expensive works would be needed; but the Author states that none of them would present any special features of difficulty in point of construction, and this question will ere long be seriously considered. In conclusion, the following matters, not strictly relating to the railway, are discussed-the Ports of Matadi and Stanley Pool, the staff of the company, and the financial considerations, together with the mode in which the capital needed for the enterprise has been obtained. The actual capital is £1,200,000, but in addition to this there is a debenture debt of £1,400,000.

Numerous photographs are given of the country traversed and of different parts of the line, and maps are appended of the Congo basin and of the route selected for the railway. One of the maps has been reproduced in the present abstract.

(Paper No. 3088.)

"The Erection of a Steel Viaduct upon the Highland Railway."

By HENRY THOMAS WHITE, Assoc. M. Inst. C.E.

IN the design of steel girder-bridges, viaducts, and roofs, as well as in their construction, it is most important to keep distinctly in view the process by which the structure is ultimately to be erected; for the actual work of erecting such framed structures will often be attended with considerable difficulties, demanding as much attention from the engineer as any other matters involved in the general problem of bridge-construction.

The method that may be chosen for the erection of any structure will depend entirely upon its situation and upon the various facilities for getting the material to the site. It may be that one or other of the ordinary processes will be eligible; otherwise, as not infrequently occurs, some special arrangements must be devised to meet the exigencies of a particular case. When the method of erection has been decided upon, it must be carefully considered, whether it does not involve some special arrangement of parts in the general design of the structure; and it will also determine how far the structure may be riveted together in the maker's yard, and what must be left to be done at the site. The Author is of opinion that a fuller acquaintance on the part of the designer with the peculiar facilities or difficulties attaching to the site of the structure upon which he is engaged, and the method of erection, which is determined thereby, would frequently save unnecessary labour and would conduce to better construction.

In the present Paper, as a contribution to the subject, the Author proposes to give a description of the method which was successfully employed upon the erection of a viaduct upon the Highland Railway in a situation that called for the employment of special appliances.

The viaduct forms part of the new direct line from Aviemore to Inverness, and carries the railway across the valley of the

River Findhorn in nine spans of 130 feet each, measured between the centres of the piers. The steel superstructure of each span consists of a pair of independent girders of uniform depth, placed 16 feet apart from centre to centre, and carrying the railway upon an upper deck, by means of cross-girders attached to the upper booms. The main girders are supported upon piers of granite masonry, whose height varies with the contour of the valley, the highest being 118 feet from ground-level to the bearings of the girders. The viaduct is built upon a curve of 40 chains radius, and upon a gradient of 1 in 60.

For various reasons it was not considered practicable in this case to erect the superstructure either by lifting the girders from the ground, or by the alternate plan of launching them across the piers from the embankment at each end, as either plan would be attended with great difficulties, and therefore some other scheme was necessary. The method that was devised was that of providing a long bridge-like structure, or travelling stage, composed of steel and timber, which was put together upon the approach-embankment at one end of the viaduct, and was then pushed across the viaduct span after span, the girders of each span being built upon it and left in their final positions, when the stage travelled forward to the next span. The total length of this travelling-stage was 193 feet, measured on the bottom boom, and it consisted of two main trusses, 18 feet deep, braced together at the top and bottom by timber transoms and steel rods. The main trusses, Fig. 1, were of a simple type, and were made of a skeleton of steel bars. These bars are flitched on each side with timber in every member, which, under any condition, can act as a strut, while the

verticals, which can only act as struts, are wholly of timber and have no steel bar. At the bottom of each vertical, folding-wedges were provided, so that the vertical could be tightened to take out any permanent deflection of the stage. On each side of the steel bar, in the bottom booms, an angle-bar was riveted, which, besides supplying a sufficient area of metal to take up the tension in the boom, acted as a path upon which the stage was rolled forward.

The steel skeleton was riveted together, and the timber was carefully fitted on to it, recesses being cut in the timber to receive the rivet-heads. The timber, which varied in section from 12 inches by 12 inches, and 10 inches by 10 inches in the booms to 12 inches by 6 inches and 10 inches by 4 inches in the vertical struts, was bolted to the steel skeleton with bolts passing through both timbers. The small intermediate struts, shown in Fig. 1, do not form part of the web-system of the truss, but were placed there merely to strengthen the bottom boom, when it had to act as a beam as the stage travelled over the rollers on which it moved. The main trusses of the stage were placed 9 feet apart, from centre to centre, and were connected at the top and bottom by horizontal systems of bracings, consisting of timber transoms framed across between the trusses at the head and foot of each vertical, and diagonal steel rods with screw connections; while the stage was further stiffened by bracing in the transverse plane, consisting of diagonal rods applied at each vertical of the truss, as shown in Fig. 2, and provided with adjustable screw couplings. Being thus braced in every direction, the stage was exceedingly rigid, which was very necessary, as it had to withstand, besides its vertical loads, a very strong lateral wind-pressure. At the back end, and again at a point 130 feet from that end, that is, the bearing point on the next pier, the stage was stiffened with diagonal struts, and special beams were fixed across it horizontally, by which it could be lifted by means of hydraulic jacks whenever it was necessary to insert or remove the rollers from underneath it. The main body of the stage, constructed as above described, was capable of travelling forward telescopically between the erected steel girders of any one span, so as to form a temporary bridge across the next opening; but, before commencing the erection of a pair of girders, the stage was extended laterally by attaching to it, on each side, a movable platform, as shown in Fig. 2.

Rolled-steel joists (6 inches by 3 inches in section), projecting from the stage on each side opposite each vertical, were attached to the lower boom by straps, which passed through and took a

bearing upon a steel plate, bolted to the outside of the lower boom of the stage. The joists were held in place by a pin passing through the strap at the back of the plate, and were supported by a 11-inch diameter chain, fastened to the upper boom of the truss by a strap-bolt passing through it, and attached to the joist by links and a pin. In each chain was a strong union-screw, by means of which the joist could be levelled when in position. The joists, which were 16 feet apart, were connected by light rods attached to their centres, which served to keep them parallel and

square with the stage, the last joist at each end being braced to the bottom boom of the stage by a chain and union-screw. On the joists were placed planks 4 inches thick, and on the outside was a strong hand-rail, supported by posts which were fitted into sockets at the ends of the joists. This formed a good and secure platform and on this the girders were built.

The materials were brought to the work upon the earthen embankment which formed the approach to one end of the viaduct, and upon this embankment the travelling stage had to be put together. The steel skeleton of each truss was built and riveted