United States. Bureau of Labor Statistics. "Hours and Earnings in Anthracite and Bituminous Coal Mining: Anthracite--1919 and 1920, Bituminous--1919 : Bulletin of the United States Bureau of Labor Statistics, No. 279,"
Wages and Hours of Labor in Bituminous-Coal Mining
(April 1921).
https://fraser.stlouisfed.org/title/4021/item/493033, accessed on May 20, 2025.