here possible the trench should have a convex
surface and should be smooth. A rough bottom means delay in reliefs,
and possible injuries. Where trenches are used for long periods
board walks should be constructed. Under these drains or sink holes
can be placed to collect water. A sink hole may be constructed by
digging a pit filled with small stones, or a barrel may be sunk into
the ground and filled with stone. Where there is not sufficient slope
to carry off the water, or at the lowest point of a drainage system, a
water hole should be dug in front of the trench large enough to handle
the drainage water.

[Illustration: Plate #18]

Communication Trench (or Boyau).--Running to the rear and joining
the different parallel resistance trenches are communication trenches
or boyaux. These are for transportation of men and material as well as
for communication. Communication trenches should be made wide enough
to allow travel for men with loads, should be at least seven feet deep
and as smooth as possible on the bottom. Rough places will delay
traffic. They are dug with turns every 20 or 30 yards to prevent their
being swept by gun fire. When boyaux are built by night sharp zig-zag
corners should be used, or the angles will not be acute and protection
will be lost. During daylight when the directions can be seen, the
construction may be a serpentine curve, with no stretch of more than
30 yards visible from one point.

Boyaux are sometimes used for lateral defence and often emplacements
for automatic guns are arranged to cover stretches of them. Bombing
stations are placed near by to protect the guns and to clear the boyau
of the enemy. At these points the boyau is left straight for a short
distance in front. Where provision is made for lateral or frontal
defence by rifle fire, firing steps are constructed. If this is
inconvenient for movement along the boyau, individual emplacements
must be made in the side wall for firing. Sentry posts are dug at
right angles to the boyau.

Arrangements for passage of men moving in opposite directions may be
made by extending short spurs at the corners, enlarging the boyau at
the bends, digging niches or passing points here and there, or
constructing island traverses with the boyau running around on each
side.

[Illustration: Plate #19]

Every boyau should be marked where it meets a trench with a sign
indicating the place and trench. Without this messengers, reliefs and
re-enforceme