A counterbalance installed on a shackle-rod line, operating a pumping jack, to pull the rod line back after its power stroke. Rod lines can only pull, so on the return stroke the line is kept taut by a counterbalance. Rockers often are in the shape of a box or crate filled with rocks. One edge of the box is attached to a fulcrum bearing on which it moves back and forth like a rocking chair.
