BRIDGE, MILITARY, is a temporary construction, to facilitate the passage of rivers by troops, cannon, and military wagons. The most efficient are described under PON TOON; but there are many other kinds. A bridge of boats is formed by small-craft, espe cially cargo-boats, collected from various places up and down the river; trestles are placed in them to bring their tops to one common level; the boats are anchored across the river, and baulks of timber, resting on the trestles, form a continuous road from boat to boat across the whole breadth of the river; the boats ought to be of such size that, when fully laden, the gunwales or upper edges shall not be less than one foot above the water. Rope-bridges are sometimes but not frequently used by military engi neers. A boat-and-rope bridge consists of cables resting on boats, and supporting a plat form or road of stout timber. A cask-bridge consists of a series of thnber-rafts resting on casks; the casks arc grouped together in quadrangular masses; at certain intervals, timbers are laid upon them to form rafts, and several such rafts form a bridge; it is an inferior kind of pontoon-bridge. A trestle-bridge is sometimes made for crossing a small
stream in a hilly country; it consists of trestles hastily made up in any rough materials that may be at hand, with planking or fascines to form a flooring, cables to keep the trestles in a straight line, and heavy stones to prevent them from floating. Raft-bridges, consisting of planks lashed together, are easily made of any rough materials that may be found on the spot; but they have little buoyancy, and are not very manageable. A swing-flying bridge consists of a bridge of boats, of which one end is moored in the center of the river, and the other end left loose; this loose end is brought to the proper side of the river, the boats are laden, and they make a semicircular sweep across the river by means of rudders and oars, until the loose end of the bridge reaches the other bank. A trail-flying bridge is a boat or raft, or a string of boats or rafts, which is drawn across a river by ropes, in a line marked out and limited by other ropes.