Figs. 74, 75, 76, and 77 are sketches of high-level railway viaducts which have been constructed with great height, or headway, to allow large vessels to pass under at all times without interruption. This description of work is very costly, not only in the deep-water foundations, but also in the heavy scaffolding and appliances requisite for building piers and girders at such an elevation above the ground-level. The hoisting of the material alone forms an important item where such vast number of pieces have to be lifted to a height of 80, 90, or 100 feet. [Illustration: Fig. 78, 79] Figs. 78 and 79 are sketches of low-level viaducts constructed with one large opening span, or swing-bridge, for the passage of vessels. The girders and roadway of such opening span are usually constructed 85 as a compact framework, which revolves on a centre placed in the middle of a circular roller path or species of turn-table. The portions of the rotating opening bridge, although not always the same length on each side of the centre-pin, are generally very carefully balanced, to preserve the equilibrium of the entire mass when swinging round for the passage of vessels. To ensure stability in working, and steadiness during heavy gales, a liberal diameter should be given to the roller path of all swing-bridges having large span and great weight. Lattice, or truss, girders are preferable to plate girders for swing-bridges of considerable opening, as they present less surface area to the action of the wind. The opening and closing of these bridges is effected by wheel-gearing actuated by hydraulic, manual, or other motive-power. The revolving machinery should be set solid and true, well protected from the weather, and, at the same time, readily accessible for constant inspection, lubrication, or repair. Figs. 80 to 85 are sketches of various types of railway bridges constructed for smaller openings across narrower rivers, water-ways, or canals. Fig. 80 is an example of what is known as a _bascule_ bridge. This particular bridge is made in two halves, meeting in the centre of the span, the tail end of each half being provided with heavy counterweights to assist in opening or tilting up the bridge for the passage of vessels, or lowering it down for railway traffic. Each half of the bridge swings on horizontal axles, and the raising or lowering is effected by means of hand winches or other motive-power, actuating wheel-gearing working into toothed vertical segments attached to the tail end of each half. The same principle has also been applied to bridges having only one leaf to tilt up to clear the passage way.