74 CLASSIFICATION 0£ XHi ruai^iO j^axnus. 



shows that some American engineers believe that coal mining will 

 be carried to those depths. 



Although, without regard to profit, there may be a physical limit 

 to the depth that coal can ultimately be mined, in general the depth 

 of mining must be determined by its cost. Given a steady market 

 the depth to which coal can be mined is a question that involves 

 diminishing profits; every added foot of depth adds to the time re- 

 quired to lift the coal, thus reducing the possible output of the mine, 

 to the cost of the lift, to the initial capital required and the interest on 

 that capital, to the size of pillars, and to other factors. Where coal is 

 mined at a certain depth with a narrow margin of profit, obviously 

 mining at a much greater depth will extinguish that margin and ren- 

 der that coal unworkable until markets are better. Of two coals at 

 the same depth, of the same thickness, and in other ways equal but 

 of different quality, one, the better coal, may yield a profit and the 

 other may not. A similar inequality of profit may exist between two 

 coals of the same quality but of different thickness. The thicker 

 coal, which can be mined at a less cost, may be minable at a profit, 

 while the thinner may not. In brief, the depth to which any coal 

 can be worked depends, on the one hand, on its quality, which de- 

 termines the profits where the cost of working is the same, and, on 

 the other hand, on the cost of mining, which, omitting differences 

 due to depth, is in general proportional to the thickness of the coal. 

 Therefore, given a certain grade of coal which sells on the market 

 at a certain price the profit will be in inverse ratio to the cost of 

 mining, and hence, other things being equal, the depth to which that 

 coal can be mined will differ for different thicknesses of coal and is 

 assumed to vary in direct proportion to the value of the coal as com- 

 puted from the cost of mining. 



The depth to which any coal can be mined is therefore assumed to 

 be directly proportional to the B. t. u. value of the coal and inversely 

 proportional to the cost of mining for different thicknesses. Thus 

 below the adopted minimum depth of 500 feet, a 10,000 B. t. u. coal 

 can be mined to only two-thirds the depth of a 15,000 B. t. u. coal, 

 and an 8,000 B. t. u. coal to two-thirds the depth of a 12,000 B. t. u. 

 coal of the same thickness. If 5,000 feet is assumed as the maximum 

 possible depth at which any coal can be mined, it is also assumed to 

 apply only to coal of the highest grade and to the thickness costing 

 the least to mine — ^in other words, to a 15,000 B. t. u. coal 6 feet or 

 more thick. By this assumption a 6-foot coal of 12,000 B. t. u. is 

 considered as workable to only 4,000 feet or a 6-foot coal of 10,000 

 B. t. u. to only 3,333 feet. 



Elsewhere in this bulletin it is pointed out that the cost of mining 

 a ton of coal is much higher for thin beds than for thick beds. There 

 is no great difference in the cost of mining coals between 6 and 10 



