SAP.

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'sap on the capital, or two single saps from the inner ends of the trench cavaliers, arethen pushed forwards to within six yards of the crest of the glacis, where thelodgements are commenced.

Though the principles observed in the former part of the attack are adheredto during the subsequent operations, yet their details will vary considerably whenapplied to fortifications of different constructions. Those, however, who clearlyunderstand the attack of one kind of fortification, will have little difficulty incomprehending the modifications applicable to the other. We will therefore selectas an example a large polygon of the French Modern System, which may beconsidered the ordinary bastioned system in its most improved form. From thegreat saliency of the ravelins, a besieger must take two of them before he canreach a bastion. We will therefore suppose our attack to have been carried as faras the salient place of arms of two ravelins, it being the besieger’s intention topenetrate to the bastion between them.

While the approaches are made from the third parallel on the two ravelins,a double sap is pushed forward on the capital of the bastion, and a fourth parallelLodgement on is constructed to connect it with the trench cavaliers. The lodgement on the glacisthe glacis. 0 f eac j i rave jj n colls i s ts of a trench, commenced by double or half-double saps,

parallel to the crest, and at a distance of six yards from it, so as to leave asufficient parapet, to protect which from enfilade and reverse fire from the faceof the bastion and the opposite ravelin, traverses are made by single sap at rightangles to it. The lodgement on each flank of the attack being extended as far as theprolongation of the face of the ravelin, is converted into a battery to breach the faceof the bastion through the opening afforded by the ditch of the ravelin. Thelodgements on the other side are extended as far as the third traverses of the covered-way, the double sap is continued on the capital of the bastion as far as the foot ofits glacis, and a fifth parallel is constructed, connecting it with the lodgements onBreaching bat- either side, which are then converted into batteries to breach the ravelin. While!enes ' these breaching batteries are constructing, the besieger commences his descent into

Descent into the the ditch of each ravelin, by means of a great gallery of a mine extending from thedllcl1, lodgement to the bottom of the ditch. The gallery of descent may he on either side

of the breaching battery, hut it is better to construct it on the side next the salient,as the ascent of the breach will then be better covered from the fire of the bastion.It should never have less than three feet of earth above its roof; its slope should notbe steeper than one in four, and should be so regulated as to reach the bottom of theditch, when dry, three feet below its surface, to meet the bottom of the trenchcrossing it. It should enter a wet ditch a foot or two above the level of the water.It will often occur, particularly with wet ditches, that from the inconsiderable heightof the counterscarp, the gallery of descent will not have sufficient earth over its roofwhen passing under the covered-way; in that case it should, if possible, be carried,under a traverse, and the passage round the traverse filled up with earth or fascines.Another mode of descending into a ditch is to drive a gallery from the lodgement onthe glacis to the hack of the counterscarp revetment, and there lodge a chargeof powder to breach it. This is called Mowing in the counterscarp. The breachthus made will form a ramp into the ditch, to which a communication may be madefrom the lodgement on the glacis.

fcchwhcn d^! When the ditch4s dry ’ a passage across it is effected simply by means of a trenchmade by sap, extending from the opening of the gallery to the foot of the breach,the flank defences being subdued by the battery on the crest of the salient place ofarms, assisted by musketry and vertical fire. After the breach has been madepracticable, the fire of the breaching battery may be employed to drive the garrison