Area of unsupported roof in coal mines :conditions and factor[s] for consideration

Author:	Paul, J. W.
Year:	1927
Document Type:	USBI
Issuing Agency:	U.S. Dept. of Commerce, Bureau of Mines,
SUDOC:	C 22.11:6040
Series:	Information circular / Bureau of Mines ;
Report Number:	6040
Subjects:	Coal mines and mining--Safety measures
	Mining engineering
Link:	https://hdl.handle.net/2027/mdp.39015086471417