Sec. 24.
That all mails of the United States shipped or carried on vessels shall, if practicable, be shipped or carried on American-built vessels documented under the laws of the United States. No contract hereafter made with the Postmaster General for carrying mails on vessels so built and documented shall be assigned or sublet, and no mails covered by such contract shall be carried on any vessel not so built and documented. No money shall be paid out of the Treasury of the United States on or in relation to any such contract for carrying mails on vessels so built and documented when such contract has been assigned or sublet or when mails covered by such contract are in violation of the terms thereof carried on any vessel not so built and documented. The board and the Postmaster General, in aid of the development of a merchant marine adequate to provide for the maintenance and expansion of the foreign or coastwise trade of the United States and of a satisfactory postal service in connection therewith, shall from time to time determine the just and reasonable rate of compensation to be paid for such service, and the Postmaster General is hereby authorized to enter into contracts within the limits of appropriations made therefor by Congress to pay for the carrying of such mails in such vessels at such rate. Nothing herein shall be affected by the Act entitled "An Act to provide for ocean mail service between the United States and foreign ports, and to promote commerce," approved March 3, 1891.