BRIDGES AND DOCKS, MILITARY. The principal types of military bridges for the pas sage of troops and their supplies are the pon toon or floating, the spar, the trestle, and the suspension bridge. In most eases they must be constructed rapidly. and of such materials as are available, there being usually little time for refined calculations and for procuring the most suitable materials. The sizes of the parts of a military bridge depend primarily, as in other engineering structures, on the loads to be carried. Only in special cases will the width differ much from S to 10 feet. This is ample to provide for the passage of infantry in fours, cavalry in half sections, and guns and wagons fully horsed. It is considered desirable that, in whatever formation the troops may pass. the structure should be made strong enough to sustain them when crowded as a result of some stoppage in that portion of the column which has already crossed. in most cases 100 pounds to the square foot may be taken as the uniform weight for which a bridge intended for the passage of infantry should be made safe. if proportioned for this load, it will also be strong enough for the passage of the various train wagons and of field guns and carriages. lf, however, it is to be crossed by steam-trac tion engines or other unusually heavy loads, additional precautions may be necessary to en able it to carry them with safety.
In the United States Army, where the pontoon bridge was extensively and effectively employed (luring the Civil \Var, the bridge material of the Corps of Engineers is included in reserve and advanee-guard trains. Where a large army is operating in the field and wide and swiftly flow ing rivers are to be crossed the reserve train must accompany the troops. Such a train is divided into four pontoon divisions and one supply di vision. Each pontoon division is complete in itself. containing all the material necessary for constructing a bridge 225 feet in length. Each division is divided into four sections—two pon toon and two abutment sections—each pontoon section consisting of three pontoon-wagons and one ebess-wagon. These four wagons contain
the material for three bays (or 60 feet) of the bridge. If it is desired to vary the length of a single division, it can be done by diminishing or increasing the number of pontoon sections in the train. The pontoons are flat-bottom boats 31 feet in length, and with a maximum breadth of 5 feet S inches, of about tons burden, and with sufficient capacity to transfer 40 men fully armed and equipped, besides its crew of pon toniers.
Pontoon bridges may be constructed either by the method of successive pontoons, by rafts, or by conversion. The first mentioned is the most usual method. In following it, the pontoons are placed 20 feet apart, at right angles to the axis of a bridge, and anchored up-stream and down stream to hold them in position. Five lines of floor-beams, known technically as 'balks,' 5 inches square, rest on the pontoons and are firmly lashed to them. On these tloor-planks, inches thick and known as chess. are laid; these are held in position by side rails lashed through notches in the chess to the balk beneath. The width of bridge between side rails, when finished, is 10 feet. Bridges of this type received the hard est kind of usage during the Civil War, and proved clearly their great value. In the month of February, 1862. a pontoon bridge composed of GO boats of a reserve train was thrown across the Potomac at Harper's Ferry. The river was then a perfect torrent, the water being 15 feet above the summer level, and filled with drift wood and floating ice. Under these unfavor able circumstances, the bridge was completed in eight hours, and General Banks's corps, with all its trains and artillery, passed over it with out accident or delay.
During the year of 1863 the pontoon trains accompanied the army in all its marches through Virginia, frequently bridging the Poto mac, Rapidan. and Rappahannock.