'['he fosse before the parapet, in field fortification, is generally not very deep, but ought never to be less than 6 feet, and always 12 if possible. The talus of the fosse may be steeper than that of the parapet, as the earth of the for mer possesses its natural adhesion. Of course the base of such slopes is very small compared to their height.
If a work is situated on ground where it is liable to be enfiladed, it must have traverses in the interior. The thick ness of these traverses depends on the attack which they may have to sustain. If this consist of small arms only, two feet will be sufficient ; but they will require 6 to 9 feet, and, in some cases, 10 to I 1 feet, to hold out against afield pieces. The talus must be as steep as possible, that they may occupy little room, and to presCrve the communica tion they ought to have passages through them. In every work, indeed, there should be openings from 4 to 5 feet, or from 8 to 9, according to the size of the work, to keep up a free passage between the different parts. The talus of these entrances must be as steep as possible, and they are to be shut up with chevaux de-frises, large branches, and other barricades. They ought also to be covered by a tra verse behind them, 6 feet longer than their width, and from 3 to 9 feet in thickness. This traverse has a ban quette, from which the soldiers can fire on the enemy, in approaching it. Before these entrances, it is usual to con struct a bridge, consisting of boards laid on scaffolds in such a manner as to be readily removed or destroyed when necessary. This bridge is about 3 feet wide in small works, and 9 to 10 in works whose guns are mounted. The gorge of a work is always to be inclosed, at least by palisa does, to prevent a surprise.
If a work is to be occupied for any considerable length of time, it should be furnished with a corfia de garde. In small works this may consist of huts covered with branches ; but in larger places it should he of timber, and have if pos sible a shell proof covering. The exterior parts of a for tification may be increased and strengthened either by ac tive or passive works of defence. A covered way, however, is in general of little use in field fortification. It is very ex pensive, it adds 3 to 4 feet to the height of the profile, weakens the rest of the parapet, requires a deeper ditch, and can be defended only by a large garrison, which, how ever, generally retires before the assault is made, and creates confusion within the place. When the work is
pretty large, a covered way may be used, and then the crest of its parapet ought to be 4 feet above the horizon ; and if it be dug out behind the glacis, the latter will require a banquette. The width of such a covered way is nine feet. Traverses are seldom used at the re-entering angles of the covered way in field works. If they are used, it is to cover an entrance, and are constructed in the same way as places of arms in permanent fortification. Such traverses, how ever, require a great command. The parapet must be at least 11 to 12 feet. A simple glacis, therefore, is perhaps better, except in very large works. A second glacis, without any covered way, is of great use in field works. Where it is used, the parapet will require only from 8 to 9 feet of command, and the obstructions that may be laid in the ene my's way are better concealed. Such a glacis is also of great advantage where small wen ks, as fleches, are used, and may be successfully employed in producing little in undations, particularly if ditches are made in it at short in tervals that cannot easily be passed. It is always of the greatest importance, indeed, to have the fosse full of water.
Of the various obstructions used to retard the enemy, trappes de buns may be mentioned. These are square or circular pits, six feet in depth, and as much in diameter, with a pole in their centre, to prevent their being a cover to the enemy. The earth dug out is heaped up among them, or dispersed around. 'frappes de loops should always be hid by a glacis, and should be arranged in three rows in echiquier, like the black squares in a chess board.
Palisades are obstructions that may be used at all times, but their advantage depends principally on the manner in which they are employed. They are constructed in dif ferent parts of the fosse, and placed in various positions, according to the expected attack, and the kind of defence which the ditch possesses. Care must be taken, however, to place them without reach of the enemy's large guns. If trappes de loops are before the fosse, the palisades ought to be behind it.