Fraises and Stockades represent another form of additional defense, and are stout posts driven horizontally or perpendicularly into the earth, in long close rows. The stockade forms likewise. at times, a good substitute for the parapet itself, particularly when the direct fire of artillery is unlikely to be brought against it, as in warfare with barbarous tribes, or in a work at the very crest of a steep hill. In this case it is usually constructed of two rows of strong palisades firmly imbedded in the ground: the outer nearly a foot square, planted with three-inch intervals between; the second about six inches in diameter, closing these spaces behind. Every second small palisade is out short a few inches, so as to leave a loophole for musketry-Sre.
CoNsTrtronox OF TEE PARAFET.—The Object of the parapet being to defend, or defilade a certain portion of ground behind it, its height must be calculated so that mis siles passing across its crest shall fail to strike the troops mustered behind. The minimum width deffiaded to allow of safe communication for troops behind, and actually defending, is 30 ft.; but if the men have to be drawn up in line, not less than 90 ft. will suffice. To ascertain the height of parapet necessary in any case, three points are selected: first, the place at which the parapet is to be constructed; second, the most ele vated spot accessible to the enemy and within range; third, a point removed from the parapet at a distance sufficient to protect the troops. If, now, a line be drawn con necting points 6 ft. over the second and third positions, and a perpendicular be erected at the site of the parapet, the intersection will give the height of the parapet.
From this, the disadvantage will be apparent of constructing a parapet within range of higher ground, as for every extra foot of elevation in the commanding rise a propor tionate addition must be made to the height of the parapet. In practice, the ordinary parapet for a level is 8 ft. high, which allows for the depressed trajectory of a spending ball. See PitcJEcTinEs. If the parapet be raised on ground above the attacking posi tion, it may be lowered, according to the angle, to about 6 ft. 6 in., the height necessary for a man standing up to be thoroughly protected. On the other hand, if the position be lower than the point occupied by the assailant, the parapet must be raised; as 12 ft. forms the limit to which a parapet can conveniently be thrown up, further height necessary for protection is obtained by sinking the ground to be defiladed before the parapet's base. In measuring for these heights, the instruments used are boning-rods, which are fixed in the ground at the point favorable to the enemy and at the limit of the ground to be pro tected by the parapet, with the normal height of a man marked on them; a third rod, at the place where the parapet is to be erected, is then marked at the point where the line of sight between the normal points on the two others intersects it, and so shows the height of the parapet.,
The foregoing parapet has been provided only as a straight breastwork, deriving its safety solely from its own fire in a direct line upon the besiegers; but in practice such a rampart would be exposed to the disadvantage of holding but little command over the scalp or escarp (part cut away) at its foot; so that, if approached under cover, an enemy could readily lodge himself therein. To guard against this, a work is flanked, so that the fire of one part shall take in flank an enemy advancing against another part. In a flanked defense of this sort, the angles which project towards the country are techni cally termed salient angles; while those which extend inwards are called re-entering angles. The flanked parapet has often, likewise, the power of defilading larger spaces than the simple line of parapet, since the salient angles can, perhaps, be brought on elevated ground; while the re-entering angles, though with less elevation, may in some degree compensate that defect by greater distance from the front. A disadvantage of flanked defenses is, that the hostile fire crosses the parapet at a less angle than in the straight line, and may, therefore, be more deadly; indeed, the object of the assailant will always be to obtain an enfilade fire along one or more parapets of the defense. To avoid this, the engineer who constructs the works must ascertain minutely the elevation of the surrounding points, and make his salients at such angles that the prolongation of his parapets towards the enemy shall always fall on low ground,whence no command can be obtained.
Now, where the salient angle becomes somewhat acute, and there is an enemy on both fronts, the soldiers defending the right parapet, and standing on its banquette, would be exposed to a reverse or back fire from the enemy in front of the left parapet, beyond the defilading of which they would doubtless be. As a remedy, an internal parapet, called a traverse, or, from its duty, a parados, is raised between the parapets of the salient, its height being determined on precisely the same principles as were made use of in regard to the original parapets.