other end, and it is necessary to make proper provision at the ends to resist this tendency, or the girders should be made discontinuous when closed by hinging them over the swing pier, and connecting the girders to a vertical post over the hinge by means of tension bars, which are brought into action when the bridge is moving, or the same method may be adopted as that described for the Raritan bridge.

It is very important that provision should be made in all swing bridges, which are not lifted mechanically before turning, for taking out the droop of the girders when turning, such as a toggle joint hydraulic lifting arrangement, as in the Ouse bridge, or a cam worked by hand-gear in ordinary hand-power bridges, or the methods adopted in the Hawarden and the Le Pollet bridges may be used.

If the end drop or deflection is not taken out by some means, the stresses due to the deflection must be added to those produced by the live load, which is unsatisfactory in every way.

The frictional resistances during turning in bridges where the whole of the load is carried on a ring of live rollers, will depend to a large extent upon the equal distribution of the load over these rollers, and distributing girders should be arranged between the underside of the main girders and the circular girder immediately above the live rollers, in order to distribute the load equally over six equidistant points of the circular girder.

On Plate IV. are shown details of a design for a swing bridge, with a roadway 18 feet wide and two openings each of 60 feet in the clear. The main girders are of the lattice type 21 feet 4 inches between centres, and 152 feet over all, the depth at centre being 7 feet, and at the ends 5 feet effective. The boo sections are formed of steel T bars and flange plates 2 feet wide of wrought iron, and the web consists of wrought-iron lattice bars and irons braced to form struts, the central 14 feet 6 inches being plated and stiffened over points of support and at the centre. At each lower apex is a wrought-iron cross-girder, resting on a saddle secured to main girder, and carrying seven longitudinal timber stringers, to which is spiked the 4-inch diagonal planking forming the roadway.

At the centre the main girders are secured each to two wrought-iron stools 12 feet 4 inches apart, while the ends, when the bridge is closed, are raised and supported on cast-iron cams.

The conditions for calculation are therefore those of a continuous girder, with two side spans of 70 feet and a central span of 12 feet 4 inches, the side spans acting as cantilevers when the bridge is swinging. The girders are designed for a distributed live load of 84 lbs. per square foot, and a concentrated rolling load of 16 tons on a 10-feet wheel base. To minimize the deflection at ends when the bridge is open, an excess of area is provided throughout.

In accordance with the most recent practice, particular attention has been given to the method of distributing the weight of the superstructure evenly on to the rollers. These are of cast iron, thirty-two in number, 18 inches diameter, and 6 inches face, revolving round a central pivot, and carrying a wroughtiron annular girder 21 feet 4 inches diameter, to which the weight of the superstructure is delivered at six equidistant points. As shown on diagrams, Plates IV., IVa, IVb, and IVe, each main girder rests on two stools, secured each to a longitudinal distributing girder, one end of which is carried by a stool on the annular girder, and the other by the transverse distributing girder. The position of the stools under the main girders is such that one-third of the load is delivered direct from each to the annular girder, and two-thirds to the transverse distributing girders, and by them to the annular girder. By this means, if W be the weight on each stool, two-thirds of W is taken at each of the six points. When the bridge is swinging, the total load on rollers due to superstructure, including the weight of distributing arrangement, etc., is 173 tons; so that the pressure on rollers is 18 cwt. per lineal inch of face. Secured to the upper annual girder and to a casting which revolves round the pivot are six radial distance girders of light section. The rollers are conical, with axes terminating at the centre of the pivot at a point in the same horizontal plane as the upper surface of rollers. The roller tracks are of cast iron in twelve segments, and special arrangements have been made for obtaining a perfectly level surface, when erecting the lower track, by means of steel adjusting wedges and iron cement. For horizontal adjustment a fine circular grove is cut in the track, and a single roller rotated until its inner edge coincides with the grove throughout. To allow of the rollers being run in or out, each roller rod is fitted with an adjustable gun-metal bush on which the roller revolves. The joints in tracks are

made diagonal in order to distribute the pressure over two segments.

The machinery for operating the swing span is carried on a platform on the outside of the downstream main girder, and is worked from the deck of the bridge, both lifting and turning gear being driven from the same handle. The machinery for lifting the ends has been designed for a deflection of 2 inches, and consists of bevel wheels and pinions operating a shaft which runs the whole length of the bridge, carried by brackets on the main girder. At each end of this shaft is a steel worm gearing into a phosphor-bronze wheel attached to a transverse shaft, with two cams of a 1-inch throw at each end working in castiron cam rollers, on which the bridge rests. These in turn are supported on cast-iron chairs over piers, provided with cast-iron bearing blocks fitted with wedges to allow of adjustment when erecting. To prevent over-winding, the second motion shaft is screwed for a portion of its length, and provided with two gunmetal stops and a gun-metal nut, with arms working in cast-iron guides bolted to the outer web of the main girder.

By means of a clutch on the first motion shaft worked by a hand-wheel on the inside of main girder, the machinery for either lifting or turning the bridge may be thrown into gear as desired. The latter consists of gearing which operates a pinion E on a vertical shaft working into a rack secured to the lower annular girder. The rack is of cast iron in five segments, and allows for a movement through 150°.

It is calculated that two men will be able to swing the bridge in four minutes, the operation of lifting occupying the same time. A tell-tale near the operating handle indicates the position of the bridge during opening and closing.

The swing pier consists of six cylinders, 3 feet 6 inches diameter, spaced equidistantly round a circle of 21 feet 1 inch diameter, and securely braced to each other and to a central cylinder carrying the pivot.

The piers at ends of span are formed each of two cylinders 6 feet in diameter, with diaphragm bracing. All the cylinders are of cast iron to high-water mark, above which they are of wrought iron, and they are filled with concrete. The bridge illustrated is suitable for light traffic which does not include vessels of large size, but the piers would require to be protected by means of dolphins and guide piles, or a wrought-iron caisson filled with

concrete might be substituted for the six cylinders with advantage. The foundations are good in the case illustrated, but, if there was any likelihood of settlement, a much more substantial central pier would have been necessary. The dimensions of the girders may be verified by the student, the calculations being similar to those already illustrated.