Section 6. The directors, before applying for locations for a railway, shall cause a copy of the agreement of association to be published in a newspaper, if any, published in each of the cities and towns where the railway is to be located, and, if, in any county, a newspaper is published in none of said cities and towns therein, in such newspaper published in said county as shall be designated by the department, at least once in each of three successive weeks; and the sworn certificate of the clerk shall be conclusive evidence of such publication.