comparatively flat seams its height is the height of the slate or rock roof of the coal bed, but in steep pitching seams it is made seven or eight feet high with a roof wholly or partly of coal. In some cases the roof and sides are so firm that no timbering is required, and in other cases the timbering must be close and heavy in order to give the necessary support and security. The floor of the gangway must be given a constantly ascending grade, usually from six inches to one foot in every hundred feet, as it is driven inward. This is to facilitate drainage and the movement of loaded cars. Where the strata are horizontal, or nearly so, as in many of the bituminous mines, the gangway may, and usually does, take a perfectly straight course. This is also true where the line of strike has but a single direction, no matter how steep the pitch of the seam may be. But both of these conditions are so rare in the anthracite regions that one seldom finds a gangway driven for any considerable distance in one direction. The surface of an inclined coal seam is not dissimilar to the surface of one side of a range of small hills. Any one who has seen a railroad track winding in and out along such a range, keeping to the surface of the ground and preserving a uniformity of grade, can understand why, for the same reasons, the gangway must often change direction in following the seam of coal. It must curve in around the valleys and hollows that indent the seam in the same manner that a surface railroad curves in around the depression where some hillside brook runs down to meet the stream, the course of which the railroad tries to follow; and it must strike out around the projections of the seam in the same way in which a surface railroad bends out around the projecting spurs of the hill range along which it runs. But the coal seam is more irregular and more uncertain in its outline than the hillside, and the curves in it are sharper and more varied. The surface railroad too may shorten its route and relieve its curves by bridging its small valleys and cutting through its narrow ridges. For the gangway this cannot be done. As a rule the coal seam must be followed, no matter where it leads. And it often leads in strange courses,――in courses that at times curve back on themselves like a horseshoe and point toward the foot of the shaft. The mining superintendent or engineer never knows in advance just what tortuous course his main artery may take. He cannot go over the ground and stake out his line as a civil engineer does for a surface railway; he must build as he advances, not knowing what the rock and coal may hide in the next foot ahead of him. He must be prepared to encounter faults, fissures, streams of water, diluvial deposits, and every other obstacle known to mining engineers.