The council may by ordinance establish the merit system in all or part of the city administration but neither the council nor any of its members shall dictate the appointment of any person to office or employment by the city manager, or any manner interfere with the city manager or prevent him from exercising his own judgment in the appointment of officers and employees in the administrative service. Except for the purpose of inquiry the council and its members shall deal with and control the administrative service solely through the city manager, and neither the council nor any member thereof shall give orders to any of the subordinates of the city manager, either publicly or privately.