more than twenty tons. The reinforcement consists of an intermediate roadway-bearer supported by the heavy side rails. The wagons used to transport the new equipage will be such that they can be hauled by animals, truck, or tractor.

Railroad Bridges

In the construction of railroad bridges, military practice follows civil practice more closely than in highway bridges. As they must usually carry heavier loads, and as variations in either horizontal or vertical alignment are much more serious, greater attention should be paid to the solidity of the structure, including its foundations. Pile trestles, girders, and trusses on piers of pile clusters are the usual types of military-railroad bridge. Most of them, except for light, narrow-gauge railways, will be in rear of the combat zone, though often bridges for standard gauge must be constructed close to the front.

Foot-Bridges

Foot-bridges, employed usually to meet tactical emergencies, exhibit a greater variety of forms, and have heretofore been less subject to standardization than either vehicle or railroad bridges. In a crisis, any design and any material that will serve, or even partially serve, the purpose must be employed. Trees, cut near the bank and allowed to fall across the stream, often have enabled a combatant force to meet a grave emergency. As traffic accidents thereon will be less serious than in the case of other bridges, less attention is usually paid to the factor of safety. If this be very uncertain, the bridge should be tested by sending a few men across, and the traffic should be regulated to prevent crowding.

The demands for the rapid passage of foot troops over streams are so frequent and insistent in modern warfare, that it is certain that in future, even more than in the past, standardized, portable foot-bridges will form part of the equipment of combatant organizations. A number of such standardized foot-bridges, including the light ponton type heretofore described, were devised during the World War. Floating types will be the ones most commonly employed, these being supplemented by light sectional trusses and, in some instances, by suspension bridges. The floats which have been successfully used include light canvas pontons, casks of wood or steel, rafts of wood or cork, and Kapok Rafts. The latter have the advantage that they are unsinkable by rifle or machine-gun fire.

The width of the roadway or path of foot-bridges is from 2 to 2 1/2 ft. A greater width is unnecessary for the passage of men, and unduly decreases the mobility of the equipage, which is an essential requirement. Such a width (2 ft.) does not permit the passage of machine gun carts, which must seek other means of crossing. Machine guns, light mortars, and one-pound cannon accompanying the infantry, with their ammunition, may be carried across by hand.