SUSPENSION BRIDGE. This kind of construction is, perhaps, the simplest and most easily erected of all bridges ; we find examples of it spoken of in remote times, and there are few countries in the world where it may not be seen under some form or other. In England we now have many splen did bridges of this description, in which the combined labours of practical experimentalists and scientific theorists have pro duced monuments of surprising skill and admirable daring.
The principle on which suspension bridges are constructed, is exactly the reverse of that on which the stability of arched bridges depend. In the latter case, the force of gravity is, by an ingenious arrangement, made to counteract the natural tendency of all bodies to IA towards the earth ; and a com pressive force is called into play, by which the several parts of the bridge are kept in their proper positions, provided the materials have been properly disposed, and are sufficiently strong to bear the requisite crushing force.
111 suspension bridges, on the contrary, the roadway is supported by ropes or chains attached to towers and abut ments. so that the tension of the materials employed is prin cipally, although not altogether, acted upon, and the platform is sustained by the excess of their tensile powers above the weight (their own included) with which they are loaded. These bridges were not constructed to any great extent until the commencement of the present century ; up to that period they were of rather a primitive character. We hear, indeed, of one built in China about the year 65 A.D., and of several suspended bridges in South America and the Indies, in which the chains were formed of ropes or barks of trees plaited or interwoven together, so that the roadway was in the form of a catenary curve ; still, even in those countries we have instances of the roadway or platform being placed horizontally, and suspended from the ropes or chains by intermediate rods. Among the first Iron Suspension bridges put up in this coun try was the Winch Bridge crossing the river Tees near Middleton. It was merely intended for foot-passengers, being
only 2 feet in width, although i0 feet long. In 1816, a Wire Suspension bridge was built over the Gala Water, of upwards of 100 feet span, at a cost of only .£40 ; and another over the Tweed at Peebles in the following year, also of iron wire, and about the same span, fur £160.
In 1820, Captain Sir Samuel Brown built a Suspension Bridge over the Tweed, in an ingenious and novel manner, for which he had obtained a patent in 1817. Instead of making the main iron chains, intended for the support of the bridge, in the form of ropes, as had been hitherto done, lie constructed those chains by links of iron several feet in length, with holes formed at either end, through which he joined them together by means of bolts. This bridge, called the Union, is situate close to Berwick-on-Tweed, and has a span of 449 feet. It is supported upon 12 chains dis posed in pairs, and placed one above the other 3 deep, formed by links of round iron 2 inches in diameter, and each 15 feet long, connected with one another by intermediate short links also of iron. The suspending rods are of an oval section, and are placed 5 feet apart, every one being attached to a joint of one of the three main chains, which are made to break joint, so as to admit of this arrangement. These sus pension rods support the roadway, which is formed of timber. This bridge cost £50,000. In the following year, this engi neer built the Newhaven Pier, which is constructed on the same principle, having three openings, each 209 feet span ; and afterwards the Chain Pier at Brighton, which runs out into the sea a distance of upwards of 1,000 feet, having four openings, each 225 feet span. In the year 1823, a bridge of the kind Was erected by Sir Isambert Brunel, with two spans, each 122 feet; and the year after (1824) Tierny Clarke constructed the much-admired Suspension Bridge over the Thames at Hammersmith, spanning the river by an open of 422 feet, which has stood remarkably well.