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.
[Illustration: Plate 9.--Vertical fougass.]
When broken up a cubic foot of stone weighs about 100 pounds.
A vertical type of fougass is also shown. A charge of 25 pounds of powder should scatter a cubic yard of stones over an area about 200 by 100 yards.
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.
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.