The armament of the forts would consist of a hundred and sixty guns of large calibre, and that of the batteries of forty-five guns of long range. With the reserve park this would give a total of two hundred and thirty pieces of ordnance. Harnessed guns would be also necessary to support the trenches of the passive defence. Each fort would cost about £48,900. The eight together £391,200 Each battery or redoubt would cost about £6,000. The thirteen together 78,000 -------- Total £469,200 ======== To make head against an invasion at all points at once has always been a difficult problem to solve; and it is still more so if we confine ourselves to the defensive, for the enemy starts from a base of operation with a view to concentrating himself upon a point unknown to the defenders. The latter has therefore only a line to oppose to the apex of a triangle of action. He must limit himself to preserving the heart of the country and certain districts that are already naturally protected, and which allow him to operate on the flanks of the invading forces; regions behind which lie extensive tracts of country from which supplies may be drawn. Let us suppose that on the zone of defence of which La Roche-Pont forms the centre, an army of a hundred and fifty thousand men were assembled, which was able to reach Belfort through Besançon or by the road from Vesoul to Langres through Gray or Châtillon-sur-Seine through Dijon, these towns of Besançon, Vesoul, Langres, and Dijon being themselves in a condition to arrest the enemy's advance: the latter in attempting to make his way to Paris would be obliged either to watch this zone with an army of three hundred thousand men, or to move, and that with extreme caution, along the Lunéville and Nancy routes. If a defensive zone of the same importance is disposed on the north, a point becomes very dangerous, especially if the capital is provided with an _enceinte de préservation_, and an _enceinte de combat_ allowing an army to manoeuvre. Three hundred thousand men therefore would be required to watch the defensive zone of Burgundy and the same number that of the north, and four hundred thousand men to invest Paris; total, one million, without reckoning the troops required for keeping open the communication between the three armies and guarding the base of operations. It follows that the slightest check might entail a serious disaster. As the army of Metz under the ramparts of the town was not able to manoeuvre, the two hundred thousand men employed in blockading it were rendered inactive for two months.