The headquarters train in 1864 was followed by an advance-guard (q.v.) train. which. when a crossing was to he made by surprise, was sent forward with the cavalry to cover the con struction of the bridge and hold the position until the main body arrived.
The advance-guard equipment is similar, in geiteral principles, to the reserve. The pontoon. however, instead of being made entirely of wood, is made of canvas held in shape by wooden frames, which can be knocked down for trans portation. For transportation purposes, the material is so packed that the pontoon-wagon contains all the materials for a single bay of 20 feet. The division may, therefore, be in creased or diminished by one or more pontoon wagons, if it is desired to increase or diminish the length of the bridge in units of 20 feet. The width of the bridge is two feet less than that of the wooden pontoon bridge. The canvas pontoon, while possessing the advantage over the wooden pontoon of being more easily transport ed, will of course not stand as hard service. At the same time, it has shown its usefulness on many occasions. The pontoon-trains with the Army of the Cumberland, in its march from Chattanooga to Atlanta. Savannah. and Wash ington, in the latter part of the Civil \Var, were of canvas. and gave excellent satisfaction. Where pontoon bridges are constructed at night, with a view to making a passage and surprising the enemy. the floor of the bridge should be covered with straw, in order to reduce the noise of construction to a minimum. Alany substi tutes for pontoons, such as floating supports, have been devised. Among the more impor tant are rafts made of casks lashed to frames, rafts of timber, and boats of commerce.
Spar bridges are made with round timber usually cut near the site of the bridge. For narrower spaces of 25 feet or less, the balks consist of single trees stretching across the river. The roadway is made of small saplings smoothed of on top or covered with dirt. For spaces of 30 to 35 feet. two trapezoidal frames are constructed of timbers lashed together and braced by diagonal braces lashed to the legs of the frame. These are put in position on the opposite banks, and then lowered so as to rest their tops on each other and form an intermedi ate support above the middle of the stream for the balk or roadway-bearers. For a still wider stream, double-loek bridges can be used. In these the frames, instead of resting on each other. rest on cross-timbers held apart by straining beams. These cross-timbers furnish two intermediate supports over the gap for the balk. For still wider openings, the frames are made consider ably longer, so that they support each other at a point not less than 10 feet above the roadway. From their junction an additional roadway bearer is suspended by ropes. This, together with the points where the roadway crosses the two frames, gives three points of support.
One of the most generally useful of all the types of bridge is the trestle bridge. Vertical frames are constructed, on which the balks carry ing the roadway are placed. ln eases where a single trestle is not high enough to come from the bottom of the gap to the desired height of roadway, trestle, can he so braced that they will hold others on top of them. This type of bridge particularly lends itself to railroad work, one of the most notable examples of the kind being the bridge across the Potomac Creek, Va., dur ing the Civil War. This bridge was 80 feet high and 400 feet long.
Suspension bridges have not been of general application in the United States military ser vice. The English have made some use of them. They arc suitable for wide spans, where it is difficult to secure intermediate supports. An army traveling in a country where its ser vice will probably require the construction of this type of bridge should make provision for carrying along the necessary wire cables, clips, and ropes. A bridge with a span of 100 feet can
be constructed with such material, having a roadway 5 feet S inches wide in the clear. With a load of infantry in file crowded, the bridge has a stress of about 63,000 pounds in the cables and on the anchorages.
The type of bridge to be used in a given ease de pends on many circumstances. An army starting out for extended service in a country such as that covered by many of the operations of the Army of the Potomac in the Civil War—that is, rath er a low, fiat country, with many broad streams —would naturally provide itself with pontoon bridges, and, having them, would use this type wherever practicable. On the other hand, as the pontoon bridge must be near the surface of the water, it is not well adapted for use in a country where the streams are narrow and the banks so high that trouble would be experienced in the construction of approaches leading to the ends of the bridge. An army expecting to operate in a country provided with poor roads, but fairly well wooded, and having streams not too deep to permit of the establishment of cribs and trestles, might well rely on the trestle-and spar bridge. If operating in a mountainous country, cut up by gorges and canyons and poorly provided with roads, it should carry the necessary cables, etc., for constructing suspen sion bridges over gaps too wide for spar bridges. Bridges have been constructed across streams just too deep for fording by running in wagons and laying the balks and roadway on top of them. Railroad bridges can be utilized for the passage of troops on foot and wagons by plank ing over the ties and sometimes covering with dirt. In ninny cases no bridge will be needed. ln cold weather the ice may be sufficiently thick to permit the crossing of an army. Six inches of good ice will suffice for the passage of the usual loads accompanying an army; 2 inches permits the passage of infantry in single file on planks: 10 to 12 inches bears the heaviest loads of an army. In other eases, as in the famous passage of the Delaware by Washington, the army may be ferried over in boats. A modifica tion of this method, known as the method by raft, has also been used. Rafts with large deck areas can be made with pontoons by lashing a number of the latter together and flooring them over with chess. On them men and horses can readily lie ferried, while rafts of bamboo were used for transporting field artillery across rivers in the Philippines. A river of shallow depth and moderate current may be forded : where the depth is too great only for short distances, the level of the bottom at these places may be raised tem porarily by throwing in brush, logs, and other material. There have also been instances in his tory of the successful swimming of rivers by small bodies of troops.
Temporary piers or docks are sometimes re quired to facilitate the landing of troops and material. Such structures are rarely made stronger than to permit small boats, such as launches and lighters, to lie alongside of them and unload. They consist essentially of a top floor and of supports, the relation between the floor and supports being similar to that existing in bridges. The supports are generally either trestles of some type, usually pile-driven, or log cribs sunk with stone. A pier-head may be temporized by securely anchoring a pontoon raft.
For a comprehensive technical treatment of the subject of military bridges, consult: The Chatham Manual of Military Engineering, Part 3; Military Bridging and Use of Spars (London, 1894). An interesting chapter on the "Passage of Rivers" may be found in Fiebeger, Text-Book of Field Fortification (New York, 1901).