2. By the principal rafters reaching from the abut ments to an intermediate part of the horizontal beam, and having a stretching piece between their tops. There arc instances of this being repeated in the same bridge below the level of the beams, and also again in the side railing of the roadway.
3. By constructing a series of short frames, and plac ing them vertically in the manner of stone voissoirs, and upon these raising the structure of the roadway and railing. This mode was pointed out by Palladio, as suitable for wooden bridges of great extent. Gautier acknowledges that he had adopted Palladio's idea, in forming a design of a wooden bridge for the Seine ; and in England, the cast iron bridges at Wearmouth, in the county of Durham, and at Boston, in Lincolnshire, are also formed on this principle.
4. A very ingenious mode has been practised by James Burn of liaddington, in a wooden bridge of 109 feet 3 inches span, and 13 feet 4 inches rise, over the river Don, about 7 miles from the city of Aberdeen, on the road to Banff. Mr Burn takes a series of frames in the form of an arch, but each set of frames is laid hori zontally across the soffit. The bridge is 18 feet in width ; and there are two rows of frames, or as it were two ribs placed four inches distant from each other. Upon these arch frames, a vertical framing is construct ed to support the bearers, the joists, planking, and gra vel ol the roadway. See Plate LXXXVIII. This bridge was erected in 1803. Mr Burn built another wooden bridge upon the same principles over the same river, at Granholm, within four miles of Aberdeen, to open a com munication with the extensive manufactory of James Madden, Esq. and Co. This bridge consists of two arches, each 71 feet 6 inches span, and 10 feet 6 inches rise : it has only 10 feet 6 inches in width of soffit, and consists of one row of frames. Here there is, of course, a pier in the middle of the river.
The same artist has liken ise built, on the same prin ciples, a wooden bridge of three arches over the river South Esk, in the park of William Maule, Esq. at Bre chin in Angus•shire. The middle arch is 58 feet span, and 10 feet rise ; the bridge is 15 feet in width across the soffit : and there are two rows of frames in each arch, with a space of one foot between them. The piers are of stone, and the spandrels of the arches arc covered with boarding, and painted and sanded in imitation of stone, so that the whole has the appearance of three neat stone arches ; but the apparent effect of the carpentry by this means is lost.
We know of no wooden bridges in Britain so judi ciously constructed as those executed by Mr Burn. Short pieces of timber may be employed. The princi pal pieces abutting endwise, little change can take place from shrinking. The principle will admit of cam.) ing an arch to a very great extent ; and by judicious arrange ment, the parts may be taken out and renewed separately.
When it is necessary to keep the roadway low, the trussing is performed chiefly above its level. When the bridge is narrow, the supporting framing is made en tirely on each side, and the roadway suspended between. When it is wider than is advisable to have supported en tirely by outside frames, another is introduced on each side of the carriage way, separating it from the foot paths ; or there are two carriage ways, with a frame or truss between them.
The following short description, extracted chiefly from a publication by Mr Taylor of Holborn, to whose judicious exertions the British artists are much indebt• cd, with the annexed Plates, will convey a distinct idea of the manner in which the ingenious Ulric Grubenman constructed the celebrated bridge over the Rhine at Schaffhausen, in Switzerland. Fig. 1, Plate LXXXIX., exhibits an elevation of one side, including the roof, which was covered with shingles. Fig. 2 is a cross sec tion at AAA, sliming the uprights which are placed on the pier, the framing under the level of the roadway, the points from whence the braces proceed, the mortices for the beams which support the roadivay, and the interior construction of the roof at these uprights. Fig. 3 is also a cross section at B, shewing in what manner the afore said roadway beams and the braces pass through the other uprights, how the uprights are connected imme diately below the roof, and also how the two pieces of which they are composed are bolted together. Fig. 4 shews the form of the roof at that place. Fig. 5 shews the manner in which the roadway beams, and those along the top of the uprights, are united. And Fig. 6 explains the nature of the points at C and D, by which the seve ral pieces which compose the beam are connected to gether lengthwise. In Plate XC. Fig. 1. is a longitudi nal section including the lower part of the roof, and in which the situations of all the uprights, beams, braces, and iron ties, are distinctly shown. Fig. 2 is a plan of the floor, with every part of its framing ; and Fig. S is a similar plan of the roof.