Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/460

446 FORTIFICATION of the wall should be of irregular form. The lateral pres sure of the earth against the revetments may be diminished by building arches in one or two tiers between the counter forts. The Cordon is a round projection of stone, about a foot in diameter, near the top of the revetment wall, which serves to throw the drip of rain off the face of the masonry. It is also a considerable obstacle to besiegers, as it prevents their ladders for escalade being placed against the escarp. The profile or section of Vauban s first system is given in Plate III., fig. 4, in order to illustrate the relative relief or height of the respective works, and to show the com mand which each has over the work in its front. When the height of the rampart, including that of its parapet, is 20 feet, and that of the parapet of the covered-way is 9 feet above the plane of the site, then the rampart will have a command of 20 feet over the country, and 11 feet over the crest of the covered-way ; and the latter, again, will have a command of 9 feet over the country. Tiiere are three sorts of command, namely, in front, in rear, and in enfilade, That in front is when any eminence which commands the work directly faces it ; that in rear when the eminence is behind the work ; and that in enfilade when the eminence is on the prolongation of any line or work. The last, which is the most dangerous kind of command, is best remedied by raising the salient of works exposed to it (see fig. 23, p. 427), or by erecting traverses. In fig. 4 of Plate III. a line, called the line of site, and supposed to be the surface of the ground on which the fortification stands, is drawn, and perpendiculars are erected on it equal to the respective heights of the different parts of the works cor responding to the lines in the figure; a showing the terrepleine, b the banquette or step to enable the soldiers to fire over the parapet, c the parapet, d the revetment, e the escarp, / the counterscarp, and so on. 2. Vauban s Second and Third Systems. The principles of Vauban s First System having been thus explained, no great difficulty will be experienced in under standing the methods of other engineers who have con structed works varying but little in essentials from this system, the variations having arisen from difference of situation and local peculiarities rather than from other causes. In his other methods of construction, as, for example, in his second and third systems, Vauban himself merely modified, according to circumstances, the principles upon which his first is based. When this cele brated military engineer was called upon to repair or improve the fortresses of Landau, Breisach, &c., and found these places surrounded with strong walls strengthened by small towers at the angles, he did not destroy these de fences, but constructed, nearly in the same proportions as in his first system, large counterguards or bastions in front of the towers which crowned the angles of the wall, just as the Italian Castriotto had clone in 1584. And by this method an important object was attained ; for, as in front of each tower, or rather tower-bastion, there ran a ditch which cnt off all communication between it and the counter- guard, so the enemy, even if they should have succeeded in establishing themselves in the counterguard, would still have another ditch to cross, and another wall to breach, before they could attempt to give the assault. There is so little difference between the second and third systems of Vauban that a description of the former will enable the reader to appreciate the peculiarities of the latter. In the second system (Plate III., fig. 2), the in terior side of the polygon, from the centre of one tower- bastion to that of the next, is supposed to be 240 yards, and at 24 yards from its extremities perpendiculars are erected equal to 36 yards for the flanks of the tower- bastions. A line is then drawn through the extremities of thess perpendiculars, parallel to the interior side AB, till it meets the oblique radius of the polygon, or line drawn from the centre of the polygon bisecting its angle, and this being done on both sides of the angle forms the tower-bastion. The oblique radius is produced 78 yards, and lines of defence are drawn to the angle where the tower-bastion joins the curtain or line AB. On these lines of defence, the faces of the counterguard, or exterior bastion, are set off equal to 128 yards, and from the point forming the shoulder, flanks are directed to a point set off on the line AB, at the distance of 70 yards from its ex tremities. From the salient angles of the tower-bastions arcs are described with a radius of 14 yards for the breadth of the ditch, and tangents to these arcs are drawn parallel to the faces of the tower-bastion, but stopped where they would meet a line drawn from the salient angle of the tower-bastions, at the distance of 20 yards from the flanks. The tenaille is the same as in the first system, excepting that at its ends it is carried down till it meets the lines of defence drawn between the flanked or salient angles of the tower-bastions. The ditch in front of the counterguards, or, in other words, the main ditch, is constructed in the same manner as in the first system. The ravelin is formed by setting off 90 yards from the re-entering angle of the counterscarp, and directing its faces to points set off on the counterguards, at the distance of 20 yards from the shoulders. A flank is formed by cutting off" the corners of the ravelin at the distance of 14 yards on its demigorge, and 20 on its face ; and this flank serves for the placing of guns in such a manner that their fire may be directed into the counterguard, or into the ditch before them, as occasion may require. Again, at the distance of 48 yards from the re-entering angle of the counterscarp, lines are drawn parallel to the faces of the ravelin for the redoubt; a ditch is formed in front of this, and parallel thereto, about 1 8 feet in breadth ; and the redoubt thus constructed has a command of 4 feet over the parapet of the ravelin, as the tower-bastions have over the counterguards. The covered-ivay and glacis are formed as in the first system. It sometimes happens that redoubts are constructed in the re-entering places of arms ; in which case their demigorges are made from 15 to 40 yards, and their faces set off at an angle of 100, as before. 3. Cormontaiyne s System. The difference between the Systems of Vauban and Cor- montaigne may easily be discovered by an examination of Plate III., fig. 3. Vauban makes the faces of his bas tions two-sevenths of the exterior side, and Cormoutaigne one-third. Vauban, in his first system, directs the faces of his ravelin to points upon the faces of the bastion distant 11 yards from the shoulders, and in his second and third systems, to points distant 20 yards; but Cormontaigne makes the capital of his ravelins about 120 yards, and directs the faces to points distant 30 yards from the shoulders ; by which means the flanks are better covered, and the bastions and ravelins are enlarged. And this is an advantage ; for he is thus enabled to construct a larger re doubt in his ravelin, the curtain and flank are also better covered, and, as the former is shorter, communications are more easily kept up between the bastions. Cormontaigne gives the same breadth to his covered-way as Vauban, but he arranges in a different manner the communication round the extremities of the traverses, as may be seen by inspect ing the plate. By this zig-zag line of communication, which resembles the cremaillere trace adopted by Speckle in his covered-way, the passage round the extremity of one tra verse may be defended by the fire of the other in its rear, or nearer to the body of the place, and the advance of