Where both the faces of the salient are unavoidably so placed as to be enfiladed, a small work, called a bonnet, is constructed at the angle, which consists in the parapet being so raised up to an extra height of 12 ft. if necessary, and at the same time widened, that the banquette shall be defiladed. If a height of 12 ft. is insufficient to defilade the whole length of the banquette, traversing parapets must be raised at right angles to the face of the work, and within it, at such distances that the whole may be safe: of course, the height of the bonnet and of the traverses must be decided on rules analogous to those explained. The increased height of the parapet of the bonnet ren ders it necessary to have two or more banquettes at that portion of the work,with steps to aid the ascent.
In inclosed works—i.e., in works entirely surrounded by parapets—the position of the parados is of vital importance; and they have often to be devised with great ingenu ity, so as to protect the defenders from reverse fire in any direction, and at the same time not to prevent necessary communication between different portions of the fortress.
Relief means the height of any point in a work above the plane of construction, which may be the line of sight or the bottom of the ditch. In the latter case, the relief of the parapet is an important item in estimating the resisting power of a work, as it represents the vertical equivalent of the obstacle which will be offered to a foe.
When the relief of the parapet's crest has been determined, its thickness becomes the next consideration. The dimensions are laid down on the ground, and depend, first, on the angle at which the material to be used will pile; and then, on the nature of the missiles against which the parapet is to afford protection. For example, an earth work of from 3 to 4 ft. suffices to resist musketry ; a thickness of 18 ft. is imper vious to the 24-pounder; while larger guns can pound through even more solid obstructions.
For convenient firing the banquette should be 4 ft. 3 in. below the apex; its width a ft., if for a single line of soldiers; 4 ft. 6 in. for a double rank; its slope should be one in twelve, that water may run freely off. The base of the slope, up which the men mount to the banquette, should be twice its height, and cut into steps with inclined sides, to allow of easy ascent; and where the height is considerable, a supplemental banquette (on which relieving-men can, if necessary, reload), is desirable. The interior slope of the parapet should be one in four; the exterior slope, or plongee, intended for the direction of the guns on an assailant, one in six, a deviation being permitted between one in nine and one in four; but the crest being more liable to destruction as• the slope is augmented, it is best to keep it as small as circumstances will allow; one in six is the ordinary slope in English F., the angle of the interior slope being constant.
In some continental services, however, the angle between the top and inside of the par apet is kept constant at 100°, by increasing the deviation of the interior slope of the parapet from the perpendicular as the plunge of the exterior is greater. The flatter, however, the crest of the parapet is the better, as saud-bags are in certain cases ranged on it to form cover for the men, while they fire through loopholes left in this additional defense. Earth of medium tenacity maintains its position properly when sloped at an angle of 45' ; and this is the greatest angle which can be counted on for the outer slope of the parapet. The scarp and counterscarp of the ditch need not have so great an incline, as the ground in which they are cut has usually had time, and the footsteps of ages, to consolidate it. In such cases, the base of the triangle is frequently made equal to half the perpendicular. Cases, of course, occur in which steeper banks are consid ered indispensable; and then, to preveut slips, the earth must have a coating to keep it up, which may be of fascines, hurdles, planks, or sand-bags, for temporary works, or those constructed in the midst of action; while the most solid masonry performs the same function in fortresses of a more permanent nature. This outer coating is denomi nated a revetement.
A glacis is formed during the excavation of the ditch, having for its object the bring ing of an advancing enemy into the best line of fire from the parapet. The base and perpendicular of its interior slope should be equal; the slope of the outer face should be one in twelve, unless the slope of the ground render some different angle desirable. An advanced glacis is sometimes adopted, in order that the enemy may the sooner be brought under fire. It is absolutely necessary that the crest of the parapet should be 5+ ft. higher than the crest of the glacis, as, otherwise, an assailant having reached the latter, would be able to pour a musketry-fire over the former into the work. No part of any glacis, whether near or advanced, should be more than 2 ft. below the line of fire from the parapet—i.e., the line joining the crests of the parapet and glacis contin ued; if more depth be allowed, the enemy may advance in a crouching posture, with out being liable to be hit. Advanced glacis are usually made of earth thrown up in prolonging beneath the ground the plane of the preceding glacis. They may be defended entirely from the parapet, in which case palisades or abattis are often fixed to delay the advancing enemy wheu at the point of greatest exposure. On the other hand, these advanced glacis are occasionally defended as a series of advanced intrenehments, and only abandoned, one by one, as the defenders are driven in towards their main work.