parapet. This arrangement is called a Traversing Platform. It is evidently quite unfit for operations in the field. More commonly a terrace of earth is raised behind the parapet, high enough for the gun placed on it to clear the crest. This terrace is called a Barbette. Short ramps lead to it from the terrepleine below. This construction is much used at salient angles, as the gun so mounted has great facility in sweeping to right or left. But the gunners are exposed from the leg upwards. Figs. 12, 13, 14 show a barbette for one gun, in Plan, Section on the capital, and Perspective. When the second method is adopted, or that of the gun firing through the parapet, the cuts made for that purpose are termed embrasures. The neck, or part next the gun, is made narrow, the embrasure splaying or widening towards its mouth, in order to give the gun some range to right and left. The lower part, or sole, inclines from the neck downwards, so much as is necessary to allow of the gun being fired at its greatest admissible degree of depression, should that be desirable; i. e., 1 in 6, or nearly 10°. But as in field batteries, generally on or below the ground level, this is unnecessary, the slope of the sole is usually about an inch in a foot. In batteries for ricochet, where the guns are elevated, the sole of the embrasures may even slope upwards to the front. The mass of parapet between two embrasures is called the merlon. The inner boundary of the sole is the sill; the sides are cheeks; the part of the interior slope left under the muzzle of the gun is called in French the genouillère. To lay down an embrasure, first draw it in profile, as in fig. 17. The height of a gun's muzzle determining the height of the sill a above the ground, make it 3 feet, and draw a b for the sole with an inclination, say of 1 in 9. In plan (fig. 15) draw c d, ef, for the positions of the sill and mouth as determined by a b in section. Draw A B, the line of fire, whether that be perpendicular or oblique to the direction of the parapet. On each side of the line of fire, on the line cd, set off 1 foot for the width of the sill. At 10 feet from the sill, raise perpendiculars of 3' on either side the line of fire; draw * Plate II. lines k m, 1 n, through the extremities of these perpendiculars to meet the line ef. The form of the sole is now traced. The cheeks slope outwards 1" in a foot at the neck, and have a slope equal to their height at the crest of the exterior slope. Guns in embrasure should not be at less intervals than 18 feet from centre to centre. The disadvantages of embrasures are, the narrow range which they allow to guns placed in them; their form, which acts as a funnel to receive the enemy's shot; and the weakness entailed on the parapet by these cuts in it, rendering it liable to rapid destruction from the enemy's artillery, as well as from the concussion of the discharges at the embrasures themselves. Sea-batteries should always be in barbette, or on traversing platforms, as a limited range of fire would be of little use against ships. Guns intended to command only a definite limited space, such as a ditch, a bridge, or the face of an adjoining work, may be suitably placed in embrasure. Figs. 18, 19, 20, and 21 are profiles of various batteries in use, and illustrate what has been said of interior and exterior excavation. In fig. 18, called a sunken battery, excavation is entirely from within, the gun standing in the trench, and the natural surface forming the sole of the embrasure. Fig. 19, the half-sunken, is, like fig. 5, excavated partly from without and partly from within. Fig. 20, the elevated battery, is excavated entirely from without, the guns standing on the natural surface. In (fig. 21) the cavalier battery, the excavation is from without, and the guns are raised on a rampart. "Ambitum muri directum veteres duci noluerunt, ne ad ictus arietum esset dispositus; sed WE must now say something of the trace, or outline in plan, given to works of fortification. It is plain that at any point of a fortification, whether it be a wall of masonry or a bank of earth, the defenders behind a parapet cannot see, and therefore cannot fire upon, the ground close in front of the work upon which they stand. Thus, in fig. 22,* the defender of the parapet at ABC can see nothing of the space DDD at the foot of the work; and if the work be traced in a straight line, a polygon, or a convex curve, as in the figure, the ground immediately along the whole of its length-i. e., the ditch, if there be one-will be unseen and undefended from the parapet; so that an enemy, having once gained the foot of the escarp, may pass freely round, and establish his mines to breach the wall, or plant his ladders to scale it, where he judges best.† But if the continuous outline of the work be broken, so that a line (as BE, fig. 23) shall project at an angle from the general direction, AB, BC, from this projecting line the ground in front of AB can be seen and defended; and troops advancing to attack the line AB will be assailed in flank by the fire of BE. Hence, such a fire, which is the most destructive to which troops can be exposed, is called a flank fire; BE is called a flank, and is said to flank AB; AB, the space between two flanks, being technically called the curtain. Suppose, then, a fortress to be enclosed by a polygonal line of works, having at each angle of the polygon such flanks looking * Plate II. + So, "14th May 1837, we advanced on the fortified town of Irun, a mile from the frontier of France. Exposed to a heavy fire from the batteries, our regiment darted forward and got under the walls of the fort, where we lay all the afternoon and night, the cannon in the fort thundering over our heads on our forces at a distance, we ready there to rush in when a breach was made.”— Autobiography of a Working Man, (who served in the British Legion.) So also at the attack of the great redoubt at Toulon, in 1793, the ditches being unflanked, the French storming party, driven into them twice, re-formed there with little molestation, and at the third attempt they succeeded in carrying the work. At the attack of the redoubt San Fernando at Lerida by the army of Arragon under Suchet in 1810, the French storming party were unable to mount the escarp of the work; but as the garrison was unprovided with grenades, and the ditch was not flanked, they remained there in security. At last, as neither party could touch the other, though at such close quarters, they came to a sort of armistice and parley, which resulted in the French being allowed to go back again unmolested.-Suchet's Memoirs, i. 128. either way, the two flanks at each angle will enclose between them a square tower BEFG.* If, now, the enemy attempts the attack of a curtain, as FC in plan, fig. 24, he is, during the whole of his advance, under the fire of one or both the flanks AK, HC. Driven back, he returns to the assault, but this time attacks the tower instead of the curtain. As soon as he reaches GE, the front of the tower, he is safe ;-he is unassailable, being invisible from any part of the parapet. There the ancient leader would establish his battering-ram, or roll up his moving turrets, from which he boarded the ramparts as frigate boards frigate; — the modern would drive his mining galleries, or plant his ladders for escalade. And this fact of the towers being the weakest points of the outline, is consistent with what we know of the especial pains which the ancients took to strengthen such towers. As most liable to attack, they were made much loftier than the curtain walls,† and they were usually crowned with projecting balconies, called in medieval language machicoulis, from which missiles could be shot vertically down on the heads of besiegers. Now, imagine the front GE of the tower to be broken into two faces, forming a salient angle GPE, as in fig. 25. The case is now changed. The ground at P is no longer hidden from the fire of the parapet; it is completely swept by the fire of two flanks, CH, AK. Such an angular tower or projection, modified by experience, is a leading feature in modern fortification. It is called a BASTION.‡ BE, FG are called the flanks, GP, PE the * Such a square tower may be seen on the remaining part of the old city wall which runs along the lane called the Vennel, in Edinburgh. Thus Diodorus, when he ascribes a height of 100 feet to the walls of Nineveh, says the towers were 200 feet high. Though this step-by-step deduction of the outline of a bastion is perhaps the clearest way of illustrating the rationale of the matter to a beginner, it is to be understood that there is no historical authority for the gradual transition here. laid down. Probably the construction in question was suggested by the form naturally assumed by a square tower at an acute angle of a city wall. See, for instance, the towers at the angles of the Moorish wall of Tarifa, (Pl. VIII., fig. 116,) which closely resemble Erard's right-angled bastions. See also Note I. |