The epaulements raised by the besiegers between the salient angle of the glacis and the first traverses on each side are to serve as counter batteries, whose use is to ruin the parapet and dismount the guns in the faces or flanks of the collateral works, in order as much as possible to prevent the enemy from opposing by musketry or artillery the passage which is to be effected across the ditch; and the epaulements between the first traverses and the re-entering places of arms serve for the breaching batteries. [BssAcu.] The crowning batteries on the glacis of two ravelins, rr, are supposed to be finished about the six teenth night from the time of opening the trenches.
While the breaches are being formed, the passages by which the descents into the ditches of the ravelins are to take place are com menced. These are either open trenches or subterranean galleries cut in inclined planes through or under the covered-way opposite the breaches; each passage thus terminates at a perforation in the couuter scarp about 3 feet above the bottom of the ditch if dry, or at the level of the water if it contain any. The sappers throw fascines into the ditch if dry, till the heap is sufficiently high and thick to secure them from the fire of the defenders, and then getting in, they form by sap a trench and parapet extending some way up the breach itself. Prepara tions are afterwards made for the assault.
For this purpose the interior of the batteries and the passages lead ing to the ditches are during the night filled with troops, in whose rear are bodies of sappers with their materials; and early in the morn ing, after a heavy fire has been for some time kept up from the batteries in order to drive off the defenders, the troops charge up, and endeavour to keep the enemy engaged while the sappers execute lodgments on the breaches by filling their gabions with the loose earth ; as soon as these are finished, the storming parties retire behind them, and from thence keep up a fire upon the enemy. These lodg ments should if possible be on the tops of the breaches, but if the interior of the ravelin is retrenched by a redout, as 8 8, whose fire com mands those spots, the lodgments must be formed on the ascent, that they may not be immediately destroyed. It is estimated that the two ravelins may be taken about the eighteenth night.
If the ditches contain water which cannot be made to flow off, there may be formed across them solid causeways consisting of fascines laden with stones to make them sink, or of casks or gabions having their axes in horizontal positions; or floating-bridges of timber-legs, casks or pontoons may be constructed; and by any of these means the troops may pass over to the assault. From the lodgments just men tioned trenches are carried on by sap directly to the top of the breach, and from thence turning to the right and left they are continued about halfway down the faces of the ravelins: their extremities being made to join the parapets of these works.
The redouts in the two ravelins are next to be taken ; and to effect this object, it being supposed that the faces of the ravelins are too narrow to allow room for forming batteries on them, either a portion of the ravelin must be blown up in order to allow the fire of the breaching-batteries on the glacis to act against the redouts through the apertures, or else the redouts must be breached by undermining them. Should the latter method be preferred, a trench or gallery is cut through the mass of each ravelin, and a sap is carried across the ditch of the redout; the miner then, being secured against the effect of the enemy's grenades by timbers placed in inclined positions leaning against the scarp of the redout, cuts through that scarp and forms chambers for the reception of gunpowder. This being fired, a part of the rampart will be destroyed and a breach formed. An assault is then made by troops, and the defenders being supposed to be driven out of the works, a battery may be raised along the gorge of each redout in order to compel them to quit the tenailles, t t, in the main ditch. The redouts of the ravelins being taken (probably about the twenty-first night), the defenders will also be obliged to retire from the rear extremities of the latter works; and the besiegers occupying those extremities, their fire from thence commanding the interiors of the redouts in the re-entering places of arms, these last must also be abandoned.
The approaches towards the bastions may now be recommenced, as the fires from the ravelins are no longer to be apprehended ; therefore a double sap is carried on from the curved trench in the third parallel directly along the ridge of the glade, till it begins to be plunged into by the fire from the bastion; it then proceeds In a kerpentino direction till its heed arrives between the portions of the fourth parallel already formed. This parallel is then completed, and under the protection of the fire from the troops stationed In It, the counter and breaching batteries before the bastion are formed. By the former the fire from the guns in the flanks of the collateral bastions is partly silenced, and by the latter the breach in the faces of the opposite bastion Is effected. The passage through the counterscarp and a trench across the main ditch are then executed, and en moult may be made up the breach of the bastion, similar to those which had been made up breaches of the ravelins ; the defenders being repelled, a lodgment may be formed, and unless the bastion is strongly retrenched, it may be expected that the place will now be surrendered. It is estimated that the assault of the bastion may tale place about the twenty-sixth night from the time of opening the trenches; but a good retrenchment [RETRENCHMENT] in a baaetion may enable the defenders to hold out ten or twelve days longer.