Estimates of Area and Tonnage.

Estimates of the tonnage or amount of coal in the ground should be made by the acre-not of coal land, but of the actual acreage of the bed itself.

As the available or contained tonnage varies with the specific gravity of the coal, it is not possible to give any fixed figure to represent the amount of coal per acre for each foot in thickness.

The specific gravity of anthracite varies widely, (see Chapter I,) but for rough and ready use we may assume an average of 1.5 to 1.6, and for Bituminous coal an average of say 1.4.

On the former basis an acre of Anthracite coal one foot thick will contain in round numbers about 1775 tons, and of Bituminous coal about 1695 tons; a cubic yard of Anthracite will weigh about 2530 lbs. or. 1.13 tons, and of Bituminous about 2360 lbs.=1.05 tons*.

In measuring the coal-beds for estimates of total contents all slate bands must be excluded, but as some thin partings (“leaves") and bony coal are nearly always included in the measurement, it is well to make a certain deduction, at least 5%,-to guard against exaggeration.

When geological sections have been prepared showing the approximate depth of the basins, and a geological map defines the limits of the bed or beds, the calculation is a simple but tedious process.

Deductions are made for soft outcrop coal not of marketable quality. When the bed is bituminous and lies flat with little covering, a margin sufficiently wide to insure at least twenty feet of cover should be deducted from the actual

acreage.

In steep-pitching anthracite coals an allowance of eight or ten yards (and often much less) will generally include all the soft coal.

* Of 2240 lbs.

CHAPTER III.

Methods of Opening Coal.

In the selection of the best means of developing a coal tract, the geological structure of the underlying rocks exercises a governing influence.

When the coal outcrops within the limits of the property it is usually opened, if flat, by a water-level drift, but if highly inclined by a slope sunk directly down upon the seam, or by a tunnel driven across the intervening meas

ures.

In the bituminous coal areas where the seams lie very flat, dipping from ten to one hundred feet to the mile, the waterlevel drift is almost universally used, but shafts of moderate depth are not uncommon in some districts. To insure free drainage, the lowest accessible point upon the outcrop is chosen as the best location for the main entry, thus at the same time securing a favorable grade for the haulage of the loaded mine cars. As the outcrop coal, as described in the preceding chapter, often dips into the hill, the drift is usually commenced a few feet below the coal terrace, and driven on a slight up-grade until the normal dip is reached. It sometimes happens that this inward dip is so strong that it is advisable to open by a shaft sunk in the center of the basin, provided that the depth of such an opening is not too great and the amount of water to be pumped comparatively small. When the inward dip to the center of the basin does not exceed twenty-five feet, drainage may be accomplished (through a drift) by means of iron pipes, two, three, four or more inches in diameter, running from a sump in the center of the basin to a point outside the mine at a lower elevation. These pipes when filled with water form a self-acting syphon and will deliver a large quantity of water. Stop-cocks or valves are placed at each end of

(55 AC.)

the pipe and a force pump attached to one end to start the flow by filling the pipe with water, an additional cock being placed at the highest point on the pipe to draw off air that may accumulate at that point.

In the anthracite regions the water level drift is used in opening inclined seams exposed in ravines or gorges eroded across the strike of the measures. It is then similar in every particular to a gangway driven underground from a shaft or slope. As drifts are driven and timbered precisely like underground gangways, they will be described in the chapter relating to that subject.

When the seam is inclined, and is accessible at no point along its outcrop low enough for the location of a drift that will command sufficient breasting area, it is opened by either a slope or tunnel.

Seams dipping with the slope of the ground can gener ally be reached by tunnels of moderate length; but when the dip is away from the hill the length will be much greater, and, unless several beds are to be opened and worked, it has seldom been found advisable to open in this way. Tunnel collieries have been found to be much more cheaply operated than either shaft or slope openings, as neither hoisting or pumping machinery is needed. But the grave fact that when the coal above water level is exhausted the tunnel becomes almost worthless, is a serious offset to these advantages.

Tunnels are most extensively used as a means of reaching beds underlying or overlying a seam in which workings have already been opened, by driving underground directly across the intervening measures. Such a gallery, strictly speaking, is not a tunnel, for a tunnel is a passage driven from day to day, and open at both ends; but custom sanctions this use of the term in the anthracite regions.

When the coal does not come to the surface within the area to be developed, it is generally necessary to open it by shaft, but under certain conditions the opening may be made either by slope or tunnel.

A slope opening may be made when the coal is brought to within a moderate distance of the surface by an anticlinal