Section 107. Classes of cities

(a) City of the first class. A city with a
  population of one hundred seventy-five thousand or more.
    (b) City of the second class.  A  city  with  a  population  of  fifty
  thousand and less than one hundred seventy-five thousand.
    (c)  City  of  the third class. Any city not included in the foregoing
  classes.
    (d) The classification herein made shall be determined by the  federal
  census  or state enumeration, whichever is the later, as last previously
  taken before the effective date of this chapter.