A fence made of seven or more wires securely fastened to trees or posts not more than eight feet apart, the first four wires being of four-inch barb and not over four inches apart, commencing with the first wire four inches from the ground, the fifth wire not over six inches from the fourth, the sixth wire not over eight inches from the fifth and the seventh wire 15 inches from the sixth, shall be a lawful fence against all stock whatsoever.