It has been objected to this structure, that by con necting ribs of different lengths and curvature, they are exposed to different degrees of expansions and contrac tions. This appears just in theory ; and that no discern ible effect has hitherto been produced, is probably from the difference being small ; but this point will be dis cussed under the article IRON. Another objection is, an apparent heaviness in the spandrels, from concealing the circular arches with iron plates. For appearance, these spaces had certainly better not been concealed, but they are not liable to the objections made in the former iron bridge, because the space around them is all closely filled up, and the roadway being formed with materials similar to this filling up matter, distributes the pressure very regularly. Upon the whole, considering the strength acquired by placing the coveting-plates with their deep Haunches in the form of an arch, we doubt whether a greater degree of strength can be had by any other dis tribution of the same quantity of cast-iron, viz. 1731 tons : it appears to us, that the upright standards, braces, and king-posts, might be made of smaller dimensions.
We have been informed, that each of these two first iron bridges, including abutments and roadways, cost about 60001.
The third iron bridge was constructed over the river Wear, near Sunderland, in the county of Durham. Its projector was Rowland Bunion, Esq. a gentleman of considerable landed property in that county, and who, for sonic time represented it in parliament. The iron work was cast at the founderies of Messrs Walkers of Rotherham, and erected under the inspection of Mr Thomas Wilson. The confidence in the use of iron, for arches 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, the span is 106 feet beyond that of the second, although its rise is only the same as that of the suspending ribs at Buildwas. The arch at Sunderland springs 60 feet above the level of the surface of low water; the span is 236 feet ; the rise, or versed sine, is 34 feet ; the width of the roadway 32 feet ; and there are six ribs. See Plates XCl. and XCIII.
In this arch, the mode of construction is very diffe rent from either of the former. Instead-of working with pieces of iron from about 50 to 70 tea in length, each rib is here composed of [25 small frames, each about two feet. in the length or curve of the rib, and five deep in the direction of the radius. In each frame there are three pieces of four inches square, which run in the di rection of the curve of the arch ; and these are connected in the direction of the radius by two other pieces, four by three inches. In each side of the larger pieces, is a
groove three inches broad by of an inch in depth ; and opposite each cross piece there is a hole in the middle of the groove. When the abutments were brought up, and a scaffolding constructed across the river between them, six of these frames were placed against the abut ments in the manner of archstones. Wrought-iron bars, of a length to embrace sundry frames, were then fitted into the grooves. I follow pipes of cast-iron, four inches in diameter, fitted to reach between each two frames, across the soffit, were introduced. Upon the ends of these pipes are !launches, in which there are holes, an swerable to the holes in the four-inch pieces of the frames, and also to those of the wrought-iron bars. Through these holes, wrought iron bolts were intro duced, which brought all the before-mentioned parts together by means of fore-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 small projecting pieces, which embrace the upper and lower edges of the frames opposite each joining. These operations were repeated until the whole of the frames were placed, and the arch keyed, forming six ribs between the abutments. Up on the ribs perpendicular pillars are placed ; and between them are cast-iron circles, which come in contact with the extra does, the upright pillars, and the bearers of the road way. The bearers and covering, we suppose for cheap ness, are made of timber. The railing is cast-iron. The inclinations each way upon the arch, probably to save weight, arc inconveniently steep.
From its great elevation and lightness of construction, this bridge is justly esteemed a bold effort of art, and a magnificent feature in the country. The wooden bridges in Switzerland, and that in America, are of greater span ; but, being placed near the surface of the water, and from the difference of material, their parts being of larger dimensions, there can be no comparison as to the fineness of effect. This arch is incontrovertible evidence of what may be accomplished by means of cast-iron, since it answers so well, charged, as it is, with the fol lowing, we conceive, material defects : I. The frames arc much too short, thereby multiply ing, very unnecessarily, the number of joinings in the main ribs to 125 x6=750 ; and, in the same ratio, in creasing the nundpr of braces, ties, and bolts. The pieces of the frams, being of unequal dimensions, is also improper.