In addition, there are four arches on the western side, and three on the eastern side of the main opening, each of 50 feet span

The roadways consist of two carriage ways, each 12 feet in breadth, with a footpath of 4 feet between them, so that the platform is about 30 feet in breadth. The whole is suspended by perpendicular iron rods, from four lines of strong cables of malleable iron, passing over a pyramidal support or pier, at each end of the main opening, the versed sine of the curve being 37 feet, or about 1/15 of the chord line or span. The weight of the bridge between the points of suspension, including the weight of the cables, is 489 tons, and the suspending power being calculated at 2,016 tons, leaves a disposable force of 1,674 tons to meet any stress the bridge may be exposed to. The-iron bar suspension bridge erected over the Thames at Hammersmith, is inferior only in magnitude to the one just described. It was designed and executed by Mr. T. Clarke, civil engineer, and was opened for public use in August, 1827. The above cut Fig. 1, represents a perspective view, and Fig. 2 an elevation of the bridge, and some of the details of its construction.

This bridge consists of two suspension piers or towers, built in the river, having an opening of 400 feet between them.

On the opposite shores are two strong abutments, over which a nearly level roadway passes through archways in the suspension towers. It is suspended by four lines of strong chains, hanging in curves from the abutments over the towers, and down between them, the roadway being supported from them by vertical rods. The principle of construction is as follows: D D Fig. 3, are the suspension chains, which consist of bars of iron 5 inches deep by 1 in thickness; these are 10 feet in length, and connected together, as shown in the plan beneath, by plates of iron having a strong bolt passing through them, and turned at the ends; these saddle pieces are placed ten feet asunder, and are so arranged that those in the upper chain are not immediately over those in the lower chain, but over the middle space between each two on the lower chain. From each saddle piece in each of the chains hangs a suspending rod C C, 11/4 inch thick, so that they are 5 feet asunder. The suspending rods are furnished with a joint where they are inserted into the opening between the chains or plates, enabling them to accommodate themselves to any extraordinary weight on the bridge.

At the height of about 40 feet from the roadway, the chains pass through the masonry of the archways before mentioned, and over friction rollers, and are secured to the ballast plates Fig. 4, which are sunk to a considerable depth behind the abutments. The roadway consists of transverse beams in two thicknesses, 44 by 12 inches, with an interval of 2\ inches between them; these are fastened at the bottom by keys to strong iron plates; along each side of the bridge extends a pair of strong beams, which are firmly bolted to the flooring joists; this connexion is shown at E E, Fig. 3. The roadway of the bridge is slightly raised towards the centre of the river, and the whole is boarded longitudinally with 3-inch planks, as shewn at Fig. 5, with a small space between each to prevent any water from settling on the bridge. There are two pairs of chains on each side of the bridge; the inner pair consists of six links or bars, and the outer of only three, as represented in Fig. 5. The total length of the bridge is as follows: -

Fig. 1.

Fig. 8.

Fig. 4.

Fig.5.

The towers are 48 feet high above the level of the roadway, 22 feet wide, and 42 feet broad. The roadway is 16 feet above the water, and consists of a carriage-way 12 feet wide, and two foot-paths, each 4 feet wide. The versed sine of the curve of the suspending chains is 29.6. Several bridges on the suspension principle, composed of iron wire, have also been erected, and on a small scale will no doubt answer; but if of great extent, the vibration becomes so great as to render them unsafe. Some of these, instead of being suspended from chains hanging in a curve between their- points of suspension, are supported by diagonal braces, proceeding direct from the supporting towers to different parts of the platform forming the roadway. The wire bridge of Dryburgh, which was constructed in this manner, had one of its largest radiating chains broken off at the point of suspension, by some mischievous persons shaking it violently. Shortly after it was repaired, a high wind again broke the chains, and completely destroyed the bridge. From the evidence of many persons, it appeared that in this gale the vertical motion of the bridge was equal to its lateral motion, and was sufficient to precipitate a person into the river.

The bridge has been since replaced by one on the Catenarian principle.

At Vienna there has been erected a steel suspension bridge over the Danube. The span is 234 feet, and the versed sins 15 feet. It is the work of M. Ignace Von Mitiz, who calculates that the total weight of steel is less than half the weight of iron which would be necessary for a bridge of the same dimensions.

A model of an iron suspension bridge, upon a new construction, was exhibited in the year 1829 It was constructed by Mr. Mottley as a design for a bridge across the Thames, from Scotland Yard, on the Middlesex side, to the King's Arms, on the Surrey shore. The annexed design for an ornamental bridge for parks or pleasure-grounds, will serve to explain the nature of Mr. Mottley's plan, which consists in a combination of the principles of tension and compression. The arch represented by the curved line is confined between two parallel lines, and those lines are connected together by vertical bars, to which the arch is inflexibly attached, and the parallel lines or bars also inflexibly jointed, and the whole combined by strong bolts at the top and bottom of every vertical bar, rendering it a skeleton of a trussed beam; thus affording an opportunity of having two paths across a river, the lower one of which may be used as carriage way, and the upper may serve for a footway or promenade. This second floor is not an absolutely necessary part of the structure; but if it be not adopted, it will be needful to have diagonal braces placed horizontally to stiffen the upper part.