trestles; or else they must be cut down to provide approaches with a practicable grade an operation usually involving much labor. The ponton bridge demands low approaches, as its height above the water surface cannot be increased. For crossings of small or moderate width that can be cleared with a single span or by using a small number of intermediate supports, high banks will be no great disadvantage; consequently, for any type of bridge, the most favorable crossing is one having low, firm banks of equal height, but above the level of any flood likely to occur during the time that the bridge is in use.

(f) Materials Available. If the bridge is to be constructed in whole or in part of materials obtained at the site, the availability of such local materials, without an excessive amount of transportation, is important. There are certain other considerations, of a tactical nature, that have a bearing on the selection of a bridge site. Amongst these may be mentioned:

(g) Defensibility. It is desirable that the bridge be located where its construction cannot be interfered with by the enemy, and where it and the troops crossing it will not be subject to artillery or rifle fire after its completion. If built in the near presence of the enemy, it will be advisable, before beginning construction, to ferry some troops across, in order to prevent the enemy's snipers from interfering with the work. It is desirable to locate the bridge in a position where it is sheltered by woods or hills from hostile fire, and where it cannot be seen for any great distance either up or down stream.

(h) Secrecy. If the crossing is to be made secretly as a surprise, it is desirable that there be a sheltered locality on the near bank, at or close to the bridge site, where the material can be assembled without the enemy's knowledge.

To sum up then, the desirable requisites for a bridge site are: accessibility to adjacent roads; narrow width of crossing; moderate depth and swiftness of current; solid bottom; favorable approaches; usually low, firm banks of equal height; materials, as needed, available near at hand; and a sheltered, concealed, and defensible locality.

In making a reconnaissance for the selection of a site, these various desiderata must be balanced against each other in coming to a decision; for it will seldom be practicable to find a location that combines all the advantages mentioned. In the selection of a site for a railroad bridge there will, of course, be less latitude than in the case of a highway structure. It is to be remembered that the thing sought is not a site for a new bridge, but a practicable crossing of the stream. A bridge should not be built if a suitable ford, or an existing bridge which meets (or may be made to meet) requirements, can be found. In anticipation of their probable future use, all existing bridges within range of the army's operations should be promptly siezed and guarded, in order to prevent their destruction by either the enemy or his sympathizers.