Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/441

FORTIFICATION 427 the soldiers standing on the banquette of one line should be secured from the fire of the ground in front of the other, which firo is called reverse fire, because it strikes them in rear. This is effected by placing a mound of earth or traverse between them, and determining its height as well as the height of the parapets in the following manner : On the commanding point C (fig. 21) is placed a boning rod CD of the normal height, and another BE of the same height at B, or at the position of the traverse ; then the height of the crest of the parapet of A is determined by the intersection of the visual line from D to E with the pole fixed at A, at the point a, which is here high, as C is so much higher than A. See preceding rules and equations. In like manner a pole of the proper height being fixed at the extent of range on the opposite side at C , the visual line from D to E determines the height, a , of the parapet of A , which is much lower than the parapet of A, as A and c are nearly on the same level. Now, to defilade the banquettes, and to determine the height of the traverse necessary for that purpose, set up on the banquette of A a pole bb of the same height as CD, C D , BE, and the visual line from D to b determines the height of the traverse at /, which is necessary to secure the banquette of A from the reverse fire of C , whilst the visual line drawn from D to b determines the height of the traverse sufficient to protect the banquette of A from the reverse fire of C. Fid. 21. Practical Measurements in Defilading. The application of these principles to a work formed of two lines (or "faces" as they are called), terminating in a salient angle is shown by fig. 22, but the application is the same whether the work is connected with a line of intrenchment as a "redan," is detached as in a "ravelin" and other outworks, or forms part of a peculiar system or arrangement of works, as in the tenaille system of Monta- lemberfc, called by its author the angular system, terms and works which will be hereafter more fully explained. Here the commanding point is supposed to be at M, and to Fig. 22. secure the defenders of the face AB from a reverse fire it is necessary to interpose the traverse cd, called from its object a " parados." The length of the traverse cd is deter mined by the line MB, beyond which it should project sufficiently to give ample security to a space about 50 feet wide behind the parapet. At the other end, the traverse is not carried up to the salient angle, as it would interfere with the communication, but is completed by la, perpendi cular to the other face, by which arrangement the space within the salient and the banquette are left free. The two lines McZB and Me/, passing through points at the FIG. 23, showing Bonnette DAD , and Traverses T, T T 2 . normal height above the banquette, determine the height of the traverse. It may, however, happen that the commands are so situated as to give an enfilade fire along both the faces AB, AB (fig. 23). In this case a small work DAD is formed in connexion with the parapet, by drawing lines parallel to the crests CB and C B at a distance from them equal to the breadth of the banquette, and then determin ing, in the manner explained, the heights at A necessary to defilade a certain length of the banquettes of CB and C B from the fire sweeping them, and assuming the greater of the two as the height of the parapet at A. This work is called a bonnette; and when the height necessary to defilade the whole of one or both faces is found to exceed 12 feet, the height of A should be restricted to that limit, and traverses T, T , T 2 should be placed at such distances as shall defilade the remainder of the work. The internal space may frequently be sufficiently defil aded by raising the salient portion of the parapet without disturbing the line of direction of the crest; but in that case em Fie. 24. the banquettes of the two faces would not be covered from enfilade fire, and hence the necessity of a bonnette. The increased height of the parapet of the bonnette renders it necessary to adopt two banquettes I, b , one below the other, and each provided with steps to facilitate ascent (figs. 23, 24). The operation of defilading may be also effected by planes of defilade ; as, for example, if the line which marks out or limits the space to be defiladed be first drawn, and a piano, be supposed to pass through a line either 6 ft. G in. or 8 ft. (or whatever height between these may be assumed as the normal height N) above the limiting line, and through a point the same height above the commanding point, this