RAILWAY CONSTRUCTION. 85 



usually constructed 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 steadi- 

 ness 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. 



Railway bridges of this pattern are now very rarely adopted. 

 They have the great drawback that when raised to the vertical 

 position, a very large area is presented to the action of the 

 wind, and this defect might lead to very serious consequences 

 in the case of a bridge situated in an exposed locality. An 

 open-work floor diminishes the wind area, but a very large 

 surface must necessarily remain. 



Fig. 81 illustrates what is known as a traversing bridge. 

 In this case the width of the opening passage-way and the 



