the soldiers standing on the banquette of one line should be secured from the fire of the ground in front of the other, which firo is called reverse fire, because it strikes them in rear. This is effected by placing a mound of earth or traverse between them, and determining its height as well as the height of the parapets in the following manner :

On the commanding point C (fig. 21) is placed a boning rod CD of the normal height, and another BE of the same height at B, or st the position of the traverse; then the height of the crest of the parapet of A is determined by the intersection of the visual line from b to E with the pole fixed at A, at the point a, which is here high,

as Cis so much higher than A. See preceding rules and equations. In like manner a pole of the proper height being fixed at the extent of range on the opposite side at C, the visual line from D' to E determines the height, a', of the parapet of A', which is much lower than the parapet of A, as A' and e' are nearly on the sanie level. Now, to defilade the banquettes, and to determine the height of the traverse necessary for that purpose, set up on the banquette of A a pole bb of the same height as CD, CD, BE, and the visual line from D' to b determines the height of the traverse at f, which is necessary to secure the banquette of A from the reverse fire of C, whilst the visual line drawn from D to determines the height of the traverse sufficient to protect the banquette of A' from the reverse fire of C.

D'

E

The application of these principles to a work formed of two lines (or "faces" as they are called), terminating in a salient angle is shown by fig. 22, but the application is the same whether the work is connected with a line of intrenchment as a "redan," is detached as in a "ravelin" and other outworks, or forms part of a peculiar system or arrangement of works, as in the tenaille system of Montalembert, called by its author the angular system,-terms and works which will be hereafter more fully explained. Here the commanding point is supposed to be at M, and to

B

Fig. 22.

secure the defenders of the face AB from a reverse fire it is necessary to interpose the traverse cd, called from its object a "parados." The length of the traverse cd is determined by the line MB, beyond which it should project sufficiently to give ample security to a space about 50 feet wide behind the parapet. At the other end, the traverse is not carried up to the salient angle, as it would interfero with the communication, but is completed by ba, perpendicular to the other face, by which arrangement the space within the salient and the banquette are left free. The two lines MdB and Mef, passing through points at the

are so situated as to give an enfilade fire along both the faces AB, AB' (fig. 23). In this case a small work DAD' is formed in connexion with the parapet, by drawing lines parallel to the crests CB and C'B' at a distance from them equal to the breadth of the banquette, and then determining, in the manner. explained, the heights at A necessary to defilade a certain length of the banquettes of CB and CB' from the fire sweeping them, and assuming the greater of the two as the height of the parapet at A. This work is called a bonnette; and when the height necessary to defilade the whole of one or both faces is found to exceed 12 feet, the height of A should be restricted to that limit, and traverses T, T', T2 should be placed at such distances as shall defilade the remainder of the work.

The internal space may frequently be sufficiently defiladed by raising the salient portion of the parapet without disturbing the line of direction of the crest; but in that case

Fra. 23, showing Bonnette DAD', and Traverses T, TT.

normal height above the banquette, determine the height of the traverse. It may, however, happen that the commands

Fig. 24.

the banquettes of the two faces would not be covered from enfilade fire, and hence the necessity of a bonnette. The increased height of the parapet of the bonnette renders it necessary to adopt two banquettes b, b', one below the other, and each provided with steps to facilitate ascent (figs. 23, 24). The operation of defilading may be also effected by planes of defilade; as, for example, if the line which marks out or limits the space to be defiladed be first drawn, and a plane be supposed to pass through a line either 6 ft. 6 in. or 8 ft. (or whatever height between these may be assumed as the normal height N) above the limiting line, and through a point the same height above the commanding point, this

plane will determine the height of the parapet, the crest of which will necessarily be in it.

Where the parapet is continued not only on the flanks but also in rear, so as to form an inclosed work, it may often be necessary to defilade it in various directions as in fig. 25. Whore two traverses or parados cross each other, they must, of course, be so placed that they shall not only completely defilade the whole interior space of the work, but shall secure from reverse fire the banquette on each side, the normal N being therefore at least 6 ft. 6 in. above the banquette. Where traverses of this kind become necessary, the engineer must take in to account the space

b

Fig. 25.

they will occupy, and plan his work accordingly; and should he be able to render the difficulty of attacking one side of his works very great, he may construct the traverses so that they may be used as retrenchments, and thus increase the means of defence; for example, S being the salient of greatest strength, bcỏ might be defended, and then bed. This subject has been enlarged upon because it is of much importance in military engineering, as the safety of a long line of works may be endangered by defective defilade. Though considered here in a practical form, it depends entirely upon geometrical principles, and instruction in descriptive geometry is therefore essential in all schools of military engineering.

Having determined the relief of the crest of the parapet in reference to the plane of site, all the other vertical dimensions follow from it, as shown in several of the preceding figures; whilst the horizontal dimensions are determined by the thickness necessary to resist the enemy's missiles and by the slopes required to ensure stability. The Penetration at a mean range into common earth after it has been dug up and well-rammed and into other materials is as stated below, and to these dimensions one half should be added for the thickness for security. English Field Artillery.

men, or difficulty of ground, for reducing the thickness of the parapet below 14, or at the utmost 12 feet. For form. ing the parapet under peculiar circumstances of difficulty the engineer will avail himself of every fitting substance which may be at hand, such as bags of wool, mattresses, fire-wood, manure heaps, and fascines, either by themselves or packed in gabions. The resistance of fascines is not great, their strength being rapidly diminished by the speedy fracture of their branches when exposed to sharp

fire.

With these data it will be easy to regulate all the dimensions of the parapet,-the height of its crest, or the relief of the work, having been first established. Thus tho banquette or step on which the men stand, when firing over the parapet, should for convenience be 4 ft. 3 in: below the crest. The breadth or tread should for a single rank be 3 ft., for a double rank 5 ft.; the surface should slope backwards 2 inches in the 3 ft., 3 inches in the 5 ft., so as to discharge water freely and keep the banquette dry; the base of the interior slope of the banquette up which the men mount should be twice its height; if the height of the parapet exceed the normal height, the banquette should have two treads or steps, the lower about seven feet below the crest, so that one rank of men may stand thereon whilst reloading the muskets of the rank on the step above them; or it should have three steps, each with a rise of 1 ft., and a tread of 1 ft. or 1 ft. 3 in., sloping slightly to the rear, by which arrangement the necessary excavation of the ditch will be diminished, and less of the interior space occupied. The interior slope of the parapet should be made very steep, so that a man firing over the parapet should be as close to it as possible; the base should not exceed one half the height; the superior slope, or “ plongée,” of the parapet, by which the fire is directed towards the point on which it is to act, should not be less than one-ninth, nor more than one-fourth of its thickness, and in service is generally made one-sixth; but as the increase of the slope facilitates the destruction of the crest, it should be kept as slight as possible. It is usual on the Continent to retain the angle of the crest, as a constant quantity, at 100°, and to increase the base of the interior slope as the plunge increases, and vice versa; but this is not satisfactory, since, the height of the soldier's shoulder remaining constant whilst the line of plunge varies, the fire will not be always in the true direction; and it is preferable to keep the base of the interior slope as small as possible, and to make the top of the parapet at the crest horizontal for one or two feet, commencing the plunge therefrom, but reducing the top as the plunge increases. This flat top will facilitate the use of sand bags (bags filled with earth), which are sometimes so arranged on the crest of the parapet as to form loopholes for the musketry, thereby adding to the cover of the men. The base of the exterior slope of the parapet should at least equal its height, though possibly in some soils-e.g., chalk-the exterior slope may stand at a steeper angle than 45°. The slopes of the escarp and counterscarp should be as steep as prac ticable, but generally they will be the same as the exterior slope. Between the exterior slope and the escarp a "berm" is left. This berm is a space at least 1 ft. 6 in. wide and slightly inclined downwards and outwards for drainage; it gives greater stability to the exterior slope and to the escarp, prevents the earth from the exterior slope from falling into the ditch, and is a standing ground for the repair of the must be retained in place by a wall called a parapet. Where steeper slopes are indispensable, the earth revetment," which may be formed of fascines (long cylindrical faggots), hurdles, sods, planks, clay puddling, and, in the interior of works, of sand bags. The base of the interior slope of the glacis should be equal to its height, and the exterior slope

of Turks, numbering about 10,000 men, collected at Plevna, and at once commenced to form shelter trenches; they were very deficient in artillery. Krudener attacked them four days after with 7000 Russians and 30 guns, and was defeated with a loss of 2800 men. Ten days later he again attacked with 30,000 men and 170 guns, but in the meantime the Turks had thrown up several redoubts, and had so extended their shelter trenches that their position was beginning to assume the condition of an intrenched camp; moreover, their strength had been swelled by reinforcements to 45,000 men. The Russians were again defeated with a loss of 8000 men. Six weeks later, upon the 30th September, Krudener, who had received 30 siege guns, having shelled the Turkish position for four days previously, attacked a third time with 50,000 Russians and Roumanians; but the Turks had in the interval received reinforcements of 10,000 men, and he was a third time repulsed with a loss of 15,000 men, though the Roumanians obtained possession of and beld a large redoubt called Gravitza. It then became evident to the Russians that Plevna could not be taken by assault, and they commenced to besiege by rule. By the 24th of October they had completed the investment by the occupation of the Loftcha and Rahova roads, all of which were previously open to the Turks, and had brought into battery against it 300 guns, 40 of which were siege guns. By the 10th of December they had collected 110,000 men and 500 guns around Plevna; and shortly after, Osman Pacha, having been repulsed with a loss of 6800 men in an attempt to cut his way out, surrendered, and his whole force, amounting to 43,000 men and 70 guns, became prisoners of war.

An army intrenched or fortified in the field is in many respects of the same effect as a fortress; the intrenchments supply the lack of numbers, and enable it to cover a country, to stop the advance of a superior enemy, or, if he chooses to risk a battle, oblige him to engage at a disadvantage. Charles V. furnishes a notable instance of the first. Opposed to a combined force of twice his strength, he at once commenced to intrench his army. In a few hours he was in a position to resist attack; in ten days he was so secure that, upon receiving reinforcements a few days later, he was enabled to assume an offensive which led in four months to the termination of the campaign in his favour. "In a war of march and manœuvre," says Napoleon, "if you would avoid a battle with a superior army, it is necessary to intrench every night, and to occupy a good defensive position. The natural positions which are ordinarily met with are not sufficient to protect an army against superior numbers without recourse to art. Those who proscribe lines of circumvallation, and the assistance which the science of the engineer can afford, deprive themselves gratuitously of an auxiliary which is never injurious, always useful, and often indispensable."

Whenever Napoleon had time and occasion for strengthening his position by field-works, he acted upon the principles recommended in the above extract, as almost all his predecessors had done. In the wars which followed the Revolution of 1688, in those of Queen Anne's reign, and during the Seven Years' War, we find the commanders of each period, William III., the duke of Marlborough, Marshal Villars, Marshal Saxe, Frederick II., and Marshal Daun, practically exemplifying their conviction of the great utility of field-works. Seven redoubts thrown up overnight saved Peter the Great at Pultowa, and enabled him to gain a decisive victory over his formidable antagonist; and at Borodino, three redoubts and eight flêches thrown up hastily by the Russians, caused the French great loss, and rendered the victory, which they gained by incredible

1 Military Maxims of Napoleon.

efforts of gallantry, fatally costly; and it is not improbable that if the main redoubt had been closed at its gorge the French would have failed to take it. In 1761 Frederick the Great, having only 55,000 men to oppose to the united Austrian and Russian forces of 130,000 men, intrenched himself in the strong position of Bunzelwitz, in Upper Silesia, not far from Schweidnitz, which he held until the united armies were forced to retire for want of supplies. It has been argued by some that intrenchments and fieldworks have oftener been carried than successfully defended, and that hence incommensurate importance has been attached to them. But it should be remembered that victory in such circumstances has been purchased at an expense which has often rendered it in effect equivalent to defeat, and that a practice which the greatest commanders of ancient and modern times have approved and followed cannot be of doubtful utility. At Austerlitz, where the contending armics were nearly equal, Napoleon wa preparing to superintend the construction of intrenchments when he found himself called upon to receive battle; and in Portugal, the duke of Wellington showed to what account the art of the engineer might be turned for influencing, not merely the fortune of a campaign, but the fate of a cause. The lines of Torres Vedras, which the powerful French army under Masséna was unable to pass, and from which the wave of war was rolled back broken into Spain, were perhaps the most remarkable works of the kind ever constructed.

"Lisbon," says Sir John Jones, "being situated at the extremity of a peninsula formed by the sea and the Tagus, it is plain that if an army be posted across the peninsula, no enemy can penetrate into the city without a direct attack on the army so posted. It was on this principle that the lines covering Lisbon were planned by Lord Wellington. Nature drew the rude outline of a strong defensive position, and art rendered it perfect. A tract of country thirty miles in extent from the mouth of the Zizandra on the ocean to Alhandra on the Tagus, was modelled into a field of battle; mountains were scarped perpendicularly, rivers dammed, and inundations formed; all roads favourable to the enemy were destroyed, and others made to facilitate the communications of the defenders; formidable works were erected to strengthen and support the weak parts. whilst numerous cannon, placed on inaccessible points, commanded the approaches and gave an equality of defence to the whole position."2 These lines were not continuous and connected works; they consisted of independent forts, redoubts, flêches, redans, batteries, so placed as to command and enfilade every approach, and to support each other by a cross and flanking fire. The first line occupied a front of twenty-nine miles between the sea and the Tagus, and by means of telegraphs intelligence could be conveyed from one extremity to the other in a few minutes; whilst the troops, disposed in masses in the rear of the works, were ready to move upon any point of attack by interior communications shorter than any by which the enemy could advance. and scope of these works," says Colonel Napier, "was to bar the passes, and to strengthen the fighting positions

"The aim

War in Spain, p. 124. The French army which invaded Portugal under Masséna consisted of three corps, under Marshals Ney and Junot and General Regnier, amounting in all to 66,000 infantry and 6000 cavalry, besides a strong body of the imperial guard, which crossed the Pyrenees after the invading force had commenced its march from the threatened invasion did not exceed 48,000 infantry and 3000 cavalry, neighbourhood of Salamanca. The force collected to oppose this of which about half were Portuguese levies, yet untried in any general action, and of which a very unfavourable opinion still continued to be entertained. In point of numbers, and still more in the composition of their army, therefore, the French had a decided superiority; but all their advantages were neutralized by the defensive position of Torres Vedras.