The second in' bridge was built upon the same river, about three miles above the thriller one, at a place called Buildc,:„. An was away by a very high flood early in 179-5, and the county of Salop was obliged to restore the communication. Mr. Tellirrd, the celebrated engineer, was at that time surveyor tin' the public works of the county, and on his recommendation, the magistrates ordered the construction of a cast-iron bridge, to be of one arch 130 feet span. 'The Colebrook Dale Company became contractors, both for the iron work of the arch, and the masonry or the abutments. Alt. Telford had sonic difficulty in making that company depart from their thriller mode of construction : but he at last prevailed in keeping the road way low, and the suspending principle, by means of a rib on each side of the bridge, which sprung from a lower base than the bearing ribs, and rose above them to the top of the railing; thus the bearing ribs were supported by the lower parts of those bethre mentioned, and were sus pended by their upper parts. The bearing ribs have a curve of 17 in 130, or nearly one-eighth of their span. The sus pending ribs rise '34 feet, or about one-fourth of their span. There, are east-iron braces, and also horizontal ties. There are 46 covering plates, each 18 feet in length, and one inch in thicknes-s. They have flaunches 4 inches in depth, and are screwed together at each joint ; so that, by taking the curvature of the bearing ribs, and being firmly secured at the abntments, instead of a load, they compose a strong arch. There being only one rib in the middle of IS f•et breadth of bridge, on etch covering plate, a cross rib or Haunch, 4 inches in depth. is cast at :in equal distance between the bearing ribs. The suspending ribs are each IS inches in depth, and in,dn, in thickness, exclusive °la moulding. The ribs are 15 inches in depth, and ;2,1- inches hi thickness, aril each of the ribs is east in three pieces only, of about 50 feet each; the braces are 5 by 3 inches. The principal king posts are 101 by 4:1 inches. The springing plates are each 3 feet broad, and 3 inches thick, with openings to save metal. The uprights against the abutments are 41 inches square. The strongest uprights in the railing arc 3 inche-s square, and those between them 1 inch. They are placed 6 inches apart, between middle and middle. The height (tithe the surface of the roadway, is 4 feet 9 incites. In each spandrel, there are three circular arches formed with hurtled bricks, which 01' the space open, but they are concealed by iron plates, one inch in thickness, which form the outside facings. Ott the eastern side ttl the river, although the banks are not so very high or steep, the quality of the ground being similar to that of the other iron bridge, particular care was bestowed upon the :Iltittilients: the space for their' was excavated down to the rock, which lay. considerably under the bed of the river, and the masonry was soul: into the solid part of the rock. It was built up chiefly of square masonry, and the most of rubble, laid very close in regular courses, and haying the back hart forrned in the shape of a wedge, pointing to the bank. The wing-walls were curved horizontally and vertically. At the height of 10 feet above the low-water, there is a path MI each side of the river. This bridge, which was completed in 1796, has never shown any appearance of failure in any of its parts; nothing can be more perfect than the iron work; it is fitted as correctly as a piece of good carpentry.
It has been objected to this structure, that by connecting ribs of different lengths and curvature, they are exposed to different degrees of expansion and etoitraetion. This appears just in theory ; and that no discernible effect has hitherto been produced, is probably from the difference being small. Another objection is. an apparent heaviness in the
the cir•ular arches with iron plates. For appearance. these spaces had certainly better not have been concealed, but they are not liable to the objections made to the former iron bridge, because the space :'mind them is all closely filled up; and the roadway being formed with mate rials similar to this tilling-up matter, distributes the pressure V cry re:mlarly. Upon the whole, considering the strength acquired by placing the covering plates with their deep flannehes, in the ft win of an arch, we doubt whether a greater degree of strength can lie had by any other distribution of the same quantity of cast-iron, viz., 1731 tons: it appears to us, however, that the upright standards, braces. and king posts, might be made of smaller dimensions.
The third iron bridge was constructed over the near Sunderland, in the county of 1)tirliain. Its projector was littwland Cordon, Esti., a gentleman of considerable landed property in that county, and who, for smile time, represented it in parliament. The iron-work was cast at the founderies of Alessrs. \Valkers, 1;mhz:1h:on, and erected under the inspection of Mr. Thuma* l'he confi dence in the use of iron for archer of great extent, was by this time established. The span of the second arch. we have seen, is 30 feet more than that of the first ; and, in this third instance, tho span is 106 feet that of the second, although its rise is only the same as that of the suspending ribs at Iluildwas. The arch at Sunderland springs CtO feet above the level of the surface of low-water ; the span is feet ; the rise, or versed sine, is 34 feet; the width of the roadway feet; and there are six ribs.
In this arch the mode of cow-tun-don is very different from either fturmer. Instead of working with pieces of iron from about 50 to 70 feet in length. each rib is here composed of 135 small fraine-t. each about two feet in the length or curve of the rib. and 5 deep in the di i eetion of the radius. In each frame there ..itte three pieces of 4 inches square, which run in the diro,rion of the curve or the ; and these arc connected in the direction of the, radius by tau other pieces, 4 by 3 inches. In each side of the larger pieces is a groove, 3 inches broad, by three-quarters of an inch in depth; and opposite each cross piece there is a hole in the middle of the When the abutments were brought up, and a scathdding constructed amiss the river een them, six of these f ames %, ere placed against the abutments in the manner of arch-stones. Wrought-iron bars, of a length to embrace sundry frames, were then fitted into the grooves. Hollow pipes I if cast-iron, 4 inches in diame ter, fitted to reach between each two frames, across the sotlit, were introduced. Upon the ends of these pipes are Haunches, in which there are holes, answerable to the holes in the fou• inch pieces I if the frames, and also to those of the wrought iron bars. Through these holes wrought iron bolts were introduced, which bninght all the bet1i•e-mentioned parts together by means of tore-locks. The frames do not meet at the upright pieces, but on the three points of the four-inch pieces only. On the ends of the hollow pipes, there are projecting pieces, which embrace the upper and lower edges of the frames opposite each joining. These operations were repeated mail the whole of the frames were placed, and the arch keyed. fit six ribs between the abutments. rpon the rib's. perpendicular pillars are placed ; and between then: are cast-iron circles. which come in contact with the extrados, the up' ight pillars., and the bearers of the roadway. The beaters and covering. we suppose for cheapness, are made of timber: the railing is cast-iron. The inclinations each way upon the arch, probably to save weight, are incon veniently steep.