Bridge, a construction of stone, timber, or iron, consisting of an arch or arches, and built over a river, canal, etc. for the convenience of passengers. A bridge built of stone is evidently the strongest and most durable: the proper situation for it is easily known; and the only circumstance necessary to be observed is, to make it cross the stream at right angles, that boats may readily pass through the arches with the current of the river.

Those, bridges built for a communication between high roads, ought to be so strong as to resist all accidents, and afford an easy passage to the waters : they should therefore be at least as long as the river is wide at the time of its greatest flood; because, by the accumulation of the waters above, too great a fall may be occasioned, and the foundation of the piers and abutments may thus be undermined.

The necessary requisites in a bridge are, that it be well designed, commodious, durable, and suitably ornamented. The pier* of stone bridges should be. equal in number, that the central arch may be -where the current is strongest. As the piers always diminish the bed of a river, it must be hollowed in proportion to the space occupied by them, especially where frequent inundations prevail, so that the waters may gain in depth, what they lose in breadth. It has been ascertained by experience, that when the height of the piers is six feet, and the arches are circular, it is sufficient to build the former two feet more in thickness than the sixth part of the width of the latter; or in other words, the thickness of the piers of an arch of 36 feet, ought to be 8 feet; those of an arch of 48 feet, 10 feet, etc. Rectangular piers are seldom adopted, except in bridges over small rivers. In all others they project by a triangular prism, which presents an edge to the stream, in order to divide the water, and prevent the accumulation of ice, as well as to hinder vessels from running against them.

When the banks of rivers are of a tolerable height, the bridge should be made quite level above, and all the arches of an equal width ; but where the banks are low, and, for the sake of navigation, a large arch is made in the middle of the stream, then the bridge ought to be more elevated in the centre than at the ends; in which case the slope should be easy and gradual on both sides, so as to form one continued curve.

The width of small bridges is generally thirty feet, but those near large towns usually have thirty feet clear carriage way, besides the foot-path : the parapet walls on each side are about eighteen inches thick, and four feet high; they commonly proje6t with a cornice underneath ; sometimes ballustrades of stone or iron are placed upon the parapet, as at Westminster-bridge; but this method is only employed where a bridge of great extent is constructed near a capital.

Where stone bridges cannot be erected on account of the expence, very strong and durable bridges may be constructed of wood : these ought to be so framed, that all the parts may press upon each other like an arch ; and thus, instead of being weakened by the pressure of a heavy body in its passage over it, they will become stronger. The method of forming a wooden bridge is so well known to every architect, that it is needless to enlarge upon the subject.

Among the Romans, the building and repairing of bridges was committed to the puntifices, or priests ; and the care of these edifices was afterwards undertaken by the Emperors themselves. In the middle ages, the constructing of bridges was classed among the acts of religion, and a regular order of hospitallers was founded by Saint Benezet, towards the end of the twelfth century, who were denominated pontifices, or bridge-builders ; their office was to facilitate the progress of travellers, by making bridges, establishing ferries, and receiving strangers into hospitals, or houses, on the banks of rivers.

Of all the bridges of antiquity, that built by Trajan over the Danube is allowed to be the most magnificent : the piers were 20 in number, built of square stone, and each 150 feet above the foundation, 60 feet in breadth, and 170 feet distant from each other. The piers of this vast structure still rere-main. Among modern bridges, that built over the Thames at West-minster may be considered as one of the most magnificent in the world ; it consists of 13 large and two small arches, together with 14 intermediate piers : the two middle piers are each 17 feet in thickness at the commencement of the arches, and contain 3000 cubic feet, or near 200 tons of solid stone : the middle arch is 76 feet wide.

Blackfriars bridge is also an exceedingly light and elegant structure ; it contains nine large arches of an elliptical form ; the central arch being 100 feet wide.

We cannot in justice avoid mentioning the patriotic efforts of Mr. Burdon, member for Durham, who in l792 obtained an aft for the erection of a bridge across the river Wear, near Sunderland. This structure is of cast iron, and consists of six ribs, at five feet distance from each other : the spandrils are composed of cast iron circles. The six ribs were put together over the river in the short space of ten days. The superstructure is of timber, planked over to support the carriage-road, which is composed of marl, lime-stone, and gravel, with a cement of tar and chalk upon the planks, to preserve them: the whole width of the bridge is 32 feet, and the arch is supposed to weigh upwards of 900 tons, of which 260 are iron.

This magnificent structure is erected on improved principles, yet differing from those adopted by the Colebrook-DaleCompany. Although adhering to the ancient construction of bridges, by the subdivision of the parts of the arch, in the manner of key-stones, Mr. BUR-don took advantage of the ductility and tenacity of iron, to produce an arch of that metal, at least fit-teen times lighter than a corresponding arch of stone, and most easily put together.

The expence of constructing this bridge amounted to 2(5, 0001. the whole of which, except 40001. was furnished by Mr. Burdon, This gentleman, in 1/95, also obtained a patent for his invention of the manner of making and applying cast iron blocks, to be substituted in lieu of key-stones, in the construction of arches, which blocks, tubes, etc. he employed in the bridge above-mentioned.

A patent was granted to Mr. Nash, of Dover-street, in 1/0/, for an invention somewhat similar to that above-mentioned. He provides hollow masses of cast or wrought iron, which are to be filled with earth, sand, etc. and have the appearance of solid bodies. According to this plan, the arch of the bridge is formed by hollow frames, or boxes, each consisting of four sides and a bottom. These boxes, after being properly arranged in the manner stated by the patentee, are then to be filled with sand, stones, etc. by which means the arch becomes like one solid body cased with iron. A farther explanation of this invention may be seen in the 6th volume of the Repertory of Arts and ManufacThe latest patent for bridges, was that granted in June 1800, to Mr. Samuel Wyatt, of Chelsea College, for his invention of a new method of construfting bridges, warehouses, etc. without the use of wood, as a constituent part. The principle of his discovery consists in the combination of pipes, tubes or hollow pieces of cast iron, in a longitudinal longitudinal direction and plates or pieces of the same material, having sockets in them to receive the ends or shoulders of the pipes, placed transversely, 3nd extending from one side of the bridge to the other, so that when the requisite number of pipes, etc. are put together, they form the arch so firmly as not to require the aid of screws, bolts, cramps, or any iron fastening whatever ; but the joints should be closed with lead or cement.

When applied to warehouses and other buildings, Mr, Wyatt's invention consists in forming arched cielings of cast iron, and supporting them and the floors by hollow pillars, or cylinders, of the same material. It ought to be remarked, that the number of sockets in the transverse iron plates, should always correspond with the number of ribs in the arch.

Various other kinds of bridges are constructed, according to the purposes for which they are designed, such as pendent or hanging-bridges, draw-bridges, floating-bridges, and those made of copper, or wooden-boats fastened with stakes or anchors, and covered with planks ; but as the description of them would swell this article to an undue length, and as they are not immediately connected with subjects of economy, we shall content ourselves with having given the preceding account of bridges in general.