The board of supervisors of any such county may at any time, by a majority vote, submit the question of purchasing any such canal, harbor or improvement, to the electors of said county; and if a majority of the electors shall decide to purchase any such canal, harbor or improvement, then the board of supervisors shall be authorized to purchase the same, and may, for that purpose, issue the bonds of said county to an amount sufficient to make such purchase: Provided, That if the property and franchises of any corporation, organized under the provisions of this act, shall be conveyed to the United States, as provided in section 21 of this act, then in such case the provisions of section shall not be operative.