FORTIFICATION. portion of ground behind it, its height must be 마----- Fig. 4. The B parapet, since the salient angles can, perhaps, be brought on elevated ground; while the re-entering angles, though with less elevation, may in some B Fig. 5. degree compensate that defect by greater distance from the front. A disadvantage of flanked defences is, that the hostile fire crosses the parapet at a less angle than in the straight line, and may, therefore, be more deadly; indeed, the object of the assailant will always be to obtain an entilade fire along one or more parapets of the defence, as (in fig. 5) an enemy posted at F, would be able to sweep the complete line of the parapet CB. To avoid this, the engineer who constructs the works must ascertain minutely the elevation of the surrounding points, and make his salients at such angles that the prolongation of his parapets towards the enemy shall always fall on low ground, whence no command can be obtained. it is required to defilade to a height throughout equal to BC. D, D1, D2, are three points, according to the supposed country round, from which fire could be had at the parapet-one, D, being on the level, the others on ground respectively higher and lower than the parapet: if lines be now drawn from these points to C, their intersection with a Now, where the salient angle becomes somewhat perpendicular, raised on the point A, will shew acute, and there is an enemy on both fronts, the the elevation necessary for the parapet protecting soldiers defending the right parapet, and standing the space AB to the height BC. From this, the on its banquette, would be exposed to a reverse or disadvantage will be apparent of constructing a back fire from the enemy in front of the left parapet, parapet within range of higher ground, as for beyond the defilading of which they would doubtevery extra foot of elevation in the commanding less be. As a remedy, an internal parapet, called rise a proportionate addition must be made to the a traverse, or, from its duty, a parados, is raised height of the parapet. In practice, the ordinary between the parapets of the salient, its height being parapet for a level is eight feet high, which determined on precisely the same principles as were allows for the depressed trajectory of a spend-made use of in regard to the original parapets. ing ball. See PROJECTILES. If the parapet be raised on ground above the attacking position, it may be lowered, according to the angle, to about six feet six inches, the height necessary for a man standing up to be thoroughly protected. On the other hand, if the position, A, be lower than the point occupied by the assailant, the parapet must be raised; as 12 feet forms the limit to which a parapet can conveniently be thrown up, further height necessary for protection is obtained by sinking the ground to be defiladed before the parapet's base. In measuring for these heights, the instruments used are boning-rods, which are fixed in the ground at D and B, with the normal height of a man marked on them; a third rod at A is then marked at the point where the line of sight between the normal points on the two others intersects it, and so shews the height of the parapet. The foregoing parapet has been provided only as a straight breastwork, deriving its safety solely from its own fire in a direct line upon the besiegers; but in practice such a rampart would be exposed to the disadvantage of holding but little command over the scarp or escarp (part cut away) at its foot; so that, if approached under cover, an enemy could readily lodge himself therein. To guard against this a work is flanked, so that the fire of one part shall take in flank an enemy advancing against another part. See fig. 5, where ABCDE is a flanked or reciprocally defensive parapet, in which it is evident that the fire from AB, DE, must take in flank any force moving on BC or CD, while the latter also, in like manner, flank AB, DE, themselves. In a flanked defence of this sort, the angles, A, C, E, which project towards the country, are technically termed salient angles; those at B and D, re-entering angles. The flanked parapet has often, likewise, the power of defilading larger spaces than the simple line of Where both the faces of the salient are unavoidably so placed as to be enfiladed, a small work, called a bonnet, is constructed at the angle, which consists in the parapet being so raised up to an extra height of twelve feet if necessary, and at the same time widened, that the banquette shall be defiladed. If a height of twelve feet is insufficient to defilade the whole length of the banquette, traversing parapets must be raised at right angles to the face of the work, and within it, at such distances that the whole may be safe: of course, the height of the bonnet and of the traverses must be decided on rules analogous to those explained in fig. 4. The increased height of the parapet of the bonnet renders it necessary to have two more banquettes at that portion of the work, with steps to aid the ascent (see section in fig. 6); as AB, the crest of the general parapet, with banquette at C; and DE, the bonnet, with banquettes at F and G. or C D E Fig. 6.-Bonnet. A B In enclosed works-i. e., in works entirely surrounded by parapets the position of the parados is of vital importance; and they have often to be devised with great ingenuity, so as to protect the defenders from reverse fire in any direction, and at the same time not to prevent necessary communication between different portions of the fortress. Relief means the height of any point in a work above the plane of construction, which may be the line of sight or the bottom of the ditch. In the latter case, the relief of the parapet is an important for convenient firing, be four feet three inches | below a; its width three feet, if for a single line of soldiers; four feet six inches for a double rank; its slope should be one in twelve, that water may run freely off. The base, be, of the slope, up which the men mount to the banquette, should be twice its height bc, 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, ac, of the parapet should be one in four; the exterior slope, or plongée, ad, 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 of ab is augmented, it is best to keep it as small as circumstances will allow; one in six is the ordinary slope in English fortification, the angle of the interior slope being constant. In some 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 defence. 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, l, 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, 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 performs the same function in fortresses of a more permanent nature. This outer coating is denominated a revêtement. 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 the sooner be brought under fire. It is absolutely | necessary that the crest of the parapet should be five and a half feet 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 parapeti. 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 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 feet; its depth need only be limited by the trouble of raising the earth; but in practice 12 feet 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, &c., 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 (as in fig. 7); but sometimes it would be dangerous to construct the work with this continuity, as damage to the scarp would jeopardise the stability of the parapet. In these cases a narrow step, called a Berm (q. v.) of from two to four feet, 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. The counterscarp, or outer sloping side of the ditch, should be somewhat steeper than the scarp. FORTIFICATION. The bottom of the ditch should slope from both sides towards the centre, 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 upon to defend the position of an army against sudden attack. Their utility has been shewn in their employment from the earliest times; and modern experience tends to prove that earth-parapets are of all fortifications among the most difficult to overcome. An army manœuvring 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 a triumph that it was useless to the victors. A few redoubts at Pultowa 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 Junôt. 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 the best troops of the civilised world. higher than the general interior, with a view to guns being fired from it over the 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 defence should be about right angles. 3. Salient angles should be as obtuse as possible. 4. Ditches should have the best possible flanking. 5. The relief of the flanking-works must be determined by the length of the lines of defence. 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. Fig. 9.-Redan. 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 may be open at the gorge in the rear. The latter are, of course, the simplest; but they are only available in positions which the enemy cannot turn, or where protected by the sweeping fire of other works behind. Of this class the Redan, a mere salient angle (see fig. 9), is the simplest and the representative form. Of the closed forts, there are Redoubts, usually square; Star-forts, now considered objectionable; bastioned forts, as in fig. 10, which flank their own ditches almost perfectly, while scarcely susceptible of being flanked themselves. To understand the nature of a single bastion, see A (fig. 10), which represents one at the corner of a square work; ab is the left Fig. 10.-Bastioned Fort. For a line, 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 fire through an embra-flank, be the left face, sure (q. v.) in the parapet, instead of over the latter. cd the right face, de The embrasure is a cutting through the solid para- the right flank; ae is pet, 20 inches wide at its inner extremity, and the gorge; af, fe are outwards half as much as the width of the parapet. the demi-gorges, being In cases where it is necessary, for proper command, continuations of the that the line of fire should not be lower than the sides, or curtains, of the work; a and e are the top of the parapet, the embrasure is made through left and right curtain angles; b and d, the left and an additional parapet-raised, as in the previous right shoulder angles, and c is the flanked angle. 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 genouillerè (from genou, a knee), and in field fortification should be three and a half feet 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 cut (as in fig. 8). The sides of the embrasures are cheeks, and require revêting. Fig. 8. A barbette is a platform raised behind a parapet, Continued lines are simple parapets, either connecting fortified posts, or covering the front or flank of an army. Redans joined by curtains (as in fig. 11) are those most easily constructed; but Fig. 11.-Continued line of as the ditches can only be defended by an oblique fire, the curtains are occasionally so broken as to form nearly right angles with the faces of the redan, as in the dotted line; they then become lines of tenailles. Redans. Lines en Crémaillière have long faces with perpendicular flanks. Lines with intervals are often a જ્ઞ C Fig. 12.-Line with intervals. as effective as continued lines. They consist of detached works, in two lines, within musketry FORTIFICATION. fire of each other. The re-entering angle, abc (fig. Tête-du-pont, is a work constructed to cover the A tenaille is the reverse of a redan, and consists of two faces forming a re-entering angle: it can only be used in connection with some other work. A flèche is a breastwork of two faces, forming a salient angle, constructed on the exterior of a glacis, usually at its foot, in order to defend the ground before a bastion or ravelin. Having now explained the principal forms which elemental works of fortification are made to assume, we proceed to describe-very briefly, of coursethe systems into which these have been incorporated for the defence of fortresses, towns, and other permanent purposes. It will merely be necessary to state, in addition to what has been already written, that a rampart is a raised structure of earth or stone, above the mean level of the country, on which the parapets, &c., can be thrown up, and which affords to the town or space protected the extra cover of its height, while it elevates the inner works sufficiently to enable them to command and fire over those situated exteriorly to themselves. It need scarcely be said that a line which can be made of earth may equally be constructed of any other material which circumstances may render desirable, the maximum resistance and minimum liability to splinter being the qualities to be chiefly considered. SYSTEMATIC FORTIFICATION WORKS.-Adverting to the most ancient fortificaFOR PERMANENT tions mentioned in history, we find Greek cities surrounded with walls of brick and rubble, and occasionally of stone in huge blocks. Babylon had a wall of prodigious circuit-100 feet high, 32 feet thick, and surmounted by towers. Jerusalem, at the time of Vespasian's siege, had similar walls with masonry of enormous solidity. These seem to represent fortification as it stood from the time of that emperor to the introduction of cannon for breaching Fortunately, however, the art of defence has To Fig. 13.-Vauban's First System; Ground-plan : A, bastion; B, curtain; C, tenaille; D, caponnière; E, ditch; F, ravelin; G, covert-way; H, salient place of arms; Cormontaigne, Belidor, Montalembert, Bousmard, Irrespective of irregularities in the form of the place to be defended, a particular polygon is selected as that on which the lines of defence are to be 414 drawn. Each side of this is a face of defence, and the outline of its ground-plan; fig. 14 displaying the bastion, the line of advance is only covered by an extremely oblique fire. To obviate this, a ravelin, F, is constructed on the further side of the main ditch, which commands the doubtful fronts, and, at the same time, forms an outwork capable of assisting in the general scheme of defence. To trace the main ditch, describe from the flanked angle of the bastion, a or b, an arc with radius 30 yards (if dry |