Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/458

 444 Plate III. SYSTEMS OF PERMANENT FORTIFICATION. 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 perpen dicular CD is drawn from the point of bisection towards th-3 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 num ber of sides, one-seventh for the pentagon, and one-eighth for the square. The Lines of Defence, AEG, BFH, are drawn from the ex tremities 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 102^- 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 connect ing 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 re peated 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 pos sible. 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 getting off the demi- gorges 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 thenumberof 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 de fence, and by means of which communications round the works may more easily be maintained, is therefore prefer able; 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 practic able. 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 Pumpler (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 1 1 yards distant from the shoulders of the bastion ; its front face is 28 yards from and parallel to the curtain ; its gorge is par allel to and 17 yards from the faces ; its ends are parallel to the flanks of the bastions. The relief or height of the ten aille is determined by that of the neighbouring flanks, and it has a parapet of 7^ feet in height and from 12 to 15 feet in thickness. The use of the tenaille is to cover the pos tern gate, which is often made in the curtain or flank; in