A _bridge-equipage_ made to follow an army in its movements in the field, is generally composed of light skiffs or batteaux, and the necessary timbers, planks, anchors, &c., for forming the roadway, and keeping the bridge in its position. All these articles are constructed especially for this purpose. All the wood-work should be of tough and well-seasoned timber, so as to impose no unnecessary weight on the wagon trains. The bateaux should also be made of strong and light materials. For convenience in transportation, these boats are sometimes made with hinges so as to fold up. The ribs are usually of oak, and the sides and bottom of pine. Instead of plank, a covering of tin, copper, India-rubber, &c., has sometimes been substituted. Floating supports of this character are often made in compartments, so as to prevent their sinking when injured by the enemy's projectiles. Indian-rubber pontons may be folded up into a small space, and their slight weight renders them convenient for transportation. On navigable streams a part of the bridge resting on one or two bateaux should be so arranged that it can be shipped out of its place, forming a _draw_ for the passage of river-craft. Indeed, it would be well, even where the river is not navigable, to form a draw for the passage of trees, and other floating bodies, sent down by the enemy against the bridge. An ordinary bridge-equipage of bateaux, or light pontons, for crossing a river of from three to four hundred yards in width, and of moderate current, will require a train of from sixty to eighty wagons.[46] Under favorable circumstances, and with a well-instructed corps of pontoniers, the bridge may be thrown across the river, and prepared for the passage of an army in a few hours at most.[47] After the troops have passed over, the bridge may be taken up, and replaced on the wagons in from a quarter to half an hour. [Footnote 46: The number of wagons in a ponton train will be greatly diminished if it be found that Indian-rubber boats may be used as supports for the bridge. The engineer department of our army are making experiments to determine this point.] [Footnote 47: In 1746, three bridges of bateaux were thrown across the Po, near Placentia, each fifteen hundred feet in length, and entirely completed in eight hours. In 1757, two bridges of bateaux were thrown across the Rhine, at Wesel, in half an hour; again, in the same year, a third bridge was thrown across this river near Dusseldorf, in six hours. In 1841, Col. Birago, of the Austrian army, arrived on the bank of the Weisgerben arm of the Danube, with his bridge-equipage, at a round trot, and immediately began the construction of his bridge, without any previous preparation or examination. In less than three-quarters of an hour the bridge was completed, and three loaded four-horse wagons passed over on a trot, followed by a column of infantry.]