referred to, the designs, now submitted for the consideration of the Institution, were subsequently executed.

There is nothing novel either in design or construction, the chief peculiarity being in the mode of erection, and the manner in which the substitution of iron for wood was effected. The restored portions consist of two longitudinal tubular girders (Fig. 2) extending from the abutments on each side, and resting upon the piers, which were cut down to the proper depth to receive them. The girders are uniform in depth, in both the Dinting and the Mottram viaducts, being one-thirteenth of the span; and the areas of the top and the bottom flanges are in the proportion of 7 to 6, the breaking weight, equally distributed, amounting to 12:58 tons per lineal foot. The girders are riveted and united by covering plates over the piers, and are thus rendered continuous from one extremity to the other. They are fixed to the two middle piers in the case of the Dinting viaduct, and on one pier of the Mottram viaduct, and are free to expand and contract in the direction of the abutments. On the top of the girders iron crossbeams are riveted to the flanges, at intervals of 4 feet, and to these are attached the longitudinal sleepers for supporting the rails. The cross-beams project 3 feet 7 inches beyond the longitudinal girders, and upon their extremities the hand-rails are raised. The spaces between the longitudinal sleepers and the railing on each side are covered with planking 3 inches thick, and upon this is spread a depth of from 3 inches to 4 inches of macadamized ballast, constituting the roadway of the bridge.

The principal condition insisted upon by the Directors was, that the traffic should not be interrupted during the progress of the works. This was agreed to, and it is satisfactory to state that, although there were from sixty-eight to seventy trains per day, no stoppage, even of a single minute, occurred, and that the whole of the work was accomplished without accident. In the prosecution of this undertaking, the Author incurred heavy responsibilities, and he was not free from anxious thought as to the results, and as to the best means to be employed. In this duty he was ably and cheerfully assisted by the Company's Engineer, Mr. Charles Sacré, and to that gentleman he is indebted for many useful suggestions.

As already stated, there were four ribs in the width of the Dinting viaduct (Fig. 1), but only three in the Mottram viaduct, the centre one of which was proportionally stronger. It was at first determined, in the case of the former, to construct one line of girders between the two ribs on one side at A, leaving the other side, B, untouched for the passage of the trains. But this could not be accomplished, as it was found that the space was too confined to permit the men to work; and the rickety state of the structure was such as to render the removal of any part dangerous,

and there was considerable risk of the whole falling into the river below. It was then proposed to remove two side ribs, a and b, but that idea had to be abandoned, on account of their being the only support, as lateral stays, to the others, which were distorted and out of the perpendicular, as well as out of line. Finally, it was resolved to construct the girders on the top of the old existing platforms, to cut down the piers and the abutments, and, by a simple mechanical apparatus, to suspend the girders at each end, and lower them into their places. This arrangement was successfully carried out, and answered every purpose, as it not only allowed the old structure to be retained intact, but it enabled the contractors to finish their work on every girder, without obstruction to the riveters and their assistants.

The apparatus for raising and lowering the girders was exceedingly simple. A strong wooden frame, A A (Fig. 3), about 16 feet high, was erected on the pier at each end of the girder. On the top of this frame there was a cast-iron plate a, with a hole in the centre, through which a square-threaded screw, 4 inches in diameter, was passed. On this screw there was a bevil wheel 6, having a brass bush, and resting on the plate a, which was increased in thickness to 4 inches round the hole; the meeting surfaces being covered with plates of steel and brass, carefully turned, so as to diminish the friction, when the girder, which weighed 80 tons, had to be lifted. Into the wheel a the pinion e worked, and received motion from the spur wheels d and e, the shaft f, and 'the crank handles on each side. On the lower end of the large screw was forged an eye, to receive the cross bar g, and at the extremities of this bar there were two links h, h, which hooked under the angle irons at the sides of the girders, at each end. From this description it will be seen that the operation was exceedingly simple, and with six men at the handles, the girders were raised, and lowered into their places, in the course of one hour from the commencement to their final settlement on the piers. When one line of girders was completed, and connected together over the piers, temporary wooden cross-beams were laid on the girders, in such a position as to allow of the permanent iron cross-beams being afterwards inserted between them; the temporary beams being partially supported on one side by the old centre arches, which had to be firmly stayed to the new girders. On these temporary cross-beams longitudinal bearers for supporting the rails were placed, and the permanent way having been fixed, the traffic was transferred to this side, during the operation of constructing and erecting the second line of girders. As soon as these were finished, and lowered into their places on the piers, the iron cross-beams were introduced between the temporary wooden beams, and when in place they were riveted to the main girders on the open side.

The longitudinal timbers were then bolted to the flanges of these cross-beams, the rails were permanently laid on that side, and the whole of the traffic was again transferred to it, until the other side was completed in the same manner. It will thus be seen, that there was no interruption of the traffic during the whole time of the reconstruction; and the last process was the riveting of the cross-beams to the line of girders first finished. The work of reconstruction of both viaducts was accomplished without accident, to the entire satisfaction of the Directors and of the public.

The communication is illustrated by three diagrams, from which Figs. 1 to 3 have been compiled.

[Mr. BATEMAN

Mr. BATEMAN said, it appeared to him that the most remarkable part of this work, was the manner in which the traffic of the line was maintained during the time the iron girders were being constructed and substituted for the wooden arches. He did not think the Paper described with sufficient clearness how that was accomplished; how in fact two iron girders could be made to take the place of four wooden arches, whilst the trains were running.

Mr. FAIRBAIRN said, his object in bringing forward the Paper was to show the way in which iron girders had been substituted for wooden arches, without obstruction to the traffic, or danger to the public, during this difficult and critical operation. The Dinting and the Mottram viaducts had been erected about twenty years, and had undergone repeated repairs. When he examined the Mottram viaduct, one of the arches was so much decayed, as to cause the timbers to rise and fall from 3 inches to 4 inches during the passage of the trains; and in his opinion it was in so precarious a state, that it would have tumbled down within eighteen months, or two years. In fact, the passengers became so alarmed, that it was seriously proposed to run an omnibus to a distant station, in order to avoid crossing over the viaducts. It was necessary that the main girders should be constructed, be fixed in their places, and be made continuous, before the removal of any portion of the old wooden arches; and as it was found that there was not sufficient room to effect the reconstruction between the arches without danger, it was decided to build the girders on the top, at the same level as the rails. During the progress of the works the old timber arches were strengthened by the introduction of tie-rods and bolts. The traffic both ways was conducted first on one line of rails, while the girders were being erected for the other, and on their completion, it was transferred entirely to the second line of rails. This was effected without the slightest stoppage or interruption of any kind, during the whole process of reconstruction. The girders were originally constructed completely at the works, and were then taken to pieces in sections, reconstructed as separate girders, and when in place, were riveted together, so as to make them continuous from abutment to abutment. They were fixed on the central piers, and there were rollers on the abutments at each end to allow for expansion and contraction. The extent of expansion from the freezing point to summer heat, was about of an inch; and as the movement was in both directions, from the centre to the extremities, it was not considerable. In the Britannia bridge the possible movement would be about 2 inches. Cutting out some of the cross braces to get the girders down was a delicate operation, owing to the want of lateral braces, the old ones being totally inoperative. It was feared that the new bridge might be ugly; but on the contrary, it had a very light and symmetrical appearance,