PONTOON (through the French ponton, from the Latin pons, a bridge) the name given to buoyant vessels used in military operations for supporting a temporary bridge. Pontoon bridges have been constructed, with greater or less skill, from the earliest times. Darius passed the Hellespont and Danube by pontoon bridges, and the former was traversed by Xerxes's immense army on similar temporary bridges, very admirably formed. A pontoon train is a necessity for every army maneuvering in a country where there are rivers, and many campaigns have proved failures from the want of this cum brous but.indispensable apparatus. In most armies the pontoons arc under the charge of the engineers; but in the Austrian army there is a distinct and highly-trained corps, called pontonieren. Marlborough used clumsy wooden pontoons. Napoleon and Well ington had them lighter of tin and copper. They were flat-bottomed, rectano-ular boats, open at the top. Anchored at stem and stern, beams were laid over from one to another, and transoms with planks crossino. these beams completed the roadway of the bridge. These open pontoons were exposed to the disadvantage that they were very liable to filled with water, and thus ceased to support the bridge. They were, moreover, very heavy, one pontoon, with appurtenances, constituting a wagon-load. As 36 were deemed necessary for the train, a pontoon equipment was a serious item in the impedimenta of an army. The open pontoons are now, however, obsolete, modern science having sub stituted closed cylindrical vessels of copper (or occasionally of India-rubber), which are far lighter, can in an emergency be rolled along, and can only be submerged if perforated.
Against the last contingency, they are divided within into water-tight compartments, so that one perforation may not seriously detract from the total of a pontoon. In the British service two pontoons arc used: the larger, with hemispherical ends, being 22 ft. 3 in. in length; and 2 ft. 8 in. in diameter; the smaller, cigar-shaped, with conical ends, 15 ft. in length, 1 ft. 8 in. in diameter. Two of the largest used to form a raft weigh 8 cwt. 7 lbs. ; the superstructure, 18+ cwt. At 24 ft. apart from center to center, this raft will carry infantry four deep, marching at ease; cavalry, two deep, and light field-guns; at 16 ft. interval, heavy guns. A raft of three pontoons, at close distances, will support siege-ordnance. The pontoons can be used in very wide rivers as rafts, in their proper sense, or they can be connected, when the width permits, to form a bridge. In the latter case each is towed into line, anchored above as it drops to its place, and a second time when its exact spot is reached. It is computed that each pontoon requires 1+ minutes to take its position, and that when the pontoons are placed, the roadway can be laid, if properly arranged previously, in 1+ minutes for each interval between two pontoons. A river of 600 ft. may thus be bridged in less than 1+ hours. The process of throwing a bridge over in face of an enemy is fraught with the utmost danger to the engineers employed. Pontoon bridges have to be passed with great care, and every measure should be adopted, as breaking step, etc., which can reduce the peculiarly dangerous