charge; and that empty galleries may be
destroyed by overcharged mines at the distance of four times the line of
least resistance.

By means of the deductions of physico-mathematical theory, and the
results of experiments, rules have been determined by which the miner
can calculate, with much accuracy, the charge necessary to produce a
required result in any given soil.

In the earlier stages of the history of this art, mines were only used
to open breaches and demolish masses of masonry; but in later times they
have been employed as important elements in the attack and defence of
places.

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 ma