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.
Plate 10.—Dam construction.