RAILWAY CONSTRUCTION. 91 



floods in the autumn or winter. No rafting can be carried 

 on when the river is in flood ; the current would be too strong 

 to permit of the raft being kept under control. 



Fig. 87 is a sketch of a similar bridge where the river is 

 confined to a regular channel between two sloping banks of 

 strong clay. 



Fig. 88 shows a bridge erected over a narrow rocky pass 

 in the river. The channel is hemmed in by the almost per- 

 pendicular sides of mountain granite, there are no banks to 

 overflow, the flood waters cannot spread laterally, however 

 much they may increase in depth, and with building-stone at 

 hand in abundance, and foundations formed in the solid rock, 

 the situation is one of the most favourable for a strong per- 

 manent bridge. The cast-iron arch of 150-feet span has a 

 graceful appearance, and harmonizes well with the surrounding 

 scenery. A small masonry arch at each end of the bridge 

 provides for communication along the banks of the river. 



With rivers which are neither under the control of naviga- 

 tion authorities nor used for rafts of timber, there is much 

 greater freedom for the designing and carrying out of bridges 

 or viaducts suitable for the actual physical conditions of the 

 locality. The headway will be guided only by the height of 

 the railway to be carried across, and by any flood-water levels 

 which may affect the work. The size of the spans will be 

 regulated by the width of the river, the depth of the water, 

 and the nature of the ground into which the piers have to 

 be built. For broad, shallow rivers with good firm river-beds, 

 piers may be built at moderate cost, and comparatively small 

 spans adopted; on the other hand, with a broad deep river 

 it will be better, as previously explained, to reduce the number 

 of piers and increase the span. In the one case, for example, 

 a river 150 feet wide may be crossed with three spans and 

 two piers in the shallow water, as in Fig. 89 ; in the other 

 it may be more prudent and economical to cross in one span, 

 without any intermediate pier, as shown in Fig. 90. 



Next in importance to the large bridges and viaducts over 

 rivers are the viaducts which have to be constructed for the 

 crossing of deep inland valleys. The occurrence of one of these 

 deep valleys between long lengths of average table-land renders 

 necessary either a series of cuttings and falling gradients to 

 get down to a low level, or the erection of high-level works 



