BRIDGE, a work of masonry or timber, consisting of one
one or more arches, built over a river, canal, or the like, for the convenience of crossing the same.
Bridges are a sort of edifices very difficult to execute, on account of the inconvenience of laying foundations, and walling under water. The parts of a bridge are the piers, the arches; the pavement, or way over for cattle and carriages; the foot-way on each side, for foot passengers; the rail or parapet, which incloses the whole; and the buttments or ends of the bridge on the bank.
The conditions required in a bridge are, that it be well-designed, commodious, durable, and suitably decorated. The piers of stone-bridges should be equal in number, that there may be one arch in the middle, where commonly the current is strongest; their thickness is not to be less than a sixth part of the span of the arch, nor more than a fourth; they are commonly guarded in the front with angular sterlings, to break the force of the current: the strongest arches are those whose sweep is a whole semicircle; as the piers of bridges always diminish the bed of a river, in case of inundations, the bed must be sunk or hollowed in proportion to the space taken up by the piers (as the waters gain in depth what they lose in breadth) which otherwise conduce to wash away the foundation and endanger the piers: To prevent this, they sometimes diminish the current, either by lengthening its course, or by making it more winding; or by stopping the bottom with rows of planks, stakes, or piles, which break the current. It is also required that the foundation of bridges be laid at that season of the year, when the waters are lowest; and if the ground be rocky, hard gravel, or stony, the first stones of the foundation may be laid on the surface; but if the soil be soft sand, it will be necessary to dig till you come to a firm bottom.
Among the bridges of antiquity, that built by Trajan over the Danube is allowed to be the most magnificent; it was composed of twenty arches, of an hundred and fifty feet in height, and their opening from one pier to another was an hundred and sixty feet: The piers of this fine bridge are still to be seen in the Danube, being erected between Servia and Moldavia, a little above Nicopolis.
Among modern bridges, that of Westminster, built over the river Thames, may be accounted one of the finest in the world: It is forty-four feet wide, a commodious foot-way being allowed for passengers, on each side, of about seven feet broad, raised above the road allowed for carriages, and paved with broad moor-stones, while the space left between them is sufficient to admit three carriages and two horses to go abreast, without any danger. Its extent from wharf to wharf is 1220 or 1223 feet, being full three hundred feet longer than London-bridge. The free water-way under the arches of this bridge is eight hundred and seventy feet, being four times as much as the free water-way left between the sterlings of London-bridge: This disposition, together with the gentleness of the stream, are the chief reasons why no sensible fall of water can ever stop, or in the least
endanger the smallest boats in their passage through the arches.
It consists of thirteen large and two small arches, together with fourteen intermediate piers.
Each pier terminates with a salient right angle against either stream: the two middle piers are each seventeen feet in thickness at the springing of the arches, and contain three thousand cubic feet, or near two hundred tons of solid stone; and the others decrease in width equally on each side by one foot.
All the arches of this bridge are semicircular; they all spring from about two feet above low-water mark; the middle arch is seventy-six feet wide, and the others decrease in breadth equally on each side by four feet.
This bridge is built of the best materials; and the size and disposition of these materials are such, that there is no false bearing, or so much as a false joint in the whole structure; besides that, it is built in a neat and elegant taste, and with such simplicity and grandeur, that, whether viewed from the water, or by the passengers who walk over it, it fills the mind with an agreeable surprize. The semioctangular towers, which form the recesses of the foot-way, the manner of placing the lamps, and the height of the balustrade, are at once the most beautiful, and, in every other respect, the best contrived.
But the most singular bridge in Europe is that built over the river Tave in Glamorgan-shire. It consists of one stupendous arch, the diameter of which is 175 feet, the chord 140, the altitude 35, and the abutments 32. This magnificent arch was built by William Edward, a poor country-mason, in the year 1756.
Bridges are either built of stone or timber, as is judged most convenient.
Stone-BRIDGES consist of piers, arches, and buttments, made of hewn stone, sometimes also intermixed with bricks.
Wooden-BRIDGES are composed of beams and joists, supported by punchions, well cramped and bound together.
Rushen-BRIDGES are made of great bundles of rushes, bound fast together, over which planks are laid, and fastened: these are put over marshy places, to serve for a crossing ground.
Pendent or hanging-BRIDGES, called also philosophical bridges, are those not supported by posts or pillars, but hung at large in the air, sustained only at the two ends or buttments.