The primary defences are in the co vert-way, but they are only for musquet ry ; as this part, owing to its laying very low, is subject to be enfiladed, and also because the saliant angles of the covert way are sometimes abandoned from va rious causes, there are at every forty or fifty yards parapets, whose slopes point towards the exterior, or saliant angle of the covert-way, so that cannon shot may be stopped, and the defenders may make a stand, from time to time, behind these parapets, (which are called traverses,) until at last forced into the out-works for safety from the pursuing enemy. Each traverse is made the whole breadth of the covert-way, namely, 30 feet; their exterior ends would touch the palisades, were not little inlets made " at right angles into the crest of the glacis, broad enough for two or three men to pass abreast. The traverses may be about six feet high within, and about five without ; there is a banquette within which raises the defenders about a foot and a half, for them to stand upon, and to fire over the parapet, of which about ten feet is generally the thickness. Another method of passing the ends of the traverses is not uncommon, and is, perhaps, at least equally good as the, foregoing ; this is by making a serrated line of palisades, as seen in the plate, in which the small black projections from the line of the ditch represent the tra verses, and the line bordering the glacis g, g, g, shews the line of the palisades ; not unlike the teeth of a key-hole saw. The vacant spaces +, in the re-entering angles, are for the assem bling of troops for the defence of the covert-way, and are called places of arms. In these, sometimes, small redoubts are thrown up. Places of arms are always near to some sortie from an out-work, so that the parties posted in them may be readily withdrawn, or be reinforced ; in some instances, however, places of arms are made in the saliant angles of the covert-way ; but they should then be in some measure entrenched, or pro tected, else they would be severely, and perhaps unnecessarily exposed, although the covert-way is so far above their heads.
The glacis is always made so as to give an inclined plane, corresponding with every change of direction in the line of the crest of the glacis ; not, however, adverting to the small inlets, or serrated appearance, required for passing the ends of the traverses. This will be seen on reference to the plate, where every such inclined plane is particularized. Such a disposition of the glacis is in dispensable ; it gives the true direction of every part, as it respectively stands fronting to the line of palisades ; so that the soldiers can scarcely fail to aim pro perly,if they fire straight before them, and rest their muskets on the upper rail. They thus graze ;the surface of the glacis, and consequently do great ex ecution.
An extensive defence, called a horn work, is sometimes substituted for a crown-work. The latter, as may be seen, is composed of a full bastion be tween two curtains, whose exterior sides are terminated by demi (or half) bas tions; whereas the horn-work, in lieu of expanding as it recedes from the prin cipal, contracts, and its front, (which should be parallel to that of the prin cipal when it covers a curtain therein,) is formed only of a curtain, terminated by two demi-bastions, The out-works beyond its ditch may be very numerous, though not so varied as those placed be yond a crown-work.
The object of both these defences, ge nerally, is much the same ; when a small diminishing tract is to be covered, the horn-work is proper, but when a round, or rather increasing, spot is to be enclos ed, the crown-work should be preferred. The intrenchments within a horn-work are, however, the strongest, because they oppose a larger front against a smaller one ; 'which is the reverse of what takes place in the crown-work, where the in trenchments, f f, are narrower than the front which can be opposed to them, be tween the flanked angles of the demi-bas tions b b.
The explanation of fig. 2 next demands our attention : it is the profile of the prin cipal, and of the proximate outwork. In
this, not only the defilement, but the de flations from the terre-pleine, or line of the horizon, whether by superstructure or by excavation, are shewn.
A. represents the rampart of the prin cipal, or body of the places of which the measurements may be in general terms, taken at the following computation. The height of the terre-pleine II, on which the cannon are mounted, 20 feet ; the ban quette I, on which the soldiers stand to fire their small arms, raised 3 feet above H; the point X, which is the crest of the parapet,being 41feet above the banquette I, and 7f above the terre-pleine H. The upper part of the parapet is lower with out than it is at the crest X. This decli nation, which is called the superior slope, is at the rate of one inch for every foot the parapet has of thickness, so as to al low the defenders to fire at an enemy al most close to the rampart, yet not to weak en the crest. K, spews a revetement of masonry, which should be five feet thick at the top of the rampart, not including the parapet, but measuring at the upper cordon 0. The exterior slope of the revetement should be one sixth of its height, taken from the foot to the cordon. The foundation should project in pro portion to the height, and to the nature of the soil. The interior slope of the parapet, and the banquette, are like wise bounded by a revetement in this figure, but such is not always the case ; when it is, the ascent ,to the banquette is made by two or three steps, as here shewn. Nor are all parapets faced with masonry ; the generality, indeed, are gazoned, or turfed, on account of the in calculable injury done by the splinters knocked off by such shots as graze up on masonry of any kind. In some in stances, only half revetments are used, that is, only for the scarp, or face of the ditch, as seen at M under the cordon 0 ; the whole exterior of the rampart itself being gazoned. The interior slope of the rampart, when made of masonry, as seen at P, where the counterscarp is carried up, or built upon, to form the interior slope of the counter-guard B, may be equal to only one-fifth of its whole height ; but where masonry is not used, the interior slope, as at L, of the rampart A, should, if the soil be firm, be equal to the height of the rampart, which would give an angle of 45 de grees: when the soil is sandy, crumbly, or apt to give way, the interior slope should be equal to a height and a half, or even more, if circumstances should require. The continuation of the revete ment M, above the cordon 0, which is level with the terre-pleine of the berm N, is a firm parapet, made in lieu of the exploded fausse-braye, to prevent the ruins of the rampart A, when breach ed, from falling into the ditch C; of which the breadth is indefinite, though from 15 to 25 toises may be consider ed as the limits for works, according to the mean and great systems of Vauban. About the middle of the ditch, but ge nerally rather more towards the coun terscarp P, than towards the scarp is the cunette, or cuvette, about 15 feet broad, rivetted throughout, and from ,six to nine feet deep. It is always kept full of water, where that may be prac ticable; and, as it goes entirely around the body of the place, serves to pre vent a surprise, to restrain from deser tion, also from an improper access to spirituous liquors, and as a drain to the body of the ditch. In many instances, very fine supplies of fish are obtained from the cuvette. In some fortresses it is cut off from before the curtains by rows of palisades, standing on a shelv. ing work, called a batardeau. Where it is continued before the curtains, there must be bridges of communication ; and small temporary plank passages are made over in various parts, when occa. sion may require. All ditches should be sown with good grasses, that they may give a supply of that valuable com. modity to such horses, &c. as may be kept in the fortress • and all gazoned facings, as well as the slopes of para.