. First century of national existence; the United States as they were and COAL. 141 worked without the necessity of removing theoverhanging slate, unless it is too unsoundto serve as a roof. Beds of ten feet thick-ness or more require much additional careover those of smaller size, both in removingthe coal and supporting the roof; and inmany cases it is found expedient to leave aportion of the bed, either at the top or bot-tom, untouched, especially if the upper lay-ers contain, as they often do, sound sheets ofslate. At the entrance of the mines, and ingeneral in all places where the c
. First century of national existence; the United States as they were and COAL. 141 worked without the necessity of removing theoverhanging slate, unless it is too unsoundto serve as a roof. Beds of ten feet thick-ness or more require much additional careover those of smaller size, both in removingthe coal and supporting the roof; and inmany cases it is found expedient to leave aportion of the bed, either at the top or bot-tom, untouched, especially if the upper lay-ers contain, as they often do, sound sheets ofslate. At the entrance of the mines, and ingeneral in all places where the cover is notsound, the materials overhead are preventedfrom falling by timbers across the top of thedrifts, rudely framed into posts set up againstthe walls on each side ; and where the strataare very loose, slabs are driven in over tliecross timbers and behind the posts. In suchground the coal cannot be excavated overlarge areas without leaving frequent pillarsof coal and introducing great numbers ofposts or props. But previous to abandon-ing the mine the pillars may be remov
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Keywords: ., bookauthorhodgejam, bookcentury1800, bookdecade1870, bookyear1874
