The _swing_-bridge arrangement is so simple in construction, convenient for inspection, and easy to maintain, that where possible it is now generally adopted in preference to any other system. The 89 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 destruction 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. [Illustration: Fig. 89, 90] 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 floods in the autumn or winter. 91 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.