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Coal Measures and Coal Mining in Iowa, including paleontology and a discussion on the coal formation; also the methods of mining by Russell T. Hartman, 1898

Coal Measures and Coal Mining in Iowa by Russell T. Hartman, 1898, Page 192

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[page]192.[/page] air works a piston to which is attached a pick. These machines do the work in driving entries about one dollar per yard cheaper than it can be done by hand, but out of this comes the maintainance of the compressed air plant and the repairs of the machines. After the main entries are driven a short distance the side entries are begun. The side entries are driven in pairs, as shown in figure 9, next page. The two entries of a pair are about ten yards apart while the distance between the pairs of entries is from one hundred to one hundred and fifty yards. The entries are driven in pairs to aid in ventilating the mine. Referring again to figure 9, it may be seen that the air from the air shaft passed in the direction
 
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