Suspension Bridges have only lately been introduced into this country, though known to the Chinese from a very early period. The iron-chain bridge of Yunnan is supposed to have been erected about a. D. ti5, in the reign of the emperor Mingus, and is described as very similar in principle to the Hammersmith Suspension Bridge near London. In Kircher's China Illustrate, it is stated, that the chord-line is of the length of 200 cubits. In the Asiatic Researches, Turner gives a very interesting account of the singular bridges erected by the natives of Bootan. These bridges are of varied construction, but admirably adapted to the circum stances for which they are intended. Over the widest river in Bootan, there is au iron bridge, consisting of a number of iron chains, which support a matted platform ; and two chains are stretched above, parallel to the sides, to support a matted border, which is absolutely necessary for the safety of the passenger, who is certainly not quite at his case till he has landed from this swinging, unsteady footing. At another place, a bridge for foot-passengers is formed by two parallel chains, round which creepers are loosely twisted, from which planks are suspended, the end of one plank a upon the resting other without being confined.
In the rude suspension bridge of South America, with its ropes of twisted bark, and its platform of cross pieces I If wood interwoven in them, or the platform attached imme diately to the sustaining ropes, the form assumed, the cate narian curve, is the same as in the more perfect structures of modern times—and one traces easily the transition from the simple but effective contrivance of the untutored Indian, to the masterpieces of the genius of a Telford. See SUSPEN SION BRIDGE.
Bridges of Boats are made of boats, either of copper, tin, or wood, fastened across the stream by means of anchors or stakes, and laid over with planks. The earliest instance upon record of this kind of bridge, was that laid by Darius I lystaspes over the lster, or Danube, in his Scythian expedi tion, 508 years before the Christian era. The same monarch also crossed the Thracian Bosphorus with 700.000 men by means of a bridge of boats ; the strait being live stadia, or 1,008 yards in width. Modern armies carry with them tin or copper boats, called pontoons, to be ready on any emer gency : several of them, placed side by side, across the river, till they reach the opposite shore, with planks laid upon them, form a plane for the soldiers to march on. At Beaucai re, Bowen, and Seville, are very fine stationary bridges of boats, which rise and fall with the tide : that at Rouen is nearly 300 yards long, and paved with stone, so that laden carriages and horses, as well as foot-passengers, go over it in safety. In the absence of pontoons, military bridges have been made • of blown bladders, hollow casks, sheaves of rushes, &c., covered over with planks.
When bridges of this kind do not extend over the whole breadth of the river, but are contrived to float from one side to the other, they are termed Flying or Floating Bridges.
A bridge of this description is generally composed of several boats connected with each other by a flooring of planks, and surrounded by a railing. This stage or raft is furnished with one or more masts, according to its dimensions, to which is fastened a strong cable, supported at proper dis tances by boats, and extending to an anchor, in the middle of the water, where it is made secure. The bridge thus becomes movable, like a pendulum, from one side of the river to the other, with the assistance only of a rudder. Such bridges were formerly sometimes constructed of two stories, lbr the more expeditious passage of a great number of men.
Another kind of flying bridge is formed of two platforms, laid one upon the other, and by means of' cords and putties the uppermost is made to run out beyond the lower platform, till its farther extremity rests against the place it was designed to reach. In the Histoire de l'Acadhnie Royale des Sciences, for the year 1713, page 101, is a description of a floating bridge, which lays itself on the opposite side of a river.
Under this head we have now to describe one of the most useful and ingenious constructions of modern science and engineering skill—the steam Floating Bridge invented by Mr. J. M. Bendel, the eminent civil engineer. The first bridge on this principle was erected by Al r. Bendel across the estuary of the Dart at Dartmouth, about the year 1832, and a similar one was established about two years after, across the Hanioaze, between Torpoint and Devonport.
A very full description of the latter, accompanied by elaborate drawings, has been furnished to the Institution of Civil Engineers by Mr. Bendel himself, and from the first - - — volume of the "'Transactions" of the Institution, we have extracted the following brief sketch.
The medium width of the river at the site of the bridge may be taken at 2.350 feet, the strength of the current alter heavy land floods is very great, and the site so much exposed, that it is not uncommon for the ships lying in the vicinity of the bridge to drag their moorings. The Inidge is a is llat-bottomed vessel, of a width nearly equal to its length. The vessel is divided in the direction of its length into three parts—the middle one being appropriated to the machinery—each of the side divisions to carriages, &c. These side divisions or decks are raised about 2 feet above the line of flotation, and by means of movable platfiirms, an easy emomunication is afforded with the shore on embarking or landing. The bridge is guided by two chains, which pass ing through it over east-iron wheels, are laid across the river anilliistencil to the opposite shores, forming as it were a road along which the vessel travels backwards and forwards.