Hours and precautions.—The most favorable hours are morning about 4 or 5 o'clock, at 2 p. m., and at midnight. All the occupants of that part of the trench must keep still, and all work in the trench and the mine must cease for a given time.
What is heard.—The inexperienced ear hears too many things, and is easily mistaken in the noises heard. A relief passing in the enemy trench or in his own trench at 40 meters sounds strangely like the noise of a pick. A man hitting a ground sill or striking it with his heel gives the idea that work is being done. The impact of bullets on the parapet at night, when a fusillade is uninterrupted, also gives the idea of underground work. A man filing a fuse at the foot of his loophole suggests the presence of an enemy revolving borer. A man who snores beside the gallery entrance imitates the noise of a ventilator and may be mistaken for it.
However, to even a partially trained ear the noise of the pickax is characteristic. It is not a harsh sound, like that of a heel striking on chalk ground, nor is it like the shock of bullets piercing the parapet. It is a low, rhythmic sound, with regular cadence. In a gallery the miner works kneeling. When he has struck five or six blows with the pickax, he takes a breath. He repeats this process about 12 times. He stops two or three minutes, and the second miner clears away the earth and fills the truck with sacks. The first miner resumes his work. It is easy to distinguish this regular cadence peculiar to the miner.
In an infantry company there are always several miners by occupation, or several men familiar with engineering. These men are selected specially for the listening service. They can give accurate information to the officers and noncommissioned officers of a sapper company. It is also a good thing for a platoon commander to descend into a gallery and train his ear by exchanging pickax signals with the miner in the adjoining gallery.
Distance at which noise can be heard.—The following indications help in determining the distance of underground work:
Four men work in a gallery. They start the work, then stop. The ear is placed against the side of the wall, the other ear being covered by the hand. If the work is heard under these conditions, it is at a distance of 25 to 30 meters.
If all noise is avoided, and the work is heard without placing the ear against the wall, the distance is 12 to 15 meters.
If there is talking or working going on, and still the underground work is heard in spite of it, the distance is 8 to 10 meters.
At six meters a man can hear all the sounds of the pickax, the chalk crumbling, the pieces rolling down on one another, the sliding and stacking of cases. These noises sound as if they were immediately below.
At four meters a man can hear talking, the scraping of buttons against the wall, the miner turning around.