Barricades should not as a rule close the road entirely to traffic. Passages are required to allow the defenders to pass through when it is necessary to do so. Hence they should be made in two parts, one overlapping the other, as shown in the illustration.
A fougass is a mine so arranged that upon explosion a large mass of stones is projected against the enemy. An excavation is made in the shape of a frustum of a cone, inclining the axis in the direction of the enemy so as to make an angle with the horizon of about 45 degrees. The sides splay outward slightly. A box of powder is placed in a recess at the bottom. This is covered with a platform of wood several inches thick, on which the stones are piled.
The fuse is placed in a groove cut at the back of the excavation, or the mine may be exploded by means of electricity.
The line of least resistance for the charge must be arranged so that the powder will act in the direction of the axis and not vertically. This is accomplished by throwing the excavated earth on the crest towards the defender's side and ramming it well.
To ascertain the powder charge for any fougass, divide the number of pounds of stone in the charge by 150. This gives the number of pounds of powder in the powder charge. Thus a fougass charged with about 70 pounds of powder will throw about 5 tons of stone over a surface about 160 yards long and 120 yards wide.
Plate 8.—Fougass.