When, on account of the steepness of pitch or a change in the direction of the gangway, or for any other reason, one set of parallel chambers is brought to a close, a new set is begun farther along with a different course. The direction in which a gangway, airway, or chamber is to be driven is fixed by the mine boss. His bearings are obtained with a small miner’s compass, and he marks on the roof, near the face of the opening, a chalk line in the direction desired. The miner, sighting back on this line, is thus able to take his course and to keep his opening straight. Sets of chambers similar to those described are driven up from the gangway along its entire length. This length may be limited by various causes. A boundary line of property, a fault, a thinning out of the coal seam, are some of them. They are usually driven, however, as far as strict principles of economy will allow. A gangway that requires no timbering and is easily kept in good working condition may be driven to a distance of three or four miles. But where these conditions are reversed, a mile may be as great a distance as coal can be hauled through with economy. Beyond that limit it will be cheaper to sink a new shaft or slope than to increase the distance for underground haulage. As the main gangway progresses inward it may separate into two branches, each following a depression in the coal seam, and these branches may separate into others; so that there may be a number of gangways all keeping the same general level, from each of which sets of chambers are driven. When the chambers tributary to a gangway have reached their limit of length, and there is still an area of coal above them to be mined, a new gangway is opened along the faces of the chambers, or is driven just above them in the solid coal, and from this, which is called a “counter-gangway,” new sets of chambers are driven up the seam. It is often necessary to raise and lower cars passing from one gangway to the other on an inclined plane, on which the loaded cars, descending, and attached to one end of a rope, pull up the light cars, ascending and attached to the other end, the rope itself winding around a revolving drum at the head of the plane. This system can be put into use on any incline where the gradient is one in thirty, or steeper. By this general system of gangways, counter-gangways, airways, chambers, and planes, the area of coal lying on the upper side of the main gangway and on both sides of the shaft is mined out, hauled by mules to the foot of the shaft, and raised to the surface. On long straight gangways the mule is sometimes replaced by a small mine locomotive, and in these later days the electric engine has been introduced into the mines as a hauling agent.