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that the polygon is at least a hexagon; but in the square and the pentagon, the length of the faces should not exceed two-sevenths of the exterior side, so that the flanks may be sufficiently long. To construct the tenaille draw bc and HI parallel to the curtain, bc being five toises distant from it, and HI from twelve to thirteen toises, according to the length of the flanks of the bastions; and if, after making Ga and Kd parallel to these flanks, with an interval of five toises between them, ab and c d are drawn parallel to GH and IK, at a distance from them equal to that between bc and HI, the tenaille will be completed. Its nearest extremities to the shoulders of the bastions may be rounded off, and the interior side of the parapet broken for a length ef of about three toises, as the figure shows.

The principal ditch is fifteen toises broad opposite the flanked angles of the bastions; but may be made narrower to advantage, provided a proper depth can be allowed to it, as well as such a direction to the superior slope of the parapet of the bastions as will render the fire of the faces effectual upon the counterscarp in front.

Set off fifteen toises from C to g, as well as from D to h, and, supposing g and h to be joined by a line, construct upon it the equilateral trian gle g Mh, so as to have the principal line of the faces ML and MN of the ravelin; then draw Pk and P parallel to these faces, directing them towards the interior meeting i of the pa rapet of the face and the flank of the bastion contiguous to them, and they will give the direction of the principal line of the faces of the redoubt.

The gorge of the ravelin and its redoubt is to be determined by means of drawing, from the flanked angle of each collateral bastion, a line Bmn passing through the extremity m of the opposite face of the ravelin; that is, of the interior side of the parapet. In this manner, the gorge will be entirely secured from the fire which the besieger might otherwise direct upon it from the crowning of the saliant places of arms in front of the bastions.

In respect to the flanks of the redoubt of the ravelin, from g, where the principal line of the face ML of the ravelin meets the face of the bastion, draw an indefinite line through the intersection k of P k and the gorge of the redoubt; and, after setting off six toises and a half from k to q upon this line, describe the flank Op forming with gq the angle gq 0 of 100 degrees.

The ditch of the ravelin is nine or ten toises broad, and that of the redoubt five toises; the counterscarp of both these works is parallel to their faces, excepting that part in front of the saliants which is made circular: the breadth of the covert-way is five toises.

To describe the re-entering places of arms, measure off five or six toises from r to upon the interior side of the parapet of the face of the bastion; and from s draw the indefinite line su, making with rs an angle of 100 degrees; take the distance vt, from the angle v to the intersection t of su and the counterscarp, and set it off from v to w; then draw wu forming the same an

gle with the counterscarp of the ravelin, as t with that of the bastion; and such parts of wu and tu as project beyond the covert-way will give the faces of the re-entering place of arms, allowing for the passages between the two contiguous traverses and the glacis.

For the redoubt of the re-entering place of arms, set off eleven toises upon the counterscarp rom t tor and from w to; as well as seven toises and a halt from a to b', and from c′ to ď, upon the collateral branches of the covert-way. Then draw the interior side rg'z of the parapet of the faces, in the direction of zb' and 'd', and cut off part of g'z in order to describe the small flank f'e', the length of which should be three and a half or four toises, and its direction such as to allow it to see in reverse the breach of the ravelin. The escarp and counterscarp of the redoubt are to be made parallel to the faces, three toises being allowed for the breadth of the ditch; whilst its gorge, on the side next to the contiguous ravelin, should terminate upon the line Mh drawn from the saliant of this work through the extremity e of the flank f'e', so as not to be exposed to the fire of the besieger from the crowning of the saliant place of arms in front of the ravelin.

Eighteen feet are allowed for the thickness of the traverses at the re-entering places of arms; but those at the saliant places of arms, as well as the intermediate traverses, are only nine feet thick, the breadth of the passages between the latter and the counterscarp being six feet.

In regulating the inclination of any glacis, care should be taken that its superior surface, being indefinitely produced towards the works behind, may not pass above any of their lower lines of fire, which are those of the artillery. Therefore, this inclination depends upon the command of the said works, as well as upon their distance from the crest of the glacis; but, in general circumstances, it is regulated at the rate of about three inches to each toise of the breadth of the glacis.

It is materially important that the glacis should have a proper inclination; for, if it be too steep, the great depression which the fire of the garrison should have, in order to defend it will render this fire ineffectual; and if, on the contrary, the inclination of the glacis is too little, the besieger's cavaliers of trenches will only require a small height; on which account they will be less liable to be destroyed by the artillery of the place. Cavaliers of trenches are works which the besieger constructs upon the glacis, for the purpose of raising the troops who occupy them, so that these troops may plunge their fire into the covert-way.

Passages c (plate VI. fig. 1) called sally-ports, are cut in the glacis, which, as their name indicates, serve for the sallies; they form ramps twelve feet wide, having a gentle slope, so as to allow artillery and cavalry to pass conveniently through them. A curved direction is requisite for these ramps, because, if they were straight, the besieger would enfidale them, and destroy the double barriers that the garrison places across them, in the direction of the crest of the glacis. for the purpose of securing the opening into th

covert-way, which they occasion, whilst the Covert-way opposite this opening would also be much exposed. The sally ports are to be placed in the faces of the re-entering places of arms, and in the branches of the covert-way of the ravelins between the second and third traverses, as these situations will allow the sallies to be effectually protected in their retreat by the saliants of the covert-way of both the bastions and ravelins. Besides the sally-ports will be better secured than if they were in the saliant places of arms, or near them. In those parts of the fortifications where a road d is made across the glacis, in order to communicate with the country, it is commonly placed between the traverse of a re-entering place of arms (on the side towards the ravelin) and the contiguous traverse of the covert-way of the ravelin. The road is made winding, and from eighteen to twenty or twentyone feet are generally allowed for its breadth, the profile of the glacis on each side being supported by a wall.

Supposing a to be the middle of that end of the sally-port (fig. 2, plate VI.), which is in the direction of the crest of the glacis, erect at a, the perpendicular ab to that crest, and make it equal to eighteen feet, or any other dimension which the requisite length of the sally-port, according to the greater or smaller elevation of the glacis above the covert-way, may render necessary. Set off one toise from a to b, as well as from a to c, for the breadth of the sally-port, and from c and bas centres, with cb as a radius, describe arcs, cutting each other in e; then from describe the arc c b. Make cf equal to the base of the interior slope of the glacis, or a little longer, and describe from f and b as centres, with fb as a radius, arcs intersecting at g, from which point, as a centre, the arc fb is to be described. In respect to the other profile of the sally-port, make d h equal to ad, and erect the perpendicular. hi of the same length as a b. Next, from d and i as centres, with di as a radius, describe arcs intersecting at m, and m describe the arc di; set off the length of cf from d to n, and from n and i as centres, with ni as a radius, describe arcs cutting each other in o; then from o as a centre, describe the arc ni.

On the subject of the communications of this system, an able commentator upon it, colonel de Malortie, observes, the gates should as much as possible be placed on the least exposed fronts, as this situation will not only render them more secure, but allow sallies to be conveniently made, consisting either of cavalry or infantry, in order to attack in flank and in reverse the besieger's approaches towards some of the other fronts; and, whenever fronts having gates are liable to be attacked, it is proper that they should be flanked by works inaccessible to the besieger. Those parts of the principal roads leading to the gates, which are beyond the glacis, should also be enfiladed by the artillery of the place, so that they may not serve for any purposes useful to

the enemy.

The gates nearest to the town are those which belong to the body of the place, and lead to the bridges constructed across the principal ditch, in order to gain the gorges of the redoubts in the

opposite ravelins. These gates should be placed in the middle of the curtains, this part of the fortification being the best covered, and the least exposed to be breached; and they have a bombproof archway, which, at the exterior opening, is twelve feet high in the clear, or a little more, and about ten feet broad; wider spaces are contrived inside of it, from distance to distance, where people on foot may retire when any car. riage happens to pass. The exterior of the entrances into the gateways generally has ornaments of masonry, but the architecture should be simple, as any superfluous decorations would augment the expense, without answering any useful purpose; and it is also to be observed, that the masonry should not be carried up higher than the top of the parapet, since, otherwise, it would become a mark for the besieger's artillery: nor should any buildings be constructed over the archway, although this has been formerly done.

In respect to the standing bridges they may be constructed wholly with stone, when the fronts where they are situated are neither liable to be attacked, or much exposed to the effect of the besieger's shells; provision should, however, be made, in two of the central piles, for placing a few mines to blow up the bridge, if, on account of any unexpected circumstances, it should become instantly necessary to destroy the communication. But it is preferable, under all suppositions, that the bridges should be so contrived as to allow them to be speedily taken to pieces, without encumbering the ditch with rubbish. In consequence, the upper part of the bridges should consist of timber, and be supported by piles of masonry fifteen feet distant from each other, from centre to centre. The length of the draw-bridges is in general twelve feet, and their breadth eleven feet.

From the redoubt of the ravelin, a gate constructed in one of its faces, with a bridge across the ditch in front, leads to the terreplein of the opposite face of the ravelin; whence another gate, also preceded by a bridge which is thrown across the ditch of the ravelin, serves to communicate with the covert-way, and from this work with the road cut through the glacis. But neither of these gateways is arched over, and each consists of an uncovered passage made through the rampart, the profile on each side being supported by a wall in which recesses are contrived for the security of people on foot, if any carriage should happen to pass.

The proper situations for the ramps depend on the nature of the works, and the localities; th are generally placed in the middle of the curtains at the gorges of cavaliers and full bastions, in the flanks and faces of empty bastions, in the faces of out-works, in barbet batteries, &c. In short, wherever this kind of communication may be usefully employed.

The arrangement of the posterns of each front is as follows: there is a postern underneath the curtain, which serves to communicate from the place with the principal ditch; and, when this ditch is dry, another postern is made under the tenaille, leading to the caponier in front. The communication from the redoubt of the ravelin

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with its own ditch takes place by means of a postern situated under each flank, near the angle of the shoulder; and the redoubts of the re-entering places of arms have each two posterns forming the communication from the redoubt with its ditch.

Before explaining the usual distribution of the pas de souris, it is necessary that the following remarks, relating to the ditches and other objects should be premised.

When the principal ditch contains water, or is dry but very deep, a smaller depth is allowed to the ditch of the ravelin, as in both circumstances this ditch will be better seen from the faces of the bastions which flank it, and therefore more effectually defended; whilst, if the principal ditch is wet, that of the ravelin will be kept dry, which is another advantage.

The ditch of the redoubt in the ravelin is made less deep than that of the ravelin, so as to impede the besieger in his attempt to penetrate into it from the principal ditch, after gaining this ditch through that of the ravelin, in order to cut off the troops which defend the ravelin. And, as a further precaution for the security of these troops in their retreat, the ditch of the redoubt is sunk a little deeper opposite the flanks than along the faces, besides being covered by halfcaponiers U, which, in addition to the coupures X in the ravelin, deprive the besieger of the means of plunging his fire, from the terreplein of the ravelin, into those parts of the ditch of the redoubt where the posterns are placed. Another advantage that is derived from allowing a smaller depth to this ditch than to the ditch of the ravelin, is to render more effectual the flanking defence which it receives from the faces of the

bastions.

The ditch of the redoubts in the re-entering places of arms is still less deep than that of the redoubt in the ravelin, by which means its flanking defence from the bastion on one side, and the ravelin on the other, is improved. It has no communication with the ditch of the ravelin, so that the besieger cannot penetrate into it from this ditch. The manner in which the pas de souris are generally distributed shall now be explained.

A double pas de souris is placed in the middle of the gorge of the tenaille, in order to mount upon its terreplein, and there is a double one also at the gorge of the redoubt of the ravelin, which serves to get up to the plane of site, whence the terreplein of this redoubt is mounted upon by means of ramps. A single pas de souris leads from the main ditch to that part of the ditch of the redoubt in the ravelin which is opposite each flank, and small ramps communicate from thence with the ditch along the faces. The communication with the ravelin, from the ditch of its redoubt, consists of a single pas de souris constructed near the extremity of each face of the ravelin, opposite the postern of the redoubt, besides a double pas de souris at the circular part of the counterscarp of this work; whilst the communication from the main ditch with the redoubts of the re-entering places of arms takes place by means of a pas de souris, either single or double, which is made at the re-entering angle of the gorge. From the ditch of these redoubts,

two single pas de souris, one at each end of the counterscarp, lead to the terreplein of the re-entering places of arms, and double pas de souris are made in the circular parts of the counterscarp at the gorge of the saliant places of arms, in order to ascend upon their terreplein.

When the ditch of the ravelin is not so deep as the principal ditch, a communication from one to the other is established by means of a single pas de souris.

In respect to the caponiers and half-caponiers: besides the caponier Q, which secures the communication from the tenaille to the pas de souris at the gorge of the redoubt of the ravelin, a halfcaponier R covers, on each side of the tenaille, the passage from the opening between its profile and the flank of the bastion, to the pas de souris leading to the ditch of this redoubt, opposite its flank. A half-caponier S also traverses the main ditch, perpendicularly to the face of the bastion, for the purpose of covering, on the side towards the saliant place of arms in front of the bastion, the communication with the pas de souris at the gorge of the redoubt in the re-entering place of arms; this communication is covered on the other side, by a half-caponier T, placed across the ditch of the ravelin. A half-caponier U is likewise constructed across the ditch of the redoubt of the ravelin, which, as has been previously said, serves to secure the postern, and the pas de souris situated in that part of the ditch which is opposite the flank.

In consequence of the manner in which the gorge of the ravelin and its redoubt is determined, the besieger can see in reverse, from the crest of the glacis in front of the bastions, not only the caponier Q, but also the half-caponiers R nearest to it; and as, according to the present method of constructing all these works, they merely consist of earth, the communications which they are intended to cover do not appear sufficiently secure. It has therefore been suggested that the caponier should form a permanent work consisting of a vaulted bomb proof gallery, A, seven feet six inches high internally, and ten feet wide; this gallery should be sunk at bottom about four feet six inches below the

ditch, its sides, as well as the crown of the arch, being protected by a covering of earth. In this manner the caponier, besides forming a communication of itself, will secure from the besieger's reverse fire two common caponiers constructed near it, and which, being open at top, may be covered by means of blinds, when circumstances require it.

It has been said that Cormontaingne contrived a retrenchment V for the bastions, which also answers the purpose of a cavalier; in order to construct it, draw i'k' and 'l' parallel to the faces of the bastion, at the distance of eighteen toises from them; and, at the same distance from the flanks, draw k'p' and 'o', also parallel to them, which should be produced inwardly fifteen feet beyond their intersections m' and n' with the prolongations Fq' and Wq' of the lines of defence of the collateral bastions; then join p' and o', and p' o' will represent the gorge of the retrenchment. The ditch is six toises broad, and the counterscarp parallel to i'k' and i'l.

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