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. The measures of defence during this period must embrace every thing calculated to retard the works of the besiegers. This may be most effectually accomplished by maintaining a constant fire of grape and musketry on the heads of the sap, and throwing grenades, shells, &c., into the trenches, to harass and destroy the workmen. As the musketry fire of the besiegers now becomes very destructive to the artillerists at the guns, strong musket-proof blinds are arranged to mask the mouths of the embrasures when the guns are not in battery, and also sloping blindages to cover the men when serving at the pieces. The possession of the outworks should be disputed inch by inch, and when the besiegers have reached the ditch of the body of the place, sorties, and every species of projectile, should be employed to drive off the sappers, and to retard the construction of their works. In fine, all the resources of the engineer's art should be put in requisition for the defence of the breach, and the final assault should be vigorously resisted by the bayonet, and by a well-sustained fire from all the collateral works.