An avant-glacis is to have at least five feet less com mand than the covered way, but must always be made so high as to cover the objects behind it. If it is situated only before a simple glacis, the parapet behind it should have at least six feet command over it.
The plonge or slope of the upper part of the parapet, should be in the same plane with the glacis, the covered way, or the avant glacis. At all events, this plane should never pass at a greater height above the crest of the glacis than two feet. The same is to be observed if the earth for an advanced glacis is dug out behind it ; and in this case, no part of the ground behind the advanced glacis should be more than two feet below a plane passing through the crests of the covered way and the parapet. The command of outworks is also the same as of the glacis of the covered way, if not more than 200 yards distant from the principal work. If, however, the prin cipal work has a covered way, this is to have a command of five feet above it ; and consequently the parapet must have from eleven to twelve feet command above the field. Should the principal work be an outwork, then that which covers it must have a command of at least twelve feet above the covered way and its glacis before it. The com mand of field fortifications varies therefore considerably from that of permanent fortification; it being in the former from five to six, and in the latter only two to four feet. The reason of this is, that the enemy can attack the lat ter without being covered, but he can only approach the former while covered by a parapet, which forces hint always to be at least seven feet lower than the crest of the work he attacks. The method laid clown for works in permanent fortification, to determine their relief, being too artificial, and requiring too much time, is not applica ble in field fortifications, and here, therefore, a more sim ple method is requisite for determining this relief. But it will frequently be found impossible to give such a relief as may have been determined upon, and in this case a more advantageous one is to be chosen, and preferred to a great command. By this means, field works can be made much stronger than by a defilement artificially chosen, as in permanent fortification, and which is per haps still more difficult to be executed than that given by the above rules.
Works which are open in the gorge, or which cannot be attacked from behind, must be at least 300 yards dis tant from any eminence which commands it, if only small arms are to be feared ; but if cannon are employed, it will be necessary that the distance should be at least 900 yards.
The defilement of a work, which is open in its gorge, if it be necessary to place it neareCto a mountain than 300 yards, is determined as follows :—Poles four feet six inches in length are erected on the highest visible sum mits of the mountain. The points which it is required to cover by the intended work are then marked, and poles erected at them of such a height as that their tops shall just be within the command of the parapet. These poles being united at top by a cord, the saliant and re-entering angles of the work arc determined, and poles erected at each, of such a length that their tops shall be in a line with the cord and the poles on the mountain. The height of these last poles will give the height of a work necessary to have a given command over a given place near an eminence. After determining the defilement of the para pet in this manner, the tpps of the poles along the line of fire of the parapet are united by a cord, and wherever the line of this cord is below the plane of defilement, it is ele vated by means of poles.
In the samc manner the defilement for works to resist artillery is determined, if the heights are not above 900 yards distant from it. This will always be most difficult where the visible heights are situated on one side of the fortifications. In this case, after fixing the plane of de filement of the parapet, it will still be necessary to deter mine a traverse, and its plane of defilement, that the face may not be taken in rear or in flank by the eminence near it. The most convenient situation for such a traverse be ing chosen, the plane of defilement is to be determined in a manner similar to that described above, so as to cover every part of the works at such a height as may be neces sary. The traverses are at the same tirrie to be so chosen as always to keep up a free communication, it being neces sary that no part of the defence of the works be obstructed. The traverse is always to be determined at first, where two planes of defilement are necessary ; but, where all the works can be laid down according to one plane of defile ment, the parapets arc determined, and then, at proper distances from them, the place for the traverse is chosen, and afterwards constructed from this point to the salient angle of the work.