Traverses are useful in the interior defence of a work, if they are so constructed and distributed as not be ex posed to an attack behind, nor liable to be enfiladed by the enemy.
A work may be defended either by cannon or small arms. The advantageous use of the former depends on the trac ing of the work, and the number to be employed. It is to be observed, however, that the guns are always to be placed so as to enfilade all the avenues ao the place, and enflank the works before them. The best situation for cannon ap pears to be the flanks and the saliant angles ; and that they may the more readily be brought to bear on the enemy, in whatever way he approaches, they should be fired there en barbette ; that is, over the parapet, without the assistance of embrasures.
Sometimes, however, guns will be more advantageously placed behind the embrasures, viz. to defend a defile, or any narrow passage ; and in this case it is proper to cover the artillerymen, in order to protect them from the enemy's tirailleurs, who always approach very near to the work, being covered by the ground, and thus make great havock before they come within reach of the fire of the small arms. The artillerymen who serve guns mounted en barbette in saliant angles, are very much exposed to danger ; for here a cannon can only fire a single shot, while the enemy's tirailleurs can molest it from all sides ; and, therefore, artillery alone will not always prevent the enemy from taking a work. Artillery should therefore never he placed in works where the enemy can approach with ease, as in such cases it will have little or no effect. From this it will be evident, that wherever cannon are employed, it will be necessary that they should not be too much exposed, as they can only be efficacious in such places as present great difficulty to the enemy in his approach The fire of small arms should defend and enfilade every part before a work, and he directed in such a manner, that the enemy may every where be received with the best ef and care must also be taken that it assist the fire of the great guns. The flanks arc also the most proper places for the small arms; but it being necessary likewise to de fend each part of the parapet, and to receive the enemy with a front fire, it will be requisite to give the small arms a place all round a parapet, but particularly to place the most numerous party of them at the flanks. To increase the defence of the saliant angles, the serrated form of the faces is the most advantageous ; but the faces of the small saliants should never be larger than for two men, or at most for three, one of which is to be placed in its saliant angle.
The relief which, in permanent fortification, is the most difficult to 'determine, depends, in field fortification, upon particular rules. In general, the height of the work dif fers according to its size and situation, and often depends wholly upon the opinion of the engineer; but there is a minimum of height, below which it never should be : That minimum is 4 feet 6 inches ; and even this only for such works as merely serve to cover men from the fire of small arms for a short time ; as, for instance, an outpost. No parapet should, however, if possible, be less than six feet high, if intended in any degree to the field round it ; and to cover the men completely to any distance behind, it will require to be 7 feet 6 inches. At this height, how ever, the soldier sees his enemy under a smaller angle, which renders his fire more nearly vertical ; a parapet, therefore, should never, if possible, be higher. If the height be from nine to twelve feet, this defect increases considerably ; and if flanks are constructed, they have a large dead angle before them, both of which arc always, if possible, to be avoided. But, on the other hand, a height of twelve feet secures the interior part of a work com pletely from the enemy's fire, and has, in this respect, great advantages. A strict attention therefore to circum stances, added to a sound judgment, must determine the most advantageous relief for any work.
The covered-way of a field fortification should have a command of at least 4 feet 6 inches above the ground ; and the terre-plein should be two feet below the latter, or 6 feet 6 inches below the crest of the covered way. This command may be increased to six feet, and even to 7 feet 6 inches, if it is to be made very large. The parapet should always have five to six feet command over the covert-way, and the crest of its glacis ; therefore, such covert-ways will not be of any use where the parapet can not be at least 10 feet high. A simple glacis round a field work should never have less than six feet command below the parapet, that the enemy may not fire from its crest with the same advantages into the work, as its garrison can at him. If, however, there are places of arms in its re-entering angles, then it is to have the same command as a covered-way, and also the parapet as much above it.