The enemy's third trench or parallel is generally at such a distance as to touch the exterior side of the glacis op posite the bastions and ravelins.
To determine the height of the angle of the shoulder.
As the distance between the saliant angle of the de mi-lune and the third parallel or exterior side of the glacis: To the distance between the angle of the shoulder and the third parallel in the direction of the saliant an gle of the demi-lune: : So is the height of the glacis before the saliant of the demi-lunc+2 feet: To the height of the angle of the shoulder.
In the former proportion three feet, and in the latter two feet, are added to the height or command of the glacis, that the fire of the body of the place may not injure the men in the covered way. The same is done in every other work.
The command of the faces being determined, that of the flanks is made the same. The command of the curtain is a straight line joining the crest of the angles of the flanks. It may be observed, however, in general, that two or three feet, according to circumstances, are frequently added to the command determined by the preceding rules.
To determine the command of the demi-lune at its gorge, or the extremity of its face on the counterscarpe of the great ditch.
As the distance between the saliant angle of the place of arms and the third parallel : To the distance between the extremity of the demi-lune at its gorge, and the same parallel : : So is the command of the glacis before the place of arms+3 feet : To the height of the demi-lune at its gorge.
To determine the command of the saliant of the demi lune.
As the distance from the saliant of the covered way to the third parallel : To the distance between the saliant of the demi-lune and the same parallel : : So is the command of the glacis before the salient+ three feet : To the height of the saliant of the demi-lune.
When the command of the demi-lune is determined by this rule, that of the enceinte, as formerly mentioned, re quires to have two or three feet more than the above rules give. The command of the reduit within the ravelin ought to be three feet.
The preceding method of determining the profile of a place will in ordinary cases give nearly the following heights : For the saliant angle of the bastion 17i feet, and the shoulder 21 feet ; for the gorge of the demi-lune about 17i feet, and for the saliant angle 14i. The interior side,
or command of the glacis, as has been already stated, is 71, feet above the covered way, and its declivity towards the field ought to be 1 foot in 24. If the declivity be greater, it co vers the enemy's trench or parallel ; and if less, the cover ed way becomes exposed.
The revetement of the rampart ought to be equal in height to the crest of the glacis, and at least 25 feet above the bottom of the ditch. This gives 13 feet for the depth of the ditch below the covered way. Sometimes the re vetment is not carried so high as the crest of the glacis, and the escarpe above is planted with thorns, or defended by palisadoes. This is called a demi-revetement ; but it has the disadvantage of assisting the enemy in the escalade, and therefore ought not to be adopted unless the ditch can be inundated with water. For the same reason, the re vetement should not terminate at top, as it often does, with a berme or round way, which serves only to afford the ene my greater facility in fixing their scaling ladders. The thickness of the wall of the revetement should be at least 4 feet at top, and 5 if the height exceed 12 feet. At bottom, the thickness ought to be one-sixth of the height. To strengthen the revetement, counterforts or buttresses of solid masonry, from 3 to 31 feet thick, are built behind it, at the distance of from 10 to 15 feet between centre and centre, which support a considerable part of the pressure of the rampart. When the ditch is dry, a row of palisadoes is sometimes constructed in the bottom along the centre.
When counterguards, lunettes, fleches, &c. are used, their command is to be determined by the general rule, that every work most be three or four feet lower than that behind it. Detached crown works and horn works are to be considered as separate fortifications, and their profile constructed by the rules laid down for that of the enceinte. The profile of a place is represented in Plate Fig. 4.