tally below the flooring, to brace the whole of the timbers beneath. 2 d a Fig. 195-Shows an elevation of the roadway-timbers. a, bottom longitudinal beam. b, b, roadway-bearers in pairs. c, platform. Cd, top longitudinal beam forming the bottom rail of the para- e, bolt, with a forked head to receive the end of the suspending- The roadway is 14 feet wide. It slopes from the centre point along the axis to the extremities, being 4 feet higher in the centre than at the two last points. The piers are in the form of towers, resembling the Italian belfry. They are of brick, 80 feet high, and so constructed and combined with the top saddles, that they have to sustain no other strain than the vertical pressure from the main-chains. The whole weight of the structure, with an additional load of 100 lbs. per square foot of the roadway, would throw about 1,000 tons on each pier. The tension on the chains from this load is calculated at about 1,480 tons; while the strain which they can bear without impairing their strength is about 5,000 tons. 676. Monongahela Wire Bridge. This bridge, erected at Pittsburgh, Penn., upon plans, and under the superintendence of the late Mr. Roebling, has 8 bays, varying between 188 and 190 feet in width. It is one of the more recent of these structures in the United States. The roadway of each bay is supported by two wire cables, of 4 inches in diameter, and by diagonal stays of wire rope, attached to the same point of suspension as the cables, and connecting with different points of the roadway-timbers. The ends of the cables of each bay are attached to pendulumbars, by means of two, oblique arms, which are united by joints to the pendulum-bars. These bars are suspended from the top of 4 cast-iron columns, inclining inwards at top, which are there firmly united to each other; and, at bottom, anchored to the top of a stone pier built up to the level of the roadway timbers. The side columns of each frame are connected throughout by an open lozenge-work of cast iron. The front columns have a like connection, leaving a sufficient height of passage-way for foot-passengers. The framework of 4 columns on each side is firmly connected at the top by cast-iron beams, in the form of an entablature. A A carriage-way is left between the two frames, and a footpath between the two columns forming the fronts of each frame. The points of suspension of the cables are over the centre line of the footpaths; and the cables are inclined so far inward that the centre point of the curve is attached just outside of the carriage-way. The suspending-ropes have a like inward inclination, the object in both cases being to add stiffness to the system, and diminish lateral oscillations. The roadway consists of a carriage-way 22 feet wide, and two footpaths each 5 feet wide. The roadway-bearers are transversal beams in pairs, 35 feet long, 15 inches deep, and 4 inches wide. They are attached to the suspending-ropes. The flooring consists of 23-inch plank, laid longitudinally over the entire roadway-surface; and of a second thickness of 24-inch oak plank laid transversely over the carriage-way. The parapet, which is on the principle of Town's lattice, extends so far below the roadway-bearers that they rest and are notched on the lowest chord of the lattice. A second chord embraces them on top, and finally a third chord completes the lattice at the top. The object of adopting this form of parapet was to increase the resistance of the roadway to undulations. 677. Niagara Railroad and Highway Suspension Bridge. This remarkable structure, like the Aqueduct suspension bridge at Pittsburgh, was constructed by Roebling; and for boldness of plan, and skill in the execution of its details, is every way worthy of the professional ability of this distinguished engineer. Designed to afford a passage-way over the Niagara river, both for railroad and common road traffic, it consists essentially of two platforms (Fig. 196), one above the other, and about fifteen feet apart; the upper serving as the railroad track, and the lower for ordinary vehicles; the two being connected by a lattice girder on each side; and the whole bridgeframe being suspended from four main wire cables, two of which are connected with the upper platform, and two with the lower, by suspension-rods and wire ropes attached to the roadway-bearers, or joists of the platforms. Each platform consists of a series of roadway-bearers in pairs; the lower covered by two thicknesses of flooring-plank, the upper by one thickness; the portion of the latter immediately under the railroad track having a thickness of four inches, and the remainder on each side but two inches. The lattice-girders consist of vertical posts in pairs, the ends of which are clamped between the roadway-bearers; and of diagonal wrought-iron rods with screws at each end, which pass through cast-iron plates fastened above the roadway-bearers of the upper platform, and below those of the lower, and are brought to a proper bearing by nuts on each end. A horizontal rail of timber is placed between the posts of the lattice at their middle points to prevent flexure. The roadway-bearers and flooring of the upper platform are solidly clamped between four solid built beams or girders; two above the flooring, which rest on cross supports; and two, corresponding to those above, below the roadwaybearers; the upper and lower corresponding beams, with longitudinal braces in pairs between the roadway-bearers and resting on the lower beams, being firmly connected by screwbolts. The rails are laid upon the top beams. A strong parapet, on the plan of Howe's truss, is placed on each side of the upper platform. Wrought-iron and wooden braces connect the posts and the two platforms. The piers (Fig. 198) consist of four obelisk-shaped pillars, which are sixty feet high; the base of each being a square of fifteen feet sides; and the top one of eight feet sides. The pedestal of each pillar is a square of about seventeen feet side at top, and having a batir of one foot vertically to one horizontally, or 12, on each of its faces. The height of the pedestals on the United States side of the river being twentyeight feet, and on the Canadian side eighteen feet. An archway below the level of the railroad connects the two pedestals. The main cables pass over saddles placed on rollers, on the tops of the piers, and they are fastened at their ends (Fig. 199) to chains formed of links of wrought-iron bars, which, passing through abutments of masonry, and down into shafts made into the solid rock below, are there each firmly attached to an anchoring-plate of cast iron. Besides the usual suspending-rods of the bridge, a number of wire ropes, termed over-floor stays, connect the portions of the upper platform adjacent to the piers with the saddles at the top of the piers; and the lower platform is in like manner connected with the rocky banks of the river by a number of like stays. The object of both being to resist the action of high winds upon the platform, and to give the bridge more rigidity. Each of the main cables is formed of seven smaller ones or strands. The whole bound together in the usual manner by a wire wrapping. Each strand contains 520 wires in its cross-section, sixty of which make an area of one square inch. The main cables to which the roadway-bearers of the upper platform are attached are deflected laterally towards the axis of the bridge, and thus limit the range of lateral oscillations. This provision, the lattice structure of the sides and the parapet, the over and under floor stays, the deep longitudinal girders of the railway track, the slight camber or longi |