ing with the aid of tackle and an A-frame derrick, or by a counter-poise of additional sections of truss.

It is certain that sectional steel girders and trusses will be used on a vast scale in future wars of any great magnitude.

Portable girder-spans, usually of timber, complete with flooring, in one or two sections, are provided for the passage of foot troops and artillery over trenches, ditches, and small shell-holes, in following up an attack. These bridges are transported on combat wagons or artillery caissons.

Cribs

Cribs may be usefully employed for piers on very unstable bottoms, especially when there is a swift current, and for abutments. They have the advantages that they require no plant, and can be built of short pieces of almost any material. Cribs have greater power of resistance to floods, ice, and drift than have simple trestles, provided they are solidly constructed and filled with stone. Cribs are often employed as foundations for framed trestles, the crib-work being carried above ordinary flood level.

Trenches, ditches, shell-holes, and small ravines can be made passable by filling them with any debris that may be at hand.

An interesting development of the World War was a small cube or crib of structural steel, any number of which could be bolted together to form a bridge abutment.

Suspension Bridges

Suspension bridges are occasionally employed in military operations. For spans exceeding 50 ft., when no plant and no sectional trusses are available, they will sometimes meet the situation. For military purposes they have the advantage that the only essential parts are the cables, which are easily transported. If these are at hand, the remaining portions, being small and light, can usually be obtained in any locality. Light suspension bridges are easily erected without plant. When materials for a fixed bridge must be carried with a rapidly-moving column, the suspension type has the advantage of requiring the least material, for a given span and capacity, of any kind of bridge. Moreover, its parts, being small, are easily transported and handled.

The suspension bridge is specially applicable to long spans combined with light loads, but even in such situations the ponton equipage or ferries will usually be preferred for stream crossings. Hasty suspension bridges may be built to carry wagons, but for motor traffic they are unsuitable. In general, they are used only for foot bridges of relatively long span. Their ideal function is for foot traffic and pack transportation over wide and deep ravines in mountainous country, where the ponton equipage or sectional trusses are not applicable.

Because of the difficulty of handling in the field, cables are generally