he unusual natural conditions necessary to a successful inundation
and the extent and character of the work required to construct the
dams make this defense of exceptional use. It may be attempted with
advantage when the drainage of a considerable flat area passes through
a restricted opening, as a natural gorge, a culvert, or a bridge.

Open cribs filled with stones, or tighter ones with gravel or earth
may form the basis of the obstruction to the flow of water. The usual
method of tightening cracks or spaces between cribs is by throwing in
earth or alternate layers of straw, hay, grass, earth, or sacks of
clay. Unless the flow is enough to allow considerable leakage, the
operation will not be practicable with field resources.

When the local conditions permit water to be run into the ditch of a
parapet it should always be done.

=1187. Obstacles in front of outguards= should be low so they cannot
be seen at night. A very simple and effective obstacle can be made by
fastening a single strand of wire to the top of stout stakes about a
foot high, and then placing another wire a little higher and parallel
to, and about one yard in rear of, the first. The wires must be drawn
tight, and securely fastened, and the stakes fairly close together, so
that if the wire is cut between any two stakes the remainder will not
be cut loose. Any one approaching the enemy will trip over the first
wire, and before he can recover himself he will be brought down by the
second. In the absence of wire, small sapplings may be used instead.
Of course, they are not as good as wire, but it does not take much to
trip up a man in the dark.

[Illustration: Fig. 17]

Lessons from the European War

What follows is based on reports from the battle fronts in Europe.

=1188. Wire entanglements.= The war in Europe has proven that the wire
entanglements are the most important and effective obstacle yet
devised. Owing to the intensity of the opposing fire and in many
cases to the short distance between the opposing trenches, it has
become necessary to construct all forms of obstacles in portable
sections which are carried or rolled quickly into place, either by
soldiers rushing out in day light and quickly staking the obstacles
down or by placing the obstacles quietly at night.

For placing wire entanglements at night, an iron post has been devised
about 1/4 of an inch in diameter, with eyelets for attaching the wire.
The lower 18 inches is made a