The length of the faces is determined by that of the cur tain and the side of the polygon, but in no case ought to be more than 350, or less than 230 feet.
The curtain should never exceed 500 feet, nor fall short of 200.
The depth of the ditch or fosse generally depends on the nature of the ground, and the quantity of earth required for the rampart. Its width must also be determined by circumstances, hut ought never to exceed 100 or 130 feet. The ditch ought always to be dry when there are no out works.
The covert way is of great use in preventing the enemy from approaching too near the ditch, and obliging them to be more watchful in guarding their works, while, at the same time, it enables the besieged the more easily to de fend their outworks, as well as to collect their troops in safety when they intend to make a sally. The covert way is parallel to the counterscarpe of the ditch, and varies in breadth from 30 to 48 feet, according to circumstances. In all cases, it should be of such a width as to allow the troops to form and manoeuvre without disturbing those who may be defending it. In order to give every part of the fortifi cation a complete command of the glacis, the angle which the plane of the latter makes with the horizon, ought to be such, that if the plane were continued, it would ]mss through the highest part both of the outworks and the en
The space A' in the re-entering angle of the covered way, is called a place of arms, of which the faces or lines forming the saliant angle are from 60 to 100 feet in length. A similar space in the saliant angle of the covered way is called the saliant place of arms. Such places were found to be of great use in defending the covered wav, but were soon rendered much more so by surrounding them with a ditch distinct from the covered way, and on the escarpe of that ditch constructing a rampart for the defence of the within. In this form, the work is called a demi•lune or ravelin, as A DCB, Plate CCLVII. Fig. 2. The original design of the demi•lune was principally to defend the gates of the fortress and the bridge before them, so as to prevent either from being taken by surprise. By enlarging their size, however, they were found to lie more extensively'use ful, and they have long been considered as essentially ne cessary for all curtains. Their principal advantages arc,
obstructing the enemy's view of the bastion, and command ing the glacis, so as to prevent the erection of works in two positions very advantageous for attacking both the flanks and faces.
Some engineers have preferred placing the demi-lunes before the bastions, as better calculated from their size to retard the progress of the besiegers, and affording, f•oan their position, the means of flanking the works constructed by the enemy for attacking the faces and flanks. This po sition, however, is liable to several objections. It leave the gates, curtains, and even the flanks more exposed. The communication with the rampart is less secure, and from the difficulty of flanking them, the demi•lunes themselves are easily taken. By modern engineers, they have accord ingly been completely rejected.
In the construction of demi-lunes, some have given them their defence from the curtain, as ADC, Fig. 2. where EP, GUI are from 48 to 60 feet, and the faces of the ravelin 130 to 180. Others have taken the defence from the orillons, as 1DK, where the faces are from 200 to 230 feet. Some times also they have been constructed in the form LIE11; but all these forms are inferior to what will be described afterwards.
Demi-lunes are surrounded in the same way as the en ceinte, with a ditch and covered way, the former being from 48 to 60 feet wide.
A second enceinte, called the fausse-brave, has some times been constructed around the first, having a terae plein of from 16 to 24. feet on a level with the field, and defended by a parapet. This work doubles the fire of the enceinte, and, from its low and grazing direction, affords a better defence to the covered way and fosse. It scarce ly holds out, however, for any length of time after the gla cis is taken, and is besides liable to many objections. It assists the enemy in mounting a breach, and the deserters in making their escape ; affords shelter to the enemy's re inforcements, and covers their miners while at work. It was formerly much used by the Dutch, but in modern for tifications is seldom adopted.