PONTOONS are vessels used in the for /nation of floating-bridges for military purposes. General Pasley has constructed pontoons in the form of canoes, with decks, each end being shaped like the head of a boat. They consist 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 per forations 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.
Sir James Colleton, some years since, in vented pontoons of wood of a cylindrical form, some of which have been occasionally em ployed in experimental operations ; and cylin drical pontoons of tin, which were subse quently invented by Colonel Blanchard, have lately been introduced into the service. These last have hemispherical or conical ends. They possess the advantages of great lightness and buoyancy. The diameter of one of the cylin
ders is 2 ft. 6 in., and its length 22 ft. A rectangular frame, rather greater in length than the breadth of the intended bridge, is made fast, by ropes, longitudinally, on the surface of each of two cylinders placed parallel to one another at about 10 ft. asunder; and on these frames rest the baulks carrying the chesses, or planks, which form the road. Two pontoons, with their platform, constitute a raft; which, when the bridge is to be formed, are rowed to their stations in a line across the river, the lengths of the pontoons being parallel to the banks, and there anchored; the distances between the nearest pontoons in two rafts being equal to that between the two pontoons in each raft. Then each raft carry. ing the materials which are to make a platform over the water between itself and the next, such platform is laid down in a manner similar to that which is employed in laying down the platform of the raft; and from each of the extreme pontoons a like platform is extended to the shore of the river.