In case of a vacancy in the Office of Mayor occasioned by death, resignation, removal from said City, permanent inability to serve, failure to elect, or disqualification of the person chosen, the President of the Board of Aldermen shall act as Mayor until the next annual meeting, and in case any of the causes above mentioned prevent the President of the Board of Aldermen from serving as Mayor when it becomes necessary for him or her to do so, the Board of Aldermen shall designate one of their number to act as Mayor until the next annual meeting and at such next annual meeting the legal voters of the City shall elect a Mayor from among their number to fill such vacancy.