RAVELIN, a work constructed beyond the main ditch of a fortress, and in front of the curtain between two bastions. It usually consists of two lines of rampart, which meet in a salient angle on a line drawn perpendicular to and bisecting the curtain ; and its form on the gronnd-plan may be seen at Q, Fig. 1, 13Asnos , and at Q Q, FORTIFICA• vtoN. Its profile, or the figure of a vertical section of its rampart, is similar to that of the enceinte. [BASTION, Fig. 2.] The ravelin was probably first constructed in the place of the more ancient hula= by the Italian engineers of the 16th century, when, on account of the general employment of cannon in sieges, the ancient towers and walls of masonry were either replaced or covered by ramparts of earth. Its original name, rirellino, indicates a derivation from regliare, watch ; " and both by Maggi (1584) and Errard (1594), rivellino, or ravelin, and bastion, are used as the names of a work beyond the walls of a fortified place. In some cases the rivellino appears to have been merely a parapet of earth covering a small place of arms in which were stationed the men appointed to guard the head of the bridge leading from a postern to the counterscarp of the ditch ; and a work of this kind, of a semicircular form, still exists on the exterior of the ditch on one side of Carisbrooke Castle. It can scarcely be doubted that a semicircular form was very frequently adopted for such parapets, and this circumstance may have given rise to the name of demi-lune, or half-moon, by which, even now, the ravelin is often designated. It ought to be observed however that Errard and other writers of that age apply the word ravelin to a work placed immediately in front of the salient angle of a bastion, where the counterscarp of the ditch (which is there in the form of a segment of a circle) constitutes the gorge of the work, and that the name of half-moon may, on this account have been applied to the work, though its faces were retilinear. A work thus situated is now invariably called a counterguard ; and the term ravelin, or demi-lune, is confined to the principal outwork in front of the curtain.
When the necessity of increasing the strength of fortresses by means of works beyond the enceinte, in consequence of the superior means employed in the attack, was strongly felt, the ravelin was made more capacious, and was provided with artillery ; and, in order to prevent it from being taken by surprise, its ditch was enlarged, and the covered way was continued on the exterior of the latter along both the faces of the work. Before the middle of the 17th century the ravelins were so small, that the exterior lines (the cordons) of their faces, if pro duced towards the rear, fell upon the curtain of the enceinte, and the lengths of the faces did not exceed 30 yards. Count Pagan then enlarged the works so that the produced faces fell at the shoulders of the bastions ; but Vauban apparently, in what has since been denomi nated his first system, made the faces of the ravelins about 110 yards long, and directed them towards points on the faces of the bastions at 10 yards from the shoulders (o, Fig. 1, BASTION]. The magnitude of
the work was then such as to render it capable of making a good defence : it covered the curtain and flanks of the enceinte, so that the enemy could not demolish their parapets by means of artillery in his distant batteries ; and one being placed on each front of the fortress, every two afforded not only a crossing fire on the approaches of the enemy towards the intermediate bastion, but they seriously impeded the formation of the counter-batteries on the crest of the glacis.
It was subsequently perceived that great advantages would arise if the faces of the ravelins were made still longer, and if they were directed to points at a greater distance from the shoulders of the bastions : by the first, as a reverse fire, as it is called, might be directed from the angle of the work upon the enemy's lodgments on the glacis before the bastions, he was compelled to take and make lodgments in two contiguous ravelins before he could proceed to attack the bastion ; and by the other, the power of breaching the shoulders of the bastions, that is, in rear of the most advantageous position for a retrenchment, by means of a battery on the glacis, before the salient angle of the ravelin, would be taken away from him. At Landau and other places, Vauban, without increasing the lengths of the faces of the ravelins, directed their exterior lines to points at 20 yards from the shoulders of the bastions ; while at Neuf Brisac he not only made the lengths of the faces about 120 yards, but he directed them to points at 30 yards from the shoulders. It should be observed however that at about 20 yards from the counterscarp of the main ditch he changed the directions of the faces, and made the portion between this point and the ditch nearly perpendicular to a line joining the salient angles of the collateral bastions, as in the work FORTIFICATION; by which means the second advantage, above mentioned, was lost. The inten tion of Vauban in thus giving flanks to the ravelin was that he might obtain a fire from thence on any breach formed in the face of the bastion, and that the difficulty of forming a lodgment on the glacis in front of the bastion might be increased so much as to oblige the enemy to take the ravelin before he could execute such lodgment : but ex perience has shown that this is not the fact; for the flanks, as he has formed them at Neuf Brisac, having no work to cover their pro longations, are enfiladed, and their guns dismounted, at an early period of the siege.