After the fire from the ricochet batteries has partly silenced that of the place, the trenches of communication are continued in zig-zag directions as before, and with the like precautions against being enfiladed from the fortress and its covered way. Those and all the succeeding trenches may be executed by full sap [Sm.], while the fire of the besieged continues in activity; but if that fire should become at any time relaxed, the opportunity may be seized of carrying on some part of the approaches in a more expeditious manner. When the heads of the oblique trenches are about half-way between the second parallel and the foot of the glacis, a deini-parallel, e e, &e., is carried out on each side of the produced capitals of the works attacked, till it meets the prolongation of the crest of the covered-way : its use is to protect the works which are to be executed in its front by a fire of musketry, which, being nearer, is more effectual than that of the second parallel. Howitzer batteries, f f, &c., are formed at the extre mities of these parallels for the purpose of enfilading the covered-way with shells, and thus destroying the palisades and traverses. As soon as the fires from the howitzers have produced some effect, the oblique trenches may be continued till they arrive at the foot of the glacis, on the capital of each of the works attacked, the branches being directed, as before, towards the exterior of any part of the covered-way from whence the enemy might enfilade them ; and each being prolonged towards the rear about 10 or n yards, in order to form places where tools and materials may be deposited out of the line of passage along the trenches. If the defenders should have established redans or redouts at the foot of the glacis, or should' have carried out counter approaches from any of the collateral works, in order to enfilade the trenches of attack, they must be assaulted and destroyed as soon as it is found that the fire from thence impedes the operations of the besiegers.
A third parallel trench, c c, may now be executed to connect the points of attack at the foot of the glaeis ; for this purpose the squads of sappers turn to the right and left from the head of each line of app roach, and meet each other, forming as they proceed a trench which is either rectilinear or curved towards the re-entering parts of the fortress, in order to enable the troops in it to fire less obliquely on the branches of the covered-way. This parallel should be made broader than the others, because considerable bodies of troops arc occasionally collected in it, and at intervals steps should be made in it long enough to allow a company of men to mount in line over the parapet. For the protection of the troops, the crest of its parapet should be furnished with sand-bags disposed so as to leave between them small intervals (loopholes) to fire through; and in this parallel, batteries, h h, for small mortars may be formed, in order that shells may be thrown from thence into the principal works. It is estimated that the third parallel may be finished by about the tenth night from the time of opening the trenches.
At this period, should any great necessity exist for hastening the surrender of the place, should also the garrison be weak, and should there be no retrenchments in the covered-way, it may be thought proper by main force to a.ssault, the latter, and immediately crown the glacis with batteries for breaching the works. In this case, the defenders should be driven as much as possible from the covered-way by a heavy fire from the mortar and howitzer batteries ; then the troops who are to make the assault, having been assembled in the third parallel, mount, at a signal a little before sun-set, over its parapet, and proceed rapidly up the glade. A party of men then extend them selves along its crest, and by their fire keep down that of the defenders on the parapets; in the meantime the sappers commence forming with gabions a lodgment on the crest, and the rest of the storming party endeavour to force an entrance through breaches made by artillery iu the palisades. When the defenders have thus been compelled to retire
behind the traverses in the covered-way, the assailants, who then become exposed to the fire from the parapets of the bastions or ravelins, retire into the lodgment which by this time may be finished on the crest of the glacis ; and during the night there may be executed trenches of communication from the lodgments to the third parallel. The lodgments thus formed may be afterwards connected together, and extended to the right and left if necessary; and in these the breaching batteries may be formed.
Should the fortress have small ravelins, like those of Vauban, the assault may take place at once on the inherits of a ravelin and of the two collateral bastions ; but if the ravelins advance far towards the country, as in Cormentaingne's system, it can take place only at the salient., of the ravelin on each side of one bastion; since if it were attempted to carry on the approaches between the two ravelins, the troops and sappers wonld become exposed to fires on their flanks and in rear as well as in front.
An assault [Armes] by main force is always attended with con siderable loss, and therefore, if time permits, it is preferred to continue carrying on the approaches by sap. For this purpose, supposing the place to be fortified with large ravelins (as in the cut), a trench is begun at about 30 yards on each side of the ridge between the two faces of the glacis before the ravelins only, and carried in a curvilinear direction till the two squads of sappers meet on the ridge about eight or nine yards in advance of the third parallel. From hence the trench is continued by double and direct sap along the ridge till it arrives at between 30 and 40 yards from the crest at the angle of the glacis; and at this spot, formerly, high breastworks, called trench cavaliers, k k, &c., about the thirteenth night from the time of opening the trenches, were raised on the prolongations of the branches of the covered-way, in order to allow a plunging fire of musketry to be directed into than branches. But as the fire of the defenders artillery may prevent the construction of such works, instead of them there are now formed batteries armed with small mortars from which balls or stones may be projected into the covered-way in order to compel the defenders to retire from thence. (Mortars for throwing stones arc called by the French pierriers.) After the fire from the cavaliers or mortar-batteries has obliged the enemy to abandon the advanced parts of the covered-way, double saps arc carried on towards the salient angle of the glacis, and when they have arrived at about 24 feet from that angle, the crowning of the glacis commences. This is performed by extending the sap along the crest on each side of the angle, and throwing up the earth towards the place in order to form epaulements for batteries. That the crowning trenches may be secured against the enfilading fires from the collateral works of the besieged, traverses are formed across them at intervals in situations where they may not interfere with the guns to be placed in the batteries; and, on account of the fire which the enemy may still keep up from the branches of the covered-way and the retrenchments in the re-entering places of arms (see the figure in art. FORTIFICATION), a portion of a fourth parallel, mm, should now be carried out in order that troops placed there may protect the sappers during the fermatiou of the batteries, and the artillerymen during the operation of breaching the ravelins. The crowning of the glaeis is sometimes extended along the faces of the re-entering places of arms ; and in this case trenches on those faces are connected with the fourth parallel by lines of com munication formed in serpentine directions.