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of the tenaillons, 10 fathoms from the front, and 15 long; and sometimes also a retrenchment, IH, which has a rampart, parapet, and a small ditch of 3 fathoms in front, and a banquette of 8 feet between the parapet and ditch, called a berm. It is perpendicular to the side FG of the tenaillons at its middle point H.
The tenaillons are seldom used unless when the ravelin is small; when they are used, the salient angle of the ravelin is larger than usual, which arises from making its capital smaller. The tenaillons also cover the shoulders of the bastions; but a large ravelin is considered preferable.
The outworks of the citadel of Tournay consist of ravelins with tenaillons, and ravelins with lunettes.
Construction of Retired Flanks and Orillons.
262. A retired flank, as the term expresses, is a flank of a bastion erected farther inwards than the common flank, and it is of a concave form outwards, and is defended by the projecting shoulder, which is made convex outwards, and called an orillon (last figure).
Let SF be the common flank; take Sr = one-third of FS; from the flanked angle B draw BrT, BFR, making rT and FR each 5 fathoms; on TR describe the equilateral triangle IRT, and from I, with a radius IR, describe the arc RT, and it will be the outline of the flank. Bisect Sr by a
perpendicular OV, and from S draw OS perpendicular to the face AS, and from O, with a radius OS, describe the arc SVT, and it will be the outline of the orillon; and describe the orillons and retired flanks of the bastion B in the same
The orillons serve to defend the retired flanks; but the latter would be less expensive and more convenient if constructed on the line of the chord TR, instead of being con
263. Counterguards are works similar to a ravelin, and are sometimes, though seldom, placed before this work, so as to encompass it in the same manner as it does its enclosed redoubt. Counterguards are more frequently erected on the opposite side of the ditch before the bastions, with their sides parallel to the faces of the bastions, in order to cover them, and thus to strengthen their defence (fig. art. 281).
264. A hornwork is a work of a quadrangular form, GHTK, constructed sometimes around the ravelin, and sometimes before the bastion.
The construction is:-On the produced capital of the ravelin lay off LM = 80 fathoms; draw HMT perpendicular
to LM, and make MT, MH, each 55 fathoms; make MN 18 fathoms, and then construct the front HT in the same manner as the front of the body of the place, making the faces HP, TQ = 30 fathoms; then the branches HG, TK, are made, so that their directions produced will fall within 5 fathoms of the shoulders of the bastions A and B. The ditch before the hornwork is 12 fathoms.
There are sometimes retrenchments made within the hornwork, as at I, I; and sometimes a ravelin is constructed in front of its curtain, with a capital of 35 fathoms. Hornworks are sometimes erected before the bastions.
Hornworks are liable to several objections: they are expensive; they require a great number of men for their defence; their salient angles are not easily defended from any other work; and when they have no ravelin, they present a nearly straight front to the enemy, which is always most difficult to defend, and therefore weakest.
265. A crownwork is an outwork of a form somewhat
similar to a hornwork, only the side next the field consists of two fronts instead of one, as RMNPS.
Produce the capital of the ravelin to N, and from its salient angle O, with a radius = 120 fathoms, describe an arc passing through M, N, P; lay off on this arc two chords from N, each 110 fathoms, and MN, NP, are the exterior sides of the fronts of the work, which are to be constructed exactly as in the hornwork, except that the branches MR, PS, terminate within 25 fathoms of the shoulders of the bastion. The breadth of the ditch before the crownwork is 12 fathoms.
Crownworks also sometimes have ravelins before their fronts, and they are constructed sometimes before the ravelin, and sometimes before the bastion.
The same objections apply to them as to hornworks, and more forcibly; and these works are consequently seldom used, unless to defend some large and important position near the fortress.
Places of Arms-Traverses-Caponiers-Arrows. 266. On the covert way at every re-entering angle of the counterscarpe of the ditch is a place of arms R, which is a
quadrilateral space on the same level with the covert way, and from which sallies are made during a siege through the sallyports S, S, which have gates that are shut during a siege.
Set off from the angle m to n and v 20 fathoms; and
from the centres n and v, with a radius of 25 fathoms, describe arcs cutting in t; then tn and to are the faces of the place of arms. The glacis is made of the usual breadth (256) beyond the place of arms.
267. Traverses are short parapets made across the covert way, to defend it against an enfilading fire, that is, a fire directed along the inside of a work at a small angle. In the adjoining figure, BPS is a plan, and CEF a section of a traverse across a covert way, and APSG is a passage between it and the glacis. The length of the traverse is equal to the breadth of the covert way; its breadth is 3 fathoms, and its height is equal to that of the crest of the glacis ; and the passage between it and the glacis is 6 or 8 feet broad.
In the figure to article 266, n, v, are two traverses in a line with the faces of the place of arms; and there are similar traverses situated before every salient angle of the fortress, which lie in the direction of the faces produced, as x, r, and y, z.
Caponiers are communications between different parts of the works, of 10 or 12 feet in breadth, covered at both sides by a parapet, which terminates in a broad slope like a glacis. When the ditch is dry, the passage between the body of the place and the ravelin is by the caponier PN. A traverse, made in a caponier, is called a tambour, as O in OU.
Caponiers are also small parapets erected at the side of the ditch before the ravelin, to dispute the passage of that ditch when the enemy threatens to cross it.
An arrow is a work erected at the salient angles of the glacis, at the farther side of it, OWUW. The faces of the arrow, UW, are 40 fathoms long, and coincide in direction with the outer side of the glacis, and are composed of parapets 3 fathoms thick, with a ditch before it 5 fathoms broad,