Referring to the general defensive system shown in Fig. 83, and required the plan of one of the forts, A, the result will be Fig. 84, giving the work at the lower level at C, and at the level of the batteries at D. The counterscarp should be cased up to at least sixteen or seventeen feet above the bottom of the ditch. The escarp should be made with tipped earth. The masonry works should all be covered and secured from being enfiladed; they form casemates inside E. Beneath the terracings, powder magazines, F, and the passages communicating with the _oiseaux_ or lower _orillons_, G, protected by the counterscarp and the covered way, and which are used only if the enemy attempts to pass the ditch. The faces and flanks on the outside are planned on angles sufficiently obtuse to cross their fires. The two faces in the plan (Fig. 84) may be armed with eight guns and the flanks with six guns. This work is separated from that of the gorge by a traverse that efficiently protects this gorge, which possesses its flanks, armed with four guns and its orillons. From the work of the gorge is a communication into the fort by a covered caponnière, forming traverse in the direction of the _capital_ or centre line. The gorge is defended by a curtain for the riflemen, and, at need, for small pieces of artillery. At need also, on the terre-plein of the work of the gorge may be mounted guns of long range, affording an indirect fire in the circumference of a semi-circle, over the great traverse H, if the parapets of the faces are damaged by the fire of the enemy. Fausse-braies consisting of palings are fixed in the ditch at ten feet from the base of the escarp, to hinder the fallen _débris_ of the escarp from filling up the ditch, and to enable its passage to be defended. The well-covered internal masonry works prevent the accumulation of earth on the interior platforms, and afford casemates, which enable the garrison to take rest in perfect security; at any rate along the two faces and the great traverse. Blindages can be set up on the traverses of the batteries, and can be easily repaired every night, as well as the escarp of tipped earth. 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.