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. The humming of a ventilator can be heard at 40 meters without taking precautions to hear it. An automatic borer can be heard all through the sector. _Directions from which noise comes._--It is easier to determine the direction of noise than the distance. There is always a chance of making observations in the galleries--one on the right, the other on the left of the noise. The exact location of the enemy underground work can be determined by intersections. The engineer companies have an apparatus for intensifying the sound (strong microphones). They reinforce the sound when the apparatus is in the direction of the source of the sound. Their greatest defect is that they magnify sound too much and too many things are heard. Why hear for a distance of 100 meters when the enemy trench is only 40 meters away? Everything is heard in a mine gallery. It is difficult to distinguish among the many noises that of the enemy miner's pick. The ear is amply sufficient.