eye. But it is also to be remarked that it was to avoid the loss of time involved in laboriously collecting the material for and constructing such bridges, that the ponton equipage and the standard types of fixed bridges were devised. The construction of a bridge of round timber is a rather entertaining recreation, and engineers occasionally overlook the loss of time thus involved; hence it should be avoided whenever possible. The collection of the material and the lamming of a pole deck, in particular, constitute a time-consuming operation. In building a bridge of mixed material, the round timber may be employed for trestle posts, sills, and caps, and, if necessary, also for stringers and side rails, leaving the dimensioned stuff for bracing, stringers, and deck—especially the deck.

In almost every town, unless it has been previously overrun by the military, a small stock of lumber can be found; and frame buildings may be wrecked, if necessary. This, in combination with standing timber, will often meet the needs of the situation. Local materials are, in general, decidedly inferior in all respects to standardized materials from the engineer depots. They must, however, frequently be employed; and the engineers should be skilful in collecting them and in adapting them to their needs.

Loading of Military Bridges

The military bridge builder should be familiar with the usual loads that bridges in various situations and for various purposes will be called upon to bear. Usually these loads are quite definite, and the standardized designs are based upon them. As the factor of safety employed in military bridging is almost always low, especially at the front, vehicles heavier than those for which the structure was designed should not be permitted to cross it. Nevertheless, it is generally impossible to foresee the extent to which any bridge may be used. Bridges on important routes of transport are almost certain to be called upon to carry the heaviest loads. Drivers of heavy trucks will proceed across any bridge they come to, whatever warning signs may be posted, unless a guard is placed at the structure.

Accordingly, it is desirable that, on any main route, or any road apt to become an important route, the bridges, whatever their immediate purpose, should be built to carry the heaviest military loads. The supports, at least, should be made of ample strength, which will allow the bridge later to be reinforced to carry heavy loads by introducing additional supports or otherwise strengthening the superstructure.

In general, the first crossing will be a hasty improvisation, or a light foot-bridge of some standard design. If the need for the crossing continues, this bridge may be replaced by the ponton equipage, and subsequently by an improvised or standardized fixed bridge. By this time the probable future needs of the route will be known with some certainty, and future steps can be taken accordingly.

There should be an ample number of through routes having bridges capable of carrying the greatest loads, including tanks and heavy artillery.