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, 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. By these means a pretty accurate knowledge may be obtained of the natural features of the ground exterior to the works; their weak and strong points; and their interior arrangements for defence.