of the facts that they are less permanent in their nature, and that no consideration is given to the appearance of the finished structures, these bridges are built in accordance with the recognized rules of good civil practice.

Structures of the second class, those at the front, are erected to meet the immediate tactical requirements of the combatant forces. They are usually in the form of hasty makeshifts of a crude character, and are built by combatant troops in accordance with the economic principles of warfare heretofore enunciated.

There are no sharp lines of demarcation between civil practice and military bridges of the first class, nor between military bridges on the lines of communication and those at the battle front. Military economics are occasionally applicable to civil practice, and vice versa.

Types of Military Bridges

The particular type of bridge to be employed in any situation depends on the nature of the stream—its width, depth, swiftness of current, and liability to flood; the character of the approaches; the labor, plant, and materials available; and the loads to be carried. Every type of bridge known to civil practice has been employed for military purposes, including pile and framed trestles, and cantilever, truss, girder, suspension, floating, and arch bridges. The last mentioned, either of steel or masonry, are very rarely used, and only on the lines of communication. Military bridges are, in general, crude, impermanent, and makeshift forms of their civil prototypes.

Any bridge for which the erection requires a long time and an elaborate and heavy plant will usually be avoided in military practice, and is out of the question for any tactical purpose. The considerations calling for such bridges in civil practice will usually have much less weight in time of war. For example, a long-span-truss or cantilever bridge may be adopted in order to avoid the expense of very deep foundations, to evade danger from floods, or to meet the requirements of navigation. Deep foundations are out of the question in emergency military bridging, and are evaded by the use of the floating equipage, by portable (sectional) trusses, by long-span suspension-bridges for very light loads, or by ferries for occasional traffic too heavy for the bridges. Danger from flood is avoided in a similar manner; but, as the period for which a military bridge will be required is always relatively short, it will frequently be better to run the reduced risk of flood damage, rather than spend much time in guarding against it. If the flood risk be really great and imminent, it is best avoided by using the floating equipage, unless a clear span of moderate length will meet the situation. In war the rights of civil navigation must give way to military necessity. Military traffic, both over the bridge and along the stream, may be regulated so as to interfere with each other as little as possible, draw spans being provided, if necessary, for the passage of vessels.