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WikiPedia: Sổ_chấm_công http://clio.lib.olemiss.edu/cdm/ref/collection/aah... http://www.thebhc.org/publications/BEHprint/v019/p... https://books.google.com/books?id=aeJZAAAAcAAJ&pg=... https://books.google.com/books?id=clkAAAAAYAAJ&pg=... https://books.google.com/books?id=tL5zHOg9pFcC&pg=... https://archive.org/stream/principlesoffact00moxei... https://archive.org/stream/sciencerailways23kirkgo... https://web.archive.org/web/20150209155653/http://...