2. If the bridge consists only of one small arch, and there is little intercourse, the road-way may be narrow and without footpaths ; if the bridge is long, and there is much intercourse, the breadth must be increased, and have proper footpaths. Since wheel carriages have been in general use, few bridges, however small or remote from towns, have been made less than fifteen feet in breadth over the parapets ; they are more gene rally made from 18 to 20 feet, in or near to considera ble towns, from 26 to 30 ; and in or near large cities, they are made from 30 to 50 feet. The roadway should have a declivity from the middle of the length each way towards and beyond the abutments, of about 1 in 24 ; and the roadway, as well as the footpaths and parapets, should, near their extremities, diverge, to suit the approaches, whether one or more, at each end of the bridge.
O. The decorations should be varied, according to the situation and accompanyments. In the country, the utmost simplicity, consistent with distinguishing the essential parts, should be preserved ; and even in the most splendid cities, or adjacent to palaces, all decora tions should be kept perfectly subservient to, and in uni son with, the essential parts : the neglect of this is a frequent error in designing bridges. Columns and enta blatures, though proper in a Grecian temple, aro ill suit ed to an edifice, where forms unknown to the Greeks arc the leading features. As columns can only be placed over the piers and abutments, the entablature, intended to represent beams of timber, cannot be supposed to be wholly upheld by supports placed at such great distances from each other. And the introduction of columns, in place of carrying up the piers, deprives the superstruc ture of powerful buttresses in situations where they would prove very beneficial. The affectation of pre
serving the entablature upon a perfect level, has led to making the roadway along the bridge also level, which is nothing less than constructing, at a vast expense, a piece of road more imperfect than what is formed by the common labourer in the open country ; and besides, this mode of construction gives an appearance of feeble ness to the outlines of the bridge. This false taste was introduced by some of the French engineers, and has of late been in some instances, copied in Britain. It cannot be too early reprobated, because bridges, when substan tially constructed, remain for many ages, and are not easily altered.
In making out the design, the engineer should furnish a plan, shewing the form and dimensions of the founda tions of the piers, abutments, wing-walls, and wharf walls connected with the bridge ; an elevation shewing the general facade ; also vertical, longitudinal, and cross sections, shewing the construction of the interior parts. In large bridges, there should be a plan or horizontal section taken at about one third way up the rise of the arches, in order to slim the mode of filling up the spandrels. There should likewise be a plan of the road way, footpaths, and parapets ; besides separate drawings of all those parts which cannot be made sufficiently dis tinct in the general drawings. A specification should likewise be added, describing in words, thquality and dimensions of every part of the work, and &he form and manner of its construction.