Land mines are placed in the line of the advance of the enemy and exploded either by electricity or fuse from the defense. They are made by digging holes from 2 to 3 yards deep, either by excavation or by boring. In the former case the charge is placed in a recess which extends into the solid earth at the side of the hole, which is then refilled and tamped. In the latter case the charge is placed in the bottom of the hole, which is then refilled and solidly tamped. In common earth the powder charge for a 2-yard hole is 25 pounds. That for a 3-yard hole is 80 pounds. The diameter of the crater formed will be about twice the depth of the charge. The mines may be arranged in one or more rows. The intervals between mines should be such that the craters will nearly but not quite join. The position of the mines should be concealed as much as possible and further sophisticated by disturbing the ground slightly at points where there are no mines and so situated as to suggest a systematic arrangement. [Illustration: PLATE 9a.—Land mine. _F_, Line from powder charge to battery. _P_, Powder charge.] Inundation Backing up the water of a stream so that it overflows a considerable area forms a good obstacle, even though of fordable depth. If shallow, the difficulty of fording may be increased by irregular holes or ditches dug before the water comes up, or by constructing wire entanglements in the water. It may be employed with advantage when the drainage of a considerable area passes through a restricted opening, as a natural gorge, culvert or bridge. Open cribs filled with stones, or tighter ones filled with gravel, may form the basis of the obstruction to the flow of the water. The usual method of tightening spaces or cracks between cribs is by throwing in earth or alternate layers of straw, hay, grass, earth, or sacks of clay. A continuous construction, as shown in the illustration, may be employed. The ends of the dam must be carried well into the solid earth to prevent the water from cutting around them. This type of dam is easily destroyed by artillery fire, and cannot be depended upon. [Illustration: PLATE 10.—Dam construction.] CHAPTER III Lookout and Listening Posts Except when the garrison are actually required to man the parapet, they will be kept under cover, with the exception of a few lookouts, whose duty it is to give timely warning of the movements of the enemy. When the opposing forces are in close proximity to each other mining operations are generally resorted to by both sides to compass the destruction of the opposing works and open the way for an attack. Lookout and listening posts serve the double purpose of having a few men at the most advantageous places for observation at the front and flanks and providing points at some distance to the front of the first line fire trenches from which listeners may be able to discover the location and direction of enemy mining operations before they really menace the fire trench.