Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/459

FORTIFICATION 445 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, Col- mar, Basel, and Strasburg, was, however, the first who re cognized 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 bas tions and their capitals, extending about 150 yards in ad vance 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 Fortifi cations existing in his time. The Ravelin is a work con structed opposite the curtain, of two faces meeting in an out ward or salifent 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 shouldej 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 G 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 po,s-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-ivay 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 pur sued 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 reform ing 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 origin ally 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 con tain 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 con structed 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 pro vide sufficient space behind it for working the guns, and for the free passage of the defenders. The Ditch is im mediately 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 masonry for revetments has always been a matter of considerable trouble and difficulty. General Sir Charles Pasley of the Royal Engineers has given the follow ing rules. (1) For full -scarped revetments without berms, and for demi-revetments having berrns 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-re vetments 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 re vetment 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