The Viaduct Works' Handbook

34 the river of 90 feet, in two openings of 45 feet each, which may serve as approximate data for others of this class. The roadway is carried at the bottom of a pair of main girders 113 feet long of the lattice principle, placed 18 feet apart upon transverse girders placed 2 feet 6 inches apart, which project outside the main girders, forming cantilevers, and upon these transverse girders the floor, which is of wood planking, is laid, and covered with wood paving. The main girders are supported at the centre upon a revolving table or pivot, which revolves on top of a central pier composed of a cluster of four cylindrical iron piles, each of 7 feet diameter, sunk deeply into the bed of the river, and afterwards firmly braced together, and filled with masonry. They are so arranged in plan that their outer circumference lies exactly within a circle of 20 feet diameter, whilst they are equidistant from each other. The re- volving table or pivot consists of a pair of massive cast-iron rings of 20 feet external diameter, one fixed and the other moveable, put together in segments, and of a section somewhat approaching that of a bridge-rail about 12 inches deep, and having a centre with radial arms, the under-side of the lower ring being formed to fit the top of the piles upon which it rests, and is attached, and has teeth cast round the outside, and the top surface is planed to suit conical turned rollers which are placed between it and the upper ring; the upper ring is similar to the lower ring, inverted, excepting it is formed to fit the under-side of the main and transverse girders to which it is attached, and it has no teeth. The rollers are 15 inches diameter, placed about 18 inches apart, the axles of which radiate from the centre of the circle upon whicli they roll, and pass through a wrought-iron ring which keeps them equidistant, and they terminate at the centre in a boss, forming a ring which revolves on a central pin. This pin is of wrought-iron, 10 inches diameter, and forms a vertical axis for the whole of the moving structure to turn upon. A pinion wheel is made to gear with the teeth cast on the outside of the lower ring, having a vertical axis, is attached by suitable bearings to the transverse girders of the bridge floor, with a large wheel fastened on its upper end, is made to revolve by a smaller pinion and vertical shaft, the end of which projects even with the surface of the floor, and is formed convenient for a horizontal tiller or removable handle, which is