Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/496

 464 FORTIFICATION less considering, as has been so frequently urged, that systems, so-called, can only be looked upon as the exhibi tion of great principles, not as rigid rules for their applica tion. His ravelin is made very prominent, and the salient is formed into a traverse, or mask, casemated and armed with artillery. Within the ravelin there is a redoubt, and within that a casemated caponniere or bastionette. The ditch of the ravelin is closed by continuing the counterscarp across it with a glacis slope into the ravelin ditch, and by this arrangement the ditch of the redoubt in the ravelin is also closed. The counterguards, the higher and the lower bastions, form almost three lines of defence, of which two, the outer and inner, are powerfully armed with artillery. The peculiar characteristic of the system is, that the parapet is thrown back, and made in its trace independent of the escarp, so that, whilst the latter retains the usual straight line, the parapet is broken into several portions not in the same line, and thus secured from the effects of ricochet an arrangement of very great merit, and largely adopted in the Polygonal system. Haxo is probably better known to English engineers as the inventor of casemated batteries a I lfaxo than from his merits as an engineer, great as they were. These batteries are formed in the parapet, and though arched over with masonry, are covered with earth. They are open in the rear to the terrepleine, and the openings in front for the guns are continued into embrasures formed in an extension of the parapet at these points beyond its ordinary retired position in Haxo s system. These batteries are thus secured from the effects of the enemy s fire, and when the embrasures are masked are equally hidden from his view, so that they may at any moment suddenly open a powerful and unexpected fire upon the besiegers, Being open in the rear, and connected to gether by arched openings between every pair, the circula tion of air is sufficient to do away with the inconvenience from smoke, so generally complained of in casemated bat teries. The batteries a I Haxo have been used at Grenoble and Lyons, in the forts of Loyasse and Sainte-Foy, and in many English works. Their value has been much dimin ished by the range, penetration, and accuracy of modern rifled artillery. The work of the commandant of engineers, M. Choumara, entitled Memoires sur la Fortification, was published in 1847. In this treatise he maintains the principle that the direction of the parapet should be independent of that of the escarp, the latter being formed in straight lines, and considered permanent during the siege, whilst the former may be broken into several lines, and may be modified dur ing the siege so as to facilitate the defence in any direction. Haxo had in his lessons or studies pointed out the import ance of this principle, but Choumara was the first to advo cate it in print. The castle of Naples exhibits an early example of the reverse operation, a new escarp having been built in front of the ancient round tower forts, so as to change them at the base only into bastions, whilst the upper portion of the towers became retired and independent para pets. Choumara, not relying on the bent trace of his re tired parapet as a security from ricochet, proposed a traverse on the capital of his bastion, placed outside of the retired parapet, and 33 yards in length. This traverse, made 26 feet high and 78 feet wide at its base, would occupy less than two-thirds of the space of the twelve ordinary traverses required to secure from enfilade the faces of Choumara s bastions 164 yards in length, whilst it would cover not only the bastions but the flanks also. In addition to the tra verse or mask on the capital, Choumara proposed high tra verses, parallel to the flanks, at about 22 yards from the salients, which would riot only secure the faces from enfil ade, but would form secure or interior flanks, as cavalier flanks, commanding and firing over those in front. By making the cavalier flanks 98 yards long, and casemating them a VHaxo, 15 guns might be placed in each, and the covered-way of the bastion attacked would be commanded by 30 guns in addition to those of the ordinary flanks, whilst the traverse of the capital would secure the flank cavaliers from ricochet. The last and most remarkable suggestion of Choumara is the proposal to widen his ditch to about 50 yards, and leaving a passage of communication of 16 yards round the base of the escarp, to form an interior glacis, sloping up from the base of the counterscarp towards the summit of the escarp, and having a base of 34 yards wide, thus constituting a continuous mask round the escarp. In respect to countermines, Choumara proposed to replace the great galleries, which are usually made 6 feet high and 3 feet wide, and which are the great arteries of a system of mines, by large vaulted galleries from 16 to 20 feet wide, pierced through the counterscarp, and continued as far as the third parallel. Six of these galleries were to be formed in each front, being placed about 55 yards apart, and con nected together by minor transverse galleries or branches. Galleries of this magnitude would, in time of peace, be use ful as stores, and in time of war would greatly facilitate the operations of the miner. The details of military mining cannot be discussed here; but it may be stated that this subterranean warfare re quires great skill on the part of both besieged and besieger. The besieged has the advantage of having had the main galleries of his countermines formed beforehand, but in spite of this a war of mines must generally be in favour of a besieger, since every explosion of the mines of the besieged, however destructive it may prove to the assailants, must destroy some portion of the works of defence, whilst every explosion of the besieger s mines must operate upon his enemy only. Starting, however, upon the assumption that a fortress, except in situations which renders regular attack impossible, must ultimately fall, the real object of defence is to occupy the enemy for at least the time for which the fortress was designed to resist ; and the destruction of the lodgment or of the battery of a besieger may materially protract the resistance of the intrenchment formed in a bastion, and thus enable the besieger to maintain his ground the longer. The most simple form of mine, and that which may be most readily applied as an obstacle in the .way of the assailant, is the fougasse. It consists of a chamber placed at the bottom of a pit about 12 feet deep. The charge is placed in a wooden box, and both the charge and size of the box may be thus estimated. When the line of least resistance, or shortest line drawn from the centre of the charge to the surface of the earth, which in this case is the depth of the pit, is 10 feet, a charge of 100ft will produce an entonnoir or excavation, the radius of which is equal to the line of least re sistance ; and it has been ascertained that the volume of the exca vation varies with the charge, the Hue of resistance and the resist ing medium being the same, and that the volume varies also as tin- cubes of the lines of least resistance ; hence, therefore, if W repre sent the weight of the charge, B the bulk of the cnlonnoir corre sponding to 100 ft of powder and a line of resistance equal to 10 feet, and b that of the entonnoir corresponding to the charge W and the line of least resistance R, we have 100:&quot;W::B:J; but as B : b : : 10 3 : R 3, we have 100 : &quot;W: : 10 3 : W, and &quot;V = -^ R 3. Now let S = sidc of cubical box to contain the charge (55 ft of powder thrown loosely in filling one cubic foot), and S = / I~Q^^^ ^ nearly R in feet. The pits for fougasses vary generally from 8 to 12 feet in depth, and from 3 to 4 feet iu width, being made square, and are revetted with planks when the earth is not firm enough to stand without support ; and for this latter purpose gabions may be used of different diameters, so that the smaller may be slipped through the larger, which have been previously fixed. The box for the powder is well tarred, and when intended to be left in the ground for some time, it should be covered with tarred canvas and put into another box, also tarred both inside and outside. The charge is ignited by Bickford s fuze, or by a saucisson or linen tube about an inch in diameter, filled with powder, and enclosed in a wooden case well tarred. The saucisson and its covering should be laid in
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