the bars by keys. The anchoring-plate is retained in its place by two strong cast-iron beams, against which the strain upon the plate is thrown. The suspending-rods (Fig. 141) are connected with both the upper and lower main-chains; to the upper by a saddle-piece and bolts, and to the coupling-bolt of the lower by an arrangement of articulations, which allows an easy play to the rods; at bottom (Fig. 142) they are connected by a joint with a bolt that fastens firmly the roadway-timbers. b d a e Fig. 142-Shows an elevation of the roadway-timbers. a, bottom longitudinal beam. b, b, roadway-bearers in pairs. platform. ed, top longitudinal beam forming the bottom rail of the para pet. e, bolt, with a forked head to receive the end of the suspending- The roadway-timbers consist of a strong longitudinal bottom beam, upon which the roadway-bearers are notched; these last pieces are in pairs, the two being so far apart that the bolts connecting with the suspending-rods by a forked head can pass between them; the flooring-plank is laid upon the roadway-bearers; and a top longitudinal beam, which forms the bottom rail of the parapet, is secured to the bottom beam by the connecting bolt. Wrought-iron diagonal ties are placed horizontally below the flooring, to brace the whole of the timbers beneath. 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 1000 tons on each pier. The tension on the chains from this load is calculated at about 1480 tons; while the strain they can bear without impairing their strength is about 5000 tons. Monongahela wire Bridge. This bridge, erected at Pittsburgh, Penn., upon plans, and under the superintendence of 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, at tached to the same point of suspension as the cables, and con necting with different points of the roadway-timbers. The ends of the cables of each bay are attached to pendulum-bars, 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 frame-work of 4 columns on each side is firmly connected at top by cast-iron beams, in the form of an entablature. 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 21 inch plank, laid longitudinally over the entire roadwaysurface; 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 top. The object of adopting this form of parapet was to increase the resistance of the roadway to undulations. MOVEABLE BRIDGES 624. The term moveable bridge is commonly applied to a platform supported by a frame-work of timber, or of cast iron, by means of which a communication can be formed or interrupted at pleasure, between any two points of a fixed bridge, or over any narrow water-way. These bridges are generally denominated draw-bridges, but this term is now, for the most part, confined to those moveable bridges which can be raised or lowered by means of a horizontal axis, placed either at one extremity of the platform, or at some intermediate point between the two ends, and a counterpoise which is so connected with the platform in either case, that the bridge can be easily manœuvred by a small power acting through the intermedium of some suitable mechanism applied to the counterpoise. The term turning or swinging bridge is used when the bridge is arranged to turn horizontally around a vertical axis placed at a point between its two ends, so that the parts on each side of the axis balance each other; and the term rolling bridge is applied when the bridge resting upon rollers can be shoved forward or backward horizontally, to open or interrupt the passage. To the above may be added another class of moveable bridges, used for the same purpose, which consist of a platform supported by a boat, or other buoyant body, which can be placed in or withdrawn from the water-way, as circumstances may require. 625. Local circumstances will, in all cases, determine what description of moveable bridge will be best. If the width of the water-way is not over 24 feet, a single bridge may be used; but for greater widths the bridge must consist of two symmetrical parts. 626. Draw-bridges. When the horizontal axis of this description of bridge is placed at the extremity of the platform, the bridge is manoeuvred by attaching a chain to the other extremity, which is connected with a counterpoise and a suitable mechanism, by which the slight additional power required for raising the bridge can be applied. A number of ingenious contrivances have been put in practice for these purposes. They consist usually either of a counterpoise of invariable weight, connected with additional animal mo tive power, which acts with constant intensity but with a variable arm of lever; or of a counterpoise of variable weight, which is assisted by animal motive power acting with an invariable arm of lever. In some cases the bridge is worked with a less complicated combination, by dispensing with a counterpoise, and applying animal motive power, of variable intensity, acting with a constant or a variable arm of lever. Among the combinations of the first kind, the most simple consists in placing a framed lever (Fig. 143) revolving on a hori platform. strut moveable around its lower end. e, bar with an articulation i, fixed pulley over which zontal axis above the platform. The anterior part of the frame is connected with the moveable extremity of the platform by two chains. The posterior portion, which forms the counterpoise, has chains attached to it by which the lever can be worked by men. When the locality does not admit of this arrangement, the chain attached to the moveable end of the platform may be connected with a horizontal axle above the platform, to which is also attached a fixed eccentric of a spiral shape, (Fig. 143,) connected with a chain that passes over its gorge and sustains a counterpoise of invariable weight. Upon the same axle an ordinary wheel is hung, over the gorge of which passes an endless chain to manœuvre the bridge by animal power. Of the combinations of variable counterpoises the mechanism of M. Poncelet, which has been successfully applied in many instances in France for the draw-bridges of military works, is one of the most simple in its arrangement and construction. The moveable end of the platform (Fig. 144) is connected by a com Fig. 144-Shows the ar- mon chain, that passes over the gorge of a wheel hung upon a horizontal shaft above the platform, with another chain of variable breadth, formed of flat bar links, which forms the counterpoise. The chain counterpoise is attached at its other extremity to a fixed point in such a way, that when the platform ascends, a portion of the weight of the chain is borne by this fixed point; and thus the weight of the counterpoise decreases as the platform rises. The system is manœuvred by an endless chain passed over the gorge of a wheel hung upon the horizontal shaft. For light platforms a counterpoise may be dispensed with, and the bridge may be manoeuvred by connecting the chain attached to the moveable end of the platform to a horizontal shaft, which is turned by the usual tooth-work combinations. When the locality does not admit of manoeuvring the bridge by a chain connected with some point above the frame-work of the platform, Fig. 145 is continued back, from two thirds to three |