There are several systems of laying out a mine for actual working after the gangway has been driven a sufficient distance. The one most commonly in use in the anthracite region is known as the “pillar and breast” system. In the bituminous mines it is called the “pillar and room,” and in the mines of Great Britain the “bord and pillar.” It will be borne in mind that the mine which is now being described is in the Wyoming region, where the seams are comparatively flat, the entrance usually by shaft, and the method of working is the pillar and breast system. The gangway and airway are not driven far, not more than two or three hundred feet, perhaps, before the openings are made for the larger production of coal. Beginning on the upper side of the airway, at such a distance from the shaft as will leave a reasonably large sustaining pillar, perhaps from sixty to one hundred feet, an opening is made and driven up the seam at right angles to the airway. This opening is called a “chamber” or “breast.” In the bituminous districts it is known as a “room.” The chamber is usually about twenty-four feet wide, though where the roof is exceptionally good its width may be increased to thirty-six feet. It is not often opened the full width at the airway. Instead of this a narrow passage, large enough to accommodate the mine car track, is driven up to a distance not exceeding fifteen feet, and it is from this point that the chamber is driven up at its full width. This narrow opening can be more easily closed in case it is desired to prevent the passage of air through it, and besides a greater proportion of coal is left in pillars along the airway to prevent the passage from becoming blockaded by falls. When the first chamber has been driven up a distance equal to its width, a new chamber is begun parallel to it and on the side farthest from the shaft. These two chambers are now separated by a wall of coal from fourteen to twenty feet thick. If, however, the workings are deep and there is danger from the weight of superincumbent strata, the wall should be made as thick as the chamber is wide. When the new chamber has been driven to a distance of twenty-five feet, or, if the mine is free from gas and the ventilation is good, to a distance of forty or sixty feet, the wall between the two chambers is pierced by an opening from six to ten feet wide. This is called a cross-heading or “entrance.” A partition is now built across the airway between the openings to the two chambers, and the air current is thus forced up into the last chamber, across through the entrance into the first, down it to the airway again, and so in its regular course back to the foot of the shaft. In the mean time progress has been made in the first chamber, and by the time the second chamber has been driven another distance of thirty or sixty feet, the entrance which will then be cut through the wall will find the first chamber still in advance. The