In fences composed of posts and wires, the posts shall be of ordinary size for fencing purposes, and set in the ground at least two feet deep and not more than twelve feet apart, with holes through the posts or staples on the side not more than fifteen inches apart, to admit four separate strands of fence wire not smaller than No. 9, and shall be provided with rollers and levers, at suitable distances, to strain and hold the wire straight and firm.