connected by another series of ribs, so that each arch consists of a series of hollow masses or voussoirs, similar to those of stone bridges the whole of the segmental castings forming each arch, as well as the transverse and diagonal tie-braces. are kept in their places by dovetailed sockets and long castiron wedges, by which the necessity for bolts is obviated. The spandrils are composed of cast-iron diagonal framing, and the roadway is formed upon cast-iron plates, resting upon the spandrils, and joined with iron cement.* The abutments and piers of the bridge are of stone, built upon platforms of timber, which rest upon piles, and are surrounded by guard or sheathing piles driven into the bed of the river. In the erection of the bridge, the ribs were commenced in the centre of the span, and continued regularly on both sides towards the piers and abutments. Upon these connecting and bedplates were secured in the masonry, and when the last segment of each rib was fixed, three wedges of cast iron, each 9 feet long and 9 inches wide, were introduced behind each rib, and nicely fitted and adjusted to them. These wedges are formed with a very slight taper, and were driven simultaneously with heavy hammers, so that the arches were nearly lifted from the centres, which were thus readily removed; and the whole of the iron-work had been so carefully prepared by Messrs. Walker, of Rotherham, and the masonry by Messrs. Jolliffe and Banks, the contractors, that when the work was completed, scarcely any sinking of the arches could be detected. By experiments made during the progress of the works, it was found that the average effect of the expansion caused by the summer increase of temperature was a rise of the arches to the extent of about 14 inch at the crown, being

* Iron cement, much used in connecting the cast-iron plates of which large tanks or cisterns are often formed, consists of clean iron borings or turnings of cast iron, 16 parts; sal-ammoniac, 2 parts; and flour of sulphur, 1 part. When used, 1 part of this mixture is added to 20 parts of clean borings, and sufficient water to reduce the whole to the consistence of a paste. This cement dries as hard as the iron itself, and forms a joint quite impervious to water.

"The two outer ribs consist of two segments of circles, each struck from different centres, the crown of one terminating immediately below the roadway, the other at the top of the parapet, so that the platform forming the roadway is both suspended and insistent; the object of this being, it is presumed, to increase the depth of the truss supporting the roadway, and thus add to the strength of the bridge but it was unnecessary, and does not appear to have been adopted in any of Telford's subsequent designs, which are nume."*Rennie constructed an iron bridge over the Witham, at Boston, in Lincolnshire, which is remarkable for boldness of design and flatness, the rise being only 4 feet, and the span 100 feet. In construction, this bridge resembles the Sunderland bridge, but has an improved arrangement of transverse and diagonal braces, and vertical spandril pieces, instead of circular ones.

rous.

The largest iron-arch bridge yet constructed is that over the river Thames at London, and known as the Southwark bridge, which was designed and erected by the eminent Rennie. This splendid bridge, which was opened on March 25, 1819 (the first casting for it having been run on January 1, 1815), consists of three arches, all segments of the same circle, the centre arch being 240 feet in span, with a rise of 24 feet, and the two side arches being each 210 feet in span, with a rise of 18 feet 10 inches. The piers are 24 feet thick; the width of the roadway over the bridge is 28 feet; and the footways on either side are each 7-feet in width. Each arch consists of eight ribs, and each rib is formed of fifteen pieces, which are of such depth that the rib is 6 feet deep at the crown and 8 feet deep at the springing. The metal is 2 inches thick in the middle, and 4 inches at the top and bottom of the ribs. The ribs are connected transversely by cast-iron tie-braces of the same depth as the ribs, but open in the centre of each, and in the diagonal direction the ribs are

*Sir J. Rennie's Address to the Institution of Civil Engineers, Session 1846.

connected by another series of ribs, so that each arch consists of a series of hollow masses or voussoirs, similar to those of stone bridges the whole of the segmental castings forming each arch, as well as the transverse and diagonal tie-braces, are kept in their places by dovetailed sockets and long castiron wedges, by which the necessity for bolts is obviated. The spandrils are composed of cast-iron diagonal framing, and the roadway is formed upon cast-iron plates, resting upon the spandrils, and joined with iron cement.* The abutments and piers of the bridge are of stone, built upon platforms of timber, which rest upon piles, and are surrounded by guard or sheathing piles driven into the bed of the river. In the erection of the bridge, the ribs were commenced in the centre of the span, and continued regularly on both sides towards the piers and abutments. Upon these connecting and bedplates were secured in the masonry, and when the last segment of each rib was fixed, three wedges of cast iron, each 9 feet long and 9 inches wide, were introduced behind each rib, and nicely fitted and adjusted to them. These wedges are formed with a very slight taper, and were driven simultaneously with heavy hammers, so that the arches were nearly lifted from the centres, which were thus readily removed; and the whole of the iron-work had been so carefully prepared by Messrs. Walker, of Rotherham, and the masonry by Messrs. Jolliffe and Banks, the contractors, that when the work was completed, scarcely any sinking of the arches could be detected. By experiments made during the progress of the works, it was found that the average effect of the expansion caused by the summer increase of temperature was a rise of the arches to the extent of about 1 inch at the crown, being

*Iron cement, much used in connecting the cast-iron plates of which large tanks or cisterns are often formed, consists of clean iron borings or turnings of cast iron, 16 parts; sal-ammoniac, 2 parts; and flour of sulphur, 1 part. When used, 1 part of this mixture is added to 20 parts of clean borings, and sufficient water to reduce the whole to the consistence of a paste. This cement dries as hard as the iron itself, and formis a joint quite impervious to water.

fixed at the abutments. The weight of metal is recorded as follows in the centre arch, 1,665 tons; in the two side arches, 2,920 tons; total, 4,585 tons.

The principle of all these iron-arch bridges is identical with that of arch bridges of stone and other materials, which derive their strength and stability by transferring the effect of the loads placed upon them to the abutments. Two requirements are therefore, common and indispensable to all of them, viz. that abutments are obtained of sufficient weight and solidity to withstand the pressure conveyed by the arch, and that sufficient height exists for such an arch-like form to be given to the structure, that the pressure shall be always safely received at the abutments, and the strength of the arch not be in any case wholly dependent upon its depth and section at that part immediately acted upon by the superincumbent load.

When the peculiar properties of cast iron had been studied with a view to its extended application in buildings, and the proportions had been correctly determined for beams of this material, intended to supersede horizontal beams of wood, their employment in the formation of bridges of limited span soon followed; and in the railway works executed during the last twenty years, we have numberless examples of cast-iron girder bridges, as we have also of cast-iron arch bridges, of considerable dimensions and great ingenuity of design and arrangement. The cast-iron girder bridge, depending for its strength upon the sectional area of the girder at that point in its length over which the weight or load acts, requires abutments to resist vertical pressure only, while the abutments of arch bridges have to resist the lateral thrust of the arch. In the cast-iron girder bridge, moreover, the depth of the structure is reduced to that of the section of material due to the maximum load; and hence the peculiar applicability of this form for railway bridges, in which it is desirable to preserve a inimum distance from the under side, or soffit of the girder, the level of the roadway above. But the limitation of span

for which girders are safely applicable has always restricted their employment in bridges, and 40 feet has commonly been considered the maximum length of bearing to which single cast-iron girders can be safely applied, liable to be loaded. with railway trains or other heavy weights.

The desire to retain this convenient form of structure, however, and to extend its use to larger spans, induced attempts to combine wrought iron with cast metal in such a manner as should impart to the compound structure the superior power to resist extension, which wrought iron is well known to possess. Malleable-iron bars or rods were, for this purpose, fitted to cast-iron girders, and thus a kind of metal trussing was formed, the depth of the truss being limited to that of the girder. Many railway bridges were erected with these additions, and were considered safely constructed when each girder was cast in two or more separate pieces, making up, when united, the total width of span, and the pieces being secured together by bolts passing through holes in flanges or projecting plates cast on the ends of each piece. One of these cast-iron compound girder bridges, trussed with malleableiron bars, erected several years since, to carry the Northern and Eastern railway over the river Lea, is formed with girders each 70 feet in length, and composed of two castings, joined at the centre by bolts passing through vertical flanges. An additional security of connection is attained by casting dovetailed projections or bosses upon the meeting ends of the two castings, and by fixing wrought-iron clips over these bosses. Each girder, thus formed of two castings, is perfectly horizontal from end to end, and the top and bottom lines parallel, the uniform depth being 36 inches; the bearings upon the abutments are 2 feet long at each end, and the clear span between bearings is thus reduced to 66 feet. The section of the castings is of the approved form, viz. with vertical rib, and projecting flanges at top and bottom. The truss-bars are arranged in sets, one on each side of the girder, passing obliquely downward from the top of the girder over the