Fig. 77.
But let us suppose the attacking body to possess projectile arms ([Fig. 77]), and instead of encountering the circular enclosure, the assailants to set up their engines between A and B within fair range. They will overwhelm the segment d, c, e, of the circle with projectiles, while the defenders will be able to oppose only an inferior number of engines to the convergent fire.
Fig. 78.
To compensate in part for this inferiority the defending party adds appendages to the enclosure ([Fig. 78] A), which allow an almost equal front of defence to be opposed to the attacking front, as regards the number of projectile weapons, and very superior in point of elevation and protection. But the attacking force will thus naturally arrange its engines as seen at B. Thus the projectiles sent from a, b, c, d, e, converge upon the salient C. The defence adds the new appendages D D, and if the engines are well protected, it can make the projectiles g, h, i, converge on the engine K and crush it, secondly the projectiles l, g, h, on engine m and destroy it, and so on.
Besides, these appendages have the further advantage of giving side views over the circumvallation itself and masking its foot.
This principle regulates and will always regulate attack and defence; distances alone modify its applications.
The more eccentric the defence is, the more distant must be the attack, and the wider the perimeter it must occupy; but it should be observed, that the more widely the defence is extended, the more open its flanks are to attack; these flanks therefore must also be capable of being defended, for every obstacle that offers only its own resisting force, without being protected by the action of a neighbouring obstacle, is soon destroyed.