BRIDGE

, a work of carpentry or masonry, built over a river, canal, or the like, for the convenience of passing from one side to the other; and may be considered as a road over water, supported by one or more arches, and these again supported by proper piers or buttments. Besides these essential parts, may be added the paving at top, the banquet, or raised footway, on each side, leaving a sufficient breadth in the middle for horses and carriages, also the parapet wall either with or without a balustrade, or other ornamental and useful parts. The breadth of a bridge for a great city should be such, as to allow an easy passage for three carriages and two horsemen abreast in the middle way, and for 3 foot passengers in the same manner on each banquet: but for other smaller bridges, a less breadth.

Bridges are commonly very difficult to execute, on account of the inconvenience of laying foundations and walling under water. The earliest rules and instructions for building of bridges are given by Alberti, in his Archit. 1. 8. Other rules were afterwards laid down by Palladio, Serlio, and Scamozzi, which are collected by Blondel, in his Cours d'Archit. pa. 629 &c. The best of these rules were also given by Goldman, Baukhurst, and in Hawkesmoor's History of | London Bridge. M. Gautier has a considerable volume expressly on bridges, antient and modern. See also Riou's Short Principles for the Architecture of Stone Bridges; as also Emerson, Muller, Labelye, and my own Principles of Bridges.

The conditions required in a bridge are, That it be well designed, commodious, durable, and suitably decorated. It should be of such a height as to be quite convenient for the passage over it, and yet easily admitting through its arches the vessels that navigate upon it, and all the water, even at high tides and floods: the neglect of this precept has been the ruin of many bridges. Bridges are commonly continued in a straight direction perpendicular to the stream; though some think they should be made convex towards the stream, the better to resist floods, &c. And bridges of this sort have been executed in some places, as Pont St. Esprit near Lyons. Again, a bridge should not be made in too narrow a part of a navigable river, or one subject to tides or floods: because the breadth being still more contracted by the piers, this will increase the depth, velocity, and fall of the water under the arches, and endanger the whole bridge and navigation. There ought to be an uneven number of arches, or an even number of piers; both that the middle of the stream or chief current may flow freely without the interruption of a pier; and that the two halves of the bridge, by gradually rising from the ends to the middle, may there meet in the highest and largest arch; and also, that by being open in the middle, the eye in viewing it may look directly through there. When the middle and ends are of different heights, their difference however ought not to be great in proportion to the length, that the ascent and descent may be easy; and in that case also it is more beautiful to make the top in one continued curve, than two straight lines forming an angle in the middle. Bridges should rather be of few and large arches, than of many smaller ones, if the height and situation will possibly allow of it; for this will leave more free passage for the water and navigation, and be a great faving in materials and labour, as there will be fewer piers and centres, and the arches &c will require less materials; a remarkable instance of which appears in the difference between the bridges of Westminster and Blackfriars, the expence of the former being more than double the latter.

For the proper execution of a bridge, and making an estimate of the expence, &c, it is necessary to have three plans, three sections, and an elevation. The three plans are so many horizontal sections, viz, first a plan of the foundation under the piers, with the particular circumstances attending it, whether of gratings, planks, piles, &c; the 2d is the plan of the piers and arches; and the 3d is the plan of the superstructure, with the paved road and banquet. The three sections are vertical ones; the first of them a longitudinal section from end to end of the bridge, and through the middle of the breadth; the 2d a transverse one, or across it, and through the summit of an arch; and the 3d also across, but taken upon a pier. The elevation is an orthographic projection of one side or face of the bridge, or its appearance as viewed at a distance, shewing the exterior aspect of the materials, with the manner in which they are disposed &c.

For the figure of the arches, some prefer the semicircle, though perhaps without knowing any good reason why; others the elliptical form, as having many advantages over the semi-circular; and some talk of the catenarian arch, though its pretended advantages are only chimerical; but the arch of equilibration is the only perfect one, so as to be equally strong in every part: see my Principles of Bridges. The piers are of diverse thickness, according to the sigure, span, and height of the arches; as may be seen in the work above mentioned.

With the Romans, the repairing and building of bridges were committed to the priests, thence named pontifices; next to the censors, or curators of the roads; but at last the emperors took the care of the bridges into their own hands. Thus, the Pons Janiculensis was built of marble by Antoninus Pius; the Pons Cestius was restored by Gordian; and Arian built a new one which was called after his own name. In the middle age, bridge-building was counted among the acts of religion; and, toward the end of the 12th century, St. Benezet founded a regular order of hospitallers, under the name of pontifices, or bridge-builders, whose office was to assist travellers, by making bridges, settling ferries, and receiving strangers into hospitals, or houses, built on the banks of rivers. We read of an hospital of this kind at Avignon, where the hospitallers resided under the direction of their first superior St. Benezet: and the Jesuit Raynaldus has a treatise on St. John the bridge-builder.

Among the bridges of antiquity, that built by Trajan over the Danube, it is allowed, is the most magnificent. It was demolished by his next successor Adrian, and the ruins are still to be seen in the middle of the Danube, near the city Warhel in Hungary. It had 20 piers, of square stone, each of which was 150 feet high above the foundation, 60 feet in breadth, and 170 feet distant from one another, which is the span or width of the arches; so that the whole length of the bridge was more than 1530 yards, or one mile nearly.

In France, the Pont de Garde is a very bold structure; the piers being only 13 feet thick, yet serving to support an immense weight of a triplicate arcade, and joining two mountains. It consists of three bridges, one over another; the uppermost of which is an aqueduct.

The bridge of Avignon, which was finished in the year 1188, consists of 18 arches, and measures 1340 paces, or about 1000 yards in length.

The famous bridge at Venice, called the Rialto, passes for a master piece of art, consisting of only one very flat and bold arch, near 100 feet span, and only 23 feet high above the water: it was built in 1591.— Poulet also mentions a bridge of a single arch, in the city of Munster in Bothnia, much bolder than that of the Rialto at Venice. Yet these are nothing to a bridge in China, built from one mountain to another, consisting only of a single arch, 400 cubits long, and 500 cubits high, whence it is called the flying bridge; and a figure of it is given in the Philos. Trans. Kircher also speaks of a bridge in the same country 360 perches long without any arch, but supported by 300 pillars.

There are many bridges of considerable note in our | own country. The triangular bridge at Crowland in Lincolnshire, it is said, is the most ancient Gothic structure remaining intire in the kingdom; and was erected about the year 860.

London bridge is on the old Gothick structure, with 20 small locks or arches, each of only 20 feet wide; but there are now only 18 open, two having lately been thrown into one in the centre, and another next one side is concealed or covered up. It is 900 feet long, 60 high, and 74 wide; the piers are from 25 to 34 feet broad, with starlings projecting at the ends; so that the greatest water-way, when the tide is above the starlings, was 450 feet, scarce half the breadth of the river; and below the starlings, the water-way was reduced to 194 feet, before the late opening of the centre. London bridge was first built with timber between the years 993 and 1016; and it was repaired, or rather new built with timber, 1163. The stone bridge was begun in 1176, and finished in 1209. It is probable there were no houses on this bridge for upwards of 200 years; since we read of a tilt and tournament held on it in 1395. Houses it seems were erected on it afterwards; but being found of great inconvenience and nuisance, they were removed in 1758, and the avenues to it enlarged and the whole made more commodious; the two middle arches were then thrown into one, by removing the pier from between them; the whole repairs amounting to above 80,000l.

There are still some more bridges in England built in the old manner of London bridge; as the bridge at Rochester, and some others; also the late bridge at Newcastle upon Tyne, which was broken down by a great flood in the year 1771, for want of a sufficient quantity of water-way through the arches.

The longest bridge in England is that over the Trent at Burton, built in the 12th century, of squared free stone, and is strong and lofty; it contains 34 arches, and the whole length is 1545 feet. But this falls far short of the wooden bridge over the Drave, which according to Dr. Brown is at least 5 miles long.

But one of the most singular bridges in Europe is that built over the Taaf in Glamorganshire, by William Edward, a poor country mason, in the year 1756. This remarkable bridge consists of only one stupendous arch, which, though only 8 feet broad, and 35 feet high, is no less than 140 span, being part of a circle of 175 feet diameter.

There is also a remarkable bridge of one arch, built at Colebrook Dale in 1779, of cast iron: and another still larger of the same metal, is now raising over the river Wear, at Sunderland, the arch being of 240 feet span.

Of modern bridges, perhaps the two finest in Europe, are the Westminster and Blackfriars bridges over the river Thames at London. The former is 1220 feet long, and 44 feet wide, having a commodious broad foot path on each side for passengers. It consists of 13 large and two small arches, all semicircular, with 14 intermediate piers. The arches all spring from about 2 feet above low-water mark; the middle arch is 76 feet wide, and the others on each side decrease always by 4 feet at a time. The two middle piers are each 17 feet thick at the springing of the arches; and the others decrease equally on each side by one foot at a time; every pier terminating with a saliant right angle against either stream. This bridge is built of the best materials, and in a neat and elegant taste, but the arches are too small for the quantity of masonry contained in it. This bridge was begun in 1738, and opened in 1750; and the whole sum of money granted and paid for the erection of this bridge, with the purchase of houses to take down, and widening the avenues, &c, amounted to 389,500l.

Blackfriars bridge, nearly opposite the centre of the city of London, was begun in 1760, and was completed in 10 years and three quarters; and is an exceeding light and elegant structure, but the materials unfortunately do not seem to be of the best, as many of the arch stones are decaying. It consists of 9 large, elegant, elliptical arches; the centre arch being 100 feet wide, and those on each side decreasing in a regular gradation, to the smallest, at each extremity, which is 70 feet wide. The breadth of the bridge is 42 feet, and the length from wharf to wharf 995. The upper surface is a portion of a very large circle, which forms an elegant figure, and is of convenient passage over it. The whole expence was 150,840l.

There are various sorts of bridges, of stone, wood, or metal, of boats or floats, pendant or hanging bridges, draw bridges, flying bridges, &c, &c, and even natural bridges, or such as are found formed by nature, of which kind a most wonderful one is described by Mr. Jefferson, in his State of Virginia; and another, but smaller, is described by Don Ulloa, in the province of Angaraez in South America.