Page:Willich, A. F. M. - The Domestic Encyclopædia (Vol. 1, 1802).djvu/386

354&#93; $54] BR1 ornamented. The piers of stone bridges should be equal in number, that the central arcli may be where the current is strongest. As the. piers always diminish the bed of a river, it must be hollowed in pro- portion 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, &c. Rectangu- lar piers are seldom adopted, ex- cept in bridges over small rivers. In all others they project by a tri- angular prism, which presents an edge to the stream, in order to di- vide 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 eighteenincb.es thick. BR I and four feet high; they commonly project with a cornice underneath ; sometimes ballustrades of stone or iron are placed upon the parapet, as at Westminster-bridge 3 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 en- large upon the subject. Among the Romans, the build- ing and repairing of bridges was committed to the pontijices, or priests ; and the care of these edi- fices 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 Benkzet, towards the end of the twelfth century, who were de- nominated pontijices, or bridge- builders ; their office, was to facili- tate the progress of travellers, by making bridges, establishing fer- ries, and receiving strangers into hospitals, or houses, on the banks of rivers. Of all the bridges of antiquity, that built by Trajan over the Da- nube is allowed to be the most magnificent : the piers were 20 in number, built of square stone, and each 150 feet above the founda- tion, 60 feet in breadth, and 1 70 feet distant from each other. The piers