Prepare for the obstruction of the communicating trenches in case the enemy should capture the fire trench: Have piles of sand bags above the entrance of each trench ready to be dumped into it. Have chevaux de frise lined up on one parapet of the trench and all held up in such a way by a single wire that when the wire is cut they will fall into the trench. Mines can also be prepared to blow up the trench when invaded. The communicating trench between the fire trench and the listening post should be covered with barbed wire screens or be tunnelled. THE WATCH FROM THE TRENCHES. 12.--Trench Warfare an outpost duty. Trench warfare, the inevitable form of modern warfare, is a continuous series of outpost duty. Hence it is based wholly on eternal vigilance. The patrols correspond to the scouts; the listening posts to the sentinels; the firing trench to the outguards; the cover trench to the supports. The safety of the sector depends entirely on the vigilance of the advanced elements and the rapidity with which supports and the reserves can be summoned. Watching is thus the fundamental duty in trench warfare. The following points should be kept in mind: AT ALL TIMES 13.--Number of men in the fire trench. There must be as many sentinels in each section as is necessary to cover completely the sector to be watched, no more, no less, each sentinel being given the exact limits (such as tree, copse, post, etc.) at each end of the line he should watch. 14.--Fix bayonets. The men on duty should have bayonets fixed as, in case of a possible surprise, they are needed for defense. Otherwise too, fixing bayonets would be an indication to the enemy of an impending raid. DURING THE DAY. 15.--Observation of open terrain. When the terrain opposite is open country, the necessary observation may be done by the smallest possible number of men. Fire only, if any of the enemy are sighted. Then, have two rounds fired, then three. But keep fire under strict control. (If enemy continues to approach. Cf. par. 52 ff.) 16.--Observation of covered terrain. When the terrain is covered (high brush wood, copses, trees, etc.) a sharpshooter in each section should fire occasionally into the trees, etc., which may be observation or sharpshooters' posts but this should not be overdone. AT NIGHT. 17.--Double sentinels. Post double sentinels in each section, each man watching in turn, the other resting but within call. 18.--Silence. They should refrain from making the least noise so as to hear and not be heard. 19.--No firing when fired upon. There should be no firing when the enemy fires since when the enemy fires, he does not advance. 20.--Look and listen. They should keep a sharp lookout but listen even more attentively. 21.--In the listening posts.