Page:The American Cyclopædia (1879) Volume III.djvu/284

 278 BRIDGE (MILITARY) minishing in breadth toward the stem, and also, but a little less, toward the stern ; the two ends rise above the gunwales and are curved like those of a canoe. The dimensions are : length, 31 ft. ; breadth at top, 5 ft. 7 in. ; at bottom, 4 ft. 4 in. The framework is of oak, cov- ered with fir planking. Every pontoon weighs 1,658 Ibs., and has a buoyancy (weight of cargo which would sink the vessel to the top of the gunwales) of 18,675 Ibs. When formed into a bridge, they are placed at intervals of 14 ft. clear space from gunwale to gunwale, and the road of the bridge is 11 ft. wide. For the ad- vanced guard of an army a smaller kind of pon- toon is used, for bridging over rivers of less importance. The Austrian pontoons are simi- lar to the larger French pontoon, but divided transversely in the middle for more conve- nient carriage, and put together in the water. Two vessels placed close alongside each other, and connected by short timbers, a longitudinal timber supporting the balks of the platform, constitute a floating pier of a bridge. These pontoons, invented by Birago, were introduced in 1823. The Russians have a framework of wood for their pontoons, so constructed that the centre pieces, or thwarts, may be unship- ped; over this frame is stretched sail cloth, covered with tar or a solution of india rubber. They are in length 21 ft. 9 in., breadth 4 ft. 11 in., depth 2 ft. 4 in., and weigh 718 Ibs. each. Breadth of road of bridge, 10 ft. ; dis- tance from pontoon to pontoon, 8 ft. The Rus- sians also have pontoons with a similar frame- work, covered over with leather. The Prus- sians are said to have been the first to divide their pontoons transversely into compartments, so as to prevent one leak from sinking them. Their pontoons are of wood and flat-bottomed. The span or clear distance between the pon- toons, in their bridges, varies from 8 to 16 ft., according to circumstances. In all continental armies small boats to carry out the anchors accompany the pontoon train. Pontoons of inflated india rubber were introduced in the United States army in 1846, and used in the war against Mexico. They are easily carried, from their lightness and the small space they take up when folded ; but besides being liable to be damaged and rendered useless by friction on gravel, &c., they partake the common faults of all cylindrical pontoons, and have been dis- carded both by the United States and England. A pontoon train contains, besides the pon- toons, the oars, boat hooks, anchors, cables, &c., necessary to move them about in the water, and to fix them in their position, and the balks and planks (chesses) to form the plat- form of the bridge. With boat pontoons, every pontoon is generally secured in its place, and the,n the balks and chesses stretched across; with cylindrical pontoons, two are attached to a raft, which is anchored at the proper distance from the end of the bridge, and connected with it by balks and chesses. Where circumstances admit of it, whole links, consisting of three, four, or five pontoons bridged over, are con- structed in sheltered situations above the site fixed on for the bridge, and floated down suc- cessively into their positions. In some cases, with very experienced pontoniers, the whole bridge has been constructed on one bank of the river and swung round by the current when the passage was attempted. This was done by Napoleon when crossing the Danube, the day before the battle of Wagram.- Pontoon trains are, however, not always at hand, and the military engineer must be prepared to bridge over a river, in case of need, without them. For this purpose a variety of materials and modes of construction are employed. The larger kind of boats generally found on naviga- ble rivers are made use of for bridges of boats. If no boats are to be found, and the depth or configuration of bottom of the river renders the use of floating supports necessary, rafts of timber, floats of casks, cotton bales, and other buoyant bodies may be used. If the river is shallow, and has a hard and tolerably level bottom, standing supports are constructed, consisting either of piles, which form the most durable and the safest kind of bridge, but require much time and labor, or of trestles, which may be easily and quickly constructed. Sometimes wagons loaded with fascines, &c., and sunk in the deeper places of the river, will form convenient supports for the platform of a bridge. Inundations, marshes, &c., are bridged over by means of gabions. For narrow rivers and ravines, where infantry only have to pass, various kinds of suspension bridges are adopt- ed; they are generally suspended by strong cables. The construction of a military bridge under the actual fire of the enemy is now a matter of but rare occurrence ; yet the possi- bility of resistance must always be provided for. On this account the bridge is generally constructed in a reentering bend of the river, so that the artillery placed right and left sweeps the ground on the opposite bank close to where the bridge is to land, and thus protects its con- struction. The concave bank, moreover, is generally higher than the convex one, and thus in most cases the advantage of command is added to that of a cross fire. Infantry are rowed across in boats or pontoons, and estab- lished immediately in front of the bridge. A floating bridge may be constructed to carry some cavalry and a few light guns across. The division of the river into several branches by islands, or a spot immediately below the junc- tion of some smaller river, also offers advan- tages. In the latter, and sometimes in the former case, the several links of the bridge may be composed in sheltered water, and then floated down. The attacking party, having commonly to choose between many favorable points on a long line of river, may easily mis- lead his opponent by false attacks, and then effect the real passage at a distant point ; and the danger of scattering the defending forces over that long line is so great, that it is pre-