Now as to the work done by these four men. Before the chamber has progressed a pillar’s length above the airway, propping will usually be necessary to sustain the roof, so large an area of which has been left without support. Hardwood props about nine inches in diameter are used for the purpose. They are purchased by the mining companies in large quantities, and are usually cut and hauled to the railroad in the winter time to be shipped at any season to the mines. By the law of 1885 the person or company operating a mine is obliged to furnish to the miner, at the face of his chamber, as many props of the required length as he may need. Having received the props the miner himself sets them on each side of the middle line of the chamber at such points as he thinks require them, or at such points as the mine boss designates. He drives the prop to its place by means of a large flat wedge inserted between the top of it and the roof, thus making the stick tight and firm and also giving it a larger bearing against the roof. Some chambers require very few props; others must be well lined with them. Their necessity depends upon the character of the roof. If it is soft, slaty, and loose it must be supported at frequent intervals. It very rarely occurs that a chamber, worked to its limit, has needed no propping from its foot to its face. Usually a good part of the miner’s time is occupied in setting props as his work at the face advances. Every seam has its top and bottom bench of coal, divided about midway by a thin slate partition, and one bench is always taken out to a horizontal depth of four or five feet before the other one is mined. If the upper bench contains the best and cleanest coal, with the smoothest plane of cleavage at the roof, that is first taken out; but if the choice coal lies at the bottom, then the lower bench is first mined. The reason for this is that a shot heavy enough to blast out effectually the section of rough, bony, or slaty coal which sticks to the roof or floor would be heavy enough to shatter the adjoining bench of clean brittle coal, and make a large part of it so fine as to be useless.