inner extremity of the chamber is called the “face.” It is sometimes spoken of also as the “breast,” though this last name is properly that of the chamber as a whole. The wall of coal at the side of the chamber is called the “rib.” A third chamber is now begun and driven up parallel to the other two, then a fourth, a fifth, and so on; as many chambers, indeed, as can be laid off in this way without deviating too greatly from a right angle to the airway. But the face of the first chamber is kept in advance of the face of the second, the face of the second in advance of the face of the third, and so on, until the limit of length is reached. This limit is determined, to some extent, by the dip of the seam. In comparatively flat workings a set of chambers may be driven in to a distance of five hundred, or even six hundred feet. Where the pitch is steep, however, two hundred or three hundred feet is the greatest length at which chambers can be economically worked. The limit of length of chambers is sometimes determined also by an outcrop, an anticlinal axis, a fault, or a boundary line. The wall of coal left between any two chambers is divided by the entrances cut through it into a line of pillars nearly uniform in size. As soon as the second entrance from the airway is cut through the wall the first entrance is blocked tightly up, and as soon as the third entrance is cut through the second is closed, and so on to the extremity of the line of pillars. This is to compel the air current to pass up to the very face of the chamber before it can find a way across to the other chambers and down again into the airway. If the air of the mine is bad, or if the coal is giving off deleterious gases with rapidity, a “brattice” or rude board partition is built from the lower side of the last entrance diagonally up toward the face of the chamber to force the air to the very point where men are working before it finds its way out through an open entrance. These boards are sometimes replaced by a sheet of coarse canvas, called brattice cloth, which is lighter, more easily handled, and answers the same purpose. [Illustration: A PLAN OF AN ANTHRACITE MINE WITH A SHAFT ENTRANCE.] From the mine car track in the gangway a branch track is built, crossing the airway and running up each chamber to its face. Up this branch track a mule draws the empty car, and when it is loaded it is let down to the gangway by the miner’s laborer. If the dip of the chamber is too steep――more than ten degrees――for a mule to draw the car up, a light car, used only in the chamber and called a “buggy,” is pushed up by hand, and when the dip is too steep for this the coal is pushed or allowed to slide down to the foot of the chamber. Chambers are often driven up obliquely in order to reduce the grade, or are curved in their course for the same reason.