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their ditches shall afford to an enemy who has carried them cover from the fire of the inner works.

Another general maxim, then, may embrace both the points on which we have now touched,—

VII.—A defensive work, as far as possible, should view every spot of ground within range of fire, but should be seen into from none. Heights, as sites for works of defence, have considerable advantages in the command of observation which they bestow, especially in a country covered with brushwood or intersected by ravines, as well as in facilities for affording cover and inaccessibility. But the popular opinion, which vaguely attributes greater strength for active defence to very elevated works, is, to a great extent, mistaken. "At the siege of Toulon, one morning Cartaux sent for the commandant of artillery, (N. Buonaparte,) to tell him he had just discovered a position from which a battery of six to twelve pieces would infallibly carry Toulon. It was a little hillock which would command three or four forts and several points of the town. He was enraged at the refusal of Buonaparte, who told him that, if the battery could command every point, it followed that every point would be able to bear upon it; the twelve guns would have one hundred and fifty to oppose, and that simple subtraction would show him his disadvantage."† Fire directed from a height on a plain below, or plunging fire, as it is called, is far less destructive than that which sweeps the surface in the whole course of its flight, and perhaps rebounds or ricochets for many hundred yards after first touching ground. The effect of plunging fire also during the night is most uncertain.‡ The rules which have been exhibited are the main general

* "I directed the Nuwab's troops to occupy some high ground on the right of our line, which not only afforded them the advantage of cover, but enabled their guns to fire from a considerable elevation down into the plain. This was a matter of no small importance in a country abounding in brushwood, behind which the enemy were seen with difficulty."-Lieut. Lake's Report of the Battle with Moolraj, July 1, 1848.

+ Las Cases. September 1815.

"Where cannon were perched, like to scarts or sea-gulls, on the top of a rock, he had ever observed that they astonished more by their noise than they dismayed by the skaith or damage which they occasioned.”—Sir Dugald Dalgetty.

principles to be attended to in designing works of fortification; but it will often be found that all cannot be observed; that in following one we shall be compelled to violate another; or that something of each must be sacrificed. In fortification, as in all other works of engineering, we are constantly called on to hit the happy mean between opposite defects. A deep ditch is a formidable obstacle; but if we dig too deep, our ditch will be invisible from our ramparts, and our elaborate flanks useless. A high wall is hard to scale; but if we build too high, our masonry will be visible, and liable to be breached from a distance, bringing down the parapet with it, and exposing the interior of our works. Powerful and well-arranged outworks are serious obstacles to an assailant, sweeping the path by which he would advance on our bastions, and hindering his progress till he shall have mastered one after the other. But if they be too large, too numerous, or too far advanced, they make large drafts on our garrison to man them, and communication with them during a siege becomes difficult. A wet ditch is no easy matter for a besieger to cross, but neither is it easy for the garrison to keep up communication with their outworks, when the bridges have been a few days exposed to shot and shell. Nor is it inspiriting for the defenders of an outwork, which must eventually fall, to know that, when the time comes, their retreat is cut off by two fathoms of water.

NOTE ON DEFILADE.

The command of one work over another has been defined to be the amount by which the height of the one above the plane of site exceeds that of the other. The plane of site is not necessarily horizontal, and the relative command of two works, or of one work and a spot of ground exterior to it, may be exactly the same whether they be on the same level or not, provided both continue on the same plane of site.

For elucidation of this, imagine a field-work or a model of one (W) (fig. 26, Plate II.) to be placed on a table, and its command to be such that an eye (E) six inches above the table is just on a level with the crest of the parapet: it is evident that this eye would see no part of the interior of the field-work. Now, suppose the table to be canted up at one end, and the eye to retain its relative position, still six inches

from the table, (fig. 27.) The eye is now at a much greater elevation than the field-work; but has it acquired any command over it? By no means. Let (W), instead of a model on a table, be a work on the slope of a hill, and (E) the position of an enemy on the upper part of the slope, the relative command and advantage of position is exactly as it was on the horizontal plane.

It is true that, if the projectile energies of the enemy at (E) were to be expended in rolling "huge round stones resulting with a bound" down upon the work, the force of gravity would give him an advantage; but in the discharge of fire-arms within reasonable range, the difference of effect between firing up-hill and down-hill can hardly be appreciable.

Again if the work (W) retain its horizontal position whilst the position (E) is elevated, (as in fig. 28,) it is plain that the latter will see the interior of three sides out of four. Whilst the base of the work remains as before, let us next elevate its parapets towards (E), until their crest occupies the position shown in (fig. 29.) The work is then screened from the observation of (E); it is defiladed from that eminence. Observe that in raising the parapet towards (E), you necessarily raise the banquette with it, and thus bring the defenders standing on the banquette into view from any point on the horizontal plane beyond (F), where it is intersected by the crest of the parapet produced. Now, if this intersection be within range of fire of the work, further steps must be taken to defilade the work on the side of the plane also. This would be accomplished by assuming a point (F") on the plane beyond the range of fire, and raising the whole parapet, till the crest lies in a plane passing through (E) and (F"). (Fig. 30.)

But as this would, in every case where the command of (E) is considerable, involve an extravagant elevation of the earthwork, it becomes necessary to defilade the two halves independently of each other, interposing a traverse or isolated screen in the middle of the work, which protects the parapet towards (E) from being taken in reverse from the plane, and the parapet towards (F) from being taken in reverse from the hill. (Fig. 31.)

The same principle will apply where the work has to be defiladed from an eminence on the side of (F) as well as (E).

When the work (W) is a lunette, or other work open at the gorge, it must be supposed to be supported from the side of (F), and its defilade, therefore, need only regard the hill in front.

Where such a work is commanded from two eminences not in the same horizontal line, (or, more correctly, not subtending the same angle of elevation from the site of the work,) each half may have its plane of defilade directed on the eminence which adjoins it, and be protected from the reverse or oblique fire of the other eminence by an arrangement of traverses either perpendicular to its face, or stretching along the capital of the salient.

As the number of commanding eminences round a work increases, defilade becomes very difficult, and can only be a slight palliative of a

B

faulty position. For a cumbrous reduplication of traverses in such a case, might perhaps be substituted an inner continuous retrenchment, following the general outline of the exterior work.

It would not suit the plan of this work to follow this subject further. Enough, perhaps, has been said to give a definite idea of the objects and methods of Defilade. For simplicity's sake, we have considered the plane of defilade as tangent to the eminence which commands the work. Strictly speaking, it must be considered as passing at a height above the eminence equal to that of artillery above the ground, or of musketry, or of a hostile parapet, as the case may be. When annoyance from musketry only is looked for, heights at a greater distance than 350 yards need not be regarded. With respect to artillery, 1200 yards is the limit usually assigned.

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OBSTACLES, MATERIALS, AND OTHER ACCESSARIES OF FORTIFICATION.

"Hostibus insidior; fossas munimine cingo."OVID, MET. xiii.

BEFORE entering on an explanation of the outlines and combinations of works, we proceed to describe various obstacles applicable as auxiliary to the defence, especially in fieldfortification, and various materials and other accessaries of engineering utility.

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