TENAILLE, in Fortification, is a rampart raised In the main ditch, immediately in front of the curtain between two bastions; and in its most simple form, it consists of two faces coinciding in direction with the faces of the bastions, and, consequently, forming with each other a re-entering angle. Generally, however, it consists of three faces, of which two have the directions just mentioned, and the third forms a curtain which is parallel to that of the enceinte. See r, fig.1, BASTION, and r (in the plan), Fon-tasselnote.
This work was originally proposed by Vauban, in order to serve the purpose, in part, of a fausse-brayo [Facssit-nnave], since the fires of musketry on its faces may bo employed, in conjunction with those of artillery and musketry on the flanks of the bastions, to oppose the pas sage of the enemy across the main ditch when about to mount a breach In the ramparts of the place.
The relief of the tenaille, or the elevation of its crest above the bottom of the ditch, is determined consistently with the intention of thus defending the main ditch ; and in order that the defenders of the tensile may not be injured by the shot fired over their heads, from the flanks of the bastions, it is usual to make the crest of that work coincide with a horizontal plane passing three or four feet below the point where a line of fire from one of those flanks would cut a vertical plane, bisecting the angle of the tcnaille or its curtain. The height thus determined will allow the parapet of the work to be elevated from two to four feet above the terreplcin of the ravelin in its front; and, consequently, from the curtain of the tenaille a grazing fire of musketry might he employed to protect the interior of the ravelin, or of its reduit, if there is one, should the defenders of either of those works abandon it (in consequence of an assault being made) before the enemy has time to cover himself in it by a lodgment : that fire will also contribute powerfully to prevent the enemy from attempting to enter the ravelin by its gorge.
Vauban, at first, gave to his tenailles abort flanks nearly parallel to those of the bastions, but he soon abandoned that construction, per ceiving that though the defenders might thus fire correctly along the main ditch, yet the parapets of those flanks were liable to be destroyed by the fire from the enemy's counter-batteries [n, fig. 1, BASTION], and they were enfiladed from the rampart of the ravelin (4), or from the glacis of the places of arms (t).
Besides affording additional fires for the defence of the main ditch, the tenaille serves to cover, in part, the revetment of the curtain in its rear, and prevent it from being breached by fire from any lodgments of the enemy on the glacis. Its parapet serves also to mask the postern in the curtain of the enceinte, which would otherwise be so much exposed to the fires from the counter-batteries, that the de fenders might be unable to communicate through it with the outworks.
On this account the breadth of the ditch between the curtain of the tenaille and that of the enceinte is made such only as to allow the parapet of the former, with the relief determined as above-mentioned, to conceal the postern from the view of the enemy on the glacis. This ditch is advantageous in preventing the defenders of the tenaille from being injured by the splinters which may be detached from the flanks and curtain behind it ; and, when dry, it serves to cover bodies of troops which may issue from thence and attack the enemy while crossing the main ditch, previously to making an assault. If the main ditch contains water, the tenaille serves to cover the boats and rafts by which the defenders of the enceinte communicate with the outworks.
The tenaille has been considerably improved by Bousmard, who, returning, in one respect, to the original idea of Vauban, has given flanks to the work in order that the main ditch may be directly de fended by them. These flanks are raised high enough to cover the revetments of the flanks of the bastions, while their upper surfaces may be grazed by a fire of artillery from thence; and, instead of being formed with open terrepleina, and parapets for musketry, as usual, each flank of the tenaille is provided with casemates, or vaults, for four pieces of artillery which are placed nearly on a level with the terreplein of the covered-way. These guns are consequently capable of being directed against the counter-batteries (e) of the enemy, as well as of defending the foot of a breach in the face of the bastion.
This construction was adopted by Chasseloup de Laubat in the tenailles of the detached works which he executed about Alessandria, In Italy, when Napoleon (after the battle of Marengo) proposed to make that city the base of his operations beyond the Alps. But, in order to avoid the mischief which results from a fire directed against casemates (the shot in striking the cheeks or sides of the embrazures detaching from them splinters, which being driven into the vault do more injury to the defenders than the shot itself), this engineer raised before each flank of the tenaille a mass of earth which was reveted with brickwork, and perforated in such directions that, in defending the ditch, the shot from the casematea could be fired through the apertures, while the mass served as a mask which would prevent the enemy from seeing the embrazures in the flanks of the tenaille.
Any work belonging either to permanent or field fortification, which, on the plan, consists of a succession of lines forming salient and re entering angles alternately, is said to be d tenaille.