The board of county commissioners of each of the several counties is hereby authorized to levy an annual tax upon all taxable tangible property in the county in an amount not to exceed the limitation prescribed by Kan. Stat. Ann. § 79-1947, and amendments thereto, for road and bridge purposes and to pay a portion of the principal and interest upon bonds issued under the authority of Kan. Stat. Ann. § 12-1774, and amendments thereto, by any city located in such county.