The precise period at which pontoons, or flat-bottomed vessels, were first employed to support a bridge is unknown. Folard observes, in his observations on the fifth book of Polybius, that for passing rivers the Germans in his time used vessels formed of timber frames covered with leather : he adds that the Dutch troops had pontoons of wood covered with tin, and that the French took some of them at the battle of Fleurns, which was gained, in 1693, by the Marechal de Luxem bourg over the Prince of Waldeck. Mention is frequently made of pontoon-bridges, both in Germany and Italy, in the beginning of the 18th century, particularly during the campaigns of Marlborough ; and, from the speed with which they are said to have been executed when required, it is probable that a corps of men was then particularly employed in that branch of service.
The pontoons employed during the war against Napoleon I. differed but little from those mentioned above. They were of metal over a light framework of wood tinned inside ; they were from 17 to 21 feet long, from 4 to 5 feet broad, and from 2 feet to 2 feet 3 inches deep. In the formation of the bridge, a rope was tightly stretched across the river ; the vessels were then rowed to their places, and each was made fast at one end to the rope; another rope was then stretched across the river, parallel to the former, at the opposite ends of the pontoons, and to this rope those ends were made fast. When a strong current set obliquely across the river, the sides of every two pontoons were sometimes connected together by rope braces stretched diagonally across the intervals, in order that the effect of the current upon each might be counteracted by a like effect on the next ; and in very rapid streams anchors were also thrown out from each or from every second or third pontoon, according to circumstances.
Timber-baulks, or joists, were then laid from the shore to the first pontoon, from that to the next, and so on, at intervals from each other depending upon the breadth of the bridge or the weight to be sup ported. Every two _ in the direction of the length were attached together by a bolt, which allowed the bridge to yield to the rise or fall of the water. Above these timbers were placed the cheeses, or flooring-planks, close together and perpendicular to the length of the bridge ; and these were kept down by a riband or a plank, which was fastened over their extremities on each side of the bridge.
General Sir Charles (then Colonel) Parley, to whom the department of military engineering in the British service is so much indebted for the improvements which he has introduced into every branch of art connected with practical fortification and siege operations, has proposed a description of pontoon. The form of the old pontoons rendering them difficult to manage in the water, and their great weight, when it was necessary to transport them from place to place by land, causing them to be a serious encumbrance to an army on its march, that officer was led to construct the pontoons in the form of canoes, with decks, each end being shaped like the head of a boat, in order that they might be easily moved through the water by rowing, with either end foremost. They are constructed of light timber frames, covered, except the deck, with sheet copper; and each vessel is formed in two equal parts by transverse partitions, so that the demi-pontoons may be separated from each other when the bridge is to be conveyed on carriages by land with the army. When in the water, the parts are connected together by a rope, which passes through two perforations in the keel, near the place of junction, and by a rectangular frame of wood, which is laid along the deck, and attached to it by lashings. Each half-vessel is also divided into two compartments by a partition ; and small pumps are provided, by which the pontoon may for a time be kept afloat, should a hole be made in its side by a shot or by any other accident.
Sir James Colleton, some years since, invented pontoons of wood of a cylindrical form, some o iwhieh have been occasionally employed in experimental operations; and cylindrical pontoons of tin, which were subsequently invented by the late General Blanchard, were introduced into the service. These last have hemispherical ends, and are divided both longitudinally and transversely into several compartments by partitions of tin, both to increase their strength and to prevent them from sinking in the water in the event of their being accidentally per forated in any part. They possess tho advantages of great lightness and buoyancy; but they have not the durability of copper vessels, and they must be very liable to be injured when transported by land, particularly if a march should take place under a hot sun.