Rafts formed of timbers, casks, barrels, &c., are frequently used as military bridges.  They may be made to bear almost any weight, and will answer for the passage of rivers of any depth and width, provided the current be not rapid.

Where the bridge is to be supported by rafts made of solid timbers, these timbers should be first placed in the water, to ascertain their natural position of stability, and then the larger ends cut away on the under side, so as to present the least possible resistance to the action of the current.  They are afterwards lashed together by strong rope or withe lashing, or fastened by cross-pieces let into the timbers, and held firm by bolts, or wooden pins.  These rafts are kept in place by anchors and cables placed up and down stream.  The roadway is formed in nearly the same manner as for a bridge supported on trestles.  Empty casks, and other floating bodies, may be substituted in place of logs in the construction of rafts.

For examples of the use of rafts in the construction of military bridges, we would refer to the passage of the Seine in 1465 by Count Charolais; the passage of the Meuse in 1579, by Alexander Farnese; the passage of the Vistula in 1704, the Borysthenese in 1709, and the Sound in 1718, by Charles XII.; the passage of the Adige in 1796; the passage of the Po in 1807; and the subsequent military operations in the Spanish Peninsula.

Military bridges are frequently made of boats, and the ordinary river-craft found in the vicinity of the intended passage.  Flat-bottomed boats are the most suitable for this purpose, but if these cannot be obtained, keel boats will serve as a substitute.  When these water-craft are of very unequal sizes, (as is frequently the case,) two smaller ones may be lashed together to form a single support; they can be brought to the same level by means of stone ballast.  The gunwales must be suitably arranged for supporting the balks, or else frameworks should be erected for this purpose from the centre of the boat.  The arrangement of the roadway, anchors, &c., is the same as before.

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.