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abutments advance Airolo arch average bars black powder blasting boring bottom brick brickwork built Canal cent centre centre-core clay coal commenced completed compound compressed air compressor connection construction cost crib cubic yard cylinder depth diameter driven dynamite engineer English system enlargement excavation explosive face feet foot glycerine Gothard Tunnel gun-cotton gunpowder heading Hoosac Tunnel hornblende inches invert iron June labor length limestone lineal longitudinal lower machine masonry material metres miners mining Mont Cenis Tunnel Musconetcong Nesquehoning nitro-glycerine open-cut patent pipe piston piston-rod Porphyry portion potash powder pressure props pumps Railroad removed rock rock-drill roof rotation Rziha saltpetre sand sandstone Scale screw shaft shown in Fig shows side side-walls sill sinking slide soft ground steam stone stroke sunk Sutro Tunnel TABLE taken thick timber tion trepan upper valve vertical walls weight