Bridges and Pontons.— When an army has to cross a stream, passage may be made on a bridge, by fording, by use of boats, rafts or ferries, or on ice. Tactical requirements will determine the general location of the point of crossing, and within the limits imposed by these requirements the site of crossing must be selected and the method adopted that is best suited to the site. The site is selected as a result of reconnaissance to verify and complete the information shown by existing maps. To meet tactical requirements the near shore in an advance should afford concealment for the pre paratory work in connection with the bridge, and should, if possible, facilitate a converging fire upon the enemy, while the farther shore should be open to favor development. The reconnaissance should furnish information as to the liability of freshets and their probable height, the rise and fall in tidal streams, the width and depth of the stream, the presence or absence of navigation, the nature of existing facilities such as roads or fords and the pres ence of bridge material such as timber, rope or wire. Where possible, the approaches should be straight and in line with the bridge.
Military bridges are divided primarily into floating and fixed. The types of floating bridges most commonly used are the ponton bridge, built with the equipage carried with the army, and bridges built with boats or cavalry. Artillery and wagons can cross water three feet deep and keep dry. The bottom should be even, hard and tenacious. A ford may be rendered impassable by a freshet or by barges. Types less frequently used are the bridges built with casks, rafts, timber, inflated skins and other means in the nature of bridg ing expedients. The fixed bridges best adapted to military use are _pile, trestle, spar and sus pension bridges. 'Other types that may be used are cribwork, steel girder, trussed and canti lever bridges. Some of the. loads (in pounds per linear foot) to which military bridges may be subjected are as follows: Infantry, single file 140 infantry, column of twos 280 Infantry, column of fours 360 Cavalry, single file 196 Cavalry, column of twos 392 In the absence of bridges a ferry may be operated by stretching a cable across the river and pulling the boat by hand along the cable. Another method is to use a long rope fastened to a point up stream, either on the bank or to an island or anchor in midstream, and navigate the boat back and forth by utilizing the force of the current acting obliquely on the boat.
This is called a flying ferry. The wooden ponton will carry 40 infantrymen armed and fully equipped, in addition to the crew, under favorable conditions. If a stream has high banks, with trees or other means available as anchorages, it may be practicable to stretch a rope across and transport men and materials in chairs, baskets or slings suspended from the rope.
Fords may be used by small bodies of troops without bridge equipage, but they are unreliable crossings and are generally unsatisfactory for large bodies. If the current is moderate a depth of three and feet may be passed by infantry and four and one-half feet by the deepening resulting from the loss of ma terial stirred up and carried away during the passage of troops. A ford may be destroyed by filling the deepest part with harrows, teeth tip, or with planks filled with spikes, with barbed wire or other obstructions.
Benefiting by the experience of the Euro pean armies, the United States has greatly in creased the proportion of engineer troops. A recent order gives the following units for each field army, in addition to the divisional engineer troops: One regiment, gas and flame; I regi ment, mining; 1 regiment, water supply; I regiment, general construction; I regiment, engineer supplies (1 battalion transport, I bat talion skilled labor, 1 service battalion to aid in this work) ; I battalion, mapping and print ing; 4 battalions transport service (6 service battalions to aid in this work, 6 truck trains of 31 trucks each, 5 wagon trains of 61 wagons each). Assigned to the line of communications and constituting an independent group, each field army has the following engineer troops: One regiment, general construction (6 service battalions to aid in this work) • 1 regiment, engineer supplies (3 service battaliops to aid in this work) ; 10 battalions forestry (9 service battalions to aid in this work) ; 1 regiment, quarrying (3 service battalions to aid in this work). The European War was one of engi neers, and upon the efficient leadership of engi neer troops depended in large measure the ultimate success of the Allies.