From its great elevation and lightness of construction this bridge is justly esteemed a bold effort of art. and a magnifi cent feature in the country. The wooden bridges in Switzer land, and that in America, are of greater span ; but, being pla•ed near the surthce of the water, and from the difference of materhd, their parts being of larger dimensions, there can be no comparison as to the fineness of effect. A east-iron bridge was also built over the river Witham, at Boston, in Lincolnshire, the design for which has been generally ascribed to .1r. Bennie. That gentleman, however, only gave the width and ri-e of the arch, and the abutments were founded and built under his direction. The iron arch itself was de signed and executed by Mr. Thomas Wilson, of Sunderland. The span of this arch is about S5 l'eet; the rise about 5 feet O inches; the breadth is 30 feet ; and there are eight ribs, each rib composed of eleven frames, 3 feet deep in the direc tion of the radius. At each joining there is a east-iron grating across the arch which connects the frames, on the same principles as practised at the Ponteysylte aqueduct. Instead of three pieces in the direction of the curve, as at Sunderland, here are only two, but they are 7 inches by 4i. These are, in each frame, connected in the direction of the radius, by pieces, 4 by 3 inches. Upon the back of the ribs, pillars, 4 by three inches, are placed perpendicularly to sup port the roadway. The superstructure resembles that of the first iron bridge at Colebrook Dale. The arch has been kept very Hat, to suit the tide below and the streets above. The frames being made about four times the length of those at Sunderland, and being connected with cast-iron _ratings in stead of wrought-iron, are essential improvements; but from the pieces in the frames, which are in the direction of the radius, being only 4 by 3 inches, while the main pieces in the direction of the curve are 7 by 41, a great proportion of the fbrmer are broken. This is a defect; and the pillars which support the roadway being perpendicular, do not cor respond with the radiated pieces of the frames. The ribs, in springing from the perpendicular thee of the masonry of the abutment have also a crippled appearance.
In the improvements made under the direction of Mr. Jessop, at the port of Bristol, it became necessary to change the course of the river Avon. and two very handsome cast iron bridges were built over the new channel. The span or the iron work of each arch, is 100 feet ; the rise 12 t 6 inches, or one•eighth of the span ; the breadth is 30 feet. There are six ribs: each rib is composed of two pieces meeting in the middle, and they are connected crosswise by nine east-iron ties, which are dovetailed, and wedged into the ribs; the cross sections of these ties arc in the form of the letter T. The ribs stand upon abutment-plates, which are laid in the direction of the radius. These plates are 32 feet in length, 2 feet 4 inches in breadth, and 4 inches in thickness. In each plate are 5 apertures, each 5 feet long, and 20 inches in width. The ribs are 2 feet 4 inches in depth in the direction of the radius, and two inches in thick ness, and have each SO apertures, one foot square, separated by bars 3 inches broad, excepting opposite the cross tics, where the solid is 12 inches broad. Where the ribs meet in the middle they have Paunches, S inches broad and 2 thick, and they are connected by east-iron screw-bolts, 3 inches in diameter, Between the ribs and the bearers of the roadway, perpendicular pillars, with cross sections formed like the letter T, are placed : the bearers are of the same form. The
whole is covered with cast-iron plates, there are rail inas of cast-iron.
is great simplicity, and much of correct principle, in this design : 1. The springing-plates being. placed in the direction tit the radius, and the abutments receding to pro duce a space behind the ribs equal to that between the upright pillars. 2. The ribs being composed of two pieces, and one joint only : and, 3. Wronght-iron tieing wholly excluded. But we regret still observing the varying dimensions of the parts of the ribs; and that the supporting pillars are still placed perpendicularly ; and which, as the arch has more cur vature, has a yet worse effect than at Boston.
In the course of his employment as engineer to the board of parliamentary commissioners tin• making roads and con structing bridges in the Highlands of Scotland, Mr. Telford erected a cast-iron bridge over an arm of the sea, which divides the county of Sutherland from that of Ross, at a part where several of those roads unite. In this bridge, the defects noticed iu the former works of this sort appear to be avoided. The arch is 150 feet span; it rises 20 feet, it is 10 feet in width, and has 4 ribs, In the abutments, not only are the springing•plates laid in the direction of the radius, but this line is continued up to the roadway. The springing plates are each 16 feet in length, 3 feet in breadth, and 4 itches in thickness, with sockets and shoulder-pieces to re ceive the ribs. hi each plate are 3 apertures, 3 feet in length and 18 inches in width. Each of the ribs, for the conveniency of distant sea-carriage, is composed of 5 pieces, 3 feet in depth in the direction of the radius, and 2h- inches in thickness. There are triangular apertures in the ribs, formed by pieces in the direction of the radius, and diagonals between them ; but every part is of equal dimensions. At every of the pieces of the ribs, a cast-iron passes quite across the arch; upon these are joggles or shoul derings to receive the ends of the ribs : the ribs have also Haunches, which are fixed to the gratings with cast-iron screw-bolts. Each rib is preserved in a vertical plane, by covering the whole with grated, flaunched plates. properly secured together, and to the top of the ribs, by cast-iron screws and pins. In the spandrels, instead of circles or upright pillars, lozenge, or rather triangular forms, are introduced, each cast in one frame, with a joggle at its upper and lower extremities, which pass into the sockets formed on the top of time ribs, and in the bearers of the roadway. Where the lozenges meet in the middle of theii height, each has a square notch to receive a cast-iron tie, which passes from each side, and meets in the middle of the breadth of the arch. where they are secured by f we-locks. Next to the abutments, in order to suit the inclined face of the masonry, there are half lozenges. Ily itil•;111S of these lozenge or triangular fornis, the points I I are preserved in the direction of the radius. The cove•ing-plates, in order to preserve a sufficient degree of strength, and lessen the weight, are, instead of solid, made of a reticulated shape ; the apertures widen be low, to leave the matter between them a narrow edge ; and contract upwards, su as to prevent the matter of the roadway from tolling through. This disposition of the iron-work, especially in the spandrels, also greatly improves the general appear:nice.