The tenaille answers several important purposes. It covers the sally ports of the curtain and flanks, by which the besieged communicate with the ditch ; preserves a communication with the ravelin, and secures the retreat of those who defend it ; protects the men in the fosse when city, and the boats when wet; affords a razing fire on the enemy when crossing the ditch, and covers the re vetement of the curtain to a certain height. It is also of great use in case of a breach being attempted in the cur tain, which is generally done when the resistance is obsti nate.
Some engineers make no ditch between the curtain and the tenaille ; but this is obviously an error, as the earth, falling from the curtain into the terre-plein of the tenaille, would greatly facilitate the operations of the enemy in making a breach; and the communication with the body of the place would also be more difficult, that communica tion being chiefly by vaults, which ought to be covered by the parapet of the tenaille.
In constructing ravelins according to Vauban's method, the faces produced terminate on the angle of the shoulder, if the bastion has flanks with orillons ; but if the flanks are straight, the faces of the ravelin terminate on those of the bastion 30 feet from the shoulder. The object in both cases is to cover the flanks completely, and to give the faces of the bastion a better command of the ditch before the ravelin. The angle formed by the face of the bastion and that of the ravelin ought to be as acute as is consistent with the best defence. In Vauban's method, this angle is always obtuse. and the face of the ravelin was at first equal to one fourth of the side of the polygon. Finding, how ever, that the shoulders were still very much exposed, he enlarged the angle by extending the faces of the ravelin to one third of the side of the polygon, and making them terminate on the faces of the bastion 90 feet from the shoul ders. He sometimes also used flanks to his ravelin as op, sufficient for mounting two guns, taking ft equal to 42 feet, and p o equal to 60. These flanks, however, are of little use, as they are easily enfiladed by the enemy, and afford little protection to the curtain.
Formerly the faces of the ravelin terminated en the counterscarpe of the ditch, in the re-entering angle of which a small harbour was constructed for the boats that were used to keep up the communication between the ra velin and the place. In this form, however, the back part
was found to be easily enfiladed, and now the faces termi nate on the lines A az, B n, drawn from the saliant angles of the bastion to the end of the terre-plein.
As an inducement to hold out in defending the ravelin, which is too frequently given up without necessity, Vau ban constructed reduits within the ravelin. These con sisted at first of a single wall, two feet thick, and from six to seven feet high, and were intended to cover the gar rison in retreating from the demi-lune in case the latter were taken. They were of the same form as the (femi nine, their faces being 30 to 120 feet, and their flanks, where the ports were, 30 to 48 feet. They were separated fiom the terre-plein of the demi-lune by a ditch 12 to 16 feet wide.
When large derni-hmes were invented, reduits were con verted into second demi-lunes, having faces 100 to ISO feet, and separated from the first by a fossb. This work has been found very useful in defending the breach, as well as the passage of the ditch. The width of the ditch before the curtain is from 100 to 130 feet, round the bastion 100 to 110, and before the ravelin 60 to SO. The saliant parts of all ditches are circular, described from the saliant an gle of the work with a radius equal to the width of the ditch.
But the greatest of all Vauban's improvements was in the covered way. He constructed the places of arms with faces from 72 to 100 feet, and forming an angle with the covered way of 100 degrees. He also constructed tra verses or parapets across the covered way, pre vented the latter front being enfiladed, and secured a re treat to those who were defending it. The traverses were perpendicular to the counterscarpe, and 90 to 100 feet dis tant from each other. A coupure or passage round these, sufficient to let two men pass together, was cut out of the glacis. By these improvements, simple as they may ap pear, Vauban contributed not a little to the great object of all fortification ; an object that the older engineers some times lost sight of, that of enabling a garrison to prolong its defence.