PONTOON, or PONTON. This term is employed by the French to signify any barge or flat-bottomed boat ; but in this country it is confined to those vessels which are used in the formation of floating. bridges for military purposes.
The conveyance of an army with its artillery and baggage across the rivers which intersect its lino of march is one of the most difficult as well as the most important operations in military tactics. The occupation of an advantageous position in a given time, when the army acts offensively, materially influences the success of a campaign ; and the favourable moment may be lost, if means should not be at hand to overcome the obstacle presented by a deep and rapid stream. But the failure or insufficiency of such means must be attended with the most fatal consequences to a retreating army, when it is prevented by a river from getting beyond the reach of an enemy ; for its safety, in this case, depends upon the power of passing the river without delay, and upon the removal or destruction of the bridge immediately afterwards.
History presents us with innumerable instances, both of the ruin of armies caused by the want of means to make good passage across rivers, and of the protection which armies have received when rivers have interposed between them and the superior forces of an enemy. To take an example from modern warfare, it may be observed, that during the retreat of Napoleon from Moscow, had it not been for the extraordinary care used by the chief of the French engineers to preserve the materials requisite for the formation of a bridge, the whole of the army must have been captured or destroyed on the banks of the Beresina.
The rapidity of warlike operations seldom allows bridges, at least those of considerable magnitude, to be constructed on piles or piers; and perhaps tho only military work of this nature which need be hinted at is that which was formed by Caesar acrosa the Rhine. [Blume.] Bridges consisting of timber platforms supported on floating vessels appear to have been in use in all ages. But those which were thrown
by Darius across the Bosphorus, and subsequently over the Danube, and that which was formed by the order of Xerxes over the Helles pont at the time of his unfortunate expedition into Europe deserve to be considered as the most famous works of that nature which were constructed by the ancients : and Herodotus, who has preserved (lib. iv., 88) the name of the Greek engineer employed on the two first, has also given a full description of the last (vim. 36). He states that 360 vessels, anchored both at the head and stern, were disposed in parallel directions across the strait with their keela in the direction of the current, in order to diminish the strain on their cables ; and that parallel to this line, but nearer the Archipelago, was another consist ing of 314 vessels, similarly disposed. Tho vessels were connected together by cables, over which was laid a platform of planks covered with a bed of earth, and there was a rail on each side. Xenophon also relates (` lib. ii., c. 4) that the Greeks, in their retreat, passed the Tigris by means of a bridge which was supported on thirty seven vessels. Of such boat-bridges for military purposes the most remarkable, in modern times, is that which was formed, by the engineers of the British army, across the Adour, in the south of France, in 1814. It consisted of twenty-five chaescanaraen, varying in burthen from 20 to 40 tons, which were moored at distances of nearly 40 feet from centre to centre. At first, hawsers resting on their decks were strained by capstans and made fast to some heavy guns which were laid behind the retaining walls on the sides of the river, and these ropes carried the planks which formed the roadway ; but after a few weeks they were removed, and replaced by timbers, which from their steadiness were found preferable. The breadth of the river at the place where the bridge was formed is 810 feet, and booms were moored across, both above and below the bridge, for its protection.