FORTIFICATION.

484

ground falls beyond the fortification, which might be advantageousto an enemy in a siege, or to cover some gate or entrance into atown.

Construction of Covert-Ways and- Glacis.

Although we have not hitherto mentioned the covert-way, ne-vertheless all fortifications whatsoever have one ; for they are es-teemed to he one of the most essential parts of a modern fortifica-tion ; and it is certain,the taking of the covert-way,w lien it is in goodcondition and well defended, is generally the most bloody actionof the siege. After having constructed the body of the place, andall the outworks which are thought necessary, lines are drawn pa-rallel to the utmost counterscarp of the ditches, at six toises dis-tance from it; and the space m n, m n, Plate LXXV, fig. 1, in-cluded betw een that line and the counterscarp, will be the covert-way ref|iiired. There is in every re-entering angle of the counter-scarp a place of arms, in: which is found by setting off 20 toisesfrom the re-entering angft a, on both sides from a to b, and from(/toe; and fro; n the points b, c, as centres, arcs are described witha radius of 25 toises, so as to intersect each other in d ; then thelines drawn from this intersection to the point b, c, will be thefaces of the places of arms. If lines are drawn parallel to the lineswhich terminate the covert-way, and the places of arms, at 20 toisesdistant from them, the space .r, x, x, between these lines and thosewhich terminate the covert-way, will be the glacis. At the ex-tremities of the places of arms, are traverses made, such as e,w Inch serve to inclose them ; these traverses are three toises thick,ami as long as the covert-wav is broad, and a passage is cut in theglacis round them, of about six or eight feet, in order to have afree communication with the rest of the covert-way. There arealso traverses of the same dimensions before every saliant angle ofthe bastion and out-works, and are in the same direction as thefaces of those works produced ; and the thickness lies at the sameside as the parapets. The passages round these last traverses arelikewise from six to eight feet wide. In each place of arms aretwosallv-ports :, which are lOor 12 feet wide, for the troops tosally out; in time of a siege they are shut up, with barriers ofgates.

Construction of Arrows qnd detached Redoubts.

An arrow is a work made before the saliant angles of the glacis, suchas A, fig. 1 It is composed of a parapet of three toises thick, and 40long ; and the ditch before it five toises, terminating in a slope atboth ends. The communication from the covert-way into thesearrows is four or five toises wide ; and there is a traverse, r, at theentrance, of three toises thick, w ith a passage of six or eight feetround it. A detached redoubt is a kind of work much like aravelin, with Hanks placed beyond the glacis ; such as B ; they aremade to occupy some spot of ground which might be advantage-ous to the besiegers; likewise to oblige the enemy to open theirtrenches farther off than they w ould do otherwise. Their distancefrom the covert-way ought not to exceed 120 toises, that it may bedefended by imiskct-'liot from thence. The gorge ah is40 toises;the flanks a c, bf, which are perpendicular to the gorge, 10; amithe faces c d, / d, 30: the {fitch before it is six toises, ending inslopes at both ends; the covert-way four; the branches of the co-vert-way 42 toises long, or thereabouts ; the faces of the places ofarms y, if, which are perpendicular to the branches, 10; and theother, w hich is parallel to them, 14. The communication fromthe covert-way into the redoubt, is five or six toises wide; a tra-verse is made just at the entrance, and another in the middle whenit is pretty long. The parapets of this communication terminatein a slope or glacis. If these redoubts are above 50 toises distantfrom the covert-way, the besiegers carry their trenches round, andenter through the gorge ; by which means the troops that are intli.-m are made prisoners of war, if they do not retire betimes ; toprevent which, some other outworks should he made to supportthem.

Construction of Second Ditches and Covert-Ways.

Plate LX XV, fig. 2. When the ground is low, and water to hefound, there is often a ditch about 10 or 12 toises made round theglacis; and opposite to the places of arms are constructed lunettes,beyond the ditch : such as D, whose breadth on the counterscarpof the ditch is 10 toises, from b to a and from c tod;, and the facesa L, dL, are parallel to those of the places of arms ; the ditch be-

fore them is from eight to ten toises wide. The second covert-wayis four toises, (he semi-gorges of the places of arms, m, about 15,and the faces perpendicular to the counterscarp ; the second glacisis from 15 to IS toises broad. This second covert-way has tra-verses every where, in the same manner as the first.

Construction of Profiles.

A Profile is the representation of a vertical section of a work ;it serves to shew those dimensions which cannot be represented inplans, and is necessary in the building of a fortification. Profilesare generally constructed upon 30 feet to an inch. It would beendless to describe all their particular dimensions; we shall there-fore lay down the principal rules only, given by M. Vaudaq, onthis subject. livery work ought to he at least six feet higher thanthat before it, so that it may command those before it; that is, thatthe garrison may lire from all the works at the same time, withgreat and small arms, at the besiegers in their approaches. Not-withstanding this specious pretence, several authors object againstit. I'or, say they, if you can discover the enemy from all theworks, they can discover, by the same reason, all the works fromtheir batteries; so that they may destroy them without beingobliged to change their situation, and thereby dismount all the gunsof the place before they come near it. Blit if all the works wereof the same height, those within cannot be destroyed, till suchtime as those before them are taken : guns might he placed in theco\ert-way and outworks to obstruct the enemy’s approach ; andwhen they come near the place, they might be transported into tiieinner works: and as the body of the place would he much lower,the expeuce would he considerably diminished. But when worksare low, tliev are easily enfiladed by the ricochet batteries, w hichis a kind of firing witli a small quantity of powder, by giving thegun an elevation of 10 or 12 degrees: this might, however, bepartly prevented, by making the parapets near the saliant angles,for the space of eight toises on each side, five or six feet higherthan the rest of the works. 2. The covert-way should be lowerthan the level giound, otherwise the body of the place must heraised very high, especially where there are several outworks -. thisis to be understood only when the works exceed each other inheight, otherwise it need not he below the level. 3. Tile hares ofall inward slopes of earth should he at least equal to the height, ifnot more. 4. The bases of all outward slopes of earth, two-thirds oftheir heights. 5. The slopes of ail walls or revetments should beone-lifth of their height; or one-6th might perhaps be sufficient;the height of a wall is estimated from the bottom of (lie ditch, andnot from the beginning of its foundation. 6. The slopes of all pa-rapets and traverses are one-sixth of their breadth ; that is, threefeet towards the field ; or the inside, where the banquettes shouldbe three feet higher than the outside. 7. \4 lieu the revetment ofa rampart goes quite lip to the top, four feet of the upper part is avertical wall of three feet thick, with a square stone at the top ofit projecting six inches; and a circular one below, or where tlieslope begins, of eight or ten indies diameter: they go quite roundthe rampart, and the circular projection is called the cordon.Where the straight part of the wall ends and the slope begins, thewall is always made live feet thick; and the counterforts or but-tresses reach no higher than that place. 8. When the rampart ispartly walled and partly turfed, then one-tilth of the height whichis turfed must be added to five feet, to get the thickness of the wallabove. And having the thickness ot any wall above, by ad-ding one-fifth of its height from the bottom of the ditch, the sumwill lie the thickness of the wall at the bottom ; but if a sixth partis only taken for the slope, then a sixth part must be added. Forinstance, suppose a rampart of 30 feet high from the bottom otthe ditch, and that 10 of these are to he turfed, then the fifth partof 10, which is 2, added to 5, gives 7 for the wall above;and as this wall is 20 feet high, the fifth of which added is 4,and 4 to the thickness 7 above, gives 11 for the thickness nearthe foundation. In fig. 1, Plate LXXIII, AL represents thehorizontal line of a profile, in which the principal elevations atmdepressions are exhibited as explained in the definitions; fig. 2, 3,and 4"represent, in military perspective, the profiles of the bodyof a place, the ravelin ami covert-way ; which gives a clear ideaof what is meant by a profile, and from which those of all otherworks may be easily conceived.

OF