The redan is a work consisting of two faces, like a demi lune, sometimes also having flanks, in which case it is call ed apiece. As the redan has an open gorge, it is easily taken in rear, and is therefore never used but to cover a road, dyke, village, castle, or other work.
The redoubt is an isolated work, and of course forming a complete enceinte. It is generally triangular, square, or circular, and sometimes also a polygon. The triangular form is seldom used indeed, its angles being too acute, and the space within too small. The circular redoubt encloses a large space, but is difficult to construct, and cannot be well flanked. The square, or pentagonal form, is there fore most commonly used.
As a redoubt is liable to be attacked on every side, its size ought to be so proportioned to the number of the gar rison, that it may be defended on all sides at once. To be sufficiently defended, a Side trial require a man to every yard, and three ranks of men. The first rank lives, the second loads, and the third acts as a body of reserve.
The weak defence of such works generally arises from the bad disposition of the men who are to defend them, and therefore great care is necessary in accommodating the work to the number of the gars icon. A redoubt, whose side is about 12 yards in length, will inclose 36 square yards, but will contain men enough only for one rank along the parapet, and is besides much exposed to the destruc tive effects of stone balls and shells. This is, therefore, the smallest redoubt that ought ever to be constructed ; if the side be 14 to 16 yards, the space within will be 60 to 80 square yards, and will contain men sufficient for one rank and a reserve. A side of 18 to 20 yards will afford room for 144 men, being enough for two tanks, and, in ge neral, the more the sides are increased in length, the greater in proportion will be the room afforded for the gar rison. This is to be understood, however, only of isolated works, as those to which succours can be readily afforded, do not require to be increased at this rate. If the side of the redoubt exceed 30 yards, there will be accommodation within for 500 men, of which 384 will be sufficient for a complete defence. In this case, therefore, the redoubt may be constructed with flanks ; and, if guns are to be planted upon it, it may also have outworks. The general rule for determining the fire lines of redoubts, is to allow three men to eight square yards of interior room, if the garrison exceeds 90 men ; but, if less, one man to two square yards. Redoubts are of great use in preserving a
communication with advanced posts, and defending a defile, height, passage of a river, a frith, or even the wings of an army ; but they are also very defective in some respects. They have unflanked angles, and a dead angle all round the parapet. The first may be remedied by placing them in such a situation that their saliants cannot be assaulted.
A fortin, or field fort, resembles a redoubt, but is larger, and has a greater variety of forms. It is called a fort when isolated, and a tte when the gorge is placed upon a river, defile, &c. When forts are not irregular, which is, however, in many cases, the most advantageous form, they are either star forts or bastion forts. Star forts, or forts 'ci tenaille, are such as form a regular suite of saliant and re entering angles. They may be considered as polygons, whose sides are broken so as to form the re-entering an gles. If possible, the saliant angles should never be less than 60 degrees, and the nearer they approach to 90 the better, as a rectangular defence is always the best. The brisures, or faces, forming the re-entering angle, should not be less than 50 feet, nor more than 100. If they are longer than this, they require a numerous garrison to de fend them, and it would therefore be better, in such cases, to construct a small fortress, especially if guns are to be used.
Star forts are seldom constructed either in the triangu lar or square form, a redoubt being almost always prefer able to either. In a triangle there can be no brisures, in a square their angles are 150 degrees. A pentagon is some what superior to both, the defence of its saliant angles be ing better, and the angles of the brisure 132 degrees. The hexagon is still better than the pentagon, though its saliants • are by no means well defended. The heptagon has sa liant angles of 128 degrees, and those of the brisures 112. This form might therefore be used with considerable ad vantage, were the construction not so difficult ; the most convenient, however, as well as the most advantageous po lygon for works of this kind, is the octagon. The con struction is made either upon the interior polygon, by placing equilateral triangles on its sides, or on the exterior side, by means of the perpendiculars from the saliant and re-entering angles.