This being so, the enemy’s artillery seeks to locate the trenches and to cover them with a steady hail of shells, so as to force the defenders to keep down under cover. If they can succeed in doing this, it is possible for the attacking infantry to advance, and the artillery fire is kept up until the last moment, so that the attack may have the narrowest possible space to cover after the defenders have manned their parapets and opened fire. Fig. 78 shows the action of various natures of projectiles.
| From Mil. Engineering, by permission of the Controller of H.M. Stationery Office. |
| Fig. 78.—Effect of Projectiles. |
We need not here discuss the rôle of the defenders’ artillery in replying to that of the enemy and playing on the attack; nor for the moment consider how far the defence of the trenches while under artillery fire can be made easier by overhead cover. The main question is—what is, in view of the nature of the attack, the best disposition of lines of trench; and do they require the addition of redoubts?
The most important point, with the object of protection, is that the trenches must not be conspicuous; this is the best defence against artillery. With the object of resistance by their own fire they must have a good view, or, as it is generally described, no dead ground in front of them. For this purpose 300 or 400 yds. may be enough if the ground is even and affords no cover.
This necessity for invisibility, together with the shallowness of the zone that suffices for producing a decisive fire effect, has of late years very much affected the choice of ground for a line of trenches.
For a defensive position on high ground, it was usually laid down until the South African War that a line of trenches should be on the “military crest” (Fr. crête militaire), i.e. the highest point on the hill from which the whole of the slopes in Siting of trenches. front can be seen. Thus in the three sections of ground shown in fig. 79 it would be at a, b and c respectively. The simplicity of this prescription made it attractive and it came to be rather abused in the text-books. There were, even before the improvements in artillery, objections to it, because on most slopes the military crest would be found at very different elevations on different parts of the line, so that by a strict adherence to the rule some of the trenches would be placed near the top of the hill, and some in dangerous isolation near the bottom. Moreover a rounded hill has no military crest.
| Fig. 79. |
Further, we have to consider nowadays not only the position of the fire-trenches, but those of supports, reserves and artillery, and the whole question is extremely difficult.
For instance, considering the sections alone, as if they did not vary along the line, the positions at a and b, fig. 79, are bad because they are on the sky-line and therefore a good mark for artillery. That at b is especially bad because the slope in front is so steep that the defenders would have to expose themselves very much to fire down it, and the artillery fire against them can be kept up until the very last moment. The position c has the advantage of not being on the sky-line, but the position of the supports in rear is exposed.
| Fig. 80. |
| Fig. 81. |
| Fig. 82. |
| Fig. 83. |