The latter are employed by the opposite party to blow up the besiegers' works of attack, and to defend the passage of ditches against an assault.

Small mines called _fougasses_ may be employed for the last named object.

The _shell-fougasse_ is composed of a wooden box filled with one or more tiers of shells, and buried just below the surface of the earth.

Sometimes a quantity of powder is placed under the shells, so as to project them into the air previous to their explosion.

The _stone fougasse_ is formed by making a funnel-shaped excavation, some five or six feet deep, and placing at the bottom a charge of powder enclosed in a box, and covered with a strong wooden shield; several cubic yards of pebbles, broken stone, or brickbats, are placed against the shield, and earth well rammed round, to prevent the explosion from taking place in the wrong direction.