The troops occupying the second and third lines should be protected against bombardment as much as possible. It is therefore in these trenches or very near them that shelters capable of withstanding heavy gun-fire should be constructed.
We say “in or near” advisedly, for it is important to hide the shelters as much as possible from the enemy, and, to effect this, it is no inconvenience to build them a little in the rear of the trench-lines, if the desired result can thus be obtained. Often it will be necessary to do this merely for the purpose of getting more favourable ground on which to erect the shelter.
The main point to emphasize is that these shelters should be connected by means of communication-ways permitting quick passage from the shelters to the trench-lines.
In all three lines, when the ground is suitable, observation posts are located, but however well they may be concealed, the enemy is not long in discovering them, and in the first line it is generally with the aid of different kinds of periscopes that the observations are made.
Communication Lines. All the trenches are connected by communication trenches or boyaux, which conceal from the enemy the movements of troops. These are mere ditches about two yards deep, the earth from which is thrown out to the left and right, or on one side only. They follow a zigzag line so as not to be exposed to enfilading fire, or, when straight, are protected at intervals by earth screens (pare-éclats).
Diagram of Campaign Intrenchments.
The depth or breadth of these “ditches” depends on the use to be made of them. For instance, those intended for carrying out the wounded must be deep and wide.
To preclude obstruction during attack, and to provide free and quick passage for reserves rushing up from the rear, communication trenches should be very numerous. Some should be designated for forward and some for rearward movement. Each one should bear a name or a number and at its extremities arrows should indicate the direction of the circulation. Thus confusion will be avoided.
Trenches of Attack. When the opposing lines are sufficiently far from one another, it is impossible to launch an attack without having brought the infantry within striking distance.