of the wall should be of irregular form. The lateral pressure of the earth against the revetments may be diminished by building arches in one or two tiers between the counterforts.

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 command 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. There 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 corresponding to the lines in the figure;-a showing the terrepleine, the banquette or step to enable the soldiers 6 to fire over the parapet, c the parapet, d the revetment, e the escarp, f 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 understanding the methods of other engineers who have constructed 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 celebrated 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 defences, 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 done 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 cut off all communication between it and the counterguard, 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 In the second system (Plate III., fig. 2), the interior side of the polygon, from the centre of one towerbastion 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 these 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 extremities. 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 18 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-way 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. Cormontaigne's System.

The difference between the Systems of Vauban and Cormontaigne may easily be discovered by an examination of Plate III., fig. 3. Vauban makes the faces of his bastions two-sevenths of the exterior side, and Cormontaigne 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 redoubt 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 inspecting the plate. By this zig-zag line of communication, which resembles the crémaillère trace adopted by Speckle in his covered way, the passage round the extremity of one traverse may be defended by the fire of the other in its rear, or nearer to the body of the place, and the advance of

W&AK Johnston.

ENCYCLOPEDIA BRITANNICA. NINTH EDITION.

Ditch.

a Terre plein, b.Banquette, c.Parapet, d-Revêtment, e-Escarp, f-Counterscarp.

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the counterscarp is traced from a point 36 yards from the shoulder of the bastion to meet the counterscarp of the ditch of the cavalier. This system originated in the school of military engineering instituted at Mézières in 1750, and was for some time called the system of the school of Mézières. It has, however, been much improved since; and the system which is now recognized as the "Modern System" is that of General Noizet. Referring, however, to fig. 2 of Plate IV., it will be seen that coupures have been introduced in the faces of the ravelin; and as the ditch of the ravelin in this system is less by 7 or 8 feet than the main ditch there is a sufficient fall be

assailants along the covered-way checked. As Speckle planned in 1589, or long before the invention of ricochet fire by Vauban had rendered traverses an essential element in fortification, his object was not the same as that of Cormontaigne, but simply to ensure a more perfect flanking defence of the branches of the covered-way than that afforded by the places of arms of his systems. The ditches are, as shown in fig. 3 of Plate III., on different levelsthe main ditch being about 23 feet deep, and the ditch of the redoubt of the ravelin only 7 feet, so that from this latter ditch there is a fall of 16 feet to the main ditch, rendering it impossible to attack the ravelin by its gorge without the aid of ladders. An examination of the several figures which represent Vauban's and Cormontaigne's systems, as also the outworks of fig. 3, Plate IV., will at once render evident the vital defect of the ordinary arrangements of outworks-that they expose through the openings of their ditches the escarp of the body of the place, or of the work on which their faces or branches are directed, to be breached. In the System of Cormontaigne, as well as in the Modern System next to be considered, the increased projection of the ravelins, by throwing the intervening bastion into a deeply re-entering position, secures it from attack by approaches until the salients of the ravelins have been taken; but this great advantage is diminished by the power of breaching the bastion from the glacis through the opening afforded by the ravelin ditch. For the purpose of covering the communication to the re-entering place of arms, a demi-caponnière, or work composed of a parapet and glacis, was thrown across the ditch of the ravelin, as shown in the figure of the modern system (Plate IV., fig. 2). This so work afforded cover also to troops assembling preparatory to a sortie upon the enemy when making the passage of the ditch, but, from the depth of the ravelin ditch, it was insufficient to mask the revetment of the bastion behind it. It will presently be shown how this object was afterwards effected; and it may be said that without any material change in system, Cormontaigne's variations from Vauban's trace are unquestionable improvements

4. The Modern System.

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The Modern System, which is shown in Plate IV., fig. 2, varies little from Cormontaigne's. Its perpendicular is one-sixth of the exterior side, and the faces of the bastions are one-third. The flanks are at right angles to the lines of defence, whereas in Vauban's System they form an angle of about 82°,-which is not so good, because, in the Modern System, the guns placed in the flanks can fire straight along the ditch without being moved or turned on their platforms. The ravelin is formed by setting off points 34 yards from the shoulder along the face of each bastion; a line joining these points is the base of an equilateral triangle, the vertex of which, opposite the centre of the curtain, forms the salient angle of the ravelin. doubt of the ravelin is formed by drawing its faces parallel to those of the ravelin from the shoulder of the parapet of the bastion; and it has flanks with a ditch about 20 yards in breadth. cavalier in the bastion is drawn parallel to the faces of the bastion, at the distance of 48 yards from them. The ditch of the faces is 10 yards in width, but there is no ditch to the flanks. In bastions strengthened by a cavalier retrenchment, a coupure is formed perpendicularly across the faces of the bastion; the ditch of the coupure is 30 feet wide, and

The re

The

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FIG. 63.-Half-Front of the Modern System. tween the two to check the enemy in his passage to the latter, whilst the demi-caponnière is raised so much

higher, and therefore begins to mask more effectually the revetment of the bastion. In General Noizet's system this demi-caponnière is formed into an elevated mask, which tends to secure the revetment from the breaching effect of the fire from the enemy's battery on the crest of the ravelin glacis. This is shown in fig. 63; and the system modified from the former modern system in this respect, as well as in other arrangements, is now the normal bastioned system of the French schools. In General Noizet's arrangement the flanks are not made perpendicular to the lines of defence, but, as in Vauban's first system, form angles of about 80° with them. In the citadel of Ghent, which is a beautiful example of this system, but with still further modifications, the retrenchment of the bastion is so formed as to take advantage of this construction; and the flanks, being casemated and pierced in both directions, become on one side the flanks of the retrenchment, whilst on the other they are the ordinary flanks of the bastion, thus giving a much longer curtain to the retrenchment, which occupies the whole gorge of the bastion, than in the form exhibited in fig. 2 of Plate IV.

5. Outworks.

Plate IV., fig. 3, shows several kinds of Outworks, as a horn-work g, tenaillons k and k, a bonnette d, lunettes a and d, an entrenched bastion e, a batardeau ƒ, and a caponnière h. These, and other works of a similar description, are constructed for the purpose of occupying ground which might otherwise be of service to the besiegers, or, as in the caponnière, to cover communications; but their application must of course depend upon localities, and the judgment of the engineer must therefore determine in each particular case, which are the best to adopt.

POLYGONAL OR GERMAN SYSTEM

The Systems of Fortification of which mention has been made were devised to meet the attack of horizontal fire, but henceforward the efforts of the engineer must be directed to devise methods of protection against curved and indirect fire of greatly increased range and power. It is true that the efforts of the later engineers have been in this direction, but the recent introduction of rifled guns and the still more recent development of indirect fire have rendered a recourse to very different measures absolutely neces

sary.

.it

Before entering upon the consideration of these measures, may be well to point out that a rigid adherence to the theory of defilade, which is based on the erroneous supposition that the path of projectiles is in a straight line, and, as a consequence, that artillery can only strike that which it sees, has contributed more than anything else to detain the art of defence behind the art of attack; indeed engineers of great weight were wont to maintain that, if the cordon of the escarp were in the plane passing through the crest of the glacis, the escarp was perfectly covered from artillery fire. Cormontaigne and Montalembert have fallen into this error, and so in more recent times have Choumara and later writers upon the Polygonal System.

The increase in the weight, range, and energy of projectiles, the greater accuracy of fire, not only direct, but indirect, curved, and vertical, the enormous increase in the numbers of armies, and the wonderful mobility afforded to them by the development of railroads and steam vessels, by the subordination of steam, machinery, telegraphy, and other scientific applications of modern life, to military purposes, have rendered necessary fortresses of an enlarged and of a more expensive nature, and have led to the adoption of a system of defence more simple, and based upon broader principles.

N.B.-The penetration of the siege howitzers is comparatively small. The lowest effective velocities against revetments are considered to be from 450 to 500 feet a second.

can now be destroyed by the fire of distant artillery, and the faces of the bastions themselves can be ricochetted from an equal distance.

Moreover, this trace, in which the relations between the length of the front, the depth of the ditches, and the command of the enceinte are obligatory, is difficult of application, and weak in its direct artillery defence, inasmuch as a portion of the rampart which should be available for the defence is withdrawn for the service of the flanks and curtains.

In the Polygonal Trace,-which requires no inter-relation between the length of the front, the depth of the ditches, and the command of the body of the place, in which the exterior sides may at pleasure be longer or shorter within the limits of musketry fire, and the line of parapet may be independent of the line of ditch,—these defects disappear, and its application to every variety of ground is more easy and more economical. As, too, the flanks are entirely protected from distant fire, and the faces are not exposed, or are far less exposed, to ricochet than in the angular traces, the polygonal trace offers greater facilities for the employment of artillery as the principal arm of defence, while it has yet another advantage, viz., that its communications are far more simple.

The main difference between the Polygonal and the Bastioned Systems lies in the arrangements for the defence of the ditches. In the latter it is provided by the flanks of the bastions, in the former by caponnières, powerful casemated works, constructed across the ditches. In the flank of the bastion the guns are exposed, and limited in number by the length of the flank; if the flank be lengthened, the face of the bastion must be shortened, thereby dimiuishing the direct artillery defence of the bastion, or the flanked angle of the bastion must be diminished, rendering the facesalready too liable-still more liable to enfilade. Moreover, a second or lower tier of casemated guns cannot be added to the flank of the bastion without the suppression of the tenaille and consequent exposure of the curtain; whereas the caponnière may be increased in length to any reasonable extent, and may be in two tiers. Again, in the bastion the flank is enfiladed by direct fire on the adjoining face, and is taken in reverse by the enfilade of that face, and by the direct fire against the opposite face and flank. Lastly, in the Polygonal System a direct fire can be given upon the capital of the angle, while in the Bastioned System this capital is undefended.

The advantage of this system in direct artillery defence