(ii) At close ranges, therefore, where the trajectories are flat, the depth of the beaten zone will be much increased if the ground behind the target falls at a gentle slope ([Fig. 9], A). At long ranges, on the other hand, a greater area will be beaten when ground falls more steeply ([Fig. 9], C).
(iii) For example, at 1,500 yards range the depth of the beaten zone is roughly increased by three-fifths when the bullets strike ground falling at 2 degrees. On ground falling at 5 degrees, which is nearly parallel to the trajectory at this range, the depth beaten is about ten times greater than on level ground.
(iv) It is clear, therefore, that falling ground far behind the objective will at times be swept by unaimed fire, and it follows that in such circumstances supports should either be under cover or, if there is no cover, in shallow columns on narrow frontages with the object of reducing target surface as much as possible.
6. Crest Line.—(i) When the objective is a crest line the depth of the beaten zone is greatest, and part of the fire is grazing, when the ground beyond the crest is parallel, or nearly so, to the trajectory of the bullets ([Fig. 9], A and B). At close ranges, in this case, there will be behind the crest-line a defiladed zone, or space not swept by fire ([Fig. 9], C), greater or less according to the distance from which fire is delivered, the inclination of the line of sight, the extent of the hill top, and the inclination of the reverse slope.
(ii) Position of Supports and Reserve.—It appears, therefore, that when the firing-line is placed on the crest of a razor-backed hill, with steep reverse slopes ([Fig. 9], C), supports and reserves will at all ranges be but little exposed to unaimed fire when posted in its vicinity. In other cases, when the crest of a hill is occupied, the vulnerability of supports and reserves will be least if, when the enemy is at long range, they are withdrawn from, and as he approached closed on, the firing-line ([Fig. 9], C).
7. Dead Ground ([Fig. 10]).—Dead ground is ground on which, owing to its conformation or to the existence of natural or artificial cover, fire cannot be brought to bear from a given locality. The term is thus a relative one, for though an area of ground may be dead in relation to one locality it may be possible to bring fire to bear upon it from another locality, in relation to which it will not therefore be dead.
Fig. 10.—Dead Ground.