the interior slope being constant. In some European continental services, however, the angle, cad, 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, m, 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 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 performs the same function in fortresses of permanent nature. This outer coating is denominated a recétement. In fig. 7, ghi, is the glacis, formed during the excavation of the ditch, and having for object the bringing of an advancing enemy into the best line of fire from the parapet. The base and perpendicular of its interior slope, gh, 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, k in fig. 7, is sometimes adopted, in order that the enemy may sooner be brought under fire. It is absolutely necessary that the crest of the parapet should be five and a half 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 two feet 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 (as in fig. 7) 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 toward 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 be limited only by the trouble of raising the earth. but in practice 12 ft. is found the greatest which can be con veniently arrived at. Having ascertained the protile of the parapet, with its banquette or banquettes, bonnets, traverses, glacis, etc., it becomes a mere inatter 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 (as in fig. 7); 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 two to four ft. is made to intervene between the foot of the parapet and top of the scarp: as a precaution, it is covered with a 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.-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 carry off the water; and obstacles should be scattered about to prevent an enemy from forming his troops in the ditch. EARTHWORKS IN FIELD FORTIFICATION.-As the most readily constructed, earthworks naturally recommend themselves to the engineer, who, in the field, is called to defend the position of an army against sudden attack. Their utility has been shown in their employment from earliest times; and modern experience tends to prove that earth parapets are of all fortifications among the most difficult to overcome. An army maneuvering before a superior force, can scarcely hope to avoid battle being thrust upon it, unless, strengthened by fieldworks, it be rendered more nearly equal to the adversary. Napoleon, Marlborough, Eugène, Wellington, have given their names as witnesses to the indispensability of such works. The Russian parapets at Borodino made the French victory so sanguinary that it was useless to the victors. A few redoubts at PuĬ towa saved Peter the Great from total defeat by his formidable Swedish rival. The world-famed lines of Torres Vedras enabled Wellington with 50,000 troops, half of whom were untried Portuguese, to withstand for five months, and ultimately to drive back, the hitherto victorious army of 70,000 French, under such commanders as Masséna, Ney, and Junot. The earthworks surrounding Sevastopol partook greatly of the nature of fieldworks for the protection of a large army, and history will not forget to recount the resistance they offered for almost a year to troops equal to any in the world. For a line of works whether of earth or masonry, to be efficient, it must combine artillery fire with that of musketry. The guns will generally be so placed as to command some specific line of approach, such as a ravine, a line of abattis, or some portion of the glacis. They should themselves be as little exposed as possible, nor should the gunners be uncovered more than is absolutely requisite. To effect this, the gun is generally made to tire through an embrasure (q.v.) in the parapet, instead of over the latter. The embrasure is a cutting through the solid parapet, 20 inches wide at its inner extremity, and outward half as much as the width of the parapet. In cases where it is necessary, for proper command, that the line of fire should not be lower than the top of the parapet, the embrasure is made through an additional parapet-raised, as in the previous case of the bonnet, above the original one. The bottom of the embrasure is called the sole, and slopes downward sufficiently to allow of a certain depression being given to the gun. The remainder of a parapet below the sole is the genouillère (from genou, a knee), and in field fortification should be three and a half ft. high; the portion between two embrasures is the merlin (Ital. merlone, battlement): and an embrasure need not cut the parapet perpendicularly, an angle being admissible, when an oblique fire is necessary. When, however, the obliquity would exceed 70°, it is usual, in order that the thickness of the parapet should not be too much diminished. to form a projecting angle in it, through which the embrasure is ent Fig. 8. (as in fig. 8). The sides of the embrasures are cheeks, and require reveting. A barbette is a platform raised behind a parapet, higher than the general interior with a view to guns being tired from it over parapet. There are certain fixed rules in all fortification, such as:-1. The length of lines must never exceed musketry range, or the flanking-works would become ineffective for their object. 2. The angles of defense should be about right angles. 3. Salient angles should be as obtuse as pos sible. 4. Ditches should have the best possibie flanking. 5. The relief of the flanking-works must be determined by the length of the lines of defense. 6. The value of almost every detached work depends on the support it can give to or receive from an army or other work or works. 7. The reduction of every fortified work is merely a question of time; and a work fairly surrounded is sure to fall, unless relieved from without. Fieldworks, which, it must be borne in mind, are intended merely to support or strengthen an army, may either have a complete circuit of parapets, or be open at the gorge in the rear. The latter Fig. 9.-Redan. are, ofcourse, the simplest; but they are available only in positions which the enemy cannot turn, or |