secure and form the curb of the roadway. In addition to the pontoon wagons a bridging unit always included wagons carrying adjustable timber trestles known as " Weldon trestles." These were an im- portant part of the equipment, being used to form the piers of the bridge in shallow water near the bank where the pontoons could not float, or to make a landing-stage when the pontoons were used as rafts on a wide river, or without the pontoons to bridge the narrow streams or dry gaps.
LIGHT BRIDGE.
n
w
MEDIUM BRIDGE.
H*****-^ ,, ,, ., .1 \-f'f<^ ,**
- Pontoon tMl or ritaojfi
Double chessed
a* 2*
Baulks or Ritende 'Saddle txam
LONGITUDINAL SECTION.
HEAVY BRIDGE.
nnoi
Pontoon ffeu/ks f
Ritenda alternately.
Tn,aru. LONGITUDINAL SECTION.
FlG. 12.
In the organization of a British division of 1910-14 were in- cluded two, and in the division of 1915 three, " Field Companies Royal Engineers," each of which, besides its other military engineering equipment, included two pontoons and one trestle wagon, the latter carrying two trestles; the three wagons among them carried also five bays of superstructure for light bridge, using five baulks to a bay. 1 This gave every division the means of crossing a river independently, the engineers being able rapidly to form three bridges up to about 75 ft. in length, or one bridge of about 200 ft.; if used to form bridge of half -pontoons capable
1 The length of bridge section between two points of support technically called a " bay " is normally 15 ft.; thus a bridge sup- ported on the two shore transoms, with piers formed of two pontoons and two trestles, would consist of five bays equal to a span of 75 ft. The width of the roadway of the bridge as normally formed is 9 ft. clear between ribands.
of carrying infantry in file and pack animals, the equipment could be extended to bridge about double this width.
Bridging trains moving in rear of the army carried each 42 pontoons and 16 trestles with superstructure, as a reserve for the crossing of wide rivers, and these were later supplemented with a superstructure of heavy steel joists, so that the pontoon equipment could be used to form medium and heavy bridges to carry mechanical transport and the heaviest guns and tractors on the road. The pontoon trains were originally drawn by horses, but to save the great number of horses a pontoon train requires, and to give greater mobility, some were adapted for mechanical transport. These consisted of " four-wheel-drive " lorries, each trailing two pontoon or trestle wagons, and were able on good roads to cover much greater distances in less time than the horse-drawn bridging trains.
The " Field Squadrons Royal Engineers " attached to cavalry divisions were equipped with a lighter form of collapsible boat, and each cavalry regiment was provided with an air-raft equip- ment. A special cavalry bridging train equipped with small steel pontoons was provided for use in Egypt and Palestine. These forms of bridging equipment could take the h'ghter natures of transport accompanying a cavalry brigade, including horse artillery guns.
On the other hand, the British army when it took the field in 1914 had no reserve of heavy bridge equipment, nor any of the portable steel-girder bridges which were found so invaluable later in the war.
The British army, unlike most European armies, had no specialized bridging units. All the field units of the engineers carried out the annual course of bridging as part of their normal duty. This course was held wherever possible on the banks of a tidal river, and work was mainly concentrated on the pontoon drill which enabled the sappers to handle the material with great celerity. But the training also included practice with vari- ous forms of light improvised bridges, and the crossing of rivers by means of barrels, tarpaulin rafts, spar and timber trestles, and the construction of light suspension bridges. Little was done in the way of heavy bridging, but all units were taught the use of spars as derricks and sheers for launching girders and moving heavy loads, and a certain amount of pile-driving and heavy trestle work was done. The officers' theoretical course included the design of timber and steel girder bridges of all types, and some gained practical experience in bridging works in India and elsewhere abroad in the course of their employment in peace on the public works. Never, however, before the World War of 1914-8 had the problem to be solved been of such a varied and complex nature. The immense advance in the use of mechanical transport of all kinds, from motor-cars to steam traction engines, the greatly increased weight of artillery in the field, and finally the coming of the tank, demanded the use of heavy road bridges not far short of railway bridges in strength.
On the other hand, owing to the ease with which destruction can be carried out by means of modern explosives, advancing troops were more frequently than ever before confronted with the problem of crossing a river or canal when all existing bridges had been destroyed, approaches broken up by explosives, and the river and its environs defended by artillery and machine-gun fire. In such a case pontooning was clearly impracticable, and other means had to be devised by which the infantry could be given a footing on the opposite bank to form a bridge-head to cover regular bridging operations.
For these fighting bridges, which were practically the most important because without them no advance could be made, no standard equipment existed. Each field company improvised its own solution to the problem after reconnoitring the crossing to be forced. Usually the material could only be carted to within a mile or so of the site, and had to be carried by hand the remaining distance across shell-pitted ground, or marshland intersected by dykes. Lightness and extreme portability were thus essentials of the design. Then the material might suffer from shrapnel fire whilst en route or when lying hidden behind a bank or wall, and might be pierced by machine-gun bullets whilst actually
