33

FORTIFICATION.-I. a, b, Outline and profile of a lunette. Intrenchments: 2. Fléche. 3. work. 8. Quadrangular redoubt. 9. Hexagonal redoubt. 10. Triangular redoubt, wi 17. Merlons and embrasures; a, raised embrasure; b, deep embrasure, with reveted profile of a parapet. 19 c. Profile of a parapet protected by gabions. 20. Platform. 21. de-loup (Wolf-traps). 25. Palisades, with ditch. 26. Stockade, with empalement. obtuse angle. 30. Different modes of fraising. 31. Wire fence. 32. Abattis. 3 36. Fusée case. 37 a. Trap. 37 b. Percussion discharger for mine. 38. Location for m camponières covering ditch. 43. Barricade and stone mines in open ground. 47. Fortified village. 48. Tète-du-pont, or bridge-head. 49. Fortified forest. through a barricade. 53. Breast works ambuscaded with hedges. 54. Wet ditch, with on swamp. 58. Heavy breastworks (of tree-trunks) on stony ground. 59. Gabion entire, double, triple. 62, 63. Tents. 64. Cook's pit. 65, 66. Lccp-holes. 67. Plat

44.

50.

Lunette. 4. Single redoubt and shoulders. 5. Tenailled line. 6. Horn-work. 7. Crownwith semi-bastions. 11. Star-redoubt. 12. Cross redoubt. 13 to 16. Forms of intrenchments. cheeks; c, with unreveted cheeks. 18. Oblique embrasures. 19 a, b. Ground-plan and Scarp. 22. Sand-bag scarp. Obstacles: 23. Cheval-de-frise. 24 a, b. Snare and round Trous27. Commcn palisades. 28. Breach in palisade, closed by cheval-dc-frise. 29. Fraising an 3. Caltrops, or Chausee-trapes. 34. Mine. 35. Mode of conveying fire to mine (fusée). ines. 39. Stone mine. 40. Block-house. 41. Gallery "covering" ditch. 42. Redoubt, with Residence fortified. 45. Garden wall arranged for fortification. 46. Tamboured gate. Profile and construction of breast-works. 51. Passage through an earth-line. 52. Passage branches as barricades. 55. Bomb-proof earthwork. 56. Powder magazine. 57. Breast works on parapet. 60. Fascine on parapet. 61. Earthworks, one-quarter, one-third, one-half, form, with gun in place. 68. Open space. 69. Dead angle.

the next consideration. The dimensions are laid down on the ground, and depend, first, on the angle at which the material to be used will pile; and then, on the nature of the missiles against which the parapet is to afford protection. For example, an earthwork of from 3 to 4 ft. suffices to resist musketry; a thickness of 18 ft. is impervious to the 24-pounder; while larger guns can pound through even more solid obstructions.

For convenient firing the banquette should be 4 ft. 3 in. below the apex; its width 3 ft., if for a single line of soldiers; 4 ft. 6 in. for a double rank; its slope should be one in twelve, that water may run freely off. The base of the slope, up which the men mount to the banquette, should be twice its height, and cut into steps with inclined sides, to allow of easy ascent; and where the height is considerable, a supplemental banquette (on which relieving-men can, if necessary, reload), is desirable. The interior slope of the parapet should be one in four; the exterior slope, or plongée, intended for the direction of the guns on an assailant, one in six, a deviation being permitted between one in nine and one in four; but the crest being more liable to destruction as the slope is augmented, it is best to keep it as small as circumstances will allow; one in six is the ordinary slope in English F., the angle of the interior slope being constant. In some continental services, however, the angle between the top and inside of the parapet is kept constant at 100°, by increasing the deviation of the interior slope of the parapet from the perpendicular as the plunge of the exterior is greater. The flatter, however, the crest of the parapet is the better, as sand-bags are in certain cases ranged on it to form cover for the men, while they fire through loopholes left in this additional defense. Earth of medium tenacity maintains its position properly when sloped at an angle of 45°; and this is the greatest angle which can be counted on for the outer slope of the parapet. The scarp and counterscarp of the ditch need not have so great an incline, as the ground in which they are cut has usually had time, and the footsteps of ages, to consolidate it. In such cases, the base of the triangle is frequently made equal to half the perpendicular. Cases, of course, occur in which steeper banks are considered indispensable; and then, to prevent slips, the earth must have a coating to keep it up, which may be of fascines, hurdles, planks, or sand-bags, for temporary works, or those constructed in the midst of action; while the most solid masonry, called a revête ment, performs the same function in fortresses of a more permanent nature.

A glacis is formed during the excavation of the ditch, having for its object the bringing of an advancing enemy into the best line of fire from the parapet. The base and perpendicular of its interior slope should be equal; the slope of the outer face should be one in twelve, unless the slope of the ground render some different angle desirable. An advanced glacis is sometimes adopted, in order that the enemy may the sooner be brought under fire. It is absolutely necessary that the crest of the parapet should be 5 ft. higher than the crest of the glacis, as, otherwise, an assailant having reached the latter, would be able to pour a musketry fire over the former into the work. No part of any glacis, whether near or advanced, should be more than 2 ft. below the line of fire from the parapet-i.e., the line joining the crests of the parapet and glacis continued; if more depth be allowed, the enemy may advance in a crouching posture, without being liable to be hit. Advanced glacis are usually made of earth thrown up in prolonging beneath the ground the plane of the preceding glacis. They may be defended entirely from the parapet, in which case palisades or abattis are often fixed to delay the advancing enemy when at the point of greatest exposure. On the other hand, these advanced glacis are occasionally defended as a series of advanced intrenchments, and only abandoned, one by one, as the defenders are driven in towards their main work.

The dimensions of the ditch depend in some measure on the amount of earth required for the parapet and glacis; but in addition to being the mine whence the materials for the latter works are drawn, the ditch must also oppose a considerable obstacle to any hostile advance. To do this effectually, the minimum width across the top is 18 ft.; its depth need only be limited by the trouble of raising the earth; but in practice 12 ft. is found the greatest which can be conveniently arrived at. Having ascertained the profile of the parapet, with its banquette or banquettes, bonnets, traverses, glacis, etc., it becomes a mere matter of mensuration to compute the area of a section, to multiply it by the length, and so to obtain the cubic feet of earth required. With the length of the ditch known, a very simple calculation then exhibits its width and depth-a small allowance being made for the fact that the earth, dug out from the ditch, where it has probably been long compressed, will occupy somewhat more space when thrown up, and broken into clods, for forming the parapet.

The scarp, or inner face of the ditch, is most difficult of ascent by the assailant, when in a continuous line with the parapet; but sometimes it would be dangerous to construct the work with this continuity, as damage to the scarp would jeopardize the stability of the parapet. In these cases a narrow step, called a berm (q.v.) of from 2 to 4 ft., is made to intervene between the foot of the parapet and top of the scarp: as a precaution, it is covered with all possible obstacles to any lodgment being effected on it by the enemy. When a berm is employed, greater steepness is usually given to the scarp. See figs. 21, 22. The counterscarp, or outer sloping side of the ditch, should be somewhat steeper than the scarp The bottom of the ditch should slope from both sides towards the center, to