Some suspension-bridges have been erected in which the main chains, instead of passing over a pier or tower at or near each end of the platform, are supported by a single tower in the centre of the bridge, and form what may be called two semi-catenaries. A bridge of this kind was built in 1823 by Sir M. I. Brunel, in the Isle of Bourbon. The bridge consists of two openings of 122 feet each. A few suspension-bridges have been built in which the chains are suspended below the platform, which is supported by framework built upon instead of hanging from them.
Some small bridges of wire have been alluded to already, and Drewry describes several of large dimensions which were erected on the Continent soon after the intro duction of suspension-bridges upon an exten sive scale in Great Britain. The first large one erected in France was that of Tournon, across the RhOne, between Tain and Tournon, in 1824-5; it consists of two openings of 278 feet each. Another remarkable wire-bridge is that.over the Sarine, at Fribourg in Switzer land, which was completed in 1834, by M. Chaney. It has a span, from pier to pier, of 870 feet, and is 167 feet above the level of the river, being much longer and higher than the Menai Bridge. The platform is suspended from four cables, arranged in pairs at the sides of the bridge, with a deflection of 55 feet. The wire of which the bridge is com posed is about one-twelfth of an inch in dia meter, and each cable consists of fifteen bundles of eighty wires each, packed together in a cylindrical form, and bound round at intervals of two or three feet with annealed wire. The wires are not twisted together like the strands of a rope, but each of them extends straight from end to end of the cable. The two pairs of cables are suspended at a distance of 30 feet from each other, but the width between the lower ends of the suspen sion rods is only 24 feet, so that their position is not quite vertical. These rods are small cables, consisting of thirty wires similar to those of which the cables are composed. The roadway is formed of fir, planks, supported by transverse beams, and stiffened by a strong oak railing, or diagonal truss. The bridge was completed at a cost of 24,0001., and was publicly opened in 1834.
The use of wire instead of bar-chains in the construction of suspension-bridges is favoured, by the simplicity of the apparatus necessary for their erection, and the superior strength, bulk for hulk, of small wires over bars of considerable dimensions. In some instances the chain system and the wire system have been combined. There is a chain bridge at Geneva, in which both the long and short links may be called skeins of wire, bound round into a cylindrical form in the centre, and spread out into broad loops at the ends, where they embrace the hollow- bolts. The structure is in fact a chain bridge, of which the chain-bars and linking-plates consist of bundles of wire instead of solid bars.
Several suspension-bridges of small span have been constructed upon an ingenious plan which combines the advantages of the oppo site principles of tension and compression.
The first of these was the Monk Bridge, across the river Aire at Leeds, which was erected in 1827, by Mr. George Leather, of that place. The platform is supported by vertical suspension-rods, the upper ends of which, instead of being attached to catenarian chains, are supported by rigid arches of cast iron, which rising between the carriage-way and the footpaths, are elevated above the level of the platform. The suspending rods area malleable iron, and they sustain transverse cast-iron beams upon which the roadway is laid. Timber bridges have been constructed on a similar principle to the above. Drewry mentions one at Eglisau, near Ziirich, in Switzerland, consisting of two arches formed of beams 15 feet long, 10 inches broad, and 1 foot deep, with their abutting ends secured by iron straps. The ends of the arches are tied together by horizontal braces, and the weight of the platform is suspended by vertical bars arranged in pairs, which embrace the wooden arch or rib, and are bolted together above it.
Much attention has been excited by the principle of constructing suspension-bridges introduced by Mr. Dredge of Bath, who ob tained a patent for his invention in 1836. The leading features of his plan are the adaptation of every part of the chain to the precise amount of strain to which it is exposed, by diminishing the number of plates, and conse quently the weight and strength of the chains, from the points of suspension to the lowest or central point of the catenary ; and the position of the suspending rods, which, instead of being vertical, are arranged in oblique lines from their points of attachment to the main chains towards the centre of.tho catenary. The Victoria Bridge over the Avon, at Bath, which was built on this plan in 1836, contains onlytwenty-one tons of it is of 150 feet span. The number of bridges since built on this plan is very considerable. An example of them on a very small scale may be seen in the Regent's Park. In 1827.8 a suspension-bridge of steel was erected over the Danube at Vienna, by an engineer named Von Mitis; the bridge alluded to at Vienna has a chord-line of 331 English feet, with a deflection of nearly 211 feet.
Perhaps the finest suspension bridge ever constructed is that built by Mr. Tierney Clark over the Danube at Pesth. In magnitude and in cost it exceeds all others. It was finished in 1840, and was shortly afterwards traversed by the Austrian and Hungarian armies.
Suspension-bridges are well adapted for many situations in which, from the limited traffic, the expense of ordinary stone bridges would prevent their adoption, and also for places in which, from the great span required, the great elevation, the unfavourable nature of the bottom, or the rapidity of the current to be crossed, the erection of any other kind of bridge would be difficult; but they are not applicable to situations of great and constant traffic, since they are much weaker than arch bridges, and very liable to injury from the vibration occasioned by what might appear slight forces.