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, and well managed during the operations of the siege, they contribute very materially to the length of the defence.
_Attack and defence_ .-- This subject admits of two natural divisions: 1st, of intrenchments, and 2d, of permanent works.
I.Intrenchments maybe attacked either by _surprise_, or by _open force_.

In either case the operations should be based on exact information of the strength of the works and the number and character of the garrison--information that can be obtained from spies, deserters, and prisoners, and confirmed by examinations or reconnaissances made by officers of engineers.