THE LOWER COAL MEASURES. 155 
position of the slack in several districts. In some the slack is sifted or 
washed in revolving screens, by which the dust is removed, and the 
resulting fuel is known as pea coal. This finds market for use in 
stationary engines, and for other like ;-urposes. The second of these 
new methods consists in coking the slack. Mention will be made of 
both processes in connection with the several districts in which they 
are used. 
The statements here made are sufficient to render intelligible the 
details that will be found in subsequent pages upon the points involved 
in this brief discussion. 
PROPORTION OF CoaL Lost IN MINING. 
This very important subject is directly connected with and depend- 
ent upon many of the points that have been already treated, but no 
proper nor profitable average can be given as to the results of coal 
mining in Ohio at the present time. The proportion of the coal gained 
in mining is increasing in most districts of the State, owing to better 
methods of mining, and better demand for the smaller coal. The em- 
pirical rule that gives 1,000 tons of coal to every acre for each foot in 
thickness of the seam, doubtless expresses the actual result in a large 
number of instances, but better figures are now obtained in many sec- 
tions, as will be hereafter shown, and far better results are certainly 
attainable in almost all of our districts. 
The specific gravity of our coals ranges for the most part between 
1.24 and 1.34. Assuming the mean, or 1.29 as the average, and this 
figure is very near the truth, every acre of such coal will contain for 
each foot in thickness 1,752 tons of 2,000 lbs. The miner’s estimate, 
as embodied in the rule above referred to, shows that he is content with 
securing four sevenths, or a little more than half, of the coal that the 
seam contains. The best foreign practice is far in advance of such 
results. In the Bristol coal field of England, it is calculated that 1,500 
tons to the foot are won for each acre, the entire loss in mining being 
reduced to one-tenth of the seam. The best practice that is fairly well 
verified in Ohio gains two-thirds of the coal, and the cases in which 
this is done are very rare. More than this is claimed in many mines, 
but it is probable that, if examined, such claims would be found to be 
untenable. 
