The principal work constructed in the bastion, as for merly mentioned, is a cavalier. The most approved form of this work is represented in Plate CCLVII. Fig. S. where E, F denote the terre-plein of the bastion 36 feet in breadth, so x y a line parallel to the faces of the bastion, and form ing the escarpe of the faces of the cavalier, from which it is 30 to 36 feet distant, and GIB the parapet. The cou mires z qv, z y are perpendicular to the faces, and 20 to 50 feet distant from where the face of the ravelin tel urinates on the face of the bastion. These coupures cover the terre-plein, from which alone the work can be taken. The communication between the cavalier and the interior., isby means of steps behind the faces, so placed as to be cover ed from the enemy's fire.
Another kind of cavalier called a barbette, is sometimes constructed in the saliant angle of the bastion, as a. ex tending about 36 feet along the faces. The terre-plein of the barbette is about 21 lower than the parapet of the bas tion ; 24 feet in breadth, and has ramps at each cod, for bringing the guns upon it. The chief advantage of the barbette is, that their fire may be directed to my point where it may be most effectual in annoying the enemy.
As the object of the parapet, in all works, is to cover the besieged on the rampart, it ought to be of a sufficient height and thickness for that purpose. It has been found that a twenty-lour pounder fired against a parapet, at a short distance, penetrates about 11 feet it, hard ground, 13 in middling, and 16 in loose earth. A parapet therefore ought to be from 18 to 20 feet in thickness. The prac tice of raising the revetement as high as the parapet is now abandoned, and the exterior talud or slope is finished with gazons or turf, having a base equal to its height, or half its height, according to the nature of the materials. The interior of the saliant angle is sometimes made circu lar, to admit a greater number of men than it could other wise do. The height of the parapet above the terre-plein ought to be 7 feet 6 inches, and the banquette about 3 feet; that is .1. feet 6 inches lower than the highest inner edge of the parapet. \Vhen the banquette is higher than 2 feet, it ought to be provided with steps for the soldiers to ascend. Its breadth should be about 4 feet, to allow two ranks of soldiers to act. As a banquette is not on all occasions useful, it may sometimes be sufficient to have a mound of earth behind the parapet, ready to be construct ed should it be necessary.
In order to give the fire of the garrison a complete com mand of every part of the field, and the enemy's works, the parapet has a slope outwards. To determine this slope, which is called the ponge, is always a problem of great importance. If the parapet be too level, the guns of the garrison cannot be brought to bear on such of the enemy's as arc very near ; and should the slope he very great, it weakens the upper part of the parapet. The plongc, that is the vertical difference between the interior and ex terior edge of the parapet, should not therefore if possible exceed 18 inches, but, at the same time, it ought to he so constructed as that the garrison may completely command the covert way and glacis.
The guns of the garrison are fired through openings in the parapet, called embrasures. These openings are not made when the parapet is at first constructed, but are ge nerally left to be cut out as the position of the enemy's batteries and other circumstances may require. The form of embrasures has undergone various changes, but that now generally adopted is from six to nine feet in width towards the field, and from 16 to 18 inches towards the place. The best method of supporting the sides of the embrasures is by means of saucissons or large fascines. Wood and stone are sometimes used, but are dangerous on account of the splinters. The part of the parapet be tween two embrasures is called a merlon. Revetements should never be carried so high as to interfere with the embrasures ; nor should trees be planted near them, as the roots become very troublesome. \Vhen, from a scarcity of earth, it becomes necessary to construct the whole or any part of the parapet of masonry, it ought to be from four to seven feet in thickness.
In former times, it was usual to carry a passage round the outside of the parapet, on the top of the revetement, from 6 to 10 feet wide, with a from three to six feet high towards the field, in which were loop-holes for the soldiers to fire through. This passage, commonly called the passage of the rounds, is now completely abandoned, having been found to afford considerable facility to the ene my in making an assault. Guerrites, or small towers be fore the saliant angles of the parapet, for centincls, have also been given up.