At some distance to the rear of the support trenches and overlooking them, the first-line system is reinforced by a line of “centres of resistance,” which is punctuated with blockhouses protected by thick wire entanglements. Well sheltered machine-guns, a certain number of which sweep the communication-ways leading to the front, compose their main armament. This blockhouse line is connected by many communication-ways with the trenches in front of it. These centres of resistance are intended to check the advance of the enemy, when they succeed in breaking into the three anterior lines, and to give the reserves time for counter-attacking. Between the blockhouses and the second-line system a fortified line is frequently organized which is called the “protecting line of the artillery.” This is intended to repel the advance of the enemy infantry, if they gain possession of the first-line system, before they reach the field batteries, or to retard it long enough to allow the batteries to fall back. This line is held by the troops of the sector which do not belong to the fighting contingent. The distance from the first-line system of trenches to the second-line system varies according to the configuration of the ground. These second lines should, when possible, overlook the first lines, so as to hold them under their fire. The same rules prevail in the location of the third lines. In the sectors which seem to particularly interest the enemy, very strong “check positions” are prepared in the rear. 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.