Start, depth, and progress of work 9 Barrage trench for miners 9 Camouflet 9 Offensive galleries 10 Superimposed galleries 10 Mine chamber 11 Launching an attack 11 Craters 11 Craters to be occupied 11 Craters that the enemy occupies 12 Craters not occupied 12 Diagrams 9, 10, 12, 13 USE OF MINES IN TRENCH WARFARE. [From the French St. Cyr School.] (Secret and Confidential. For Official Use Only. Not to be Taken into First-Line Trenches.) USE OF MINES. In sectors where the distance between the two lines is below 150 meters, mine warfare must be used. When the trenches are farther apart, underground warfare is seldom employed. In special cases, however, when there are strong ventilators and the line is stable enough to permit of it, advance may be made underground. SPECIALLY MENACED POINTS. The most vulnerable points evidently are the following: The outposts in advance of the line, machine-gun positions approximately located by the enemy, and the junction points of the communicating trenches with the first line. SURFACE OBSERVATION. Underground activity, either offensive or defensive, is first observed from those points in our lines nearest to it. All enemy trenches facing a salient of our lines will be the object of particular attention and closest daily observation. This observation of the first-line trenches should disclose the presence of enemy underground works and their approximate location. One of the difficult questions in mining is the removal of the earth. Expert miners sometimes remove the earth as far as 100 to 200 meters from the entrance to the gallery. They throw it on old ruined shelters, in shell holes, on the reverse of the trenches. But these precautions are not always rigidly observed. When the noncommissioned officer is absent, or the enemy bombards a little strongly, some one in the working party not wanting to work overtime throws several clods of earth on the parapet. INDICATIONS REVEALING THE ENEMY GALLERIES. Freshly placed earth coming from underground is white and less dull in color than that of the parapets, which have been washed by the rain and blackened by explosives. The difference in color for the first day or so is striking. In chalk, large white spots are seen on the reverse of the German trench, daily growing larger. Without doubt they are working near by, and should be watched.