THE MINERAL DEPTHS 



Pennsylvania mine hauled about 150,000 tons of coal 

 in a year at a cost of less than one-tenth of a cent per 

 ton for repairs. The usual train was made up of 

 thirty-five cars, each loaded with about 3,700 pounds 

 of coal, which was hauled up a three- per-cent grade. 

 The cost of such haulage was only about 2.76 cents 

 per ton, as against 7.15 cents when hauled by mule- 

 power. These figures may be considered represent- 

 ative, as other mines show similar results. 



A particular advantage has been gained by the use 

 of electric locomotives over older methods in the proc- 

 ess of "gathering" the cars. In many coal mines, 

 even when the main hauling is done by electricity, 

 the gathering or collecting of cars from the working 

 faces of the rooms was formerly done either by mule- 

 power or by hand. In some low-veined mines, hand 

 power alone was used, on account of the low roof. 



In such places, low, compressed-air locomotives were 

 sometimes used; but these were very expensive. 

 These have now been very generally replaced by 

 "turtle-back" electric locomotives, operated at a dis- 

 tance from the main trolley wire by means of long, 

 flexible cables, so geared that they can be paid out 

 or coiled as desired. 



On the main line these locomotives take the current 

 from the trolley wire by means of the trolley pole, but 

 when the place for gathering is reached, the connection 

 is made by means of the flexible cable, and the trolley 

 pole fastened down so as not to be in the way. This 

 allows the locomotive to push the little cars into the 

 rooms far removed from the main line, with passages 



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