This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
A simple form of timber bridge is shown in Fig. 1400, in which stout beams a are supported on posts b, duly fixed and strutted, with a flooring c carrying posts d and handrails e braced as at f. When the stream admits, central posts may be dispensed with, and the beams supported only at the ends. The arrangement must be adapted to meet the requirements of the force of the stream, height in flood, liability to change of course, silting up of the bed, and probability of ice, fallen trees, etc, being carried down against the structure. Usually the narrowest point on a stream is the best for a bridge.
An efficient substitute for a bridge, often used in India on watercourses which contain little water during a great portion of the year, and are only flooded occasionally, consists of a paved causeway. The banks are cut down to a gentle slope on each aide, and a pavement or solid flooring of masonry or concrete is built to afford a firm roadway for vehicles, at such a level that the water does not enter them. One across the river Soane is a mile long and 12 ft. wide. Boat bridges are useful under some conditions, and are constructed by laying a plank platform on balks of timber resting on the banks of the stream and on boats lashed together. In hilly districts foot passengers can cross rivers in travelling cradles suspended from a single cable; or there may be a rope for 2 x 2 the feet and 2 others for the hands, kept in position by triangular sticks; or 2 foot-ropes may be laid parallel and support a platform of bamboo. In India, beams weighted with stones are made to gradually project from each side till they meet in the centre; these are called sanghoos.