TIMBER BRIDGES.
This historical sketch respecting bridges, has hither to been confined to those constructed of stone, but other materials have been successfully employed. Bridges of great extent have been constructed of wood.
With the exception of drawings made by Palladio and others, from the descriptions given in Cxsar's Com mentaries, of his bridge over the Rhine, we have no satisfactory account of any ancient wooden bridge. Of those of more modern times, there is one described by Palladio, said to be situated upon the Cismonc, at the foot of the Alps, between Trente and Bassanc in Italy. It is of very simple construction ; the whole being sus pended by the framing, which forms the sides ; the open ing between the abutments is 109 feet. Palladio also gives sundry designs for wooden bridges formed in different ways, some of which are supported by the sides only ; and one is in the form of an arch. See Plate LXXXVII E. Gautier gives designs of his own, and one from ;lathourin Jousse ; the last mentioned consists of one passage over it for cavalry, and another for infantry, and is protected by a roof. But the boldest and most ingeniously constructed wooden bridge on the European continent, was that at Schauffhausen, in Switzerland.
1Ve are informed that there was formerly a stone bridge at Schau ifhausen, that the Rhine injured the piers, and that in the year 1754 three arches fell ; that the depth of water immediately on the upper side of the old piers being, during summer, from 18 to 20 feet, and from 28 to 30 feet below them, the idea of rebuild ing a stone bridge was abandoned, and that the old piers, excepting one near the middle, were taken away ; that Ulric Grubenman, a common carpenter of Tueffen, produced a model for a wooden bridge, supported only by the abutments on the banks of the river that after some hesitation on the part of the committee of Schauff hausen, his proposal was adopted, and that he completed this truly extraordinary work in the year 1758. The total length of the bridge was 364 feet, and its breadth ill feet. It was eight feet out of a straight line, and the angle pointed down the river ; the distance from the abutment next the town to the angle was 171 feet, and from the angle to the opposite shore 193 feet.
This magnificent and ingenious bridge was destroyed by the French in April 1799. See Plates LXXXIX. and XC.
About the same time that Ulric Grubenman was engaged at Schauffhausen, his brother John construct ed a bridge of the same kind at Ruiehenau, 240 feet in length ; and some years afterwards they jointly erect ed one near Baden, 200 feet in length, over the river Limmat.
We know of no wooden bridges in Europe of an extent equal to those constructed by the beforemen timed ingenious men. But one upon nearly the same principles, and of 250 feet span, has been constructed over the Portsmouth river in North America, by a Mr Bludget. Yet, though of inferior magnitude, several upon principles equally simple and effective, have been erected upon rivers in Scotland by James Burn, of Had dington, in East Lothian. The largest is over the river Don, about seven miles from the city of Aberdeen, upon the road which leads from that place to Banff ; the extent between the abutments is 109 feet 3 inches, and the breadth 18 feet. The frames which support the roadway are composed of short pieces of timber, but instead of being elevated above the level of the road way in order that it may be suspended from them, they here support it after the manner of stone voussoirs. This bridge was erected in 1803. Sec Plate LXXXVIII.
There is an elegant wooden bridge in the park at Wotton, a scat of the Marquis of Buckingham, con structed precisely upon the principle of one given by Palladio. The span of it is 87 feet, the versed sine 13, and the breadth across 20 feet.
Small timber bridges, being, in all countries abound. ing in wood, so obvious a means for crossing streams, it is impossible to trace their origin and progress ; and those consisting of rows of piles driven into the bed of a river, and supported by common trussings and bracings, being found in most countries, and being familiar to every body, it is only necessary, in what regards them, to refer to the Plates, and to what is said under the head of Practice.