(1)    Cities shall be divided into 4 classes for administration and the exercise of corporate powers as follows:

      (a)    Cities of 150,000 population and over shall constitute 1st class cities.

      (b)    Cities of 39,000 and less than 150,000 population shall constitute 2nd class cities.

      (c)    Cities of 10,000 and less than 39,000 population shall constitute 3rd class cities.

      (d)    Cities of less than 10,000 population shall constitute 4th class cities.

   (2)   Population of cities shall be determined by the last federal census, including a special federal census taken of such city, except in newly incorporated cities when a census is taken as provided by law. Cities shall pass from one class to another when such census shows that the change in population so requires, when provisions for any necessary changes in government are duly made, and when a proclamation of the mayor, declaring the fact, is published according to law.