as to precautions against gas.

Selection of Site.

(a) Fire trench should be selected with due regard to tactical
requirements and the economy of men.

(b) Every fire trench should have a good field of fire, at least 250
yards.

(c) The trenches should have the best possible cover.

(d) The forward position on a slope for the first line has the best
advantage. The support trench should be on the reverse slope from 100
to 600 yards in rear of the first line to prevent direct observation
and to be practically free from artillery fire. The reserve trench is
usually from 1/2 to 1 mile in rear of the first line. Remember that
the first line, the support line, and the reserve line are all fire
trenches. Do not put them on a crest.

(e) The communicating trenches (boyaux) should be zigzagged, wide and
deep, and should follow the low ground. The longest straight trench
should not exceed thirty paces. The angle made by each turn should be
less than 140 degrees.

(f) The fire trenches should have salients and re-entrants so as to
flank the wire entanglements. The bays are usually 27 feet long with 9
feet of traverse.

(g) There are two problems in the siting of trenches, one for those to
be constructed under fire and another for those that will be
constructed without any danger from fire. Trenches built under fire
are usually made by connecting up individual shelters made by the
front line when forced to halt. Trenches built under quiet conditions
can be laid out according to the best possible plan.

Trench Construction.

Several kinds of difficulties face the trench digger: Sand, clay,
water and bullets. In order to overcome them he must be familiar with
the general arrangement of a trench, the principles which govern its
construction and the standard trench as it has been worked out in the
present war at the cost of thousands of lives.

General Arrangement.--A position is a combination of trenches,
consisting of: The fire trench, or first line, nearest the enemy; the
cover trench, just behind the first line, where all but sentinels of
the fire trench garrison are held in dugouts or shelters; the support
trench, from 150 to 200 yards in rear of the cover trench, and the
reserve, from 800 to 1,200 yards still further to the rear.

The support trench is placed far enough from the first line to prevent
the enemy from shelling both trenches at once. By a concentration of
artillery fire and a determined advanc