Towards the close of the eighteenth century some bridges were erected, the arches of which were constructed entirely of cast iron. The honour of this invention is due to this country, and the first of these structures was the bridge over the Severn, at Colebrook Dale, erected by Mr. Darby, in 1779. Some years after (about 1790,) the celebrated Thomas Paine had one constructed at the same place, which he intended for America, and which wa put together in a meadow near that place ; but he not being able to pay for it, it was taken down, and part of the materials were employed in the construction of the Sunderland Bridge, erected by Mr. Bunion over the river Wear, in 1793. Subsequently, numerous iron bridges have been erected: the principal ones are Buildwas (Colebrook Dale Company), 1795-6 ; Tame, Herefordshire (failed), 1795-6; Parret, at Bridgewater, 1796 ; Staines (twice failed), 1800 ; Tees, at Yarm (failed) ; Boston, in Lincolnshire; New River at Bristol (two); Vaux hall Bridge, over the Thames; but the most celebrated of these structures is the magnificent one over the Thames, called the Southwark Bridge, designed and erected by the late Mr. Rennie. This noble bridge consists of three circular arches of cast iron, supported by piers of granite; the centre arch is of 250 feet span, and the side arches 210 feet each. The piers arc 24 feet thick. The bridge is 718 feet long between the abutments, and 42 wide between the parapets. There is a dry arch over the road on the Southwark side. The work was begun in 1814, and completed in 1824. The annexed cut is a view of the centre arch.
Iron Suspension Bridges were in use in Europe at the time of Scamozzi, the architect, who mentions them in his work Da Idea Archi, 1615 ; but the knowledge requisite to determine the properties of this kind of bridge was not published till the time of Bernouilli. Suspension bridges are described as existing in various parts of Asia, Africa, and America, before this species of construction began to be practised in Europe. The first chain bridge erected in England is supposed to be that over the Tees, forming a communication between the counties of Durham and York. Since that time the number of these structures has been increasing, some of which, in size and magnificence, exceed any thing of the kind in the known world. Of these, the most cele brated is the one recently erected over the Straits of Menai, by Mr. Telford. The bridge consists of one opening of 560 feet between the points of suspension, and 100 feet in height between the high-water line and the under side of the road-way, which is horizontal. 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 foot path 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 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 road way 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 con sist of bars of iron 5 inches deep by 1 in thickness ; these are 10 feet in length, and con nected 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, It 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, ena bling them to accommodate them selves to any extraordinary weight on the bridge. At the height of about 40 feet from the roadway, the chains pass through the ma sonry of the archways before men tioned, 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, 4} 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 lon gitudinally 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 :— The towers are 48 feet high above the level of the roadway, 22 feet wide, and 42 feet broad. The road way 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 sus pending 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 vibra tion 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, pro ceeding direct from the supporting towers to different parts of the platform forming the road way. The wire bridge of Dryburgh, which was constructed in this manner, had one of its largest radiating chains broken off at the point of suspen sion, by some mischievous persons shaking it vio lently. 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 sine 15 feet. It is the work of M. Ignace Von Mitiz, who calcu lates 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.