Page:The American Cyclopædia (1879) Volume VII.djvu/338

 330 FORTIFICATION the plan consists of a tenailled line, the reen- tering angles being between 90 and 100, and the salient angles not less than 60. The bas- tioned consists generally of two faces and two flanks, the extremities of the flanks being con- nected by curtains. A work consisting of an enceinte alone would restrict the garrison to a passive defence, and would be more or less exposed to surprise. To provide against the latter, and to enable the garrison to make a more active defence by operating on the exte- rior of the place, engineers have devised cer- tain exterior defences called outworks without the enceinte. Others have been placed within the enceinte, called interior works, more par- ticularly for the purpose of defending any breach that may be made in the main work. When an interior work is detached from the enceinte and is organized to receive the garri- son and rely on its own resources after the main work has fallen, it is called a citadel. Owing to the form and height of the parapet, its fire can take effect only at some distance beyond it. The enemy having gained the ditch will not be exposed to the fire from the works unless some arrangement has been made for this emergency. Such points where the enemy can find shelter are called dead angles or spaces. These may be removed either by arranging the lines of the work with this object in view, as in the bastioned system, or by means of auxiliary works, termed capon- meres, scarp galleries, counterscarp galleries, &c., as in the polygonal system. Bastioned System. If, supposing the place enclosed by a bastioned system, we connect the salients of the bastions by straight lines, these will form a polygon of as many sides as there are bas- tions. If the curtains be produced, they will form a second polygon within the first. They are respectively called exterior and interior polygons. Either of these may be taken as a general outline to enclose the place to be forti- fied. The exterior polygon is generally used, and sometimes called the polygon of the for- tification. It may be regular or irregular. We will suppose the site to be fortified to be level or approximately so, and the polygon to be regular. There is an intimate relation be- tween the length of the side of the polygon and the lengths of the lines of the fortification, which will be referred to hereafter. Assume the length of a side, called the exterior side, to be 380 yards. To lay off the lines of the work, some one line must be selected as the directing one. In permanent fortifications the line of intersection of the front face of the scarp wall with the top or upper surface of coping is taken, and receives the name of ma- gistral. In giving the method for locating the lines on one side or front, all the information necessary for the entire work will be known. Bisect the side of the polygon by a perpen- dicular, and lay off on it inside the polygon a distance equal to |-, ^, or of the side, accord- ing as the polygon is a square, a pentagon, or a polygon of a greater number of sides. In this particular case lay off, or 63 -33 yards; this will be the distance for the hexagon or any greater polygon. Lines drawn through this point and the extremities of the exterior side determine the directions of the faces and the lines of defence. We may assume the lengths of the faces and then deduce the flanks and curtain, or assume the curtain and deduce the others. If we take the first plan, we lay off from the salient a distance equal to f of the exterior side, which gives us the length of the face and the positions of the shoulder angles ; then draw the flanks, making an angle of 110 with the lines of defence or 100 with the curtain. Lay off on each flank a distance of 50 yards and join their extremities by a straight line. This will give the curtain, 140 yards in length. This length of curtain admits of the flanks having a relief of 44-50 ft., and at the same time thoroughly sweeping the ditch in front of the curtain by the fire from them. Let X X, fig. 2, be the exterior side; then, fol- lowing the foregoing construction, we have X Y the magistral of the face, Y Z of the flank, and Z Z the curtain. The line X Y produced to Z, the opposite extremity of the curtain, is the line of defence. From their positions it is evi- dent that an intimate relation exists between these lines ; any change in one affects all the others. The angle at X is called the salient angle of the bastion ; X Y Z, the shoulder an- gles ; Y Z Z, the curtain angles ; and X X Y, the diminished angle. From an examination of the figure, it is seen that we have now laid out the plan of the enceinte. The heavy line paral- lel to the one constructed is the interior crest. Although drawn parallel in the figure, it is not absolutely so in practice. To explain these details would extend this article to a degree that is not admissible. The other lines are easily understood by looking at the profile (fig. 1) taken on m' m' n' n'. Communication from the interior to the exterior is made by a postern through the middle of the curtain that comes out 6 ft. above the bottom of the ditch. A wooden ramp is used to descend from the postern to the ditch. In front of the curtain is placed the tenaille O, its form being a cur- tain parallel to that of the enceinte with two wings the scarps of which are on the prolonga- tions of the scarps of the faces. It is separated from the curtain by a ditch 13 yards wide, and from the flanks by ditches 11 yards wide. It is intended to mask the masonry of the curtain and flanks and cover the postern. It is ar- ranged for defence having its fire to bear tipon the ditches. The object of the double capon- mere P is to afford a secure communication across the ditch, and to be a defensive work for the main ditch. The object of the demi- lune G is to secure the gates of the place from a surprise, to mask the flanks and curtains of the enceinte from the enemy's batteries, and to give cross fires on the salients of the bas- tions. They favor sorties by the strong reen-