"Section 18-A.     The President, upon prior investigation by and recommendation of the commission, is hereby empowered to decrease by not more than sixty per centum or to increase by not more than ten times the rates of import duty however established when in his judgment such reduction or increase is necessary in the interest of national economy, general welfare, and national defense, any other provision of this Act to the contrary notwithstanding: Provided, That such rates shall be effective until modified by Congress: Provided, further, That the authority herein granted to the President shall expire on December thirty-one, nineteen hundred and fifty-five."