BRIDGE (Ang.-Sax. brycg; Dutch, brag; Ger. brucke) is a structure for carrying a road over a stream, river, ravine, low ground, or other impediment to its course. A bridge for carrying a canal or other water-course, is called an aqueduct (q.v.); one for carrying a railway is sometimes called by the recently coined, though not very correct, word viaduct (q.v.). Bridges are formed of stone, brick, cast-iron, or timber arches; of timber beams or frame-work, supported on piles or on masonry; of iron rods or chains, in which case they are called suspension-bridges; of lattice-work; or of cast or wrought-iron girders. Sometimes a combination of beams and suspension-rods is used. Of late years, the plan of tubular or hollow wrought-iron girders has been frequently and successfully employed, the first great example being the Britannia bridge (q.v.).
Bridges are either fixed or movable. Of movable bridges there are various kinds. and are, in fact, mere ferry-boats (see FERRY) with gang ways attached, ane other provisions for safe and ready transport, and which are drawn across the stream by ropes. and are constructed in two parts, that turn on pivots—in the former, the parts are lifted vertically; in the other, they are moved round horizontally. A runs backward and forward on wheels or rollers. Another kind is much in use in low districts like Norfolk, where the water flaws lazily, and almost on the land-level. These are sometimes called pontoon-bridges, from the movable roadway being balanced at a small height above the water-level on a pivot working in a large pontoon or hollow cylinder sunk in the bed of the river—the ends of the roadway of the B., when laid across the river, resting freely on piers on either side. There are several such bridges in use over the Ouse. The pivot is set in the center of the stream, and, when necessary. the B. is turned round on it by machinery, till it lies parallel to the banks, and permits the passage of barges on either side. fn a flat district, these bridges are exceedingly appropriate. See also BRIDGE, 311LrrAnv.
Convenience must have led men in a very rude state of society to form bridges, in order to the easier communication between districts separated by rivers. On most streams there occur fords, but often these are not to be found where they would be most desirable. The most rudimentary form of a B. may be assumed to be a series of stepping-stones, such as are yet almost everywhere to be found on river-courses at some point. Large stones deposited in the streams at the shallows or fords, would first give a chance to a passenger of getting across dry shod; by and by, where one or two stones were wanting to complete the steps in the passage, they would be supplied. Next, it
would naturally occur to give greater security to the passage, by laying planks or trees across the stepping-stones, so as to avoid the risks attending stepping or leaping from the one to the other. In the arrangement of planks resting thus on stones, we have the first advance iu the art of bridge building, the suggestion at once both of piers and road ways; and beyond this stage, the art would appear not to have advanced for a very long period. From the Greeks, we have accounts of bridges built by Semiramis, Darius, Xerxes, and Pyrrhus; and in Egypt, necessity early compelled the formation of bridges in connection with the canals constructed for the purpose of irrigation. But all these would appear to have been rudimentary in form, and to have consisted simply of piers, with the intervals between them spanned by beams of tiinber or large flat stones. Sometimes boat ; moored in the stream served the purpose of piers, as was the case with the famous 13. of Xerxes across the Hellespont. l3ridges of boats are in use to this day. The principle of the arch was long known before it was applied to the art of bridge-building. See ARCH. That application we owe to the Romans, whose first great work in which the arch was employed, the Cloaca Maxima (q.v.), is referable to the time of the Tarquins. The Ponto de Rotto, or Senators' 13. (127 B.c.), erected by Caius Flavius, appears to have been the first instance of its application to bridges. In the course of the great engineering undertaking of the Roman empire under Augustus Ctosar for the formation of roads and supply of water to Rome, its application became general; and afterwards, the empire having extended its bounds, the necessity for ready communication between its provinces, led to the erection of numerous splendid bridges therein, many of which, indeed, surpassed in their greatness those of Rome itself. But although the Romans have unquestionably the merit of having originated the art of bridge-building proper in Europe, yet it seetns doubtful whether the principle of the arch was not applied by eastern nations to bridge-building long before the dawn of the greatness of the Roman empire. The Chinese are said to have been before the west in this as in other arts, though the antiquity of some of the bridges on which this assertion is rested may well be doubted, considering the uncertainty which pervades the chro nology of that extraordinary people.