In the anthracite regions, where the seams dip at a high angle, slopes dug in the seam and following its dip are generally made use of in the manner set forth in figure 139, and as represented in several illustrations on other pages. This mode is the most available one that can be adopted under such circumstances, in our deep basins. The cost of labor in sinking slopes is less than that of tunnelling, or perhaps about equal, when timber and the cost of drainage, &c. are included, and is only one-fourth the cost of shafting as an average.
The cost of labor in sinking slopes ranges from 25 to 50 dollars per yard, and that of shafting from 100 to 300 dollars per yard. If we take the minimum, the cost is four times greater ; but if the maximum, the cost is six times greater. A slope 100 yards deep may cost $5000 for labor, but a shaft of the same depth would cost $30,000 in the same proportion.
At a dip of 45° it would require a slope 1500 feet long to reach a basin 1000 feet deep, and the difference in cost would be $75,000 in favor of the slope ; and as 100 yards of a lift is as much as can be economically worked in seams of this pitch, the coal could be mined by the sloping process with much more availability than in the shaft. The shaft must go down to the basin, and the coal, when mined, is with much difficulty brought down the long range of dip or "breasting," by inclines or otherwise. All the coal and all the water must therefore be lifted a thousand feet through the shaft, when one-fifth of it would only be lifted that distance vertically in the slope, though the increase of length would be nearly one-third ; but this does not increase the cost in the same proportion, since the same power in machinery will lift water 1000 feet high through 1500 feet of pipe at 45° as easily as 1000 feet high through 1000 feet of vertical column, while less powerful machinery is required to lift the coal 1000 feet high at an angle of 45°, and a distance of 1500 feet, than 1000 feet by perpendicular lift ; on the same principle that it is easier to draw a load on a railroad of 20 feet grade to the mile than on one of 100 feet grade. In a slope of 60° or less, the car alone or a
small additional weight for following truck is lifted with the coal on a railroad-track, but in the shaft not only the car and coal must be lifted, but the additional weight of a heavy " cage," which is frequently one-third of the load.
But, in addition to those advantages, while only one-fifth the coal is to be lifted the full perpendicular height of 1000 feet in the case under consideration, the other four-fifths are respectively lifted only 800, 600, 400, and 200 feet: and this, of course, means the water as well as the coal. To be more explicit to the inexperienced, we may state that the first lift of a slope should not be sunk over 300 feet on a seam pitching enough to " run," or over 35°. The coal is then worked out to that depth from one end to the other of the property on the "strike" of the seams. When this is done, the slope is sunk another lift of 300 feet, and the same process repeated, and so on successively until the basin is reached. It is thus evident that the coal is not only mined with more economy, since 100 yards is of sufficient length for any breast in which the coal runs from the miner to the cars in the gangway, but that only a small portion of both coal and water is required to be raised the full distance specified of 1000 feet, in the case of a shaft.
The same arguments hold good in nearly all pitching seams from 30° to the perpen dicular. If shafts are used of less depth than 1000 feet, or the depth of the basin, only that portion of the coal which is above the point of intersection with the seam can be mined from the level of the pit; and, if a greater depth is required, the shaft must either go down through and below the seam, and the coal be again cut by a right-angled tunnel, or a slope must be made use of from the bottom of the shaft: therefore, under such cir cumstances, a slope on the underlay of the seam is always more available than a shaft.