Ignorance of the point to be selected for attack, and the great expense of permanent countermines, also require those prepared beforehand by the defence to conform to simple and economical systems. For this reason it is not necessary to give in detail the systems proposed by the older writers. They are described in most of the extended treatises on military engineering. =77. The Attack.=--The object of the attack is to advance his galleries in the most rapid manner possible, with the best available system of ventilation, and to place his mines in such position as to break up the galleries and destroy the men, materiel, and works of the defence, both above and below ground; or to form connecting craters which may be occupied and converted into parallels, trenches, etc. =78.= To accomplish this, when no natural ravine exists, a deep trench or “_lodgment_” is made, usually connecting the entrances of all the galleries and serving as a communication between them, and as a depot for such supplies as must always be at hand. From this lodgment the galleries are started by a shaft, blinded descent, or mining-gallery; the method depending upon the depth to be reached and the thickness of cover required. The entrance of each gallery is protected from horizontal and vertical fire, and from splinters, by a bomb-proof cover and traverses of sufficient thickness and strength. The galleries are generally driven in lines nearly parallel, and at such distance apart that the hostile miners working at any point between them will be heard, either from the main galleries or from returns called “_listening-galleries_” or “_listeners_.”[20] Depending upon the depth at which they are placed and other circumstances arising in different cases, the main galleries in various sieges have been placed at distances apart varying from about 8 to 30 yards. These galleries are connected at intervals by “_transverse galleries_” or “_transversals_,” which assist the ventilation very much and give additional communication between them. Branches for placing mines are driven in prolongation of the gallery or obliquely to the right or left, and, when the gallery is at a low level, inclining upward so as to shorten the line of least resistance, economize powder, and diminish the injury to the gallery and branches, resulting from the explosion of the mine. =79.= When the hostile miners come within striking distance of each other, each strives to run his galleries directly toward the other in order to avoid exposing its flank to the hostile mine; thus diminishing as much as possible the injury resulting from its explosion. =80.= The mines of the attack are generally overcharged in order to do the greatest possible injury to the mines of the defence, and to open large craters, but undercharged mines and camouflets are also used at times.