will greatly reduce the time required for construction. For example, the building of a pole floor is the slowest item of an improvised bridge. If dimensioned flooring material (decking) alone can be sent up from the rear, a material loss of time is eliminated, and the engineer will be able to improvise the rest of the structure. Bolts, spikes, wire, cordage, etc., form a very small part of the weight of the bridge—they come in small packages and are easily transported. Without them the time and difficulty involved in building the structure would be greatly increased; hence, these fastenings, at least, should be made available wherever needed.

Selection of Site

In many cases there will be little room for choice in the selection of a site. New bridges must frequently be built at localities where former bridges have been destroyed. In any case accessibility to adjacent roads on both sides will be a controlling factor in fixing the site, except for foot-bridges. The location must fulfill the tactical requirements; but, as a rule, the chief tactical requirement is rapid construction, and, accordingly, the site is selected with this in view. Where there is room for choice, the following conditions, affecting the time required for construction, will be given consideration.

(a) Width of Stream. This, of course, directly affects the length of the bridge and the time required for its construction. Other things being equal, the narrowest point is preferred.

(b) Depth of Water. The time and effort required for the construction of any type of fixed bridge increases rapidly with the depth of water, unless the stream can be crossed with a single span (truss or suspension). For the ponton bridge there should be sufficient depth to float the boats. Any greater depth does not increase the time required, except that very great depths make the anchorage of the pontons somewhat more difficult. Its independence of the depth of water is one of the principal advantages of the floating equipage.

(c) Swiftness of Current. A swift current greatly increases the difficulty of constructing and maintaining any type of bridge, except one with a single span from bank to bank. The slowest current is found in wide, straight reaches of the stream.

(d) Nature of Bottom. A very soft, yielding bottom will demand pile or crib construction, whereas a hard bottom will support framed trestles, which can be more rapidly placed. A ponton bridge, or a fixed bridge of a single span, is, of course, independent of the nature of the bottom, as it is of the depth.

(e) Nature of Banks and Approaches. A low, flat, and marshy approach, or one subject to overflow, will demand either a very long bridge, or a causeway, the construction of which is apt to require much more time than the bridge itself. On the other hand, high banks will necessitate tall