The ground on the enemy’s side and within the entanglement, as a matter of courtesy, is strewn with broken glass and tangled wire. The whole system of entanglement should be under well-controlled machine gun fire from special points and should be widest where the fire of the defenders is least effective by night. There should be one sentry at least to each 50 yards of entanglement. ABATIS. A form of obstacle made by trees cut down and laid side by side as close as possible with their branches towards the enemy is used. These should be in a hollow and screened from view to make it of any value. The butts of the trees should be firmly secured by burying them in the earth, or by laying logs of timber across several butts. Wire and barbed wire must then be interlaced between the boughs, which should also be sharpened to points on the enemy side. Some of the lower branches may be pegged to the ground, if thought necessary, to insure the maximum resistance. BARRICADES. These are used for the defense of streets, roads, bridges, etc., and are made of any available materials, including furniture and vehicles, either overthrown or with wheels removed, carts filled with earth, railings, bales of goods, etc. Where trees are growing along the roadside, fell them across it and entangle with wire. It must be kept in mind, though, that passages are required through these barricades to allow outposts to advance or retreat, that these passages, viewed from the front, must not appear as openings. To avoid this the barricade should be made in two parts, one overlapping the other. Sometimes, where there is a gap in a row of houses, or a sharp bend in the road, a barricade may be made in one part and a passage round one end left for traffic. INUNDATIONS. They may be formed by damming streams at convenient points, specially in the valleys, or by damming up the arches of bridges. In the latter case, care must be taken not to endanger the stability of the bridge. The ditches of field works form a good obstacle when flooded. Destroyed trenches in front of a breastworks may be filled with water, and with barbed wire thrown into it, will prove an effective obstacle. FOUGASSES. These are used in connection with obstacles and are really land mines loaded with stones, bricks, etc. An excavation is made in conical shape with an axis inclined to about 40 degrees toward the enemy horizon. A box of powder is then placed in a recess at the bottom and on the box is placed a wooden platform or shield 3 to 4 inches thick, over which stones are piled. A fuse is placed in a groove cut at the back of the excavation. A line of least resistance must be so arranged that by placing the excavated earth on the back edge of the fougasse, the powder will act in the direction of the axis and not vertically. A fougasse charged with 80 pounds of powder may be constructed in this manner to throw five tons of brick and stone over a surface about 160 yards long by 120 yards wide.