It would be difficult to say how many projectiles it would require to render such a work untenable; since we have seen in the siege of Paris, that a marine battery erected on tipped earth on the military road between the forts of Rosny and de Noisy, armed with three guns, had been a mark for the German shells during twenty-four days, without any of its guns having been dismounted, or its escarp suffering more than could be made good each night.
Permanent works should, however, only be established with the utmost circumspection:—1. Because they require a very considerable outlay. 2. Because they are necessarily familiar to, and for a long while studied by the enemy, who takes his measures accordingly.
The important point is to possess an accurate acquaintance with the ground to be defended, and only to establish permanent works in second line, and on points incontestably favourable for defence, supposing an artillery of even a still longer range than the present.
Every centre to be defended should therefore possess works sufficient to prevent a surprise; and in addition at a distance of six to eight thousand yards, a line of forts, crossing their fires if possible, or at any rate connected by strong batteries; and lastly, at a distance of about four thousand yards, positions previously examined and known, suitable for placing very simple works of field fortification, but which at a given moment may offer a resistance sufficient to permit movements on a grand scale—and delay the formation of an enemy's batteries.
Fig. 85.—Defensive System of the Great Intrenched Camp.
In applying these principles to the defence of the town of La Roche-Pont, whose strategical position is one of major importance, since it connects Besançon with Dijon, and forms a salient on the flanks of an army which manœuvres from Epinal and from Vesoul upon Langres and Chaumont, it would be necessary to construct around this place, whose old fortifications are no longer of any value ([Fig. 85]) eight forts, A, on the brows of the plateaus which surround the town, and thirteen batteries or redoubts, B, a little behind or to command the meeting of the river Abonne with the Saône and enfilade the valleys. Thus the group of roads which from the point C lead to Langres and Champlitte, from D to Dijon and Beaune, from E to Besançon and Dôle, from F to Gray and from G to Saint-Jean-de-Losne, would be occupied. A tête du pont H, protected by batteries which would dominate the Saône, would enable an army to manœuvre on both banks.
This passive defence would occupy a perimeter of forty miles, and the zone of action of the permanent works, a perimeter of sixty miles.
The purely passive defence would require, for the eight forts, four thousand eight hundred men; for the thirteen batteries two thousand men; for the guard of the intermediary trenches and the service of the enceinte de préservation, not including the forts and batteries, eighteen thousand men; reserve in the fortress, two thousand five hundred men; total, twenty-seven thousand three hundred men, whilst the effective investment would require an army of one hundred thousand men. But if this great intrenched camp contained an army of a hundred and fifty thousand men in addition to the troops necessary for the passive defence, this army could besides, on a very extended perimeter or on some advantageous points, occupy within the zone of action of the forts, an enceinte de combat defended by field works, which would enable it to assume the offensive at the opportune moment.