462 FORTIFICATION better representative of a mine than it now is. At present a 13-inch shell weighs, when loaded, 200 pounds, and can carry a charge of 11 Ib of powder, which is ample for break ing the shell and scattering its fragments, but is insufficient for producing the effects of a mine. The Dutch System of Coehorn deserves especial attention, and is represented in Plate IV., fig. 1, which exhibits his first system. The great characteristic in this system is the combination of wet and dry ditches, and the use of covering A works, or couvre-faces, intended to preserve the body of the place from injury till an advanced period of the siege. These envelopes were first proposed by Diirer, and it is worthy of note that the remarkable orillon of Coehorn is a reproduction on a modern scale of the complicated case- mated structure of one of Diirer s basteien. Coehorn was well acquainted with the principles and the systems of the eminent German engineer Speckle, and adopted them when applicable to his purpose. The profile (fig. 69) may enable FIG. 69. Profile of Coehorn s Bastion A, inner or upper t>asti6fl, B, outer or lower bastion; D 1 , dry ditcli between the two; D 2 , wet ditcli. the student to appreciate the difference between the dry ditches of Coehorn and the narrow passage afforded by the fausse-braye of the older engineers. The profile exhibits also the loopholed galleries of the counterscarp, by which a reverse musketry fire is directed on the revetted escarp of the inner rampart, a system of defence which has since been generally adopted, and is most valuable when a secure communication can be kept up between the galleries and the work which they are in tended to defend. After the great siege of Corfu by the Turks, and its successful defence by Schullemburg, some Dutch engineers who had been invited to Corfu by him, and had taken part in the defence, were employed in adding detached forts to the old Italian bastioned fronts. In these works the ruins of numerous examples of loopholed galleries and loopholed traverses may be observed ; and they de monstrate that though Coehorn adopted in his writings his reliefs to the aquatic sites of Holland, he developed prin ciples of defence which were equally applicable to other sites and other countries. It has been argued by Bousmard and others that an opening would be formed by shells through the couvre-face, and that the flanks of the bastions would be thereby exposed to the fire of the counter batteries on the glacis ; but it remains to be proved whether such an opening through an earthen mass could really be effected by the horizontal fire of shells ; and the French translator of Zastrow, M. Neuens, captain of artillery, justly remarks that, " if shells fired horizontally into earthen works are so efficacious in destroying them, such shells must be a still more powerful instrument in the hands of the defenders for destroying the besiegers batteries." Zastrow observes that, if we admit with Coehorn and others that, though the besieger may succeed in destroying a few feet of the parapet of the lower or outer face of the bastion, he would in vain, by firing shells horizontally into its mass, endeavour to lay open the counterscarp galleries, it must be admitted that he would, on mounting the low face, find himself in a most critical position, as all the defences, both direct and reverse, of the dry ditch, would remain uninjured, and be in full action against him. These dry or inner ditches, which facilitate the war of sorties, and the reverse or counterscarp galleries, are defensive arrangements of great merit, and may, by modification of profile, be adapted to any site ; though the advantage dry ditches offer in such countries as Holland, where the besiegers cannot excavate in them with out coming to water, cannot be expected in other sites, and must be made up for by stone pavements, or other contriv ances likely to embarrass the besieger in his excavation. Coehorn assumes the plane of site to be 4 feet above the level of the water, and the dry ditch of his bastion is at its centre on the level of the water, so that a passage by sap becomes impossible, as the spade sinks at once into water ; but near the escarp and counterscarp the ditches arc 2A feet below the plane of site sloping on each side towards the central portion. The breadth of the dry ditch of the bas tion is 98 feet, and that of the wet ditch before the salient 148 feet. All the slopes are at an angle of 45. The whole breadth or thickness of the couvre-face, measured at the water level, is 52 feet, so as not to afford space for the besiegers batteries, and its relief of construction only 12-^ feet. In the ravelin the relief of the low face is 10 feet, of the high face 18 feet, and the height of its revetment 8 feet; in this work also the thickness of the low face would not afford space for batteries. The width of the dry ditch is the same as that of the bastion. These few details, with an examination of Plate IV., and of woodcut 69, will enable the student to comprehend the general principles of this great engineer; and it is rather by tracing out the ideas of a master mind, as exhibited in the peculiarities of his plans, than by studying the plans themselves as wholes, that the engineer will acquire practical knowledge which will enable him to vary his own projects, so that they may really be the best suited for the ground he is working upon. Coehorn never restricted himself to the rules even of hi3 own system. In fortifying^ Groningen he was required to construct works on an eminence which commanded the tovn and he adopted a trace towards the exterior of ten- ailles, the gorges of which were closed by small bastioned fronts constructed by walls which should be easily breached from the main works when the enemy had succeeded in gaining possession of any one of the intervening redans. By this curious combination of the tenaille and bastioned systems, Coehorn gave an undoubted proof of his superiority to the narrow prejudices which often prevent the adoption of the system best suited for the place. METHODS OF BOUSMARD, CARNOT, CHASSELOUP, DUFOUR, NOIZET, HAXO, AND ClIOUMARA. It would be wrong to dismiss the subject of Bastioned Systems without at least some more reference to the works of these distinguished engineers than has been given in tracing the history of this subject. Bousmard makes the faces of his bastions as well as their flanks curvilinear, the former convex, and the latter concave outwards ; but though by this arrangement the effect of ricochet fire may be dim-
