Plate 13.—A listening and observation post.
The post may be connected with the first line fire trench by a narrow zigzag trench or by an underground passage. If the former, it must be thoroughly concealed and have no excavated earth visible. If it can be located along a hedge or some other natural feature its location may remain unknown to the enemy for a considerable length of time. Where a communication gallery is constructed the roof and walls must be suitably shored up by casing and supports.
The sentinel in the listening post carries no accouterments. It has been found that the creaking noise made by equipment when the sentinel moves has been taken for mining operations of the enemy by his comrades.
Listening galleries should never be left without a sentinel. There should be a depot of arms and hand grenades near the entrance to the gallery in case men are attacked while on duty from either above or below ground.
Listening will be conducted at specified times, or on some prearranged signal, and for a definite period. During this time all within the listening area, including the trenches, must remain absolutely motionless.
Infantry manning a trench can assist listening by digging a small pit, 6 feet deep below the trench, and running a bore-hole out 20 feet or more.
The enemy is always listening for indications of the direction and position of gallery heads. Work must therefore be carried on with a minimum of noise. Shouting down the shafts of galleries is absolutely forbidden.
When the mining operations of the enemy are detected a report should be made at once to the officer in charge of that section of the trench.