The President of the Council shall preside at all meetings of the Council when the Mayor is not present; whenever there is a vacancy in the office of the Mayor, or he is absent from the city, or unable from any cause to discharge the duties of his office, the President shall act as Mayor and exercise all his authorities and be subject to all his duties. The President Pro Tem of the Council shall have all the powers of the President of the council during the session of the council at which the Mayor Pro Tem is presiding.