y 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 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. These mines are fired by means of powder hose, or
by wires connected with a galvanic battery.

The defensive mines employed to blow up the besiegers' works, are
generally common mines with the lines of least resistance seldom greater
than fifteen feet. All the main galleries and principal branches of
mines for a permanent fortification are constructed at the same time
with the other portions of the work, leaving only the secondary
branches, chambers, &c., to be made during the siege. For the general
arrangement of these galleries, and the precautions necessary for their
protection from the operations of the besiegers, reference must be made
to treatises specially devoted to the discussion of this subject.

Mines can seldom be employed with advantage in works of slight relief,
and liable to an assault. But if judiciously arranged in the plan of
their construction,