From and after the effective date of this Act in all counties having a population of not less than fifteen thousand, seven hundred and sixty (15,760) and not more than fifteen thousand, seven hundred and ninety (15,790), and having an area of three hundred and twelve (312) square miles according to the last preceding Federal Census, the county board of trustees shall make provision for the salary of the county superintendent of public instruction in the sum of Four Thousand Dollars ($4,000) per year and no more. The amount allowed shall be paid in the manner and in accordance with now existing laws governing the payment of maintenance and salaries of county superintendents.