by the trials and examinations to which a bridge of this kind, erected by Colonel Dufour, at Geneva, was subjected by him after twenty years' service. It was found that the undulations were greater than when the bridge was first erected, owing to the shrinking of the roadway-frame; but the main cables, and suspending-ropes, even at the loops in contact with the timber, proved to be as sound as when first put up, and free from oxidation; and the whole bridge stood another very severe proof without injury.

The following succinct descriptions of the principal elements of some of the most celebrated suspension bridges of chains, and wire cables, of remarkable span, are taken from various published accounts.

672. Bridge over the Tweed near Berwick. This is the first large suspension bridge erected in Great Britain. It was constructed upon the plans of Capt. Brown, who took out a patent for the principles of its construction.

Span...
Versed sine.

449 feet.
30"

Number of main-chains 12, six being placed on each side of the roadway, in three ranges of two chains each, above each other.

The chains are composed of long links of round iron, 2 inches in diameter, and are 15 feet long. They are connected by coupling-links of round iron, 1 inch diameter, and about 7 inches long, by means of coupling bolts.

The roadway is borne by suspending-rods of round iron, which are attached alternately to the three ranges of chains. The roadway-bearers are of timber, and are laid upon longitudinal bars of wrought iron, which are attached to the sus pension-rods.

673. Menai Bridge, erected after the designs of Mr. Telford. Opened in 1826.

Span..

Versed sine..

579.8 feet.

Number of main-chains 16, arranged in sets of 4 each, ver

tically above each other.

Number of bars in each link, 5.

Length of links, 10 feet.

Breadth of each bar, 31 inches; depth, 1 inch. Coupling-links, 16 inches long, 8 inches broad, and 1 inch deep.

Coupling-bolts, 3 inches in diameter.

Total area of cross-section of the main-chain, 260 square inches.

The main-chains are fastened to their abutments by anchoring-bolts 9 feet long and 6 inches in diameter, which are secured in cast-iron grooves. The abutments, which are underground, and reached by suitable tunnels, are the solid rock. Upon the tops of the piers are cast-iron saddles, upon which the main-chains rest. The base of the saddle, which is fitted with grooves to receive them, rests upon iron rollers placed on a convex cylindrical bed of cast iron, shaped like the bottom of the base of the saddle, to admit of a slight displacement of the chains from movable loads or changes of temperature.

The roadway is divided into two carriage-ways, each 12 feet wide, and a footpath 4 feet wide between them. The roadway-framing consists of 444 wrought-iron roadwaybearers, 3 inches deep and inch thick, which are supported at the centre points of each of the carriage-ways by an inverted truss, consisting of two bent iron ties which support a vertical bar placed under the roadway-bars at the points just mentioned. The platform of the roadway is formed of two thicknesses of plank. The first, 3 inches thick, is laid on the roadway-bearers and fastened to them. is covered by a coating of patent felt soaked in boiling tar. The second is two inches thick and spiked to the first.

This

The roadway is suspended by articulated rods attached to stirrups on the roadway-bearers and to the coupling-bolts of the main-chains.

The piers are 152 feet high above the high-water level. They have an arched opening leading to the roadway, and the masses on the sides of the arch are built hollow, with a cross-tie partition wall between the exterior main walls.

The parapet is of wrought-iron vertical and parallel bars. connected by a network.

This bridge was seriously injured by a violent gale, which gave so great an oscillation to the main-chains that they were dashed against each other, and the rivet-heads of the bolts were broken off. To provide against similar accidents, a framework of cast-iron tubes, connected by diagonal pieces, was fastened at intervals between the main-chains, by crossties of wrought-iron rods, which passed through the tubes, and were firmly connected with the exterior chains. Subse

quently to this addition, a number of strong timber roadwaybearers were fastened at intervals to those of iron, as the iron roadway-bearers were found to have been bent, and in some instances broken, by the undulatory motion of the bridge in heavy gales.

The total suspending weight of this bridge, including the main-chains, roadway, and all accessories, is stated at 643 tons 15 cwt.

674. The Fribourg Bridge of wire thrown across the valley of the Sarine, opposite Fribourg, was erected in 1832, by M. Chaley, a French engineer.

Span.

Versed sine...

870.32 feet.

66 63.26

There are 4 main cables, 2 on each side of the road, of the same elevation, and about 1 inch asunder. Each cable is composed of 1056 wires, each about 0.118 inch in diameter, which are firmly connected and brought to cylindrical shape by a spiral wire wrapping. The diameter of the cable varies from 5 to 5 inches. The cables pass over 3 fixed pulleys on the top of the piers, upon which they are spread out without ligatures, and are each attached to two other cables of half their diameter, which are anchored at some distance from the piers, in vertical pits, passing over a fixed pulley where they enter the mouth of the pit.

The suspending-ropes are of wire a size smaller than that used for the cables. Their diameter is nearly one inch. They are formed with a loop at each end, fastened around a crupper-shaped piece of cast iron, that forms an eye to connect the rope with the hook of the stirrup affixed to the roadwaybearers, and to a saddle-piece of wrought iron, for each rope, that rests on the two main cables.

The roadway-bearers are of timber, being deeper in the centre than at the two ends, the top surface being curved to conform to a slight transverse curvature given to the surface of the carriage-way; they are placed about 5 feet between their centre lines, every fourth one projecting about 3 feet beyond the ends of the others, to receive an oblique wroughtiron stay to maintain the parapet in its vertical position. The carriage-way, which is about 15 feet wide, is formed of two thicknesses of plank. The foot-paths, which are 6 feet wide, are raised above the surface of the carriage-way, and rest upon longitudinal beams of large dimensions, the inner one of which is firmly secured to the roadway-bearers by stirrups which embrace them, and the exterior one is fastened to the same pieces by long screw-bolts, which pass through the top rail of the parapet. The roadway has a slight curvature from the centre to the two extremities, along the axis, the centre point being from 18 inches to about 3 feet higher than the ends, according to the variations of temperature. The main

cables at the centre are brought down nearly in contact with the roadway-timbers.

The parapet is an open-built beam, consisting of a top rail, the bottom rail being the longitudinal exterior beam of the footpath, and of diagonal pieces which are mortised into the two rails; the whole being secured by the iron bolts that pass through the roadway-bearers and the top rail. This combination of the parapet with the inclination towards the axis of the roadway given to the suspending-ropes, gives great stiffness to the roadway and counteracts both lateral oscillations and longitudinal undulations.

The piers consist of two pillars of solid masonry, about 66 feet high above the level of the roadway, which are united, at about 33 feet above the same level, by a full centre arch, having a span of nearly 20 feet, and which forms the top of the gateway leading to the bridge.

675. Hungerford and Lambeth Bridge, erected over the Thames, upon the plans of Mr. Brunel.

This bridge, designed for foot-passengers only, has the widest span of any chain bridge erected up to this period.

The main chains are 4 in number, two being placed on each side of the bridge, one above the other. These chains are formed entirely of long links of flat bars; the links near the centre of the curve having alternately ten and eleven bars in each, and those near the piers alternately eleven and twelve bars. The bars are 24 feet long, 7 inches in depth, and 1 inch thick. They are connected by coupling-bolts, 4 inches in diameter, which are secured at each end by cast-iron nuts, 8 inches in diameter, and 2 inches thick. The extremity of each chain is connected with a cast-iron saddle-piece, by bolts which pass through the vertical ribs of the saddle-piece, of which there are 15. The bottom of the saddle rests on 50 friction-rollers, which are laid on a firm horizontal bed of castiron. The saddle can move 18 inches horizontally, either way from the centre, and thus compensate for any inequality of strain on the main chains, either from a load, or from variations of temperature.

The side main-chains are attached in like manner to the saddle, and anchored at the other extremity in an abutment of brickwork. The anchorage (Fig. 193) is arranged by passing the chains through a strong cast-iron plate, and securing the ends of 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. 194) 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 the bottom (Fig. 195) they are connected by a joint with a bolt that fastens firmly the roadway-timbers.

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 horizon