\Ve consider it our duty to introduce these observa tions, in order to shew the necessity for great precaution in similar works, and how liable first attempts are to be defective ; but they derogate nothing from the merit of projecting a great arch of cast iron, introducing a ma terial almost incompressible, which is readily moulded into any shape, and which is peculiarly applicable in the British isles, where the mines of iron are inexhaustible, and the means of manufacturing cast iron unrivalled.
The second iron bridge was built upon the same river, about three miles above the former one, at a place called Buildwas. An old stone bridge was carried away by a very high flood early in 1793, and the county of Salop was obliged to restore the communication. (See Plate XCI1 ) Mr Telford, who was then, and is now, sur veyor for the public works of that county, perceiving, that although, in a former repair, the middle pier of the four arches had been taken away, and that space, as well as the two adjacent arches, converted into one arch, yet that the waterway had still been too much confined ; and being aware that a few years previous to that time, the extensive low lands in to Montgomeryshire, which for merly acted as a reservoir, had been embanked, so that the flood-waters passed off more hastily, and in a greater body than formerly ; in order, therefore, to remove all obstacles out of the way of future floods, and on account of being within two miles of the best founderies in the world, he recommended a cast-iron arch of 130 feet span. (See Plate XClI.) The magistrates of the county agreed to this, and the Coalbrookdale Company became con tractors, both for the iron work 01 the arch, and the masonry of the abutments. Mr Telford, we understand, had some trouble in making that Company depart from their former mode of construction ; but he at last pre vailed in keeping the roadway low, and adopting the sus pending principle, by means of a rib on each side of the bridge, which sprung from a lower base than the bear ing ribs, and rose above them lo the top of the railing : thus the bearing ribs were supported by the lower parts of those before •mentioned, and were suspended by their upper parts. The bearing ribs have a curve of 17 in 130, or nearly one-eighth of their span. The suspending ribs rise 34 feet, or about one-fourth of their span. There are cast-iron braces, and also horizontal ties. There
are 46 covering plates, each 18 feet in length, and one inch in thickness. They have flaunchcs four 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 abutments, instead of a load, they compose a strong arch. There being only one rib in the middle of 18 feet breadth of bridge, on each covering plate, a cross rib or flaunch, four inches in depth, is cast at an equal distance between the bearing ribs. The sus pending ribs are each 18 inches in depth, and 24- inches in thickness, exclusive of a moulding. The bearing ribs arc 15 inches in depth, and 21- inches in thickness, and each of the ribs are cast in three pieces only, of about 50 feet each ; the braces are 5 by 3 inches. The prin cipal 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 ments are 4, inches square. The strongest uprights in the railing are 3 inches square, and those bete een them 1 inch. They are placed six inches apart, between mid dle and middle. The height of the railing above the sur face of the roadway, is 4 feet 9 inches. In each span drel there are 3 circular arches formed with hard burned bricks, which preserve most of the space open, but they are concealed by iron plates, one inch in thickness, which form the outside lacings. On the eastern side of 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 abutments : the space for them was excavated down to the rock, which lay considerably under the bed of the river, and the masonry was sunk into the solid part of the rock. It was built up chiefly of square masonry, and the rest of rubble, laid very close in regular courses, and having the back part formed in the shape of a wedge, pointing to the bank. The wing walls were curved ho rizontally and vertically. At the height of 10 feet above the low water, there is a hauling path on 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 ironwork; it is fitted as correctly as a piece of good carpentry.