Part 20 (1/2)

Fig. 66 presents on a larger scale the plan of the outwork constructed by Vauban. Before the three fronts of this work, demi-lunes with tenailles behind defended the approaches.

Four barracks were built at _a_. Cavaliers arose on the bastions, and the covered ways, with their _places d'armes_, were furnished with traverses. In the event of this work being taken, the main body of the fortress could still hold out some days.

Fig. 67 gives the sections of these works, which were cased with masonry, presenting a strong defence which only a regular siege could affect.

[Ill.u.s.tration: FIG. 67.]

But it is desirable to point out the reasons that determined the plan of this work, and the method adopted by the ill.u.s.trious engineer.

Vauban fortified according to the nature of the position, and was not one of those _esprits routiniers_ who, when once a certain system has made good its claims, insist on applying it on all occasions.

The fortresses, which, like that of La Roche-Pont, are situated at the extremity of a promontory and present only a narrow front to the besieger, a.s.suredly give certain advantages to the defence, since they have scarcely to fear more than one attack and are accessible only on one side; but this position is not without its drawbacks, especially if, as in the present instance, a fan-shaped plateau spreads outside the fortress; for then the besiegers sweep the defences with converging fires, to which the besieged can oppose only a narrow front unprovided with considerable flankments. On the east side the large bastion, in the middle of which Vauban had left standing the fifteenth-century tower, which thus gave him a good revetted cavalier, sufficiently flanked the eastern brow of the outer plateau; but on the western side such a flankment failed entirely, on account of the outward bend caused by the promontory. To obviate these disadvantages Vauban inclined his capital some paces eastwards.[58]

He had thought at first of suppressing the south flanks of the two extreme bastions, but in that case the exteriors of the east and west faces of these bastions would have been too slanting to sweep the crests of the plateau effectively, while the two curtains answered this object.

Besides, the enemy could not then, without risk, commence his trenches on the slopes of the plateau and rapidly approach fronts insufficiently flanked. Vauban therefore set out the plan of the great outwork according to the following method (Fig. 68):--To the outside he gave a length of 180 toises, or 1,156 feet. To the western side, _a c_, 1,120 feet; to the eastern side, _b d_, 1,054 feet--that is, he placed the points _c_ and _d_ according with the edge of the plateau; the two angles _a_ and _b_ being equal to one another. On the centre of the side _a b_ of the polygon he erected the perpendicular, _e f_, having a length equal to one-sixth of _a b_. From this extreme point, _f_, were drawn the lines of defence, _a g_, _b h_, on which the lengths of the faces of the bastion, _a k_, _b i_, were set off equal to two-sevenths of the outer side, _a b_. To find the flanks of the bastion, according to the method usually adopted in these defences, points _k_ and _i_, he described arcs of a circle, _k l_, taking _i k_ as the radius. The point of intersection of this arc with the line _b h_ gave the length and the direction of the flank of the bastion; but, not having been able to trace a regular half-hexagon, and the angles _a_ and _b_ being less obtuse than those of a regular hexagon, by proceeding in this manner, the gorges of the bastion would have been too contracted. Therefore, to determine the flank of the bastion, from the points _i_ and _k_, he let fall perpendiculars to the lines of defence, _a g_, _b h_, and the point _h_ gave the re-entering angle in the curtain, _h g_, parallel to the side _a b_. This exposed the flanks a little too much, but enabled them to sweep the outsides more effectively, and in this particular case that was the princ.i.p.al consideration.

[Ill.u.s.tration: FIG. 68.]

The width of the ditch of the main work was fixed at 112 ft. 7 in., to the rounding of the counterscarp; and this was determined by a tangent to this rounding, drawn from the angle of the epaule of the opposite bastion.

The ditches being dry, Vauban sunk cunettes in the middle, 23 ft. 4 in.

wide and 6 ft. 8 in. deep. Double caponnieres connected the tenailles with the demi-lunes. The demi-lune was set out as follows:--Taking _g k_ as radius, the arc _k m_ was drawn. Its meeting with the perpendicular, _e f_, prolonged, gave the point of intersection _m_, the salient of the demi-lune. From _m_, the face _m n_ was directed upon a point, _o_, taken on the face of the bastion at 31 ft. 3 in. from the angle of the epaule, _i_. The width of the covered way was fixed at 31 ft. 3 in., and that of the glacis at 124 ft. 4 in. The internal _places d'armes_ were 100 ft. across the demi-gorge and 133 ft. 4 in. along the faces. These _places d'armes_ were closed by traverses. The ditch of the demi-lune was 89 ft. 7 in. in width. The tenailles, _g_, constructed in the direction of the lines of defence, were 43 ft. 9 in. wide at the base.

Cavaliers were made on the bastions to obtain convenient views over the slopes of the plateau. Their faces and sides, parallel to those of the bastions, had to be placed at a considerable distance from the epaulements, in order that the un-cased external foot of the talus might leave the necessary room for the easy working of the pieces of artillery.[59]

The same method was followed for the sides _a c_ and _b d_. The width of the ditch was increased to 100 ft., and the great demi-lune, _t_, was so formed that its faces had a length of 332 ft. and its narrow sides 66 ft. The old bastions restored and enlarged, _u v_, were armed with cavaliers, and the escarp of these bastions was 6 ft. 6-1/2 in. higher than that of the bastions of the outwork,[60] which difference, for that matter, was favoured by the conformation of the ground.

All the escarps and counterscarps were cased with strong masonry, with counterforts in the terre-plein; as shown in the sections of Fig. 67 and Fig. 69.

[Ill.u.s.tration: FIG. 69.]

The works communicated with each other by posterns. As the fortress of La Roche-Pont was only a.s.sailable on one side, the number of cannon necessary for its defence might be, it appeared, in proportion to its extent, inconsiderable. The number amounted to thirty twenty-four pounders, ten twelve-and four-pounders to arm the demi-lunes.

At the end of the reign of Louis XIV., the efforts of the coalition were directed towards the north-eastern frontiers, and the garrison of La Roche-Pont did not get a view of the enemy. However, during the course of the eighteenth century, this fortress was kept in pa.s.sable condition.

FOOTNOTES:

[Footnote 57: See Fig. 60.]

[Footnote 58: See Fig. 65.]

[Footnote 59: See Fig. 67, the section on E, F.]

[Footnote 60: See Fig. 67, the sections on G H and I K.]

CHAPTER XVI.

_THE SEVENTH SIEGE._

On the 31st of December, 1813, the grand army of Bohemia, one hundred and eighty thousand men strong, and commanded by Prince Schwartzenberg, crossed the Rhine at Bale, entered Switzerland, and penetrated into France. Its right bore down on Belfort, Colmar, and Strasburg, its centre marched for Langres, its left for Dijon. The same day the Prussians were crossing the Rhine at Mayence. The invasion of the eastern frontier had to be met by sixty thousand men, at most, echeloned between Epinal and Langres; and this force consisted only of dispirited soldiers and of recruits scarcely knowing how to use their arms. Prince Schwartzenberg's army--which had some reason to fear being attacked on the side of Savoy by the forces commanded by Prince Eugene, and knew that the _debris_ of the army of Spain had orders to make for Lyons with all speed--wis.h.i.+ng to secure its base of operation, left detachments before Besancon, Dole, along the Saone and between Dijon and Langres, with injunctions to occupy the most favourable strategic position. La Roche-Pont was to be taken; and the generalissimo of the Bohemian army imagined that it was not in a condition to hold out eight-and-forty hours, for he knew that it had no garrison, and that the citizens were little disposed to defend themselves.