ehind 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.

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