In counties having a population of not less than eighteen thousand, seven hundred and sixty (18,760) and not more than eighteen thousand, nine hundred and sixty (18,960), according to the last preceding Federal Census, the salary of the County Superintendent of Public Instruction shall be not less than the sum of One Thousand, Nine Hundred Dollars ($1,900) per annum nor more than the sum of Two Thousand, Four Hundred Dollars ($2,400) per annum, the amount of which salary shall be fixed by order of the County Board of Education for the respective counties, and the County Board of Education for each county coming within this Act shall by order entered in its Minutes, set the salary for each of the respective counties. The salary allowed shall be paid in the manner and in accordance with existing laws governing the office of the County Superintendent of Public Instruction.