FORTIFICATION 425 throw the earth from the ditch ; it is very important to keep this space clear by throwing forward or backward Fia. 13. Epaulement. the earth as quickly as it is raised. The distinguishing characteristic of an able engineer is the power of varying his appliances thus in his hands the abattis may become ths fraise, or may take the place of the palisade, as in fig. 14. If this be not borne in mind, evil rather than good Fid. 14. Use of Abattis as Fraise and Palisade. may result from adherence to systematic instruction ; as the engineer who has acquired a knowledge of one con trivance may be found crippled by his constant efforts to conform to it rather than to seek some other better fitted to the circumstances of the case. In this profile a berm is represented, as it would be difficult to arrange the abattis and to build the parapets without it. The arrangement of the trous-cle-loup, combined with stakes driven into the ground is shown in fig. 15, an ad- FIG. 15. Trous-de-loup in front of triangular ditch. vanced glacis having been formed of the earth thrown out of the excavations. The ditch is in this case triangular ; and it is scarcely necessary to add that the particular form of ditch must be determined by the nature of the ground, re membering that the contents of the ditches must supply material for the parapets; and their depth, as it adds to the difficulty of assault, should not be diminished except from necessity. After these preliminary remarks, the student should be prepared to enter on the consideration of Field Fortification. Rules for determining the Dimensions of Parapets. Determination of the Relief of a Parapet. First, where the ground is horizontal, for the protection of troops in a normal position. The minimum for a simple parapet may be here stated at 6 feet 6 inches, as a musket ball would penetrate the parapet for a few inches below its crest, and the maximum at 8 feet, a height which gives the defenders perfect security under almost every circumstance of fire, including that from mounted soldiers. Defilade. Secondly, where the ground is uneven, and it is necessary to defilade the work from a point or points which command it. Fig. 16 explains the first case, in which the points A, B, C, are on the same level, the distance AB being the space intended to be protected by the parapet at C. The line CF represents the supposed normal height at which it is presumed the assailants may tire, in this case 8 feet; BE will be the same; and AD cut by the lino drawn from F to E will be 8 feet. In fig. 17, A, B are still considered to be in one horizontal piano, but C is considerably elevated ; and hence, adopting the same data
Hftf Fig. 19. FIGS. 16- 19 illustrate rules for different heights of Parapets. as to height, and drawing the line FE and the line CB parallel to it, AD, or the height of the parapet, is equal to AI + ID, ID l)eing equal to BE, or CF, or N, the normal height. Calling also AB, or the distance to be covered, d ; AH, or the distance from the commanding point, D; and HC, or the height of C above A and B, II, we have AI : HC ::AB :HB; or AI : H ::d :d+D; and hence so that the necessary height of the parapet increases as the height of the commanding point increases, or as the distance AB to bo defiladed increases, and diminishes as the distance from the com manding point increases. Taking D = 600 ft. , d = 30 ft. , H = 60 ft. , fiO then AI = -, or 2 ft. 10 in., and AD, the height of the crest of the parapet =8 ft."+ 2 ft. 10 in. -10 ft. 10 in. ; or taking D-1200 ft., or 400 yards, then AD = 9 ft. 6 in. Fig. 18 represents A lower than B by a quantity AOGH A ; hence AD -AO + 01 + ID, and 01 - 2?. CG __ <L (II-A),or ~ . BG D + d which shows that the deeper A is sunk below B and C the more ele vated must be the parapet, and hence that this is a very unfavourable condition of parapet. For example, let A be 2 ft. below B, and all other data the same as before, then - . (H-#) = 2 ft. 9 in., and AD = 8 ft. +2 ft. 9 in. +2 ft. -12 ft. 9 in.; or when D = 1200 ft., AD = 11 ft. 4 in.; and if it should be necessary to defilade a distance of 90 ft. instead of 30, the heights of the parapet would necessarily become 18 ft. 8 in., and 14 ft. 2 in. Again, in figure 19, A is higher than B, and C is lowest of all ; and if H still represents the difference of level of A and C, and h the difference of level of A and B, then (H-A) (3); and, of course, so far as concerns the height alone of the parapet, this is the most favourable condition of all. Any other case is easily resolvable by one or other of the for mula; ; thus, when A and C are on the same level, and B higher than A, H is 0, and equation (2) becomes AD = X + A- ----- . A. D -f~ u> And in equation (3), if B be higher than A, k becomes positive, and IX. -- 54
