Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/442

 428 FORTIFICATION Fi plane will determine the height of the parapet, the crest of which will necessarily be in it. Where the parapet is continued not only on the flanks but also in rear, so as to form an inclosed work, it may often be necessary to defilade it in various directions as in fig. 25. Where two traverses or parados cross each other, they must, of course, be so placed that they shall not only completely defilade the whole interior space of the work, but shall secure from reverse fire the banquette on each side, the normal N being therefore at least 6 ft. 6 in. above the banquette. Where traverses of this kind become necessary, the engineer must take in to account the space they will occupy, and plan his vork accordingly ; and should he be able to render the difficulty of attacking one side of his works very great, he may construct the traverses so that they may be used as retrenchments, and thus increase the means of defence ; for example, S being the salient of greatest strength, bob might be defended, and then bed. This subject has been enlarged upon because it is of much importance in military engineering, as the safety of a long line of works may be endangered by defective defilade. Though considered here in a practical form, it depends entirely upon geometrical principles, and instruction in descriptive geometry is therefore essential in all schools of military engineering. Having determined the relief of the crest of the para pet in reference to the plane of site, all the other vertical dimensions follow from it, as shown in several of the preceding figures ; whilst the horizontal dimensions are determined by the thickness necessary to resist the enemy s missiles and by the slopes required to ensure stability. The Penetration at a mean range into common earth after it has been dug up and well-rammed and into other materials is as stated below, and to these dimensions one half should be added for the thickness for security. English Field Artillery. Field Guns. SI.L.R. Penetration at 1000 yards with common shell. Remarks. Nature. Earth. Soft Wood. Brick in Mortar. 16 Prs. 9 Prs. Shell. ft. in. 4 6 3 6 ft. in. 3 2 3 ft. in. 1 9 1 6 These shells break up on hard ma sonry. The pene tration into con crete is about 7 Prs. &quot; 2 10 6 two - thirds that into brick-work. English Rifled Small Arm. Weapon. Projectile. Penetration at 300 yards. Inches. Earth. Soft Wood. 5 Brick. Iron. Steel. Martini- Henry. 480 grs. 14 0-5 0-25 0-13 The penetration into sand is about two-thirds the penetration into earth, and about one-half that into clay. The penetration into oak is about half that into fir. With light, sandy, or gravelly soil, or, when ramming can only be imperfectly performed, greater thickness should be allowed; and as a wide and deep ditch must always materially strengthen the work before which it is placed, there can be no reason other than want of time, want of men, or difficulty of ground, for reducing the thickness of the parapet below 14, or at the utmost 12 feet. For form ing the parapet under peculiar circumstances of difficulty the engineer will avail himself of every fitting substance which may be at hand, such as bags of wool, mattresses, fire-wood, manure heaps, and fascines, either by themselves or packed in gabions. The resistance of fascines is not great, their strength being rapidly diminished by the speedy fracture of their branches when exposed to sharp fire. With these data it will be easy to regulate all the dimensions of the parapet, the height of its crest, or the relief f the work, having been first established. Thus the banquette or step on which the men stand, when firing over the parapet, should for convenience be 4 ft. 3 in. below the crest. The breadth or tread should for a single rank be 3 ft., for a double rank 5 ft. ; the surface should slope backwards 2 inches in the 3 ft., 3 inches in the 5 ft., so ai to discharge water freely and keep the banquette dry ; the base of the interior slope of the banquette up which the men mount should be twice its height ; if the height of the parapet exceed the normal height, the banquette should have two treads or steps, the lower about seven feet below the crest, so that one rank of men may stand thereon whilst reloading the muskets of the rank on the step above them; or it should have three steps, each with a rise of 1 ft., and a tread of 1 ft. or 1 ft. 3 in., sloping slightly to the rear, by which arrangement the necessary excavation of the ditch will be diminished, and less of the interior space occupied. The interior slope of the parapet should be made very steep, so that a man firing over the parapet should be as close to it as possible ; the base should not exceed one half the height ; the superior slope, or &quot; plongee&quot; of the parapet, by which the fire is directed towards the point on which it is to act, should not be less than one-ninth, nor more than one-fourth of its thickness, and in service is generally made one-sixth ; but as the in crease of the slope facilitates the destruction of the crest, it should be kept as slight as possible. It is usual on the Continent to retain the angle of the crest, as a constant quantity, at 100, and to increase the base of the interior slope as the plunge increases, and vice versa ; but this is not satisfactory, since, the height of the soldier s shoulder remaining constant whilst the line of plunge varies, the fire will not be always in the true direction ; and it is preferable to keep the base of the interior slope as small as possible, and to make the top of the parapet at the crest horizontal for one or two feet, commencing the plunge therefrom, but reducing the top as the plunge increases. This flat top will facilitate the use of sand bags (bags fillel with earth), which are sometimes so arranged on the crest of the parapet as to form loopholes for the musketry, there by adding to the cover of the men. The base of the exterior slope of the parapet should at least equal its height, though possibly in some soils e.g., chalk the exterior slope may stand at a steeper angle than 45. The slopes of the escarp and counterscarp should be as steep as prac ticable, but generally they will be the same as the exterior slo[ e. Between the exterior slope and the escarp a &quot; berm&quot; is left. Tltis berrn is a space at least 1 ft. G in. wide and slightly inclined downwards and outwards for drainage ; it gives greater stability to the exterior slope and to the escarp, prevents the earth from the exterior slope from falling into the ditch, and is a standing ground for the repair of the parapet. Where steeper slopes are indispensable, the earth must be retained in place by a wall called a &quot; revetment,&quot; which may be formed of fascines (long cylindrical faggots), hurdles, sods, planks, clay puddling, and, in the interior of works, of sand bags. The base of the interior slope of the glacis should be equal to its height, and the exterior slope