=58.= The first is accomplished by forcing air through pipes, which may be of tin, wood, or common hose, leading to the point where ventilation is required. The air is forced in by the use of the miner’s bellows or other apparatus already described. This method is simple in its application and places the fresh air where it is needed, but drives the foul air back into the galleries occupied by other miners. It is the only practicable method of ventilating single, long, narrow galleries and branches. =59.= The second method may be applied to a system consisting of a number of galleries connected by transversals, by so placing an exhausting fan as to draw the air out through one gallery, while by light wooden or canvas doors and screens the other galleries are so arranged that the fresh air, entering from the exterior, sweeps through the galleries occupied by the miners, and escapes through the unoccupied gallery leading to the fan, carrying the gases with it. In this method a single large gallery may be ventilated by using a canvas partition placed near the top or on one side, so that the fresh air will go in on one side around the end of the partition and back by the other side to the fan. This method has the advantage of carrying the gases away from the galleries occupied by the men, and supplying fresh air throughout those which are occupied. The exhaust may be produced by a fire constantly burning at the foot of a shaft instead of by a fan. The method is, however, complicated in its application, and can seldom be used for military mines. =60.= The third method, or assisting natural ventilation, is carried out by cutting numerous cross-galleries connecting those which are near each other, by making air-shafts and bore-holes connecting the galleries with the surface of the ground, and, when practicable, by placing the openings of the shafts and galleries at different levels. This method will serve for a few men working leisurely in preparing countermines before an attack, but is entirely inadequate during active mining operations. =61.= By the use of masks covering the face, and supplied with fresh air either through hose or from a reservoir of compressed air carried with him, a miner may work in galleries in which the air is irrespirable. The advantages which may frequently result from the time thus saved justify providing apparatus of this kind for use in mining operations. MINE-CHAMBERS. =62. Mine-chambers= to contain the charge of explosive are preferably nearly cubical in form, and if not charged at once, or if of large size, must have sufficient lining to support the roof and sides.