An isolated wall, only two or three feet thick, may readily be demolished by exploding one or two casks of powder placed in contact with its base.  If the wall be five or six feet thick, the charges should be placed under the foundation.  For walls of still greater thickness it will be best to open a gallery to the centre of the wall, a foot or two above its base, and place the powder in chambers thus excavated.  Revetment walls may be overturned by placing the charges at the back of the wall, about one-third or one-quarter of the way up from the base.  If placed too near the base, a breach will be made in the wall without overturning it.

To demolish a bridge of masonry the powder should be lodged in chambers excavated in the centre of the piers.  When there is not time for excavating these chambers in the piers, a trench may be cut over the key of the arch, in which the powder is placed and exploded; or, the casks of powder may be suspended immediately under the arch, with the same results.  Where a saving of powder is of consequence, small chambers may be excavated in the haunches of the arch, and the mine carefully tamped before firing it.

Bridges of wood may be destroyed by suspending casks of powder under the principal timbers, or attaching them to the supports.

Palisading, gates, doors, &c., may be destroyed in the same way, by suspending casks or bags of powder against their sides; or still more effectually, by burying the charges just beneath their base.

To demolish a tower, magazine, or house, of masonry, place charges of powder under the piers and principal walls of the building.  In wooden structures the powder should be placed under, or attached to the principal supports.  Where time is wanting to effect these arrangements, a building may be blown down by placing a large mass of powder in the interior.  The powder may be economized, in this case, by putting it in a strong case, which should be connected with the walls of the building on all sides by wooden props.

Special treatises on military mining contain full instructions for regulating the size and position of the charge for the various cases that may be met with in the practical operations of field-engineering.

As applied to the attack and defence of a fortified place, mines are divided into two general classes—­offensive and defensive mines.  The former are employed by the besiegers to overthrow the scarps and counterscarps of the place, to demolish barriers, palisades, walls, and other temporary means of defence, and to destroy the mines of the besieged.  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