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Architettura Militare (1759), that several small bastions | had been constructed in the preceding century, and that the ruin of a large bastion which had formed part of the fortifications of Turin, built for Duke Louis of Savoy, still existed in the royal gardens at that time. The bastions on the enceinte of Verona, built by the Italian engineer Micheli, in the year 1523, are generally supposed to be the oldest extant; and the next, probably, are those of the citadel of Antwerp, which were constructed for the emperor Charles V. in 1545, by the Italian engineer Paciotto D'Urbino. These bastions are small, with narrow gorges and short flanks and faces, and are placed at great distances from one another, it being the invariable practice, at the time when they were built, and for a considerable time afterwards, to attack the curtains, and not the faces of the bastions.

Errard of Bois-le-duc was the first in France who laid down rules respecting the best method of fortifying a place, so as to cover its flank. He was one of the principal officers of the engineers in ordinary to the king, a corps formed by Sully, who was grand master or master general of artillery in the reign of Henry IV., out of the best instructed and most experienced military men. This corps is now the Corps de Génie, and to this day maintains its scientific character. At the command of the minister, Errard wrote a book on the subject, which was published in 1594, and in which the details of his method are explained. As a writer on fortification, he was preceded in France by Beril de la Treille, who published his work on fortifying towns and castles in 1557. Errard fortified inwards; and in the square, pentagon, hexagon, heptagon, and octagon he made the flank perpendicular to the face of the bastion; but in the enneagon, and in polygons of a greater number of sides, he made it perpendicular to the curtain. In his method, however, the gorges are too small, the embrasures are too oblique, and the ditch is almost defenceless. This engineer constructed part of the enceinte of the citadel of Doutens, the citadel of Amiens, and some works at Montreuil and Calais.

Antoine de Ville, who succeeded Errard, published a treatise, dated 1629, in which he completed much that his predecessor had only sketched, and rectified various defects in the method of the latter. He was employed under Louis XIII., and constructed new enceintes for Montreuil and Calais. His method of fortifying has been denominated by some the French Method, and by others the Compound System (Système à trait composé), because it united the Italian and Spanish methods, from the latter of which it differs only in having no second flanks and fichant lines of defence, and in not confining the flanked or salient angle of the bastion to ninety degrees. The leading maxims of De Ville were to place the flanks perpendicularly to the curtain, to make them equal to the demigorges, or a sixth part of the side of the interior polygon, and, in the hexagon and all higher polygons, to confine the flanked angle to ninety degrees. But this method is liable to nearly the same objections as that of Errard, as the embrasures are too oblique, especially in the polygons, and the ditch is ill defended.

In 1645, sixteen years after the publication of De Ville's, appeared the treatise of the Comte de Pagan, which contained the development of a system that, in a short time, entirely superseded those of his predecessors. It was the Comte de Pagan who first disengaged the science of Fortification from a number of suppositions which custom had consecrated, and which, resting more on abstract mathematical reasoning than on practical experience, had hitherto retarded the progress of the art. This engineer served at twenty-five sieges with great reputation; but having become blind at the age of thirty-eight, he was

obliged to retire from the service, in which he had already obtained the rank (then second only to that of marshal of France) of mareschal-de-camp, and he died six years after completing the treatise above mentioned. He had from his earliest years devoted himself to the study of mathematics and fortifications, and he published several works on astronomy besides his celebrated work on Fortification, Pagan made the flank perpendicular to the line of defence, in order as much as possible to cover the face of the opposite bastion; and he devised a method of building casemates peculiar to himself. Vauban borrowed from Pagan the length of his perpendicular, and Allain Manesson Mallet, whose construction has found much favour, proceeded upon the principles Pagan had laid down.1

The Mareschal de Vauban was born in 1633; and at the time of the Comte de Pagan's death, he had already acquired reputation at several sieges. Vauban followed up the principles suggested by Pagan, and employed them extensively, with consummate skill and judgment. He constructed 33 new fortresses, repaired and improved 100, and conducted about 50 sieges. His extensive works, especially the treatise De l'Attaque et de la Défense des Places, published in 1737, speak for themselves. From these works have been compiled the systems which, in the military schools, are denominated Vauban's first, second, and third systems of Fortification, and which the reader will find developed in the sequel. Had the genius of Vauban been applied to the discovery of a method for securing a permanent superiority to the defence of fortified places, posterity would have been greatly indebted to him, and even humanity would have had cause to rejoice in such a triumph of military art. But, being engaged in the service of the most ambitious monarch of modern times, Louis XIV., he applied his great talents to forward his master's views, and perfected that irresistible system of attack2 which has ever since been so successfully followed. Before his time the superiority was on the side of the defence; but he so completely reversed the case that the success of an attack, conducted scientifically and with adequate means, has been until the present time a matter of certainty.3 Vauban was no ordinary man in any sense. As the inventor of parallels in sieges, and of ricochet fire, he stands in the first rank of military engineers; and as he conducted 53 sieges, and took part in 140 battles and skirmishes, it must be admitted that in respect of experience he stands in no inferior position. At fiftyfive years of age he attained the highest honour of the French army, being created marshal of France; yet amidst his stirring and successful military life he never ceased to turn to account the geometrical knowledge for which he was distinguished when a youth, and which had obtained for him the early notice of the Prince de Condé. His mind was never idle, and was constantly directed to projects of public utility, civil as well as military; and he left behind him records of such labours in 12 folio manuscript volumes, entitled Mes Oisivetés, a wonderful monument of his ability and industry.

M. Minno, Baron de Coehorn, first a general of artillery, then a lieutenant-general of infantry, and ultimately direc

demigorge equal to a fifth part of the side of the interior polygon or 1 Mallet constructed outwards, making in every figure or polygon the figure, the capital of the bastion equal to a third part of the same side, the curtain equal to three-fifths or thrice the demigorge, and the angle of the flank equal to 98°. The faces of the bastions and the flanks are determined by the lines of defence, which are razant. From these data all the other lines and angles are easily found.

2 See his work De l'Attaque et de la Défense des Places, passim. 3 The protracted siege of Sebastopol is no exception to this rule. The length of its resistance was due to the fact that the resources at the disposal of the allied armies did not suffice for its investment, and that the Russians were in consequence able to introduce at their pleasure reinforcements of men, material, and provisions; indeed, at times they were more numerous than the attacking force.

tor-general of all the fortified places belonging to the United Provinces of Holland, was the contemporary and rival of Vauban. This able engineer, convinced that, however expensively the rampart of a town may be constructed, it cannot long resist the shock of heavy ordnance, invented three different systems by which such obstacles are thrown in the way of a besieging force that, although the place be not thereby rendered impregnable, it can only be reached with great difficulty and hazard. But these systems, with'but modification, are only applicable to low and swampy situations, such as are to be found in Holland, and are therefore not available in localities of a different description. Nevertheless, Bergen-op-Zoom, Mannheim, and other places fortified by this engineer, particularly the two named, have very great merit, inasmuch as it is impossible for a besieger to penetrate into any of the works without being exposed, on all sides, to the fire of the besieged, who are under cover, and from whose artillery and musketry fire it is scarcely possible for an assailant to shelter himself. In fact, Coehorn was a great master, and combined with the bastioned trace, as will be explained when his system is noticed, many of the means of defence springing from another source. He published his first work on Fortification before he had acquired much experience; and in fortifying Bergen-op-Zoom, which is allowed to be his masterpiece, he did not reproduce, except in fragmentary details, any of his published systems.

Since Vauban's time several improvements have been suggested, particularly by Cormontaigne, who entered the corps of French engineers in 1716, nine years after Vauban's death, and died a mareschal-de-camp in 1750. Some account of the system of Cormontaigne will be found in a subsequent part of this article. The three methods enunciated by Belidor are all applicable to an octagon of 200 toises. Scheiter distinguished his systems as great, mean, and little, in imitation of Pagan, requiring the exterior sides of the polygon to be 200, 180, and 160 toises respectively. He adopted from Castriotto detached bastions, and made use of a continuous fausse-braye. Fritach, a Pole, proposed two methods, which he exemplified on different polygons. Dogen, a Dutchman, after enumerating, in a large volume on Fortification, various modes employed by different writers for determining the salient angle, selected three as most worthy of approval, and proposed as many methods of construction, one of which is borrowed from Fritach, the Pole. Pietro Sardi, an Italian, suggested a peculiar method of construction on a hexagon. The Sieur de Fontaine found the flanked or salient angle of the bastion by adding 15° to half the angle of the figure, from the square up to the dodecagon, in which last it becomes 90°, and at this he continued it in all the higher polygons. He also constructed outwards, and, in every regular figure, made the curtain equal to 72 toises, the face of the bastion to 48, and the flank, which he placed perpendicularly to the curtain, to 18 toises, or a fourth part of the curtain. Ozanam and Müller delivered each four methods of construction, the particulars of which will be found in their respective works. In 1751 Charles Bisset, who, as engineerextraordinary, served with the duke of Cumberland in the Netherlands, and was present during the siege of Bergenop-Zoom by Marshal Lowendahl, published a treatise on the theory and construction of Fortification, in which there are many sensible and judicious remarks; and this may also be said of an Essai sur la Fortification, ou Examen des Causes de la grande supériorite de l'Attaque sur la Defense, published anonymously in 1755. In a work entitled Science de la Guerre, which appeared at Turin in 1747, a new method of construction is proposed, in which the principal novelties are mines under all the works, and regular communications with them by means of subter

raneous galleries, to be resorted to as the enemy approaches the body of the place. The third volume of the Euvres Militaires contains useful observations and maxims relative to irregular Fortification; and in the supplement to the Réveries of Marshal Saxe, by Baron d'Espagnac, the subject of Fortification is amply discussed, and an accurate description given of the different means of attack and defence. Besides the writers above enumerated may be mentioned the Chevalier St Julien, an able engineer, who published a method by which, he asserts, works may be constructed at less expense, yet in such a manner as to render the defence more formidable; Francisco Marchi, of Bologna, who in 1599 furnished no less than 139 different methods of constructing fortifications, many of which are valuable, and from which subsequent engineers have greatly profited; Bombelle, who established three kinds of Fortification, called the grand royal (grand royal), the mean royal (moyen royal), and the little royal (petit royal); Blondel, who published a system divided into two principal heads, the great and the little, whose exterior sides are respectively 200 and 170 toises; Donato Rosetti, a canon of Leghorn, who wrote on the method of constructing works in what he calls Fortification à rebours, or Fortification in reverse, so denominated because the re-entering angle of the counterscarp being opposite to the flanked angle, it will, according to him, be necessary to attack it from the reverse side of the other works; and Antonio de Herbart, major of artillery in the duke of Würtemberg's service, who published a treatise on Fortifications with what he calls angular polygons. The treatise entitled Nouvelle Manière de fortifier les Places, tirée des méthodes du Chevalier de Ville, du Comte de Pagan, et de M. Vauban, avec des Remarques sur l'ordre renforcé, sur les desseins du Capitaine Marchi, et sur ceux de M. Blondel, which appeared in 1689, is full of strong reasoning, whence the author deduced a new system; but it contains little that is original, though it gives numerous references to what had previously appeared, and disposes the different parts in a judicious manner. M. de Montalembert's system of casemated and reverse fire has been in part adopted in the splendid fortress of Alessandria, in Italy, which was constructed under the direction of Napoleon.

Of the more recent treatises on fortification, that of M. de Bousmard, entitled Essai Général de Fortification, d'Attaque et de Défense des Places, dans lequel ces deux Sciences sont expliquées et mises l'une par l'autre à la portée de tout le monde, is very elaborate and complete, and enjoys a deservedly high reputation for accuracy and research.

Carnot's Traité de la Défense des Places Fortes was written to serve a temporary purpose; and the exaggerated celebrity which it acquired on its first appearance has been succeeded by an equally unfounded neglect. The more prominent innovations recommended in this treatise were first, an alteration, which, however, was not original, in the trace or outline of the polygon; secondly, the suppression of the exterior revetment of the covered-way, known as the counterscarp; thirdly, the detachment of the escarpwall from the rampart, and the construction of the latter without revêtement; fourthly, destructive personal conflict with the besiegers by means of frequent sorties; and, lastly, vertical fire as the basis rather than the accessory of the defence. With regard to these innovations, all of which the reader will find ably discussed in Jones's Journals of Sieges in Spain and Portugal, vol ii., it may be remarked that by means of an increased expenditure for retrenchments and casemates, as recommended by Carnot, the strength of particular portions of the polygon may be increased; and that, if he has failed in tracing a perfect front, founded on the basis of Montalembert's system of casemated and reverse fire, he has at least rescued a valu

able suggestion from unmerited neglect, and rendered an | of earth, the Italians completed the system of bastioned important service to science by directing the attention of defence, which, notwithstanding all the modifications of In military men to means likely to create a barrier against the French, ought to be called the Italian system. the growing powers of the attack. this system, whilst imitating the construction of the old towers by using casemated or masonry-vaulted chambers for artillery, in addition to the guns mounted on the rampart, the Italians placed the musketeers on the banquette of the parapet, and made them fire over it. Now this arrangement of the musketry fire is an essential characteristic of the Italian system, and the reliefs of the several works are by it restricted within certain limits, as it is necessary so to determine the levels of the opposite flanks that the fire along their superior slopes shall defend the whole of the intervening curtain: but there are other modes of using musketry as well as artillery fire in the defence of the ditches, and on these were founded other systems of Fortification.

The first Italian writer on Fortification was Tartaglia, whose work was published in 1546; but the really first writer on the science was Albrecht Dürer, a great painter, sculptor, and engraver, and a civil and military architect, whose work is dated 1527, being published one year before his death. This remarkable man founded his system on the old circular tower-forts C, fig. 62, with which the dead space is much less than with square tower forts, and, enlarging the towers to an enormous extent, he adopted the name "Bastei," or in the plural "Basteien," for his new work. As attention was at this early period more directed to cannon than to the rude musket as an instrument of defence, it was natural that the latter should be less considered in these arrangements than the former. Dürer based his systems on the principle that the defences of basteien or other works which depend only on the cannon placed on their terrepleines may be effective whilst the enemy is at a distance, but cannot be so when, under cover of his epaulements, he has reached the ditch; and

leaving therefore to the cannon on the terrepleine the task of firing upon the enemy's troops and batteries at a distance, he placed cannon and musketry either in vaulted galleries running along the base of the escarp, or in caponnières, also vaulted or casemated works, built across or transversely to the ditch. The great circular bastei of his third and most improved system was no less than 130 yards in diameter, with an escarp 120 feet high, a ditch in front 100 feet wide, and a massive envelope, about 80 feet thick and 100 feet high, formed of earth with thick masonry revêtements both in front and in rear, as a mask between the main work and the counterscarp. Such gigantic proportions as these have led many to consider Dürer as little more than a speculative writer, but this would be an unjust estimate of his real merits. Reducing his works to more reasonable dimensions, they would, with proper modifications, have become practicable, and they have afforded many useful hints to the scientific engineer. The defects of the circular form were compensated by the grazing fire of the caponnières in his system, and the main work was retained in an effective state by the cover afforded to it by the envelope. We shall have occasion to refer again to Dürer, but in the meantime it may be said that whilst the Italians are properly considered the originators of bastioned systems with an earthen parapet over which the musketry fire is directed, Dürer has an undoubted claim to be considered the author of the systems of Fortification in which casemated defence in the main works and in the caponnières becomes the essential characteristic-a system which has proved more fertile in results in modern times than the bastioned system.

The Traité de Fortification Souterraine, suivi de quatre Mémoires sur les Mines, by M. Mouzé, lieutenant-colonel of engineers in the French service, published at Paris in 1804, is the most complete work on the subject of which it treats which has yet been given to the public. Subterranean Fortification is a branch of the art which, until recently, was wholly neglected in England, and in which British engineers were far behind their brethren of the Continent. We learn from Colonel Jones's work on the Peninsular sieges that the duke of Wellington's army in Spain was without a trained sapper or miner until late in the year 1813. In this respect things are now changed, and in the Engineers the army has the assistance of a body of men well instructed in the duties of the trench, the sap, and the mine, though it cannot be said even now that the corps of Engineers is of sufficient strength. Nor can this corps, as some contend, be supplemented by civil labour in time of war. Discipline is the foundation upon which the whole military system rests, and the highest skill, unless strictly subordinated to it, will avail little in war. The undisciplined labourer, who, under ordinary circumstances would have worked with the greatest effort, would be found worse than useless if forced to work on his knees in a sap, exposed to an enemy's fire, or upon his back in a countermine, with the knowledge that the enemy's miner, though he cannot fix his precise position, is probably within striking distance.

Before going further it is desirable to recur to the earlier methods, and to investigate the manner in which the ancient arrangement of a wall, with round or square towers, passed into the present systems of defence. Fig. 62 will explain the natural and probable manner in which


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FIG. 62.-Illustrating the Growth of the Bastion. the old tower or rather tower-fort (baluardo) grew into the pentagonal bastion. If, for example, lines of defence be drawn from the extremes of two adjacent curtains to the angles a and c of the square tower-fort A, a space would be left, cba, unseen from the adjacent forts B and C, and therefore undefended, except by downward or vertical fire from machicoulis, or projections from the walls supported by corbels made for the purpose. Such a space would be turned to account by the besiegers in fixing their scaling ladders; and the change of the straight line ac into the two faces cb, ab seems but the result of a self-evident necessity. As the work became enlarged, the portions of the fort within the connecting walls m, m were omitted, and the flanks dc, fa alone remained of the old wall, forming with the faces the bastion fabcd, which only required to be improved in proportions to become the bastion of modern times. It is, however, said that the towers were sometimes placed with an angle salient as in B, and if so, omitting the portions nn, the resulting bastion has a strong analogy to those of Errard before mentioned. By using the old wall merely as a retaining wall, and as an obstacle against escalade, and adding to it a rampart and a parapet

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Plate III.

1. Vauban's First System.

Before commencing to draw a plan of Fortification, it is usual to determine the polygon on which to describe it. In Plate III., fig. 1, the angle taken is that of an octagon, and the length of the side is 360 yards. In constructing a fortification, a figure is determined on, as near a regular polygon as possible, within which the enceinte or chain of main works is to be contained. The enceinte or body of the place consists of as many bastions, connected with curtains, as there are sides to the figure, and each of these is made about 400 yards.

The Principal Line or Outline denotes the line by which the figure of the work is defined. This line is supposed to pass along the cordon of the escarp, and is that from which all the other parts of the work are set off.

The Exterior Side, or side of the polygon above mentioned as equal to 360 yards, is that upon which the Front of the Fortification is described, and it extends from the flanked angle of one bastion to the corresponding flanked angle of the next, as AB. These lines are bisected, and a perpendicular CD is drawn from the point of bisection towards the place, its length being proportional to the length of the exterior side and adjacent angle of the polygon, that is, one-sixth for the hexagon and all figures of a greater number of sides, one-seventh for the pentagon, and one-eighth for the square.

The Lines of Defence, AEG, BFH, are drawn from the extremities of the exterior sides through the point D, and produced to an indefinite length; and upon the lines so drawn are set off two-sevenths of the exterior sides, equal to 1029 yards, which mark the points for the shoulders of the bastions, E and F. The distance between these points is then laid along the continuation of each line of defence, viz., from E to G and from F to H, and the line connecting G and H is the curtain GH, from the extremities of which lines are drawn to the points marked off for the shoulders of the bastions, and these form the flanks. In this manner is drawn a Front of Fortification, and this being repeated upon each side of the polygon, completes the works of the enceinte, or body of the place.

Vauban divided his first system into three classes, namely, the little, the mean or intermediate, and the great. The first he used for small forts of four or five sides, citadels, horn-works, and crown-works, making the exterior side from 120 to 240 yards, the perpendicular equal in the square to one-eighth, and in the pentagon to one-seventh, and the faces of the bastions in each equal to two-sevenths, of the exterior side. In the mean or intermediate, which is adapted for all sizes of towns, the exterior side varies from 250 to 360 yards, the perpendicular is one-sixth, and the faces of the bastions are two-sevenths. In the great the exterior side varies from 360 to 520 yards. This was never adopted for all the sides of a place, but only in the case of one which happened to be near a river or a marsh, when the distances of the bastions should be so regulated that they may not be out of musketry range from one another. When the curtain becomes unavoidably too long, this defect is in part remedied by erecting on it a flat bastion, which is not so high as the rest of the works.

Ground which will admit of being regularly fortified throughout is seldom met with; nevertheless, the rules of regular fortification must be observed as nearly as possible. These are that the flanked angles should not be less than 60°, that the lines of defence should not exceed musketry range, and that the sides should be lengthened or shortened so as to obtain a well-proportioned front upon each. After an irregular place has been reduced to as regular a figure as possible, lines are drawn parallel to

the sides of the figure at the distance of about 50 yards from the houses, in order to give sufficient space for the rampart; and these lines form the interior polygon, which may be fortified outwards, by setting off the demigorges of the bastions, and raising their flanks at angles of 100° with the curtain. Or the exterior side may be formed and fortified inwards by drawing a line parallel to each of the interior sides; and when the angle is that of a polygon of more than five sides, the distance from the exterior to the interior sides should not be less than 100 yards. If a side extend from 360 to 520 yards, the perpendicular should be diminished to about 50 yards, and the faces of the bastions be made from 100 to 120 yards. All these dimensions should, however, be now much increased, and placed in relation to the range of the modern rifled small arm.

The Ditch or Fosse is an excavation of from 15 to 30 feet in depth, and from 30 to 50 yards in breadth, surrounding the rampart on the exterior side; the earth dug out of it serves for the rampart and parapet. The side of the ditch next the place is called the escarp, the side next the country is called the counterscarp, and is made circular opposite the salient angles of the works. In the figure arcs are described with a radius of 30 yards, opposite the salient angles of the bastions, tangents to which are drawn upon the shoulders of the neighbouring bastions, and thus the width of the ditch is fixed. The general dimensions of a ditch should be such that its excavation, or deblai, would produce sufficient earth, or remblai, for the formation of the works. The breadth varies from 30 to 50 yards, in order that, in passing across it to the assault, the enemy may, for a considerable time, be exposed to the fire of the works; and its depth must be such as to render difficult the escalade of the parapet, as well as to prevent the besiegers at the crest of the glacis from being able to see the lower part of the revetment of the escarp. The line of the counterscarp is drawn from the rounding at the salient angles of the bastions upon the shoulders of the bastions next adjoining, in order that the whole of the ditch may be defended by the fire of the flanks of the bastions. Ditches are of three kinds,-wet, dry, and such as may be rendered either wet or dry. The wet ditch is effectual against surprises or sudden assaults, excepting during hard frost, for it may be passed when frozen, as in the attempt to surprise Bergen-op-Zoom in the year 1814; but the number of bridges of communication, which require continual repair, and the difficulty of making sorties, render a wet ditch extremely inconvenient. A dry ditch, which is capable of containing works for its own defence, and by means of which communications round the works may more easily be maintained, is therefore preferable; but the third kind, which unites the advantages of the other two, is to be preferred to either. It is only in particular situations, however, that such a ditch is practicable.

The Tenaille, in the form given to it by Vauban, does not appear in the works of earlier engineers, but it seems to be naturally derived from the trace of Rimpler (1673), in which the middle flank is analogous in function to the tenaille, and occupies its position. It is a work placed in front of the curtain, and is thus formed:-its side faces are upon the lines of defence, and commence at points 11 yards distant from the shoulders of the bastion; its front face is 28 yards from and parallel to the curtain; its gorge is parallel to and 17 yards from the faces; its ends are parallel to the flanks of the bastions. The relief or height of the tenaille is determined by that of the neighbouring flanks, and it has a parapet of 71 feet in height and from 12 to 15 feet in thickness. The use of the tenaille is to cover the postern gate, which is often made in the curtain or flank; in

dry ditches it protects the troops formed behind it for the defence of the ditch; in wet ditches it covers the boats collected for the same purpose. It adds materially to the defence, as its fire, from being more horizontal and nearer to the bottom of the ditch, is of course very effective. It fires into the ravelin, and acts as a partial traverse to the escarps of the curtain and the flanks.

The Ravelin or Demilune was originally designed to cover the entrance to the fortress, but it soon developed into a most important work of defence, appearing as such in some early Italian Traces. Speckle, the great German engineer, who fortified Schlottstadt, Hagenau, Ulm, Colmar, Basel, and Strasburg, was, however, the first who recognized fully its importance, and laid down the rule that "great ravelins materially augment the defensive power of a bastioned system." Constructed upon this principle, the ravelins of Speckle were even larger than the ravelins of Cormontaigne's system, and covered nearly the whole of the bastions, their faces being directed on the salients of the bastions and their capitals, extending about 150 yards in advance of the exterior side of the polygon. Speckle was a man of science, having studied mathematics and military engineering in his youth, and perfected his knowledge by visiting and inspecting the most remarkable Italian Fortifications existing in his time. The Ravelin is a work constructed opposite the curtain, of two faces meeting in an outward or salient angle, with two demigorges formed by the counterscarp. Its use is to cover the curtain, the gates, and the flanks of the bastion, The Ravelin is constructed as follows. At 11 yards from the shoulder along the faces of the bastion, points are set off; an arc is described from the angle of the flank upon the perpendicular produced, with a radius of 160 yards; from this intersection lines are drawn towards the points set off, but not further than the lines of the counterscarp; at the intersection of the lines of the counterscarp or re-entering angle 6 yards are set off on the capital or line bisecting its angle, whence lines are drawn parallel to the lines of defence till they meet those of the counterscarp. Stairs, called pos-de-souris, are constructed here in order to facilitate the entrance into the ravelin from the ditch. The ditch in the ravelin, which is 24 yards in breadth, is made circular at the salient angle, and drawn parallel to the faces till it joins the main ditch.

The Covered-way was first described by Tartaglia in 1554, so that it must have been used at a very early epoch of Italian Fortification. Some of the first bastioned fortresses were, however, without this highly important work; and it is recorded that at the siege of Vienna by the Turks, the garrison having made a sortie, some companies were pursued by the Turks up to the counterscarp, and forced over it into the ditch. The necessity of being able to assemble under cover from the enemy's fire the troops intended for a sortie, and to afford them when repulsed a place for reforming and checking the enemy's progress, and thus insuring an orderly retreat into the body of the place, soon became apparent, and a Covered-way was supplied to works originally constructed without one. It is a space of 10 yards in breadth, extending all round the work between the edge of the counterscarp of the ditch and the parapet which covers it; this parapet is from 7 to 9 feet in height, and has a banquette for musketry. The superior part of this parapet forms a gentle slope towards the country, which terminates at the distance of from 40 to 70 yards; and this slope is called the Glacis. The Covered-way serves for drawing up troops in order to make sorties, and costs less than any other part of the works in proportion to the difficulty of taking it. In its salient and re-entering angles are open spaces called Places of arms.

The Salient Places of arms are the open spaces between the circular parts of the counterscarp and the prolongation

to intersection of the branches of the covered-way. The Re-entering Places of arms are constructed with two faces, forming a salient angle of 100° with the covered-way. The demigorges of the re-entering places of arms are generally from 24 to 30 yards; but when they are intended to contain a redoubt or intrenchment, they are from 40 to 48 yards. The Re-entering Places of arms are meant to flank the branches of the covered-way, and to contain the troops for its defence. The Salient Places of arms also serve for the assembly of troops for the defence of the covered-way. Traverses are constructed across the covered-way, upon the prolongation of the sides of the ravelins and bastions, perpendicular to the line of the counterscarp; they are from 18 to 20 feet thick, and serve to cover the troops from the enfilading fire of the enemy. Other traverses are constructed between these, so placed that the distances between them should not exceed 40 yards. Passages 10 or 12 feet wide, which are formed by cutting into the parapet of the covered-way round the ends of the traverses, afford thorough communication; the openings are closed when necessary by gates. In the more improved systems of Cormontaigne and others, these openings are constructed in such a manner that each can be defended by the fire from the traverse in rear of it.

The Glacis, as already stated, forms a gentle slope from the parapet of the covered-way towards the country, and varies from 40 to 70 yards in length. Its parapet cannot be ruined by the fire of the enemy; it covers the revetment of the body of the place; and being an inclined plane, it can be easily seen and defended from any part of the works.

The Rampart is an embankment of earth, and is the part of the works next to the town which it surrounds. It must be broad enough to carry the parapet and to provide sufficient space behind it for working the guns, and for the free passage of the defenders. The Ditch is immediately in front of the rampart, which is sustained by a revetment-wall of masonry or brickwork, called the escarp, strengthened by interior buttresses, called counterforts, placed at intervals regulated by the height of the wall and their own thickness. The Rampart is divided into the terrepleine, the banquette, the parapet, and the escarp. See Plate III., fig. 4.

The Revetment or face of masonry around the work on both sides of the ditch is intended to prevent the earth above from falling into the ditch. To ascertain the proper thickness of niasonry for revetments has always been a matter of considerable trouble and difficulty. General Sir Charles Pasley of the Royal Engineers has given the following rules. (1) For full-scarped revetments without berms, and for demi-revetments having berms equal to one-fourth the height of the masonry, the thickness of the wall should be seventeen-sixtieths, and the length of the counterforts or buttresses one-fifth of their height. (2) For demi-revetments without berms, the mean thickness of the wall should be three-tenths, and the length of the counterfort one-fifth of the height. (3) For counterscarp revetments, having only to retain simple terrepleines, the mean thickness should be one-fourth, and the counterfort one-sixth of the height. In all these cases General Pasley supposes the revetment to be countersloped, that is, to have the exterior slope in a vertical plane, and the interior face inclined, so that the base of the wall may be broader than its top by one-fifth of its height; and he also supposes the counterforts to be rectangular, and the intervals between their centres to be equal to four times their width. (4) He recommends that the foundations be made deeper in rear than in front,-that the courses of masonry form an angle with the horizon of about 10° till within a foot from the exterior, where they should be horizontal, to prevent the rain from penetrating, and that the interior face

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