When the mining operations of the enemy are detected a report should be made at once to the officer in charge of that section of the trench.
The next element of the defensive position is the _first line fire trenches_. These are located so as to have a good field of fire to the front for several hundred yards and so constructed as to give the greatest cover and protection from the fire of the enemy.
An unbroken, continuous trench would be exposed to enfilade fire. A shell, shrapnel or grenade bursting therein would have widespread effect. To overcome these elements the trench is constructed in short lengths, with traverses between them, and technically known as the _traverse type_.
Better defilade is thus secured and the material effect of any burst is confined to narrow limits.
The trench interval between the traverses is known as the "bay," which should not ordinarily be longer than 18 feet. Longer bays invite heavy casualties in case the trench is enfiladed or a high explosive shell finds its mark.
The illustration, Plate 14, shows a trace of the traversed type of fire trench.
[Illustration: _PLAN OF TRAVERSED TYPE OF FIRE TRENCH_ _PLATE 14._]
Formerly, protection from the enemy's fire was obtained by thickness of parapet. In the trench warfare of today it is obtained by completely concealing the riflemen in a deep, narrow trench with a very low parapet.
The height over which the average man can fire is about 5 feet or about five-sixths of his own height. This factor determines the height of the parapet above the firing banquet of the trench or the height of bottom of loophole above the same point, when the latter is employed.
[Illustration: Plate 15.--Simple standing trench.]
The type of trench in general use today is the simple standing trench shown in Plate 15.
If a trench is to be occupied for any length of time, especially if much ground or falling water is to be encountered, drainage becomes of prime importance. Many years ago a celebrated military authority asserted that "nothing so saps the courage of a soldier as to wet the seat of his breeches." This may be accepted as a true maxim, especially in cold weather. The trench should therefore be made as dry as possible. The floor of the trench should be given a sufficient slope to the rear where an intercepting drain should carry the water to prepared sumps or to a point from which it can be disposed of by drainage. Provision should also be made to exclude surface drainage from the trenches.