In counties with a population according to the last preceding Federal Census of not less than thirteen thousand, four hundred and fifty (13,450) and not more than thirteen thousand, six hundred (13,600) and of not less then nineteen thousand, nine hundred and fifty (19,950) nor more than twenty thousand, one hundred (20,100), the salary of the county superintendent of public instruction shall be fixed by the Commissioners Court at not less than Eighteen Hundred Dollars ($1800), nor more than Twenty-two Hundred Dollars ($2200) per annum, and office and travelling expenses not exceeding Four Hundred Dollars ($400) per annum may be allowed, when necessarily incurred by the superintendent in the discharge of the duties of the office, out of the State and county available school funds, provided further, that the Commissioners Court may expend from the general fund of the county not exceeding Two Hundred Dollars ($200) per annum for the purpose of supplementing the office expenses of the county superintendent of public instruction. The compensation of each county school trustee in such counties shall be Five Dollars ($5) per day, but not more than Sixty Dollars ($60) shall be paid any trustee in any one year, payment thereof to be made in the manner provided by existing law.