The art of constructing temporary bridges for the passage of troops across large rivers and narrow arms of the sea, was well known to the ancients. Darius passed the Bosporus and Danube, and Xerxes the Hellespont, by bridges of boats. The army of Xerxes constructed two bridges across the latter strait, the first of 360 vessels, anchored head and stern alongside each other, their keels in the direction of the current, the vessels connected with each other by strong cables, over which planks were laid, fastened by a rail on either side, and covered in by a bed of earth. The second bridge had 314 vessels, and was similarly constructed. According to Arrian, Alexander had a regular pontoon train of light boats attached to his army. The Romans had wickerwork vessels, covered with hides, destined to support the timber platform of a bridge; these formed a part of the train of their armies until the end of the empire. They, however, also knew how to construct a more solid kind of military bridge, whenever a rapid river had to be crossed; witness the famous bridge on piles, on which Caesar passed the Rhine from Gaul into Germany in 55 B. C. The bridge was built, according to the best authorities, somewhere in the region between Ooblentz and Andernach. Its construction occupied the army for ten days.

During the middle ages we find no notice of bridge equipages, but during the thirty years' war the various armies engaged carried materials with them to form bridges across the large rivers of Germany. The boats used were very heavy, and generally .made of oak. The platform of the bridge was laid on trestles standing in the bottoms of these boats. The Dutch first adopted a smaller kind of vessel, flat-bottomed, with nearly vertical sides, pointed head and stern, and both ends projecting, in an inclined plane, above the surface of the water. They consisted of a framework of wood, covered with sheets of tin, and were called pontoons. The French, too, according to Fo-lard, claim the invention of pontoons made of copper, and are said to have had about 1672 a complete pontoon train. By the beginning of the 18th-century all European armies had provided themselves with this kind of vessels, mostly wooden frames, covered in with tin, copper, leather, or tarred canvas. The latter material was used by the Russians. The boats were small, and had to be placed close together, with not more than 4 or 5 ft. clear space between them, if the bridge was to have any buoyancy; the current of the water was thereby greatly obstructed, the safety of the bridge endangered, and a chance given to the enemy to destroy it by sending floating bodies against it.

The pontoons now employed by the continental armies of Europe are of a larger kind, but similar in principle to those of 100 years ago. The French have used since 1829 a flat-bottomed vessel with nearly vertical sides, diminishing 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, covered with fir planking. Every pontoon weighs 1,658 lbs., and has a buoyancy (weight of cargo which would sink the vessel to the top of the gunwales) of 18,675 lbs. 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 advanced guard of an army a smaller kind of pontoon is used, for bridging over rivers of less importance. The Austrian pontoons are similar to the larger French pontoon, but divided transversely in the middle for more convenient 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 unshipped; 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 lbs. each. Breadth of road of bridge, 10 ft.; distance from pontoon to pontoon, 8 ft. The Russians also have pontoons with a similar framework, covered over with leather. The Prussians 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 pontoons, 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, etc, they partake the common faults of all cylindrical pontoons, and have been discarded both by the United States and England. - A pontoon train contains, besides the pontoons, the oars, boat hooks, anchors, cables, etc, necessary to move them about in the water, and to fix them in their position, and the balks and planks (chesses) to form the platform of the bridge.

With boat pontoons, every pontoon is generally secured in its place, and then 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 constructed in sheltered situations above the site fixed on for the bridge, and floated down successively 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 navigable 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, etc, and sunk in the deeper places of the river, will form convenient supports for the platform of a bridge. Inundations, marshes, etc, are bridged over' by means of gabions. For narrow rivers and ravines, where infantry only have to pass, various kinds of suspension bridges are adopted; 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 possibility 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 construction.

The concave bank, moreover, is generally higher than the convex one, and ihus in most cases the advantage of command is added to that of a cross fire. Infantry are rowed across in boats or pontoons, and established 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 junction of some smaller river, also offers advantages. 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 mislead 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 preferred to keep them concentrated at some distance from the river, and march them in a body against the real point of passage as soon as it has once been ascertained, and before the enemy can have brought over all his army.

It is from these causes that in none of the wars since the French revolution has the construction of a bridge on any of the large rivers of Europe been seriously contested.