Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/453

FORTIFICATION 439 fended by two ranks should have a side (x) 54 yards in length, and a garrison of 324 men, the total length of parapet required for this force being 162 yards ; whereas a square redoubt of 38 yards side, which has a total length of parapet of only 152 yards, will accommodate u garrison of 456 men, admitting a " two-rank" defence, with a reserve of one-third the whole garrison. Another objection to Triangular Redoubts is the great amount of dead-angle space before 3 the salients. Keeping these figures in recollection, the engineer will readily be able to determine his arrange ment of the proposed garrison, and to limit himself to the least amount of work in parapet. If, however, he has to provide, in addition, for guns, for traverses, and for other constructions, he must increase the side of his square, bear ing in mind that in the square of 38 yards side the augmentation of 1 yard per side adds only 12 men to the garrison, raising it to 468, whilst it adds 549 square feet to the interior space; so that, as the 12 men require only 216 square feet, there will be an addition of 333 square feet towards the additional provision specified. And if the side be increased to 45 yards, and the garrison to 540 men there will be a surplus interior space of 2709 square feet, being sufficient for barbettes, for three guns and one howitzer, and for a traverse; this size may therefore be assumed as the best for a normal redoubt. It has been usual to assume that a redoubt should not have more than 180 yards of parapet, but there does not seem to be any reason for this assumption ; and it may be said with confidence that a redoubt with double this extent of parapet will be superior to a field fort of the same strength of garrison, even though the fort has the theoretical advantage of reciprocal defence. The ditches of larger redoubts are sometimes, when circumstances admit, defended by Caponnieres (fig. 58) placed either at the angles or in the middle of the sides. SECTION TMRO JCH C D. FIG. 58. Caponniore. These caponnieres are built of timber and roofed with logs or fascines, covered with earth ; occasionally the ditches are defended by counterscarp galleries at the angles, con structed of the same materials as the caponnieres, but buried in the counterscarp. Access is obtained by galleries revetted with timber descending from the interior of the work into the caponniere, and beneath the ditch to the counterscarp galleries. In fig. 40, one of the angles is shown with an indented parapet as a means of correcting the defect of a dead salient; but this is difficult of construction, and it will generally be preferable either to cut it off as in another angle of the figure, or to round it as in a third, or to occupy it by a gun en barbette, as in the fourth. Of Forts, fig. 41 exhibits a Star-Fort of eight points formed upon a square, which is far preferable to one of six points formed upon a triangle, as giving com paratively more available interior space. The defect, how ever, in this trace is that, though the intermediate angle F is sufficiently open as it exceeds 60 the angles A and B are less than 60. Down to the lowest limit of such forts, in which AB of fig. 41 is taken at 60 yards, and the sides AE, EF, EB at 20 yards, there is still sufficient space to accommodate the necessary garrison, which should be in that case about 900 men. It is useless to describe those forms of star-forts which would not accommodate the re quired garrisons; but that represented in fig. 59 is well Fia. 59. Trace of Star-Fort. fitted for a large garrison. The following is a simple construction on a pentagon. Bisect AB in C ; make the perpendicular CP = - 5 - AB ; join AP and BP ; make Bt/ and AtZ each -jV of AB, and join dc, do. In this case the angles at A and B will be 64, and the short sides (with an exterior side of 120 yards) each 23 yards. From what has been already said, Demi-bastioned Forts (fig. 43) will rarely be used ; but when the proposed garrison requires a large amount of accommodation, and there is time to under take such works, the bastioned trace should be adopted, as it introduces a principle not observed in the preceding traces, namely, that of defending the whole by a part, the op posite flanks EG, FG (fig. 42) of the two bastions GEAEG, GFBFG defending the intervening curtain GG between them, as well as the faces AE, BF of the bastions- whilst the fire of one flank necessarily sees the escarp of the opposite one and defends it. The bastions are indeed, like their analogous lunettes, works in themselves, the curtain being only a connecting line, forming several bastions into one connected whole. Fig. 42 is a square bastioned fort, but the pentagon is a better form, and should be adopted when practicable. The bustioned form of field forts has been derived from the more massive struc tures adopted in the permanent defences of fortresses, and its history, so interesting in itself, will be given in a future page. Little further need be said on this sec tion of the subject, except to point out the great import ance of field-works in securing a base of operations for an advancing army. Fig. 49 (page 435) exhibits, for ex ample, a bridge head consisting of a bastioned front, with either simple straight branches or branches with a short flank, as shown in the figure (see also fig. 60 below) This is technically called a Horn Work; and if there had been two such fronts so placed as to throw a bastion in the centre, and connected as before with the river by straight branches, the work would have been called a Crown Work. Lunettes and even lledans may also be used, as in left side of fig. 36, for a similar purpose, where the object is first to secure the bridge from the enemy s attack and fire, and secondly to allow the defending army