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In this profile the dimensions are all given in feet, except the breadth of the ditch and glacis. The dimensions are written along, not across the lines, to which they belong. Thus, from m to 0 is 12 feet, from 0 to n is 11, from 0 to 1 is 18, and from 3 to 11 is 30, and 30 +18= 48 feet = 6 fathoms, the breadth of the rampart.
253. The dotted line LV represents the natural level of the ground, or plane of site, the bottom of the ditch being 12 feet below it, and the top of the parapet 18 feet above it, and that of the rampart 12 feet. The inner side of the top of the parapet is 18-(11+3)=18-144 feet higher than the outside of the top, giving a slope of 4 feet. The parapet is always 6 or 7 feet higher than the rampart, but the latter may be at a less or greater height above the plane of site, according to circumstances. In the figure (p. 153), the plane of site coincides with that of the covert way; but the covert way should generally be about 3 feet lower, to prevent unnecessary labour in raising the works. Only a part of the glacis is represented in the figure.
254. Permanent fortifications are generally of the form of a regular polygon, one side of which is called the front, as in the following figure, next page.
255. At the angular points of the polygon are projecting parts, consisting of four sides, called bastions, as A, A'. The sides of a bastion next the field are called its faces, as AE, AH; and the other two sides are called its flanks, as EF, HG; the entrance to the bastion is called the gorge, as FG; the salient angle of the bastion next the field is called the flanked angle, as A; the other two salient angles at its sides, the angles of the shoulder, or simply the shoulders, as H, E; and the two re-entrant angles, the angles of the flank, as F and G.
256. The line A A' is called the exterior side of the polygon; and the line bisecting it perpendicularly, CD, the perpendicular. The line of the bastion and curtain, AEFF'E'A', is called the tracing or magistral line. A line from the centre to the salient angle of the bastion, as AG, is called
the capital of the bastion. The capital line of any work is an imaginary line that divides it into two parts, which are equal when the work is regular. The lines AG, GC, are also respectively called the oblique and right radius.
The direction of a face of a bastion produced, as AF', is called a line of defence; for the face AE is defended from the flank E'F', which is said to be a flank defence to the former.
The most approved construction, by Vauban's method, is the following:
AA′ = 180 fathoms, CD = 30, AE = A’E'
EF' EE', and E’F = EE'.
Construction of a front.
covert way, QR= 6
mn = 8
Sometimes a fine line is made within the parapet, to represent the banquette, about 13 fathoms broad.
257. Forts are generally square fortifications, the exterior side being between 80 and 130 fathoms, the perpendicular being only about of the exterior side, otherwise the gorges of its bastions would be too narrow. Forts are intended to defend passes, to command rivers, or to guard a fortified town, in which case they are erected at some distance from the town.
The little fortification is intended chiefly for citadels, and is commonly a pentagon with an exterior side between 140 and 170 fathoms, the perpendicular being about of this side.
The mean fortification is the most common kind, and may have any number of sides not less than five, its exterior side being from 180 to 190 fathoms.
The great fortification is used only in irregular fortifications, its exterior side being from 200 to 260 fathoms. Some of the sides of a fortification are made of this kind when they could not be made less without much unnecessary expense; as, for instance, when a town lies close upon a river, the sides of the fortification at this part are of this kind; and as they are less exposed, the perpendiculars of these sides are made less, so that they can thus be constructed at less expense.
The faces of the bastions are all about two-sevenths of the exterior sides.
All works erected on the outside of the body of the place are called outworks.
Ravelin or Half-Moon.
258. Before the curtain is erected an outwork called a ravelin or half-moon, LKL' (fig. in art. 256).
The sides KL, KL', are called its faces; the parts LP, L'P, the demi-gorges; and the bisecting line PK, the capital.
The dimensions of the parts of the ravelin are:-PK = 50 fathoms, and from K draw KL, KL', so that their direc
tions produced will fall on the faces of the bastions within 3 fathoms of the shoulder. The breadth, KO, of the ditch before the ravelin is 12 fathoms, and the dimensions of the covert way and glacis beyond it are the same as when there is no ravelin.
The ravelins protect the gates of the fortress, and the bridges before them, and also prevent the enemy from erecting works in those positions that would be most favourable for attacking the bastions. In the ravelin a small work is sometimes erected exactly similar to it, called a redoubt, IkI' (fig. to art. 259), thus constructed :-Make LI = L'I' - 16 fathoms, and draw the faces Ik, I'k, parallel to the faces of the ravelin. The ditch before the redoubt is 6 fathoms broad.
259. A tenaille is a work erected in the ditch on the side next the curtain, the parapet of which is only 2 or 3 feet higher than the level ground of the ravelin.
There are several kinds of tenailles, possessing different advantages and disadvantages. One of these is represented in the annexed figure.
The faces ae, a'e', are in the lines of defence, and 16 fathoms long, with a passage of 3 fathoms between them and the shoulders of the bastions. From e as centre, with radius ee', describe an arc, and lay off on it a chord = 10 fathoms for the flank e'f', and similarly construct ef; then draw ff'.
Tenailles are intended chiefly for the defence of the ditch before the redoubt of the ravelin; they also defend with a
grazing fire the level ground of the ravelin, and the bottom of the ditch, when dry, before the curtain and bastions. They serve also, when the ditch is dry, for a place of arms from which to make sallies; and they give more security to the communication between the body of the place and the ravelin; and when the ditch is wet, they serve as harbours for boats to carry men and arms to any of the outworks, or to defend the passage of the ditch.
260. Lunettes are works erected at the sides of the ravelin, T and T' (fig. in art. 259). Their parapets and ramparts are of the usual breadth, and sometimes they have no ramparts.
They are constructed thus :- -The face MN is perpendicular to the face of the ravelin LK, and its direction produced bisects that face; and its length, MN, from the counterscarpe, M, of the ditch 30 fathoms; the other face PN meets the ditch at P, so that the demi-gorge PQ = 25 fathoms. The breadth of the ditch before it is 12 fathoms. The other lunette T is similarly constructed. Were the face PN of the lunette perpendicular to the face of the bastion, it would then have an efficient flank defence.
Another work, called a bonnet, is sometimes erected to defend the salient angle of the ravelin, K, having its sides parallel to those of the ravelin, and terminating about the points G, G'.
The lunettes are useful in sorties, and by obliging the enemy to open his approaches at a greater distance, they thus tend to prolong a siege.
261. Tenaillons are works sometimes added to the ravelin, as LFGK. The face LF is formed in a line with the face ME of the ravelin, and is made 30 fathoms from the ditch at L to F; for the face FG, take from the re-entering angle K, a distance KG = 15 fathoms, and draw FG; the ditch before it is 12 fathoms; the other two faces of the counterpart S are similarly formed.
Sometimes a retired battery, R, is formed within the front