Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/447

FORTIFICATION 433 the height of the barbette. The lateral slopes of the bar bette and of its ramps should be revetted whenever it is possible to obtain sods, fascines, hurdles, or other materials suited for the purpose, in order to economize space in the work, as the base of the slope may be then reduced to one-fourth or one-sixth the height ; when unrevetted they have slopes of one-sixth. The terrepleine of the barbette may require, as at C, to be covered in flank by a traverse. The mode of con structing a barbette in a salient is ex hibited in fig. 33. At any point g of the face AB raise a perpen dicular gh, either 18 or 20 feet in length, to in clude the amount of re coil; at the points thus determined raise a per pendicular to gh, and prolong it to its inter section with the other face AC at E, then set ting oiF AF on the other face equal to AE. On the capital AD set off a double perpendicular at any point i, prolonging it both ways, and mak ing ik and il each at least equal to 4 feet to represent the half breadth ef the platform on which the gun-carriage is intended to stand and move ; through the points k and I draw parallels to the capital, cutting the faces on the points m and n; join TO, n, and parallel to the line mn draw the line OP at 18 or 20 feet distant from it, when mn~PO forms the platform. Join FO and EP, and AEPOF will represent the contour or trace of the barbette. The manner in which the crest of the parapet is formed above the salient A, and the mode in which the firing may be effected in directions perpen dicular to the faces, as well as in the direction of the capital, are shown in fig. 34 conversion of stores of information to the best practical uses. In passing through a country, it requires an experienced Fid. 33. Construction of a Barbette in a Salient. Fig. 34. In the case of a partly sunken parapet, in which the por tion above the banquette is raised above and the portion below the banquette excavated below the plane of site, as in fig. 35, the barbette constructed in the hollow portion will enable the gun to fire over the parapet ; and it should be protected by forming a bonnetted embrasure, which may be sometimes made large enough, as here represented, to hold two guns. It need scarcely be added that an engineer ought to be ready to adapt any of the expedients here briefly noticed to the circumstances of the case before him, and that a mind stored with resources against any contin gency is the highest endowment of a good officer. It must be obvious that in Field Fortification even a limited know ledge of the art of war opens a wide field for the exercise of the talents and resources of engineers; but the possession of a military coup-d ceil, which comprehends at a glance the true bearing or character of objects and events is necessary for the Fig. 35. eye to seize quickly on whatever it presents calculated to prove advantageous or disadvantageous for attack or defence ; to appreciate the value of villages, inclosures, and broken ground ; to know where to dam up rivers, to scarp heights, to form abattis, trous-de-loup, and other obstacles ; to select the best situations for field-forts and redoubts, and the best sites for batteries ; and to arrange the defensive measures with reference to the number of troops disposable, so that the movements of the defenders may not be obstructed or retarded, and their communications may be short and easy. The variety of ground upon which military operations must be carried on precludes the possibility of laying down fixed rules in regard to this subject ; the accidents of the ground, and the peculiar circumstances of each individual case, must determine the extent and description of the works to be constructed, as well as the obstacles most proper to be formed for retarding the advance of an enemy. 1 But though the observance of fixed rules be imprac ticable, general principles are of universal application ; and certain maxims founded upon them hold good in the con struction of field-works as much as in the erection of the complicated works of a fortress. These maxims, which are of invariable application, are (1) that the works to be flanked must never be beyond the range of the projectiles used in the works flanking them, or, in other words, that the length of the lines of defence must never exceed the effective range of the weapons in use ; (2) that the angles of defence must be about right angles; (3) that the salient angles of all works must be as obtuse as possible ; (4) that the ditches must be efficiently flanked ; (5) that the relief of the flanking works must be determined by the length of the lines of defence ; and (6) that the works must be con structed with reference not only to the direct and immediate obstacles which they present to the enemy and to the positive effect of their fire on the approaches to them, but also to the support they derive from or afford to other works. Field-works are either open at the gorge as in figs. 36, 37, 38, 39, or inclosed all round as in figs. 40, 41, 42, 43 They are of the following kinds : Redans, or simple heads fig. 36. Tenailled heads 38. Bastioned heads 39. Redoubts 40. Star forts . . 41. Bastioned forts 42. 1 Shaw s Course of Field Fortification, p. 9 scq. Much assist ance has been derived from this useful work in the compilation of this article, as well as from the admirable treatise on Field Fortification by Fischmeister, translated by Rieffel into French, the figures of which have been freely used. IX. - 55