“ By virtue of the authority vested in me by section 179 of the Revised Statutes of the United States (5 U.S.C. 6) , I hereby authorize and direct the Assistant Secretaries of Labor, in the order of the date of their commissions, to perform the duties of the office of the Secretary of Labor in case of the absence, sickness, resignation, or death of both the Secretary of Labor and the Under Secretary of Labor ”