In all counties of the State of Texas having a population of not less than fourteen thousand, five hundred and thirty-five (14,535) and not more than fourteen thousand, five hundred and fifty-five (14,555) inhabitants, according to the last preceding Federal Census, the salary of the County Superintendent of Public Instruction shall be not less than Eighteen Hundred Dollars ($1800) and not more than Twenty-four Hundred Dollars ($2400) per annum, the amount of which salary shall be fixed by the order of the County Boards of Education for the respective counties; and the County Board of Education for each of the counties coming within this Act shall, by order entered in its Minutes, set the salary for its respective county.