Having established, by this exposition, the absolute necessity for adding exteri orly to the defence of the principa!, we shall now proceed to give a general in sight into the various modes of construct ing the other outworks ; all being so de signated, which do not come within the principal or body of the place. The read er should understand, that every outwork, as it is placed more distantfrom the prin cipal, must have a less elevation from the terre-pleine, or level of the area, on which the walls of the principal are found ed. Thus we find, that a line drawn from the foot of the glacis g, at 45, opposite to the flanked angle of the bastion C, in the horn-work M, carried through the cen tre all the way up to E, which is the cen tre of the polygon, should graze thb crests or inner summits of all the parapets standing in that line : this is called the de filement of the ramparts. By such a con struction it must be obvious, that every added work is a screen to, but is com manded by, that within it: thus, the bas tion C, in the crown-work M, is a screen to the raveline N, and that again to the curtain TZ v; while the intrenchmentsf correspond, in height with the flanks of the horn-work, so as to be under the command of the raveline, though they command all that portion of M which is in their front; and would continue to do so, until the beseigers should construct batteries in the gorges of the bastions 6, C, 6, or elsewhere, and render f f un tenable.
• The angle C, of bastion 4, is covered by the counterguard 7, which not only has that effect, but protects the adjacent tenaillon li, and can plonge upon the ad joining flank of the crown-work M As a still further covering to the bastion 4, a fleche 9.is added, parallel to the counter guard 7, at the foot of its glacis, serving to render the attack more tedious and difficult, by compelling the beseigers to commence their approaches at a greater distance, where they are more generally subject to the fire from the bastions, &c. of the crown-work. The fleche e. arrow-head,) should properly extend equally each way, having both faces alike, but that is not of any moment, and might have a second glacis ; it is connected with the counter-guard, or with the crown work, or with the tenaillon, by means of a sortie, or winding passage, cut through the glacis, or by a caponnier, as in the plate, intercepted with traverses, which will be duly explained when treating of the covert way.
The raveline K is defended within by the redoubt I., surrounded by a dry ditch. This redoubt should not be to» high, because it would else serve to shel ter the enemy in case they should suc ceed in silencing the faces of C 6, and m D, of the corresponding bastions 4 and 5. The small work S is a lunette, which must be carried, or silenced, before the rave line can be breached in that part ; and, indeed before any lodgment can be made opposite to the flanked angle D, of the bastion 5. The lunette must be lower
than the raveline, from which it properly derives its defilement, as will be here after explained.
The tenaillon IL is a very important conjunctive to the raveline K; it, in fact, doubles its force on that side, and pro longs the battery of its other face ; it flanks the counterguard, and its direct fire is a great protection to the demi bastion 6, on that side of the horn-work, as well as to the whole face of its rave line a; it commands the fleche ; and be ing itself commanded by the raveline K, and by the face C 6, and the counter guard 7, cannot be occupied by an ene my while any of these three works re main in force.
With respect to the construction of the counterguard,lunette, and tenaillon, they are not upon any exact scale in propor tion to the principal, as the raveline is, but though not perfectly arbitrary, their formations depend on some general rides, which should invariably be had in view. The counterguard is always placed on the counterscarp, its front immediately behind the glacis, and its rear, generally, being a continuation of the revetement of the counterscarp, so that the passage lays along its terre-pleine, or battery. This kind of work may be of any extent, that is, it may proceed from raveline to raveline without interruption ; or it may break off where it enters a lunette, a te naillon, or a redoubt ; or it may be only formed of two parallels equal in length with the faces of the bastion. On account of the number of men required for the defence of extensive out-works, counter guards are advantageously made hollow, having casements covered with bomb proofs, their parapets being solid mason. ry : their entrances, at each end, are se. cured by harriers and drawbridges; and their walls may, in places, be pierced with loopholes, through which musque try may be discharged against assail ants.
Casemates are likewise made on each side of the posterns, or arched passages through the faces of ravelines ; there are always drawbridges and barriers in such situations, as also at the cuts through the lunettes, &c. which lead through the co vert way to the esplanade, and are call ed sorties. The necessity for casemates must, generally, depend on the quantity and distance of out-works from the body of the place : it should be a rule never to place an out-work so that it could be cut off, without receiving aid from some suf ficiently strong and contiguous part. Were this neglected, the enemy would not fail to surround such ill-judged de tachments, and to a certainty carry them off during the night. Admitting this principle, the fleche 9 would be subject to the foregoing evil, if there were not a strong body of troops stationed in the counterguard 7, or the flank ofthe crown work, from which detachments could be sent without delay.