Third period.—­After the completion of the third parallel, the crowning of the covered way may be effected by storm, by regular approaches, or (if the work is secured by defensive mines) by a subterranean warfare.

In the first case stone mortar-batteries are established in front of the third parallel, which, on a given signal, will open their fire in concert with all the enfilading and mortar batteries.  When this fire has produced its effect in clearing the outworks, picked troops will sally forth and carry the covered way with the bayonet, sheltering themselves behind the traverses until the sappers throw up a trench some four or five yards from the crest of the glacis, high enough to protect the troops from the fire of the besieged.  It may afterwards be connected with the third parallel by boyaux.

When the covered way is to be crowned by regular approaches, a double sap is pushed forward from the third parallel to within thirty yards of the salient of the covered way; the trench is then extended some fifteen or twenty yards to the right or left, and the earth thrown up high enough to enable the besiegers to obtain a plunging fire into the covered way, and thus prevent the enemy from occupying it.  This mound of earth is termed a trench cavalier, (O).  Boyaux are now pushed forward to the crowning of the covered way and the establishing of breach batteries, (J).  Descents are then constructed into the ditches, and as soon as these batteries have made a breach into the walls of the bastions and outworks, the boyaux are pushed across the ditches and lodgments effected in the breaches.  The demi-lune is first carried; next the demi-lune redoubt and bastion; and lastly, the interior retrenchments and citadel.  In some cases the breaches are carried by assault, but the same objection is applicable here as in the storming of the covered way; time is gained, but at an immense expense of human life.

If the place is defended by mines it will be necessary for the besiegers to counteract the effects of these works by resorting to the slow and tedious operations of a subterranean warfare.  In this case a fourth trench is formed in front of the third parallel; shafts are sunk in this, about six yards apart, for establishing overcharged mines; as soon as the galleries of the besieged are destroyed by the explosion of these mines, the covered way is attacked by storm; other mines are established on the terre-plain of the covered way to destroy the entrance to the galleries, and thus deprive the besieged of the use of their entire system of mines.