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terminating in a slope at both ends. The caponier OU, leading to the arrow, is about 5 fathoms wide, having a traverse, O, at its entrance, of 3 fathoms in thickness, with passages round its ends, of the usual breadth of 6 or 8 feet.
The figure to article 266 represents a front of a fortification; the part on the right is not finished in the figure, for it is exactly similar to the part on the left.
268. A detached redoubt is a work similar to a ravelin, BEFC, placed beyond the glacis.
The gorge, BC, is 40 fathoms; the flanks, BC, 10 fathoms; the faces, E and F, 30 fathoms; and the ditch before it is 6 fathoms broad, sloping gradually to both ends; the covert is about 4 fathoms broad, with places of arms, W, W, the sides of which are perpendicular.
The redoubt being supposed to be placed beyond the ravelin, in the direction of its capital, the caponier AD is continued from the salient angle of the ravelin (see fig. art. 266). This caponier is 5 or 6 fathoms broad, having a traverse at its entrance (fig. art. 266) E, and another in the middle A, when it is of considerable length.
Redoubts are made for the purpose of occupying some position that might be favourable to the enemy. Their distance from the Covert way should not exceed 120 fathoms, otherwise they could not be defended from this place by musketry.
Some other outworks ought to be erected near detached redoubts, in order to support them; for otherwise, if they are upwards of 50 fathoms from the covert way, the enemy can
easily take them by carrying his trenches round to the interior side, and then entering them by the gorge.
Sometimes there is a second ditch made round the glacis, with a covert way and glacis beyond it—that is, a second enceinte with places of arms at the re-entrant angles of the covert way, and lunettes along the sides of its salient angles, directly opposite to the places of arms in the first covert way.
The preceding constructions are those of the first method of Vauban. There are other two methods of Vauban employed in particular circumstances; but his first method is of most common use. Various other methods have been invented, and there is no limit to the variety of constructions that may be proposed, but unnecessary complexity and expense ought to be avoided. Of all the various constructions, when their advantages and disadvantages are considered, none possesses such superior merits as entirely to supersede Vauban's first method. There are, however, two methods that differ somewhat from Vauban's, that are by some good judges considered to be in some respects superior: these are Cormontaigne's and the modern system; the latter of which, however, is almost identical with the former, and both are similar in their leading features to Vauban's.
CORMONTAIGNE'S AND THE MODERN SYSTEM.
269. In Cormontaigne's system, the exterior faces of the bastions are longer than in Vauban's, being one-third of the exterior side, and the and redoubt within it are also considerably larger.
Let the exterior side AA' be the usual length of 180 toises. Construct the front as in Vauban's system, with this difference, the length of the faces of the bastions are to be one-third of the exterior side, and the direction of the flanks perpendicular to the lines of defence.
The breadth of the principal ditch is 15 fathoms opposite to the flanked angles of the bastions, and its exterior line is directed towards the shoulder of the next bastion; and the covert way is 5 fathoms.
To construct the tenailles, draw yy' and YY' parallel to
the curtain, and at the respective distances of 5 and 13 fathoms; also draw Xx parallel to the flank of the bastion, and 5 fathoms distant; draw XY in the line of defence, and xy parallel to it, and at the same distance from it that YY' is from yy'.
To construct the ravelin, set off 15 fathoms from H to K, and find K', in the same manner; on KK' construct an equilateral triangle, and its vertex, E, will be the salient angle of the ravelin, and ER one of its faces.
Parallel to the face of the ravelin, draw the line DZ,
directed to the point w, where the interior edges of the parapets of the face and flank of the contiguous bastion meet, and it will be one of the faces of the redoubt.
The gorge of the ravelin and its redoubt are determined by drawing a line, as A/C, from the salient angle of the contiguous bastion, A, through the extremity, 7, of the face of the ravelin. By this means, the gorge is defended from the fire which the enemy might direct from the crowning of the salient place of arms before the bastions.
To determine the flanks, VZ, of the redoubt, through K and k, draw a line Kz; make kz: 61 fathoms, and draw VzZ, making the angle KzZ=100°.
Make the breadth of the ditch of the ravelin 10 fathoms, and that of the redoubt 5 fathoms. The breadth before the covert way of the ravelin is 5 fathoms.
The re-entering place of arms, TBW, is thus constructed: From the interior flanked angle of the parapet u, lay off uv = 5 fathoms, and draw the line vB, making Bvu = 100°; make SW = ST, and draw BW, making an angle of 100°, with ER, the face of the ravelin, or BWS 80°; and these lines determine the faces of this place of arms.
For the redoubt of the preceding place of arms, lay off TL and WI each 11 fathoms, and make ab and cd each 7 fathoms; then draw bl and dL, and these lines determine the interior sides, LO, OI, of the parapets of the faces. The breadth of the ditch is 3 fathoms.
The traverses (at b and d) of the re-entering place of arms are 3 fathoms broad, and those at the salient places of arms are only 9 feet.
To construct the retrenchment in the bastion, draw PQ, parallel to the face of the bastion, and distant from its exterior line by 18 fathoms; and at the same distance from the flank, and parallel to it, draw QM, and produce it, so that SM shall be 15 feet within the produced line of defence Y's. The ditch is 6 fathoms broad. Draw Ko in a line with ER ; from Q to m, lay off 4 fathoms, and make mm' perpendicular to PQ, and = 6 fathoms; draw Pm'; make m'f 12 fathoms, and draw fR. The part mm'o is called the retirade. The other half of the retrenchment is constructed in the
270. Any work raised within another for the purpose of impeding the progress of the besiegers, or of securing a retreat to the besieged, is called a retrenchment. The retrenchment described above serves also for a cavalier, that is, a work generally constructed within the body of the place several feet higher than the rest, for the purpose of commanding the adjacent works and country around. Its most common situation, when used, is within a bastion; and a full bastion is better adapted for it than an empty one. A full bastion is one whose interior is wholly on a level with the terreplein of the rampart; and when the rampart is formed in the usual manner within a bastion, so that a portion of it is on a level with the plane of site, it is called an empty
The preceding figure affords a view of the method of constructing a front, one half of the front containing the lines of construction, and the other half representing the work when completed.
271. In the modern system, which is just Cormontaigne's system slightly modified, the construction of the front is the same as in the preceding system, and also that of the ditch and ravelin, except that 17 toises instead of 15 is laid off from the shoulder H to K. The faces of the redoubt of the ravelin are formed in the same manner, but its gorge is found thus:-Bisect the thickness of the ends of the ravelin, which will be found to be 101 fathoms; join the points of bisection, and the connecting line will be the gorge of the redoubt. The flanks of the redoubt are made by laying off 8 fathoms from the angular points of the gorge, as already determined, along the gorge, and also 9 fathoms along the faces from their extremities, and joining the points thus obtained for the line of the flanks. In the ravelin are retrenchments, called also coupures. The lines of the counterscarpes of the ditches before them are perpendicular to the faces of the ravelin, and in a line with the escarpes of the ditch of the redoubts in the places of arms. There are also cavaliers in the bastions. There are embrasures in the flanks and faces of the bastion, and in the faces of the ravelin and the flanks of the redoubt.
The base of the exterior slopes of the parapet of the rave