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being placed, hence strength and impermeability were required. Lastly, the bridge had to be put together in the dark in perfect silence, exposing as few sappers as possible on the bank, so that simplicity and interchangeability of parts were essential.

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ELEVATION OF CORK FLOATS

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ELEVATION OF PETROL TINS

FIG. 13.

The lightest and least vulnerable pattern evolved was probably the cork-float footbridge with light wooden footboards hooked over the saddles of the float and interlocking. A pattern of this type is shown in fig. 13, which also shows the employment of captured German canister floats and of petrol tins to support these light foot- bridges. A petrol-tin raft was used by the engineers of the British 25th Div. for the crossing of the Sambre-Oise canal near Landrecies in 1918; in this case each raft consisted of two floats each of eight petrol tins laid flat and built into a wooden crate for carriage. Eighty of these rafts were carried for 3,000 yd. under fire to the canal bank, and each when launched carried across a man with full equipment. When sufficient men had been ferried across by this means to secure a foothold on the far bank the rafts were connected by light foot- boards to form a bridge 55 ft. in length.

A form of light ferry-boat which was very useful was made as shown in fig. 14 by tying the standard-size waterproof trench shelter, or bivouac sheet, measuring 13 ft. by io ft., over a light wooden fram- ing made in parts for easy transport. In the little boat thus formed six men could squat, and be pulled across b'y a rope worked by a sapper who had swum to the far bank or paddled across in the first boat, another man on the near bank pulling the empty boat back; and considerable numbers of infantry could thus be put across even before a light footbridge could be constructed. The boats also formed a very serviceable footbridge when connected together as illustrated in fig. 3 (plate). I n a case where a crossing could be effected at a canal lock or other point where the width to be spanned was not more than about 20 ft., a light trussed timber bridge was built up complete, and carried or rushed forward from undercover on wheels, and launched across the gap by the sappers, somewhat as a fire- escape is handled. Similar devices have often been used in the storm- ing of a fortress for the crossing of the ditch. A notable example of this method was the crossing at a lock on the Sambre-Oise canal made by the British 1st Div'. on Nov. 4 1918.

Another notable piece of front-line work was the construction of a crib causeway, built of railway sleepers bolted together and sunk in the bed of the river, to carry tanks across the river Selle in the first line of the assaulting troops (1918). This was kept just below water- level for concealment, and was built in the nights just preceding the attack under the nose of the enemy holding the opposite bank.

As soon as a foothold on the opposite bank has been gained by the infantry, and the enemy's machine-guns put out of action, the next step for the engineers is to establish the crossings more

strongly so that pack-animals can be got across with ammunition and supplies; these pack-bridges usually took the form of rough improvised trestle or pile bridges, but in some cases tarpaulins lashed round a wooden framing were used as floating supports in the same fashion as the waterproof sheets above mentioned.

Not e :- Framing about 5'*%. Bottom same form 3s"B"bvt wider.

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For the crossing of minor streams and dykes often met with bsfore or after the main crossing, various devices were used to suit the varying conditions. Plank or light footbridges of the pattern shown in fig. 13 were often sufficient to carry the infantry, but where the span exceeded 10 ft. light trussed bridges of timber, strutted and tied with hoop iron or stout wire, were made up to about 15 ft. in span. Above this limit some form of intermediate support in the form of a float or trestle became necessary. For marshland, muddy ravines, or shell-pitted ground, mats of canvas and wire netting stiffened with wood battens and rolled up for convenience of carriage were found very useful to give a foot- hold. For horse traffic, corduroy mats of timber bound together with wire and picketed down in place were used, as also were the artillery " trench bridges," 12 ft. in span with timber bearers and ij in. flooring, made up in sections 3 ft. 6 in. wide to be laid side by side. These were a little heavy for hand carriage; but in most cases they were issued to the artillery before the advance and carried by them in their limbers to be laid down where required.

Next, it becomes necessary to bring forward the field artillery into position on the far bank. For this work the pontoon equip- ment is invaluable, as it enables a bridge for horse transport to be made across a river more quickly than it is possible by any other means, and the peace training of the British engineers in pon- tooning work justified itself in the fine work done, notably in the advance across the Aisne in Sept. 1914. The field companies of the New Army were likewise instructed in and equipped for pontoon- ing work, and the material was used to advantage on nearly every waterway on the entire front in France, on the Piave, on the rivers of Palestine, and in Mesopotamia.

Figure I (plate) illustrates the type of bridge built with pontoon equipment across a tidal estuary in which the standard service trestle with adjustable transom is used for the bays nearest the shore; that part of the bridge which will ground on the fall of the tide is carried on barrel-piers strong enough to carry the load when grounded, and the floating portion is composed of pontoons. A " cut " is formed in the bridge by disengaging the central floating portions and allowing it to swing on the tide or stream so that vessels may pass freely along