The dimensions of the ditch depend in some measure on the amount of earth required for the parapet and glacis; but in addition to being the mine whence the materials for the latter works are drawn, the ditch must also oppose a considerable obstacle to any hostile advance. To do this effectually, the minimum width across the top is 18 ft.; its depth need only be limited by the trouble of raising the earth; but in practice 12 ft. is found the greatest which can be conveniently arrived at. Having ascertained the pro file of the parapet, with its banquette or banquettes, bonnets, traverses, glacis, etc., it becomes a mere matter of mensuration to compute the area of a section, to multiply it by the length, and so to obtain the cubic feet of earth required. With the length of the ditch known, a very simple calculation then exhibits its width and depth—a small allowance being made for the fact that the earth, dug out from the ditch, where it has 'probably been long compressed, will occupy somewhat more space when thrown up, and broken into clods, for forming the parapet.
The scarp, or inner face of the ditch, is most difficult of ascent by the assailant, when in a continuous line with the parapet; but sometimes it would be dangerous to construct. the work with this continuity, as damage to the scarp would jeopardize the stability of the parapet. In these cases a narrow step, called a berm. (q.v.) of from 2 to 4 ft., is made to intervene between the foot of the parapet and top of the scarp: as a precaution, it is covered with all possible obstacles to any lodgment being effected on it by the enemy. When a berm is employed, greater steepness is usually given to the scarp.
The counterscarp, or outer sloping side of the ditch, should be somewhat steeper than the scarp. The bottom of the ditch should slope from both sides towards the center, to.
carry off the water; and obstacles should be scattered about to prevent an enemy from forming his troops in the ditch.