Tenaillcs are generally 50 feet in width, having a para pet on the esearpe, and a terre•plein of 25 feet. Under neath, they have posterns similar to those in the rampart, in the outer extremity of which may be constructed a con venient harbour for small boats, when the ditch is wet. In the gorge of the tenaille, ern each side of the postern, is a flight of stairs.
Demi-lunes are called simple when the interior is empty, and composed when they have reduits within. A simple demi-lune consists of a parapet only, or a rampart with a parapet upon it. The width of this rampart ought to be about 50 feet, which will leave 30 feet for the terre-plein, after a parapet has been constructed. The ramps of the terrc-plein are in the saliant angle. Demi-lunes are some times furnished with posterns, which serve for a communi cation with the places of arms when the ditch is wet, and for sally ports when the ditch is dry. They are generally in the saliant angle. The gorge of the demi-lune ought to have a revetement with a small harbour, and ramps or stairs.
The reduit of a demi-lune ought to have a rampart of 50 feet wide, with a parapet, and ramps at the extremities of its faces. If the ditch before it be dry, it ought to be from 9 to 12 feet higher than that of the enceinte. The stairs between the two ditches ought not to reach to the bottom of the lowest, lest they facilitate the attack of the enemy. The communication for the garrison may be com pleted by wooden stairs, to be removed when not to be used. On the ramparts of demi-limes and reduits, trees ought to be planted, as formerly mentioned, with regard to the body of the place. They render it more difficult for the enemy to effect a lodgment there. In the saliant angles of the ra velins. there ought always to he barbettes ; and the coon tersearpe should have a revetement of from 10 to 12 feet.
With regard to the communication between the higher and lower parts of a fortification, it may be observed, in ge neral, that ramps arc more advantageous than stairs, the latter being more easily destroyed. Sometimes, however, it is impossible to have ramps.
Traverses ought to he about 10 feet in thickness, with two passages, one towards the glacis, and another in the counterscarpe. To prevent the covered way from be
ing enfiladed, the glacis has a crotchet c d ef. Traverses have in general banquettes, from which the covered way is defended, and which are as high as the crest of the glacis. Sometimes the banquette is carried across to the counter scarpe, leaving only one passage at the extremity of the traverse next the glacis. In the faces of the places of arms, there are ramps to facilitate the sorties, about 10 feet wide, and so steep as to prevent their being enfiladed. These ramps ought to be at some distance from the saliant angle, where they would be too much exposed. The passages from the gates through the glacis are from 12 to 18 feet wide, the sides of the glacis being lined with walls, and the whole constructed so as nowhere to expose the cover ed way.
It has been proposed by some to cover the glacis with stones, or plant it with brush-wood, to prevent the enemy front making excavations. Perhaps the same purpose would be better acomplished by planting it with trees. This last method was adopted by the French engineers subsequently to 1792 A,iother improvement in the construction of the giacis, is to round off all the angles, except such as are saliant, by which means the places of arms are rendered more capacious, and the lines are less liable to be enfiladed. This practice w_s first recommended by Fontalard.
The postern in the places of arms which communicates with the covered way, ought to look towards the bastion, because, if it were on the opposite side, it would be ex posed to the enemy's fire, whenever they get possession of the saliant angle of the ravelin. The communication between the covered way and the ditch is formed by ramps in the counterscarpe of the places of arms.
Such is a general outline of a complete fortification ac cording to the modern system of defence. A fortified place, however, when finished, seldom presents so simple a form as what we have now described, being generally surround ed with a great variety of other outworks. The nature and use of these we shall now proceed to consider.