61.170.  Qualifications (certain third and fourth class counties). — In all counties of the third and fourth class, having a population of less than fifty thousand or an assessed valuation of less than seventy million dollars, the county highway engineer shall be a resident of the state of Missouri, and shall be skilled in the laying of drains, in bridge, culvert and road building and general road work, and he shall have a practical knowledge of civil engineering, and he shall be active and diligent in the discharge of his duties.