and abutments should be made in all cases suffi ciently strong to resist the thrust of the arch. In cases of small foot-bridges, where even the distance between the supports should be as much as 20 or :30 feet, longitudinal scarfed girders may be laid upon the caps of the piles. Under such circumstances, as we have seen, there is nothing but the weight, or perpendicular pressure, to be provided tbr; and the same may be said of timber-bridges of greater width, fur roads, and even for railways, provided the distance between the piers does not greatly exceed 10 or 15 feet; beyond that opening, however, bridges are usually sustained by struts or tension rods, or the roadway-timbers are trussed so as to exert an oblique pressure upon the supports; indeed, in all instances of the kind, where the bays arc formed upon the principle of compression, or tension, the piers must be so thrilled as to counteract the tendency constantly exerted to force them out of their perpendicular position; this must be done either by making them of sufficient weight and strength so as to overcome, by their inertia, any force that may be exerted against them, or else to counterbalance the efforts of one bay, or arch, acting in one direction, by a similar and similarly-acting arch, or timber-frame, exerting an equal and like force in a contrary direction. The tbrme• of these methods is used in the abutments of a bridge, while the latter is invariably adopted with respect to piers.