That the salary of County Superintendents of Public Instruction in all counties in Texas having not less than two thousand eight hundred and sixty-five (2,865) and not more than two thousand eight hundred and seventy (2,870) population and in all counties in Texas having not less than three thousand eight hundred and fifty (3,850) and not more than three thousand nine hundred (3,900) population, and in all counties in Texas having not less than four thousand nine hundred and fifteen (4,915) and not more than four thousand nine hundred and twenty (4,920) population, each, according to the last preceding Federal Census from and after the effective date of this Act shall be paid from the Available School Fund of such County.