Mechanization, employment, and output per man in bituminous-coal mining / by Willard E. Hotchkiss ... [et al.]. Employment and related statistics of mines and quarries, 1935 / by F.G. Tryon ... [et al.].
Imprint
Philadelphia, Pa. : Work Projects Administration, National Research Project in cooperation with Dept. of the Interior, Bureau of Mines, [1937-1939]