Page:The American Cyclopædia (1879) Volume VII.djvu/343

FORTIFICATION 335 at hand. Works of a greater command than this have been constructed ; the length of time consumed in building these, their importance, and the many interior arrangements devised for the safety and comfort of the troops using them, cause them to approach more nearly in detail works of a permanent character ; and they have been called for these reasons semi- permanent works. The ditch that surrounds the work affords the earth for the parapet, and should be wide and deep enough (not less than 12 and 6 ft.) to form a considerable obstacle to the assailants. The slopes are the same as given for the permanent work, viz. : superior slope, I ; exterior slope, | ; and interior slope, f . The sides of the ditch are not ordinarily revetted ; if so, however, it is by some mate- rial of a perishable nature, as plank, timber, &c. The interior slope is revetted by sods, or some material hastily gathered for the pur- pose, as logs, boards, fascines, gabions, &c. The banquette is placed 4 ft. 3 in. below the interior crest, and is from 2 to 4 ft. wide. It is connected with the ground by a slope of $-, which is sometimes replaced when interior space is needed by steps whose tread is 12 in. and rise 9 in. In laying out the plan of the work the line of the interior crest is adopted as the directing line. The plans of these works vary according to their object and site. Sup- posing the site level or approximately so, the relief is generally uniform throughout. Such works may be classed into three kinds: 1, those that are arranged to sweep only the ground in their front; 2, those that do the same for their flanks in addition; 3, those that are arranged to fire on all sides. A work consisting of a simple straight line, or of two faces, the rear, called the gorge, being open, is an example of the first class. This is called a redan, and is used in front of a defile, at the exit from a bridge, or for the defence of an outpost, where the attack is expected only from the front. A redan with flanks, called a lunette, is an example of the second class, and is hardly ever used alone. In this case the attack is not expected from the rear, but an assault made on the flanks would be liable to work around and carry it at the rear. Both in this case and the first, when they are used alone, the gorge is closed by palisading or a stockade. A work of a polygonal figure, with- out reentering angles, in plan as a square or rectangle, arranged to fire in all directions, is used when liable to be attacked on all sides. It is called a redoubt. The ditch of a redoubt cannot be swept by the fire from the work it- self; therefore the bastioned trace has been sometimes used, but the great length of time required to construct it prevents its general adoption. When these works are joined to- gether or placed along a given front with in- tervals, they form either a continued line or a line with intervals. A number of redans j oined by curtains is a simple case of a continued line. If the line be formed entirely of redans, 329 VOL. vii. 22 it is called a tenaille line. The usual plan of a line with intervals is to place lunettes and simple redoubts, or either, on a given front at distances apart from 250 to 500 yards, and in rear of them, opposite the intervals, redans or other field works, arranged to sweep by their fire the ground in front of the salients of the first line. In field works all accessory de- fences, both artificial and natural, should be freely used. On irregular sites it will not be possible to preserve the relief uniform, and the same problems for defilement will be met with as in permanent works. The only general rules that can be laid down are to lay out the principal lines so as to obtain a direct and cross fire on the approaches of the enemy, and to place them as nearly as practicable parallel to the general crests of the commanding heights, so that the enemy will have only a direct fire on them. Where the defilement would in- volve considerable labor in raising the interior crest to intercept the fire, traverses should be resorted to in preference. Field works re- quire to be simple and easy of construction. All lines and fronts should have their flanks protected by natural obstacles if possible. The relief of a work should be obtained by its position and not by the construction of em- bankments, and if possible the necessity of de- filement should be avoided. The class of works already named will require several days and often weeks to construct. In an active cam- paign they are freely used, especially in the defence of defiles, the crossing of rivers, de- pots of supplies, intrenched camps, &c. An- ticipating the movements of the enemy, time enough may be had to finish them, or at least to put them in such condition as to render them useful for defence if an attack be made. When, however, an engagement is imminent, or has begun, in the absence of such works, resort is had to what may be called hasty in- trenchments for opposing the enemy's advance, sheltering troops, or strengthening the weak parts of a line. These are shelters that may be executed in a short time, say within an hour, certainly not more than three hours. They are not limited to defensive positions, but are used on the battle field itself. By excavating a trench 2 ft. wide and 1 J ft. deep, throwing the earth to the front, building up the slope next to the trench as steep as possible by using clods of earth, sods, fallen trees, fence rails, &c., shelter will be given to two ranks, one kneeling in the trench, the other lying flat in rear of it. By placing a man at every 4 ft., the trench can be dug in 20 minutes. This trench can be widened to 4 ft. in 20 min- utes more, when both ranks can enter it ; in 20 minutes more it may be widened to V ft., increasing the thickness of the mound of earth between the men and the enemy, and allowing freedom of movement to the limbs of the sol- diers. Thus it is seen that in an hour good shelter can be obtained by active troops. These are known as shelter trenches, and upon