But if said charter be rejected, then within six months thereafter, the mayor and council or governing authorities of said city may call a special election at which fifteen members of a new charter convention shall be elected to be called and held as above in such city, and they shall proceed as above to frame a charter which shall in like manner and to the like end be published and submitted to a vote of said voters for their approval or rejection. If again rejected, the procedure herein designated may be repeated until a charter is finally approved by a majority of those voting thereon, and certified (together with the vote for and against) to the secretary of state as aforesaid, and a copy thereof deposited in the archives of the city, whereupon it shall become the charter of said city. Members of each of said charter conventions shall be elected at large, and they shall complete their labors within sixty days after their respective election. The charter shall make proper provision for continuing, amending or repealing the ordinances of the city.