=Sentinel.= A sentinel is a soldier placed in the first line or in an advanced post for the purpose of security. The duty of the sentinel is to watch and guard against hostile attack and to warn the chief of the platoon of every action of the enemy in “No Man’s Land.” At night, one sentinel is posted about every ten yards in the firing trench. During the day, only a sufficient number are posted to assure that all parts of the enemy’s line and “No Man’s Land” are observed. Sentinels are relieved every two hours except under bad weather conditions when the length of the tour of duty is reduced. Every sentinel is regularly posted by a non-commissioned officer who explains to him his duties and ascertains that both the sentinel and his relief are aware of the position of the platoon commanders and of the sentinels, on either side. At night, the sentinels must observe over the parapet. As little challenging as possible is done and then only in a very low voice. If the sentinel receives no answer to his challenge, he shoots, thus giving the alarm. During the day, the sentinel uses a periscope or observes through a loophole. The sentinel always has the bayonet fixed on his rifle, which is loaded and ready for use. He must be on the alert for every noise, and for this reason he is not allowed to wear any ear covering. Sentinels must be informed of the sending out of patrols, the approximate hour of departure and return, and the general direction that the patrol is to follow. =Watcher.= Each platoon in the trenches, from the first line to the rear, must have some watchers, whose duty it is to give the alarm both day and night against attack. These watchers do not observe the enemy as do the sentinels. They walk in the trench in which the platoon is placed, especially at the entrance of the dugouts or shelters occupied by this platoon. When they hear a signal of alarm given by a sentinel or by a messenger, they wake the platoon as quickly as possible. In general, they give to the chief of the platoon all the information that they have obtained during the time of their duty. When an officer or private from another sector enters the trench, it is the duty of the watchers to ascertain the purpose of his visit. Watchers receive special orders to give the alarm against a gas attack, and must sound the alarm gongs and bells for this purpose. The watcher may have his rifle, without bayonet, resting on the side of the trench at the entrance of a dugout. =Observer.= The observers are specially trained men whose duty is to observe the hostile position and to gain all the information that they can obtain of the enemy’s activities.