BRIDGE. Most countries can exhibit specimens of bridge-building. The Chinese lay claim to high antiquity in the construction of arched bridges. One at Fou-tcheou-fou is 400 yards long, about 12 yards wide, and had formerly ranges of shops upon it ; and one at the estuary of Suen-tcheou-fou is 2500 Chinese feet in length, 20 in width, and has 252 stone piers, on which is laid a roadway of huge stone blocks. Chinese bridges have pointed, semi-circular, polygonal, and semi elliptical arches. The Romans executed many good specimens of bridge-building, some of which remain in use to the present time. Trajan's bridge over the Danube, the most stupendous work of the kind constructed by the Romans, had 20 stone piers, 60 Roman feet broad, and 150 feet, without the founda tions, above the bed of the river. The Pont du Gard, near Nismes, is a remarkable exam ple of their bridge-aqueducts. It consists at the base of a series of six arches, forming a bridge of 465 feet long ; this is surmounted by a longer series of arches, extending 780 feet, to the slope of the mountains on each side ; and above these is a third series, of 35 smaller arches, extending to the length of 850 feet, on the top of which is the aqueduct or channel for water. The entire height is 190 feet.
The bridges erected by the Romans in the provinces under their sway afforded models from which the art of constructing bridges extended throughout the north and west of Europe. No country possesses finer or more numerous examples than our own ; in which, even before the extraordinary impulse given to this department of civil engineering by the introduction of the railway system, it has been carried to the utmost perfection. The oldest bridge now existing in England is the triangular bridge at Croyland, in Lincolnshire, which is said to have been erected about A.D., 860; it consists of three semi or half-arches, whose bases stand equidistant from each other, in the circumference of a circle, and unite at the top. Old London Bridge, which, with numerous alterations and patchings-up, re mained in use until 1831, was begun in 1176, and was for several centuries covered with houses. The modern bridges across the Thames at the metropolis form noble speci mens of bridge architecture and engineering ; but those constructed in the last century, Westminster and Blackfriars bridges, have entailed heavy subsequent charges on account of the insufficiency of the foundations. New London Bridge is of granite, 928 feet long between the abutments, and consists of five semi-elliptical arches, of which the centre one has a span of 152 feet, while the next pair and the abutment arches are 140 and 130 feet respectively. It was commenced in 1824, and
completed in 1831. Southwark Bridge is 718 feet between the abutments, and consists of three cast-iron arches each forming a seg ment of a very large circle ; the span of the centre one being 250 feet, and of the others 210 feet each ; they are supported by granite piers. Waterloo Bridge is a fine example of a flat or level bridge, with a horizontal road way, supported by a series of arches of equal elevation. It is of granite, has nine elliptical arches of 120 feet span, and is approached by elevated roads, supported upon brick arches. The length between the abutments is 1380 feet, the river being wider at this point than it is lower down ; but, including the arched approaches, the total length is 2456 feet. The cost was upwards of 1,000,0001. Some very !me specimens of equal-arched bridges have been erected by French engineers, of which the Pont de Neuilly, built by M. Perronnet, between 1768 and 1780, over the Seine, has five arches of 128 feet span.
The first iron bridge in England was erected in 1770 over the Severn at Coalbrook-dale ; it consists of a single arch of about 100 feet span. Bishop.Wearmouth iron bridge, com pleted in 1798, is a single arch of 240 feet span; and Sunderland iron bridge, built about the the same time, is an arch of 236 feet span, and has a clear height from low water of 60 feet to the spring and 04 feet to the centre of the arch, so that ships of 300 tons pass under it by lowering their top-gallant masts. The very important class of iron bridges in which the suspension principle is adopted are treated of under SUSPENSION-BRIDGE. Swing-bridges, and other contrivances for removing the road way when it is necessary to allow a passage for masted vessels, constitute another class of iron bridges on which much ingenuity has been expended. In many cases such a bridge consists of two parts, which when closed meet in the centre, and which are either raised or turned aside out of the way when the water way has to be opened. Another curious con trivance for a similar purpose is the telescope. bridge introduced by Mr. Rastrick, on the Brighton and Chichester Railway; in which a massive platform, more than twice as long as the water way is broad, is so mounted on wheels as to be capable of having one half either projected across the river Arun, or drawn back out of the way ; an adjoining por tion of the railway being laid on another moveable platform which may be rolled to one side to make way for the larger platform when it is thus pushed back from the river.