lying upon its flat, and then the timber was bolted on. When this was finished, the completed truss was turned up on edge, by means of five derricks attached to it at intervals along the top boom. The second truss was treated similarly, and after the cross and diagonal bracing had been put in, the stage was ready for launching.

For launching the stage, rollers were put under each truss in two places. These rollers were 4 inches in diameter, and were made in sets of eight, fixed in a steel frame; two sets, that is, sixteen rollers, were placed end to end, affording a bearing 9 feet in length at each of the four points of support. The rollers were set in their frames to the gradient of 1 in 60, in order that the frames could be bedded level.

The clear span between the piers was 120 feet, and the length of the stage was 193 feet, so that when the stage was pushed forward from the abutment across the first span there would be 73 feet of the stage on the bank to serve as a counterbalancing arm at the moment when the forward point of the stage was just landing on the first pier. It should have been mentioned that the first 130 feet of the stage was made without any camber at all, but the 63 feet at the point had 6 inches of camber, to compensate for the deflection which would occur when the whole 120 feet between the piers was overhanging without support. When the building of the stage was completed and the rollers were put under it, the rear end was loaded with about 40 tons of kentledge to prevent the forward end from overbalancing, and the whole was launched forward by means of hand-winches and block-tackle till the forward end was over the first pier. This end was then temporarily packed up off the pier (the end of the stage arrived usually about 1 foot 6 inches above the pier), and then the rollers, which had been used at the rear end, were taken across and were fixed under the point on the first pier. The launching was then continued till the back 130 feet length of the stage bridged the first span of the viaduct and the forward 63 feet was overhanging into the second span. This was the final position of the stage for the building of the first span of the viaduct. The weight of the stage, when stripped of its side platforms and all encumbrances, was about 84 tons, which, with the 40 tons of kentledge, made a total load of 124 tons, which had to be moved across the first span.

The stage being in position across the span, it was lifted at each end and flat plates were inserted between the rollers and the angle bars which formed the roller-path on the stage, in order to prevent abrasion of the rollers. The platforms were then

fixed on each side of the stage to receive the steelwork of the girders.

For building the girders a hand-crane was provided, which travelled upon rails on the top of the stage. The lower booms were first laid down upon slack-blocks lying upon the planking of the side platforms, and then the remaining members of the girders were built in the ordinary way. This part of the work calls for no especial remark, except that two points had to be noted: firstly, a considerable excess of camber had to be put in the bottom booms when first laid down, in order to allow for the ever-increasing deflection of the stage as weight was added to it; thus, although 1 inch was the camber specified for the girders when finished and under full permanent load, it was found necessary to put in 3 inches when first laying the booms down. The second point to be observed was that the stage had to be loaded as equally as possible upon each side-platform; that is to say, that as soon as one member had been fixed on one side, the corresponding member was fixed upon the opposite side. The total weight of each girder was 45 tons. The maximum deflection noted in the stage was about 2 inches, and a large part of this remained after the weight of the girders had been taken off. It was, however, always found possible to get rid of this permanent deflection by tightening the wedges under the vertical struts of the stage.

As soon as the cross-girders and rail-bearers had been fixed upon the main girders, the side-platforms of the travelling stage were removed and stacked upon the cross-girders, and preparations were then made for moving the stage to the next span. This operation required much care, on account of the great length and weight of the stage and the slenderness of the tall piers over which it had to travel. Before the commencement of the movement the stage was between the girders of the bridge, and was resting on rollers on the two piers, 130 feet apart, while the remaining length of 63 feet overhung into the next span forwards. At the back end of the stage, which was specially stiffened, a wheel and axle were now fixed in bearings upon each side, at such a height as to run upon the inside flanges of the bottom booms of the girders. As there were rivet-heads along these flanges, steel strips, of the thickness of the heads and perforated to receive them, were fixed upon them, and thus a smooth path was provided for the wheels to run upon. At the beginning of the movement the weight upon these two wheels was approximately 30 tons, but it is obvious that the load on the trailing wheels was gradually

reduced by the forward motion of the stage; and, when the centre of gravity arrived at the roller frame upon the pier, no further support was needed at the rear end. To allow of the further movement of the stage without tipping, a pair of flanged guide-wheels was mounted over each truss of the stage, running in bearings which were attached to timber framing on each side of the central cross-girder, so that the whole weight of the span last erected was called into play as a counterbalance. The guide-wheels were mounted immediately over the rails, which have before been mentioned as forming a road for the travelling hand-crane, and which bore against the guide-wheels, as the stage travelled forwards. The motive power for moving the stage was obtained by block-tackle worked by hand-winches; a set of blocks was fixed on each side, one block being fixed to the bottom boom of the girder at its forward end, and the other block being made fast to the bottom boom of the stage; the leading parts of the falls being taken directly, or, when necessary, through snatchblocks, to the winches, which stood on the cross-girders on the top of the bridge. It has been roughly calculated that the tractive force necessary to move the stage forward was about 13 tons.

As soon as the point of the stage reached the next pier, men were sent across and the point was temporarily packed up, and then the rollers which had been left behind on the backward pier were taken across and were fixed under the point. The point was now eased up with jacks and the wheels were removed from between the cross-girders at the back end of the stage, and then the point was lowered on to the rollers just fixed. The stage was now again resting upon rollers upon two piers, and the movement was continued forwards for the remaining 60 feet, until the stage occupied the right position (in point of distance) for building the girders of the next span upon it. Owing, however, to the viaduct being built upon a curve, the stage had now to be moved sideways upon the forward pier. For this purpose the stage was lifted again by four hydraulic jacks, the rollers were removed and were replaced by a planed bridge-rail bolted to a timber which was laid upon the pier transversely under the stage. When resting on this rail, which was well greased, it was comparatively easy to move the stage sideways by means of jacks, aided by union-screws attached to the holding-down bolts which were built into the pier. As soon as the forward end had been moved to the centre of the pier it was again lifted, the rail was removed, and the rollers replaced ready for the next move forwards.

The side platforms were then fixed and the stage was ready to receive the girders of another span.

When the stage was moved forward in fairly calm weather, it was not difficult to keep it travelling in a straight line upon its rollers. On the other hand, it was impossible to move it during a strong side wind, owing to the large surface offered by the stage, and the leverage at which the wind-pressure acted when a large length of the stage was overhanging without support. A distinct vibration was felt in the piers during the moving of the stage.

In conclusion, it may be remarked that the appliances which have here been described were perfectly successful in the accomplishment of the work for which they were designed; and the method of erection was found to be convenient, expeditious and well adapted to the situation.

The Paper was accompanied by two tracings from which the Figures in the text have been prepared.

(Paper No. 3126.)

"The Failure of the Embabeh Bridge, Cairo."

By FREDERICK EWART ROBERTSON, C.I.E., M. Inst. C.E.

THE Embabeh bridge over the Nile connects the railway system which centres in Cairo with that of Upper Egypt. It is a singleline through bridge of eight continuous spans and a swing-span ; the ordinary spans are 243 feet, the end spans 203 feet 4 inches long, and each arm of the swing-span is 94 feet 3 inches long between centres. There is a corbelled-out passage 14 feet wide for cart traffic on each side. The bridge was built by contract under open competition, the general conditions only being specified; the designs and the tenders were examined by a committee of engineers in the service of the Egyptian Government, on which the railway was represented, and the work was allotted to a wellknown French firm. An ordinary span and the swing-span are illustrated in Figs. 1 and 2, Plate 3, respectively. The lateral bracing at the top and bottom was of adequate design.

The unit stresses in all members required by the specification

The process by which the steel was to be made was not mentioned, nor were chemical restrictions imposed in regard to its composition. Hot punching and bending tests were specified, and the minimum tensile strength prescribed was 28.6 tons, with an elongation of 20 per cent. in 200 millimetres, subsequently revised to 26-29 tons, and an extension 26 per cent. to 22 per cent. It was prescribed that for pieces less than 10 millimetres thick the punched hole must be 3 millimetres less than the finished hole, the rest being drilled out, and for pieces thicker then 10 millimetres, 4 millimetres less; and that in the case of the metal not behaving well under the punch the contractor might be required to drill the holes entirely.