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that the polygon is at least a hexagon; but in the gle with the counterscarp of the ravelin, ast square and the pentagon, the length of the faces with that of the bastion; and such parts of wu should not exceed two-sevenths of the exterior and tu as project beyond the covert-way will side, so that the flanks may be sufficiently long. give the faces of the re-entering place of arms, alTo construct the tenaille draw bc and HI pa- lowing for the passages between the two contigurallel to the curtain, bc being five toises distant ous traverses and the glacis. from it, and HI froin twelve to thirteen toises, For the 'redoubt of the re-entering place of according to the length of the flanks of the bas- arms, set off eleven toises upon the counterscarp tions; and if, after making Ga and K d parallel irom t to r and from w to s; as well as seven to these flanks, with an interval of five toises toises and a halt from a' to b', and from c' to d', between them, ab and cd are drawn parallel to upon the collateral branches of the covert-way. GH and I K, at a distance from then equal to Then draw the interior side rg'z of the parapet that between bc and HI, the tenaille will be of the faces, in the direction of zb' and ad', and completed. Its nearest extremities to the cut off part of gʻa in order to describe the small shoulders of the bastions may be rounded off, flank fé, the length of which should be three and the interior side of the parapet broken for and a half or four toises, and its direction such as a length ef of about three toises, as the figure to allow it to see in reverse the breach of the rashows.
velin. The escarp and counterscarp of the reThe principal ditch is fifteen toises broad op- doubt are to be made parallel to the faces, three posite the flanked angles of the bastions; but toises being allowed for the breadth of the ditch; may be made narrower to advantage, provided a whilst its gorge, on the side next to the contiguous proper depth can be allowed to it, as well ravelin, should terminate upon the line Mh. as such a direction to the superior slope of the drawn from the saliant of this work through the parapet of the bastions as will render the fire extremity é of the flank fé', so as not to be exof the faces effectual upon the counterscarp in posed to the fire of the besieger from the crownfront.
ing of the saliant place of arms in front of the Set off fifteen toises from C to g, as well as ravelin. from D to h, and, supposing g and h to be joined Eighteen feet are allowed for the thickness of by a line, construct upon it the equilateral trian the traverses at the re-entering places of arms ; gle g Mh, so as to have the principal line of the but those at the saliant places of arms, as well as faces M L and M N of the ravelin; then draw the intermediate traverses, are only nine feet Pk and Pl parallel to these faces, directing thick, the breadth of the passages i between the them towards the interior meeting i of the pa latter and the counterscarp being six feet. rapet of the face and the flank of the bastion In regulating the inclination of any glacis, contiguous to them, and they will give the di- care should be taken that its superior surface, rection of the principal line of the faces of the being indefinitely produced towards the works redoubt.
behind, may not pass above any of their lower The gorge of the ravelin and its redoubt is to lines of fire, which are those of the artillery. be determined by means of drawing, from the Therefore, this inclination depends upon the danked angle of each collateral bastion, a line command of the said works, as well as upon Bmn passing through the extremity m of the their distance from the crest of the glacis; but, opposite face of the ravelin; that is, of the in- 'in general circumstances, it is regulated at the ierior side of the parapet. In this manner, the rate of about three inches to each toise of the gorge will be entirely secured from the fire which breadth of the glacis. the besieger might otherwise direct upon it from It is materially important that the glacis the crowning of the saliant places of arms in should have a proper inclination; for, if it be front of the bastions.
too steep, the great depression which the fire of Il respect to the flanks of the redoubt of the the garrison should have, in order to defend it ravelin, - from g, where the principal line of the will render this fire ineffectual; and if, on the conface ML of the ravelin meets the face of the trary, the inclination of the glacis is too little, the bastion, draw an indefinite line through the in- besieger's cavaliers of trenches will only require tersection k of Pk and the gorge of the redoubt; a small height; on which account they will be and, after setting off six toises and a half from k less liable to be destroyed by the artillery of the to q upon this line, describe the flank Op form- place. Cavaliers of trenches are works which ing with g9 the angle g 90 of 100 degrees. the besieger constructs upon the glacis, for the
The tch of the ravelin is nin or ten toises purpose of raising the troops who occupy them, broad, and that of the redoubt five toises ; the so that these troops may plunge their fire into the counterscarp of both these works is parallel to covert-way. their faces, excepting that part in front of the Passages c (plate VI. fig. 1) called sally-ports, saliants which is made circular: the breadtk of are cut in the glacis, which, as their name indithe covert-way is five toises.
cates, serve for the sallies; they form ramps To describe the re-entering places of arms, twelve feet wide, having a gentle slope, so as to measure off five or six toises from r to s, upon allow artillery and cavalry to pass conveniently the interior side of the parapet of the face of the through them. A curved direction is requisite bastion; and from s draw the indefinite line su, for these ramps, because, if they were straight, making with rs an angle of 100 degrees ; take the besieger would enfidale them, and destroy the distance vt, from the angle v to the intersec- the double barriers that the garrison places across tion t of su and the counterscarp, and set it off them, in the direction of the crest of the glacis, from v tu w; then draw w u forming the same an for the purpose of securing the opening into the
covert-way, which they occasion, whilst the opposite ravelins. These gates should be placed covert-way opposite this opening would also in the middle of the curtains, this part of the be much exposed. The sally ports are to be fortification being the best covered, and the least placed in the faces of the re-entering places of exposed to be breached; and they have a bombarms, and in the branches of the covert-way proof archway, which, at the exterior opening, of the ravelins between the second and third tra is twelve feet high in the clear, or a little more, verses, as these situations will allow the sallies to and about ten feet broad; wider spaces are conbe effectually protected in their retreat by the sa trived inside of it, from distance to distance, liants of the covert-way of both the bastions and where people on foot may retire when any car. ravelins. Besides the sally-ports will be better riage happens to pass.
The exterior of the ensecured than if they were in the saliant places of trances into the gateways generally has ornaarms, or near them. In those parts of the fortifi- ments of masonry, but the architecture should cations where a road d is made across the glacis, be simple, as any superfluous decorations would in order to communicate with the country, it is augment the expense, without answering any commonly placed between the traverse of a useful purpose; and it is also to be observed, re-entering place of arms (on the side towards that the masonry should not be carried up the ravelin) and the contiguous traverse of the higher than the top of the parapet, since, othercovert-way of the ravelin. The road is made wise, it would become a mark for the besieger's winding, and from eighteen to twenty or twenty- artillery: nor should any buildings be couone feet are generally allowed for its breadth, structed over the archway, although this has been the profile of the glacis on each side being sup- formerly done. ported by a wall.
In respect to the standing bridges they may Supposing a to be the middle of that end of be constructed wholly with stone, when the the sally-port (fig. 2, plate VI.), which is in the fronts where they are situated are neither liable direction of the crest of the glacis, erect at d, to be attacked, or much exposed to the effect of the perpendicular ab to that crest, and make it the besieger's shells; provision should, however, equal to eighteen feet, or any other dimension be made, in two of the central piles, for placing which the requisite length of the sally-port, ac a few mines to blow up the bridge, if, on according to the greater or smaller elevation of the count of any unexpected circumstances, it glacis above the covert-way, may render neces- should become instantly necessary to destroy the sary. Set off one toise from a to b, as well as communication. But it is preferable, under all from a to c, for the breadth of the sally-port, and suppositions, that the bridges should be so confrom c and b as centres, with cb as a radius, trived as to allow them to be speedily taken to describe arcs, cutting each other in e; then from e pieces, without encumbering the ditch with rubdescribe the arc cb. Make of equal to the base bish. In consequence, the upper part of the of the interior slope of the glacis, or a little bridges should consist of timber, and be supJonger, and describe from f and bas centres, with ported by piles of masonry fifteen feet distant fb as a radius, arcs intersecting at g, from which from each other, from centre to centre. The point, as a centre, the arc fb is to be described. length of the draw-bridges is in general twelve
In respect to the other profile of the sally-port, feet, and their breadth eleven feet. make dh equal to ad, and erect the perpendicular.
From the redoubt of the ravelin, a gate conhi of the same length as ab. Next, from d and i structed in one of its faces, with a bridge across as centres, with di as a radius, describe arcs the ditch in front, leads to the terreplein of the intersecting at m, and m describe the arc di; set opposite face of the ravelin; whence another off the length of cf from d to n, and from n and gate, also preceded by a bridge which is thrown i as centres, with ni as a radius, describe arcs across the ditch of the ravelin, serves to commucutting each other in 0; then from o as a centre, nicate with the covert-way, and from this work describe the arc ni.
with the road cut through the glacis. But neiOn the subject of the communications of this ther of these gateways is arched over, and each system, an able commentator upon it, colonel de consists of an uncovered passage made through Malortie, observes, the gates should as much as the rampart, the profile on each side being suppossible be placed on the least exposed fronts, as ported by a wall in which recesses are contrived this situation will not only render them more for the security of people on foot, if any carriage secure, but allow sallies to be conveniently made, should happen to pass. consisting either of cavalry or infantry, in order The proper situations for the ramps depend on to attack in flank and in reverse the besieger's the nature of the works, and the localities; th approaches towards some of the other fronts; are generally placed in the middle of the curtains and, whenever fronts having gates are liable to be at the gorges of cavaliers and full bastions, in attacked, it is proper that they should be flanked the flanks and faces of empty bastions, in the by works inaccessible to the besieger. Those faces of out-works, in barbet batteries, &c. In parts of the principal roads leading to the gates, short, wherever this kind of communication may which are beyond the glacis, should also be be usefully employed. enfiladed by the artillery of the place, so that The arrangement of the posterns of each front they may not serve for any purposes useful to is as follows: there is a postern underneath the
curtain, which serves to communicate from the The gates nearest to the town are those which place with the principal ditch; and, when this belong to the body of the place, and lead to the ditch is dry, another postern is made under the bridges constructed across the principal ditch, in tenaille, leading to the caponier in front. The order to gain the gorges of the redoubts in the communication from the redoubt of the ravelia
with its own ditch takes place by means of a two single pas de souris, one at each end of the postern situated under each flank, near the angle counterscarp, lead to the terreplein of the re-enof the shoulder; and the redoubts of the re-en- tering places of arms, and double pas de souris tering places of arms have each two posterns are made in the circular parts of the
counterscarp forming the communication from the redoubt at the gorge of the saliant places of arms, in orwith its ditch.
der to ascend upon their terreplein. Before explaining the usual distribution of the When the ditch of the ravelin is not so deep pas de souris, it is necessary that the following as the principal ditch, a communication from one remarks, relating to the ditches and other objects to the other is established by means of a single should be premised.
pas de souris. When the principal ditch contains water, or is In respect to the caponiers and half-caponiers : dry but very deep, a smaller depth is allowed to besides the caponier Q, which secures the comthe ditch of the ravelin, as in both circumstances munication from the tenaille to the pas de souris this ditch will be better seen from the faces of the at the gorge of the redoubt of the ravelin, a halfbastions which flank it, and therefore more effec- caponier R covers, on each side of the tenaille, tually defended; whilst, if the principal ditch is the passage from the opening between its profile wet, that of the ravelin will be kept dry, which and the flank of the bastion, to the pas de souris is another advantage.
leading to the ditch of this redoubt, opposite its The ditch of the redoubt in the ravelin is made flank. A half-caponier S also traverses the main less deep than that of the ravelin, so as to im- ditch, perpendicularly to the face of the bastion, pede the besieger in his attempt to penetrate for the purpose of covering, on the side towards into it from the principal ditch, after gaining this the saliant place of arms in front of the bastion, ditch through that of the ravelin, in order to cut the communication with the pas de souris at the off the troops which defend the ravelin. And, gorge of the redoubt in the re-entering place of as a further precaution for the security of these arms; this communication is covered on the troops in their retreat, the ditch of the redoubt other side, by a half-caponier T, placed across the is sunk a little deeper opposite the fanks than ditch of the ravelin. A half-caponier U is likealong the faces, besides being covered by half- wise constructed across the ditch of the redoubt caponiers U, which, in addition to the coupures of the ravelin, which, as nas been previously X in the ravelin, deprive the besieger of the said, serves to secure the postern, and the pas de means of plunging his fire, from the terreplein souris situated in that part of the ditch which is of the ravelin, into those parts of the ditch of opposite the flank. the redoubt where the posterns are placed. Ano In consequence of the manner in which the ther advantage that is derived from allowing a gorge of the ravelin and its redoubt is detersmaller depth to this ditch than to the ditch of mined, the besieger can see in reverse, from the the ravelin, is to render more effectual the flank- crest of the glacis in front of the bastions, not only ing defence which it receives from the faces of the the caponier Q, but also the half-caponiers R bastions.
nearest to it; and as, according to the present The ditch of the redoubts in the re-entering method of constructing all these works, they places of arms is still less deep than that of the merely consist of earth, the communications redoubt in the ravelin, by which means its flank- which they are intended to cover do not appear ing defence from the bastion on one side, and sufficiently secure. It has therefore been sugthe ravelin on the other, is improved. It has no gested that the caponier should form a permacommunication with the ditch of the ravelin, so that nent work consisting of a vaulted bomb proof the besieger cannot penetrate into it from this ditch. gallery, A, seven feet six inches high internally,
The manner in which the pas de souris are ge- and ten feet wide; this gallery should be sunk nerally distributed shall now be explained. at bottom about four feet six inches below the
A double pas de souris is placed in the middle ditch, its sides, as well as the crown of the arch, of the gorge of the tenaille, in order to mount being protected by a covering of earth. In this upon its terreplein, and there is a double one manner the caponier, besides forming a commualso at the gorge of the redoubt of the ravelin, nication of itself, will secure from the besieger's which serves to get up to the plane of site, whence reverse fire two common caponiers constructed the terreplein of this redoubt is mounted upon near it, and which, being open at top, may be by means of ramps. A single pas de souris leads covered by means of blinds, when circumstances
from the main ditch to that part of the ditch of require it. • the redoubt in the ravelin which is opposite each It has been said that Cormontaingne contrived
flank, and small ramps communicate from thence a retrenchment V for the bastions, which also with the ditch along the faces. The communi- answers the purpose of a cavalier; in order to cation with the ravelin, from the ditch of its re construct it, draw i'k' and il parallel to the doubt, consists of a single pas de souris con- faces of the bastion, at the distance of eighteen structed near the extremity of each face of the toises from them; and, at the same distance from ravelin, opposite the postern of the redoubt, be- the flanks, draw k' p' and lo', also parallel to sides a double pas de souris at the circular part them, which should be produced inwardly fifteen of the counterscarp of this work; whilst the feet beyond their intersections m' and n' with the communication from the main ditch with the prolongations Fq' and Wq' of the lines of deredoubts of the re-entering places of arms takes fence of the collateral bastions; then join p and place by means of a pas de souris, either single oʻ, and pó will represent the gorge of the reor double, which is made at the re-entering angle trenchment. The ditch is six toises broad, and of the gorge. From the ditch of these redoubts, the counterscarp parallel to i'k' and i'l.