At Wittengen, in Switzerland, is a very curious bridge, the contrivance of Ul•ick Grubenhamm, an uneducated car penter of Tutfen, in the canton of Appenzel, celebrated fur several works of the sane nat tire. It consists of two wooden arches parallel to each other, with the roadway hanging between them. The span is 230 feet, and rises only five feet. The arches approach the eatenarian shape, and are built of seven courses of solid oak logs, in lengths of 12 or 1.1 feet, and 10 inches and upwards in thickness. By picking these logs of a natural shape suited to the intended curve. the wood is nowhere trimmed across the grain. The logs being laid one upon the other, with their abutting joints carefully alternated, have the appearance of a wooden wall : instead of being pinned together, they are surrounded with straps of iron, at every distance of five feet, and fastened by bolts and keys. The abutments are the natural rock. The roadway intersects these arches at about the middle of their height, and is supported by cross joists, resting on a long horizontal beam, connected with the arches on either side by uprights bolted into them. Three of the spaces between these uprights have struts or braces, giving the upper work a sort of trussing in that part. The whole is covered with a roof, prgieeting over the arches on each side of the roadway, to defend the timbers from the injuries of the weather. This bridge is of more than sufficient strength to bear any load that can be laid upon it, though the attempt to truss the ends demonstrates that the builder was ignorant of true architec tural principles.
In 1754, Grubenhamm erected another bridge, upon a plan nearly similar to the foregoing, at Sehaffhausen, where the river (the lZhine) is nearly 390 feet wide. The current is very rapid at this spot, and had destroyed several stone bridges, when Grubenhamm offered to throw a wooden bridge across. of a single span ; but the magistrates were alarmed at the proposition, on account of the breadth of the river, and would scarcely listen to it : at last they consented that lie should build a bridge, provided he would divide it into two spans, and use the middle pier of the late stone bridge as a support at their junction. Grubenhamm complied with the wish of the magistrates so far as to divide his bridge into two unequal parts, the span of the one being 172 feet, and of the other, 193, both appearing to rest upon the old pier, though he contrived to leave it doubtful whether they really did so or riot. This structure cost £8,000 sterling, and travellers inform us, that though it sustained the most heavily laden waggons in perfect security, yet the weight of a single tbot-passenger caused it to tremble under him. It was destroyed by the French, when they evacuated Schaffhausen, in A oril, 1799.
Among wooden-bridges, the Schuylkill bridges at Phila delphia, in America, are very remarkable.
Wooden-bridges, unsupported by posts or pillars, and sus tained only by butments at the ends, have obtained the deno mination of l'endenl or Hanging Bridges,by some also called Philosophical Bridges, of which Palladio has described three modes of erecting ; such is the bridge over the Cismone, already described. Doctor Wallis has likewise given the design of a timber bridge, 70 feet long, without any pillars, which may lie useful where supports cannot be conveniently erected ; and Doctor Platt assures us, that formerly there was a large bridge over the castle ditch at Tutbury, in Stafford shire, made of short timbers, none of them above a yard in length, yet not supported from beneath, either by pillars or arches. The Spaniards use bridges of this kind for crossing
the torrents of Peru, over which it would be difficult, not to say impossible, to throw more solid structures, either of wood or stone. Some of these hanging-bridges are sufficiently strong and broad for loaded mules to pass along them with safety. In China, these flying bridges are constructed of an almost incredible magnitude; the Philosophical Transactions contain the figure of one, consisting °fa single arch, 400 cubits long, and 500 in height.
A great change in modern bridge-building has been effected by the introduction of iron, and the use of chain or suspen sion-bridges. The invention of Iron Bridges is said to be exclusively English, hut Duhalde gives the merit of it to the Chinese ; be that as it may, there is no country where there has been so extensive an application of the discovery, or in which has been erected so many fine bridges of iron as in Great Britain. The first was set up at Colebrook Dale, Shropshire, in the year 1'797, and was speedily succeeded by numerous others in all parts of the United Kingdom ; for a description of which we must refer to IRON BRIDGE.
Draw Bridges are of wood or iron, sometimes of both, with stone abutments. They are placed over navigable canals and rivers, or used in fortified places for the purpose of shutting out the enemy, and are of varied construction. Some are fastened at one end by hinges, so that the other end may be raised or lowered at pleasure. The most common method of doing this is by a kind of balance, called plyers, in which case the bridge NS hen drawn up stands erect, to preclude a passage across a moat, &e. Others are so constructed as to be drawn back, or thrust forward, as occasion may require. On small canals, &c., draw-bridges consist of one leaf only ; but on larger navigations, wet-docks, &c., they are of two pieces, meeting in the middle, and forming an arch, which are raised or lowered by means of balance frames, movable on the tops of uprights suited in height to the magnitude of the bridge ; such as that at Bristol, over the Frome. Such bridges, how ever, having been found inconvenient from their tackling catching the yards and rigging of vessels passing through them, a kind of bridge, diverse front all the preceding, has been in vented, called a Swivel Bridge: these, on small rivers, are only of one frame, or leaf, and turn on a centre, or series of balls or rollers ; but when made on a wider scale, they consist of two parts, one on each side of the channel, and meeting in the centre. The most complete of this kind are those con structed at the West-India and London Docks ; the latter spans 40 feet, and is 15 feet wide in the road-way. It con sists of eastiron ribs, about q- inch thick, turning on a num ber of' concentric rollers, which move between two circular cast-iron rings, very nicely turned : each leaf has a flap, which lets down by a screw, and abuts upon the stone-work on either side, forming the whole bridge, when shut, into an arch capable of bearing any weight that can possibly pass over it. The whole apparatus weighs 85 tons ; but it moves with so much ease, that it can be opened and shut iu less than three minutes.