=81. The Defence.=--The object of the defence is to retard or stop the advance of the attack, by the destruction of his mines and miners, without forming craters which will assist him in making his parallels and approaches. =82.= For this purpose his galleries must satisfy nearly the same conditions as those of the attack. They usually start out from the counter-scarp gallery or from a parallel gallery a little in advance of it, and extend to a greater or less distance from the work, according to the time and expense allowable for their construction. For permanent works they are frequently prepared in time of peace, and lined with masonry. It is particularly for this class of countermines that many elaborate systems have been designed for completely covering the ground, and for throwing up the same earth several times by mines placed at different depths and exploded in succession. For reasons previously given, these cannot be recommended. =83.= A simple system of galleries placed as far below the surface as practicable, parallel or slightly diverging, connected when necessary by transversals whose lines prolonged pass inside the enceintes, and with branches fulfilling the same conditions driven out for listening-galleries, will, under the direction of an energetic officer, fulfil the conditions of defence as well, probably, as any that can be devised. The branches leading to mine-chambers can be driven out from the main galleries, transversals, or listeners, as may be desired; and if the hostile miners obtain possession of any part of the system and blow it up, the lines of craters formed will be so swept by the fire of the work that they can hardly be occupied by the enemy. =84.= As a rule, the mines of the defence will be undercharged or camouflets, to avoid the formation of exterior craters, but the rule is not without exceptions. =85. Shaft Mines=, mines placed in vertical shafts, are used by both attack and defence for destroying galleries, etc., in their vicinity. By the attack they are usually placed in craters already formed, or in other places protected from hostile fire. A shaft is sunk rapidly, generally “à la Boule,” heavily “overcharged,” filled up with earth, and fired. The defence may use the same method or may sometimes prepare them beforehand, tamping them and leaving a tubular opening through the tamping for loading and firing them. MINE TACTICS. =86.= The tactics of mine warfare result directly from the consideration above given. The special details of attack and defence vary in each particular case. The reports of mining operations in different sieges[21] supply precedents and give suggestions for future operations of a like character. =87. Todleben’s Rules.=--The general principles of mine tactics have been laid down by General Todleben from his experiences at Sebastopol (in Royal Engineers Occasional Papers, vol. i., 1877). They may be summarized as follows: