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.