A structure with one or more transverse apertures, raised for the convenience of passing a river, canal, valley, etc, and formed of various materials, as timber, stone, iron, etc. It is highly probable that the first bridges were composed of lintels of wood stretching from bank to bank, or, if this were impracticable, resting on piers or posts fixed in the bed of the river; and in China many considerable structures of this kind are still to be seen. As experience showed the defects of these early attempts, improved modes of construction naturally followed. In a strong current, the frequent piers or posts necessary for the support of lintels, would, by contracting the water way, increase it to a torrent, obstructive of navigation, and ruinous to the piers themselves. In constructing bridges over rapid rivers, it would, therefore, be found essential to their stability that the openings between the supporters should be as wide as possible, and every facility given to the free passage of the water; and as this could only be effected by arches or trusses, there can be no doubt that these inventions were perfected before bridges became common.

The most ancient bridges which we know of, are the work of the Romans, unless we except some of the stone bridges in China, with whose antiquity we are unacquainted; some of these latter are turned on arches in the usual manner, and others built with stones from 5 to 10 feet in length, so cut as each to form the segment of an arch. The Roman bridges generally consisted of a horizontal road, supported on one or more semicircular arches. Of the bridges of antiquity, that built by Trajan across the Danube is allowed to have been the most magnificent. It is described by Dion Cassius as consisting of twenty piers of squared stone, each of them rising 120 feet above the foundations, 60 feet in width, with a water way between every two of 170 feet, which was consequently the span of the arch, so that the whole length of the bridge was nearly 1,500 yards. It was destroyed by Adrian, lest it should afford a passage to the barbarians into the empire, and some of the piers are still to be seen near the town of Warkel, in Hungary. The next considerable Roman work of this kind is the Pont du Garde, which serves the double purpose of a bridge over the Gardon, and an aqueduct for supplying the people of Nismes with water.

The bridge, which consists of six arches, is 465 feet in length, and supports a second series of eleven arches, which are continued beyond the extremities of the bridge, and form a junction with the slope of the mountain on each side; it is about 780 feet long. Over these is a third series of thirty-five arches, much smaller than those below, 850 feet in length, supporting a canal on a level with two mountains, along which the water is conveyed to Nismes by a continued aqueduct. This extraordinary edifice is built with very large stones, held together by iron cramps without cement, and remains in excellent preservation to the present day. The whole height is 190 feet above the lower river. We may also briefly notice the bridge of St. Esprit, near Lyons, which is of Roman origin, and is 800 yards in length; it is very crooked, and bends in several places, forming many unequal angles in those parts where the river has the strongest current. The bridge over the Tajo, at Valenza de Alcantara, about 25 miles from Madrid, built in the time of Trajan, is raised 200 feet above the water, is 670 feet in length, and consists of only six arches. Near the old town of Brionde, in the department of the Upper Loire, is a stupendous bridge of one arch, the largest with which we are acquainted.

The span of the arch is 181 feet; its greatest height from the level of the water to the intrados 68 feet 8 inches; and the breadth 13. It is attributed to the Romans The following are amongst the most celebrated bridges of modern date: The bridge of Avignon, over the Rhone, begun in 1176, and finished in 1188. It consisted of eighteen arches, and was about 1000 feet in length. It was destroyed by a violent inundation of the Rhone in 1699; many of the ruinous decayed arches still remain. The Rialto, at Venice, was begun in 1588, and finished in 1591, after a design of Michael Angelo Bonarotti. It consists of a bold flat arch, nearly 100 feet wide, and only 23 feet in height from the level of the water. The aqueduct bridge of Alcantara, near Lisbon, begun in 1713, and finished in 1732, consists of thirty-five arches, of various dimensions; the eighth is the grand arch, which is 108 feet 5 inches in the span, and 227 feet in height; the other arches run from 21 feet 10 inches to 72 feet in width; the total length of the piers and arches is 2464 feet. The bridge of Neuilly, which crosses the Seine, built between the years 1768 and 1780, by M. Perronet. It is level on the top, and consists of five equal arches, 128 feet (English), with a rise of 32 feet (English).

The arches, which are elliptic, are composed of 11 arcs, or circles, of different diameters. The upper portion of the arch was formed with a circle of 160 feet radius, but after removing the centres, became flattened to an arc of a circle of 259 feet radius, the rise of the curve in a length of 33 feet amounting to no more than 6 inches. The bridge of Orleans, over the Loire, built by M. Hupean, between the years 1750 and 1760. It comprises nine oval arches, described from three centres which spring at 12 inches above low water. The centre arch is 106 feet span, with a rise of 30 feet; the others gradually decrease in width as they approach the shores. The whole length of the bridge is 1,100 feet We shall now proceed to notice some of the most remarkable bridges in our own country, beginning with those of the greatest antiquity. The Gothic triangular bridge of Croyland, in Lincolnshire, is supposed to be the most ancient structure remaining entire in this country, and for singularity of construction and boldness of design may vie with any bridge in Europe. It was erected about the year 860, and is formed by three semicircles, whose bases stand in the circumferences of a circle, equidistant from each other, and uniting at top.