RAILWAY CONSTRUCTION. 89 



system. The weights on centre pin and roller path may be 

 distributed as considered most expedient, and by means of 

 suitable appliances the weight may be altogether taken off 

 the centre and rollers when the bridge is closed for the passage 

 of trains. 



There are many wide rivers which, although not navigable 

 in the ordinary acceptance of the term, nevertheless require 

 bridges of large spans to provide free waterway for the floating 

 down of rafts of timber. Away in the high ground, in the 

 timber-growing districts, trees are felled, sawn or cut into long 

 poles, logs, or scantlings, and hauled to the banks of the river. 

 The timbers are then formed into large rafts of the most 

 convenient form for floating down to the place of distribution 

 or port for shipment. Even with old experienced floaters, using 

 their long sweeps in the most skilful manner, it is difficult to 

 take anything but a very irregular course down the stream. 

 Under the most favourable circumstances one of these large 

 rafts is an unwieldy, awkward craft to manage ; but in a river 

 full of twists and turns, with reaches varying from comparative 

 smooth water to miniature rapids, the current carries the huge 

 mass surging along, and only a clear, unobstructed channel will 

 enable its navigation to be carried out with safety. The 

 presence of a pier in the main waterway might cause destruc- 

 tion to the rafts and loss of life to the men. The vested 

 interests in floating rights are tenaciously guarded, and no 

 new bridge would be sanctioned which would in any way 

 interfere with the waterway or endanger the passage of rafts 

 down the river. Bridges of this description are much less 

 costly than those over deep water navigable rivers. Excepting 

 the large spans, the rest of the work is comparatively simple. 

 The water is generally shallow, and much reduced in quantity 

 during the summer months. Good foundations can generally 

 be obtained without going to any great depth. The headway 

 may be kept low, or of such height as may best suit the 

 purposes of the railway, and be sufficiently well up out of 

 the way of the floods which may take place from time to 

 time on the river. 



Fig. 86 is a sketch of a bridge constructed over a river much 

 used for rafting purposes. The large span is over the main 

 channel, and the small spans are over a wide gravelly foreshore, 

 which is only covered with water during exceptionally high 



