BRIDGE, a work of masonry or timber, consisting of one or more arches, built over a river, canal, or the like, fur the con venience 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 para pet which incloses the whole, and the butments 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 an gular 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 stink or hollowed in pro portion to the space taken up by the piers, (as the waters gain in depth what they lose in breadth,) otherwise the current may wash away the foundation, and en danger 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 re quired, 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. For the particular mode of constructing stone bridges, the reader is referred to the latter end of the article But LMNG.
The triangular bridge at Croyland, in Lincolnshire, England, which was erect ed about the year 86U, is said to be the most ancient Gothic structure remaining entire in the kingdom. There are two circumstances in the construction of this bridge which render it an object of great curiosity. First, it is formed by three
semi-arches, whose bases stand in the cir cumference of a circle, at equal distances from each other. These unite at the top ; and the triune nature of the structure has led some to imagine that it was in tended as an emblem of the Trinity. Se condly, the ascent on each of the semi arches is by steps paved with small stones set edgeways, and is so steep, that none but foot passengers can go over the bridge : horsemen and carriages fre quently pass under it, as the river in that place is but shallow. For what purpose this bridge was really designed, it is diffi cult, if not impossible, to determine. Uti lity, it is obvious, was one of the least mo tives to its erection. To boldness of de sign and singularity of construction it has more powerful claims ; and these quali ties it must be allowed to possess, in as great a degree as any bridge in Europe. Although this bridge has been erected so many centuries, it exhibits no marks of decay.
London bridge is in the old Gothic style, and had twenty small locks or arch es; but there are now only nineteen open, two having been lately thrown in to one in the centre. It is 940 feet long, 44 high, and 47 clear width between the parapets. The piers are from 15 to 35 feet thick, with sterlings projecting at each side and end, so that the greatest water-way, when the tide is above the sterlings, is 545 feet, scarcely half the breadth of the river ; and below the ster lings the water-way is reduced to 204 feet, causing a dangerous tall at low wa ter. London bridge was first built with timber in the reign of Ethelred, between the years 993 and 1016 ; it was repaired, or rather rebuilt, of timber in 1163 ; and the present stone bridge was begun un der King Henry II. in 1176, and finished under King John in the year 1209. It is probable there were no houses on the bridge for upwards of 200 years, since we read of a till and tournament held on it in 1395. Houses were erected upon it afterwards, but being found a great in convenience and nusiance, they were removed in 1758, the avenues to the bridge enlarged, and the whole made more commodious : the two middle ar ches were then thrown into one, by re moving the pier from between them. The expense of the repairs amounted to above 80,0001.