DETAILS OF CONSTRUCTION.

*

Let A B C D be a part of the polygon,-in this case, a hexagon-having each of the sides 360 yards in length. Bisect the sides in a a a by the perpendiculars O a, O a, O a. Make the perpendicular a b equal to of the exterior side B C, and through b draw lines from the angles of the polygon. Along

these lines set off C H, BE, &c., each equal to of the exterior side. From E and H, with the radius E H, describe arcs cutting Bb, Cb, produced in G and F. Join E F, G H, F G. This being completed, on every side of the polygon we recognise a bastion trace of which (in the front BC) BE, CH, are the faces of the bastions; E F, G H, the flanks; FG, the curtain; BG, C F, the lines of defence.

The angle of the bastion at C or B is called the flanked angle; BEF or CH G, the shoulder angle; EFG or HGF, the curtain angle; BCH or CBE, the diminished angle; EFC or HG B, the angle of defence.

GK is the gorge of the bastion; and where the curtains are produced to meet in the middle of the bastion, as in bastion B, cach prolongation is called a demigorge of the bastion.

Moreover, O a is called the right radius, O B the oblique radius, and the curtain line extended both ways to meet the

*The French dimensions is 180 toises, which would be 383 yards, the toise being greater than 2 English yards by .131. But as in several English textbooks the dimensions have been rendered at 2 yards to the toise, which gives round numbers, and makes no practical difference to a beginner, we follow that example here. In the construction of the modern system which follows, the more accurate rendering of the French dimensions is adopted.

oblique radii is called the interior side. But some of these technicalities are of little or no use.

We have said that the perpendicular in this case is of the exterior side. It is plain that the flanked angle of the bastion diminishes as the perpendicular increases, and that it also diminishes as the number of sides in the polygon diminishes. Hence, in a low polygon with long perpendiculars, the salients of the bastions would be excessively acute. The rule therefore is,—to a square, make the perpendicular the exterior side, (which will give salients of about 60° opening;) on a pentagon, make it, (which will give salients of 76°;) in a hexagon or greater polygon, make it †, (which will give in a hexagon 83° for the salients, in a heptagon 92°, in an octagon 98°.)

It will be observed that the angles of defence are in a slight degree acute. In this Vauban has gone back in a measure towards the old construction of the flanks, from that laid down by Pagan. His object may be conceived to be, that the fire of the outer flank guns may bear more directly on the salient of the bastion and the probable situation of a breach. Later authorities have adopted the right-angled trace for the lines of defence.

The main ditch is 30 to 36 yards wide before the bastion. The counterscarp is represented by arcs of a circle described with a radius of that length from the salients of the bastions as centres; its outline being completed by tangents to those arcs drawn from the shoulder-angles of the collateral bastions.

The main line of rampart and parapet, as distinguished from outworks, is termed the Enceinte.

Cut 2 is an outline plan of the enceinte and outworks of one front of fortification.

The great triangular work A is called the Ravelin, or sometimes Demilune.*

* This name of Half-moon appears to have been originally applied to a sort of bonnet or detached bastion thrown out to cover the towers of old fortresses. This was generally of a cocked-hat shape with the gorge in the form of a crescent. There are, however, instances of semicircular works in the position of the ravelin, an example of which (according to the Penny Cyclopædia) exists at Carisbrooke Castle.

DF is the Covered-way, and E the glacis. B is called the Tenaille, and C the Caponière.

These works are traced as follows:

Ravelin.-From the re-entering angle of the counterscarp before the curtain, set off 100 yards along the perpendicular produced outwards; the point so fixed is the position of the salient of the Ravelin. Its faces are directed upon points on the faces of the bastions 10 yards distant from the shoulderangle.

The ditch of the ravelin has a uniform width varying in different instances from 20 to 24 yards, the counterscarp being rounded at the salient like the main ditch.

Covered-Way and Glacis.-The covered-way is 10 yards wide, extending all round the counterscarp, which forms its inner edge. Its outer edges meeting in an angle at each salient whilst the counterscarp is rounded, enlarged spaces are

This makes the ravelin somewhat larger than the outline deduced by Noizet de St Paul from actual works of Vauban. The rule he gives is, to make the face of the ravelin of the exterior side, the same as that of the bastion. Bonsmard's version of the construction still further contracts the ravelin. He fixes its salient by describing an are from the curtain angle as centre, and the distance from that point to the opposite shoulder as radius, to intersect the produced perpendicular. But this method, according to his editor, is only applicable where the face of the ravelin is directed immediately on the shoulder angle, so as to leave the whole fire of the bastion unmasqued, the flank and shoulder itself being covered by an orillon.

so enclosed, which are called the Salient Places of Arms. Places of arms are also provided at the re-entering angles of the covered-way by the following construction:-From the angle of the counterscarp b measure b c, b c, equal to 36 yards each way. Raise perpendiculars c a, c a, and from the points a a, where they intersect the outer edge of the covered-way, draw a d, a d, forming angles of 100° with that line. The space so formed, is called the Re-entering Place of Arms.

The outer edge of the covered way with its places of arms, which we have thus defined, is the crest line of the Glacis (E), the latter serving as a parapet to the covered way. slope of the glacis is 40 yards in width.

The

To protect the covered-way from enfilade fire, Vauban introduced traverses at right angles to the direction of its branches. There are usually two of these on each of the branches before the bastion, and three on those before the ravelin. The lines a c, a c, mark the crests of the traverses adjoining the re-entering places of arms. The faces of the bastion and ravelin produced to intersect the counterscarp, will determine the point at which the base of the exterior slope of the traverses adjoining the salients is to be drawn. In the long branches before the ravelin, a third traverse is inserted midway. As the traverses extend the full width of the coveredway, it is necessary to provide passages round their ends by cutting into the glacis. These passages are called Crotchets, and are either double as at e, or single as at f. They are 9 feet in clear width each way.

The Tenaille (B) has its faces on the lines of defence. The thickness of the work is 16 yards, and spaces of 8 yards in width are left between its extremities and the flanks of the bastions. These are called in French Trouées of the Tenaille.

The Caponière (C) consists of two broad glacis-like parapets, with a passage between them, running athwart the main ditch. The breadth of each slope is 20 yards, and the width of the passage from crest to crest is 11 yards. A clear way of 9 feet wide is left between the slopes of the caponière and the gorge of the ravelin.

The ravelin serves to bring a powerful fire on the ground

before the salient of the bastion; it covers in a great degree the flanks, curtain, and tenaille from distant fire; and it forms a strong additional work to be captured before a breach in the bastion can be stormed, thus compelling the enemy to employ increased force and labour on his attack. The ravelin, also, by its salience, creates two re-entering angles suited for the establishment of places of arms for the defence of the coveredway. Short flanks are sometimes given to the ravelin, in order to bring a more direct fire to bear on that part of the face of the bastion which is most liable to be breached; but this construction, applied in Vauban's outline, exposes the shoulder angle to be directly battered from the glacis.

In many old European fortresses, (and in very many places in India,) we find a second line of parapet called fausse-braye encircling and attached to the enceinte, generally on a level with the plane of site, and intended to afford a fire on the counterscarp. This, from its breaking the height of the escarp into two stages, facilitates capture by escalade; and its lines, being below the crest of the glacis, are peculiarly liable to enfilade from the enemy's batteries established there, whilst its fire upon the country is masqued by the glacis in the front. It also encroaches greatly on the space of the bastions. Hence, it has been long disused in Europe; but the tenaille, introduced by Vauban,* at Lille in 1670, may be taken as representing a partial fausse-braye. It is detached from the enceinte, so that the escarp of the latter retains its full height, and the defenders of the tenaille are not liable to be impeded and overwhelmed by splinters and rubbish detached by the enemy's shot from the upper part of the wall of the curtain. At the same time it hides more than two-thirds of the escarp of flank and curtains (with their posterns) from the fire of the enemy's breaching batteries established on the glacis or ravelin, and it gives a close fire of musketry on the interior of the ravelin. It also serves in a dry ditch to protect the assembly of troops for a dash at the enemy descending into the ditch, or for the support of the outworks; and in wet ditches it affords a covered harbour for boats. As there is scarcely any remblai * See NOTE P, the Tenaille.