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 advance of the hostile infantry the first line may be captured. The support trench must be so organized that it will then act as a line of resistance upon which the enemy's advance will break. Lieutenant Colonel Azan of the French army says: "As long as the support trenches are strongly held, the position is not in the hands of the enemy." [Illustration: Plate #16] The reserve is usually a _strong point_, so organized that it can maintain independent resistance for several days if necessary, should the enemy obtain control of adjacent areas. Where possible trenches should be on reverse slopes, with the exception of the first line; but usually the outline of a trench is determined in actual combat, or is a part of hostile trench converted. Under these circumstances it cannot be arranged according to tactical ideals. Artillery and the automatic gun are the determining factors in trench warfare to-day. The effect of artillery fire must be limited in its area as far as possible, and trenches are, therefore, cut by _traverses_, which are square blocks of earth not less than nine feet square, left every 27 feet along the trench. They should overlap the width of the trench by at least one yard, thereby limiting the effect of shell burst to a single _bay_, the 27-foot length of firing trench between two traverses. Sharp angles have the same effect as traverses, but angles of more than 120 degrees cannot be utilized in this way.