PONTOON, in War, a kind of flat-bottomed boat, the body of which is composed of wood lined without and within with tin. Pontoons serve for the construction of bridges over rivers, to facilitate military operations. The French pontoons, and those of most other powers, are made of copper on the outside; and though these cost more at first, yet they last much longer than those of tin, and, when worn out, the copper sells nearly for as much as it originally cost; whereas when ours are rendered useless, they sell for nothing. Our pontoons are twenty-one feet in length, five feet in breadth, and in depth within two feet 1.5 inches.