Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/440

 426 FORTIFICATION or if h &quot;be 0, A and B being on the same leve], AD = 3T - In equation (4), if the station C, though below A and B, falls between the horizontal line drawn through A and the line Bit, or BA be prolonged till it cuts the surface of the ground sloping from B towards D, then -. (11 + ft) is less than h, and AD is greater than N ; but should C be below the line BE, then &amp;lt;1 . (H+fc) is greater than h, and AD is less than N ; or, in other words, should the line of defilade passing through BA meet the ground at R within the prescribed limits of defilade, or the effective ranges of musketry and artillery, which may be now assumed as 800 yards for the first and 3000 yards for the second, then if the point C be above that line the parapet at A must be made higher than the normal height, and if below it may be made lower. A comparison of the examples under equation (2) will exhibit the great dis advantage to the defenders of simple lines of having any ground near to their own moderately elevated, and care should be there fore taken either to occupy such ground or to throw back the lines opposite to it as far as possible, and, should it have the character of a ridge, to bring some portion of the fire of the lines to act in the direction of its length. The space AB to be defiladed must depend upon circumstances; the minimum, to allow safe communication for the troops behind and actually defending the parapet, ought not to be less than 30 feet, but when it is necessary to draw up troops behind the parapet it ought not to be less than 90 feet ; in cases of double lines or of inclosed works the distance must of course vary, as the object will be to protect, not only the troops near to the enemy from a direct fire, but the troops arming the more distant parapet from a reverse fire. In assuming the normal height as 8 feet, on the supposition that protection is required from the fire of mounted soldiers, a condition is assumed which is not generally likely to occur in the attack of intrenchments. Never theless, the command of a work should be sufficient to cover the defenders standing on the ground inside. Infantry are assumed to be 6 feet high, and to protect them, within a reasonable distance of the parapet, from projectiles clearing the crest and tending down wards in the latter part of their trajectory, the command of a para pet on level ground should be at least 8 feet. Shrapnel fire being effective at 3000 yards, and the elevation of the heaviest gun of position for a range of 3000 yards being 7, the balls clearing an 8-feet parapet will strike the ground 18 yards in rear of the crest of the parapet. In equation (3), and in one case of (4), as explained, the height of the parapet becomes less than N, but should the diminution extend so far as to reduce the height of the parapet below 7 feet, the relief should be restored to its proper amount by excavating the ground behind the parapet, or, in other words, forming a terrepleine below the level of the plane of site. In a similar manner, in equation (2), and in one case of (4), where the parapet becomes greater than N, it would be very inconvenient to augment the height above 12 feet, and it is preferable therefore to excavate behind the parapet, whenever the defilade requires so great an increase of height. In the preceding observations the parapet lias been con sidered as a simple straight line, deriving its defence solely from its own direct fire ; but such a condition would most frequently be found inapplicable, in respect of form, in consequence of the natural inequalities of the ground, and unsatisfactory, in respect of defence, in consequence of the imperfect operation of direct fire from the top of a parapet, which can strike the ground in front only in the pro longation of its superior slope, leaving everything within this defenceless ; the line in which the superior slope meets the ground is called the &quot; Limiting Line of Defence.&quot; A disposition of trace has therefore been adopted by which the fire of one portion of the parapet takes in flank an enemy advancing directly against the adjoining portion; and a work is said to be flanked when the arrangements are such that an enemy cannot advance against any portion of it without being taken in flank by the fire of some other portion. In lines of intrenchments this arrangement leads to a bent line, having angles projecting towards the exterior called &quot; salient angles,&quot; and angles retired from the exterior called &quot;re-entering angles ; and it is evident that in this arrangement the lines AB and AC (fig. 20), which are flanked by BD, CD, in their turn flank BD and CD ; such flanking defence is called &quot; reciprocal defence.&quot; Referring back to the subject of defilade, it is evident that a bent line of this kind affords more facility for de filade than a straight line, as it is often possible so to arrange the position of the angles that the salients shall occupy higher ground, while the re-entering angles, though placed lower, shall be compensated for this disadvantage by being further removed from the commanding ground of the enemy. Though a simple straight line has the disadvantage of depending for its defence solely on direct fire, it is not exposed to be swept along its whole length by an enemy s fire. Fire of this kind is called &quot; enfilade&quot; fire, and is very destructive ; it produces the same effect upon the defence as flanking fire upon the attack, viz., it takes the defenders of the line in flank. To guard against this evil, should it be necessary to take up a position in front of ground of a superior command, the long lines AB, AC (fig. 20) should be so directed that then prolongations &quot;E FIG. 20. Illustrating defence against Enfilade Fire. may fall on low ground at E, F, and not as they would do in the case of AB, AC , on the high ground at E , F ; and, if possible, as where the high ground is not continuous the prolongations of both short and long lines should fall on low ground between the commanding eminences, an arrangement which will be especially advantageous should the low ground be marshy or otherwise difficult of occupa tion by an enemy. Such observations as these are only suggestive, since no fixed rules can be laid down to meet all cases. The engineer should examine the ground and adjust his works so as to make the most of its advan tages, and to neutralize, as far as possible, its disadvan tages. To determine the height of the parapet by the rules for defilade formulated above, it is necessary to have a correct plan of the ground, and to know the exact levels of the points A, B, C, in every case; but where there is no such plan, the defilading may be effected in practice by levelling poles or boning rods. In this case, the inner boundary of the ground within the parapet to be defiladed being staked out, a boning rod of 7 or 8 feet high, according to the intended normal height of the parapet, should be placed at Bon the staked-out line, and anotherof equal height on the commanding point or ground C (fig. 21), supposed to be at a distance equal to the range of the projectiles from the fire of which the work is to be defiladed. A rod about 1 2 feet high is then fixed at A, and a cross piece or marker is raised up or down it until it meets the point where the visual line from the top of B to the top of C intersects the pole at A; this operation is simply the mechanical determination of the height obtained in the other method by calculation. If it be required to defilade the whole space between two parallel lines, or that included between the two lines forming the salient angle in fig. 20, it is evident that the work must be defiladed from both sides, and further, that