rocking bob

from The Century Dictionary.

• Same as balance-bob.
• Synonyms and Rock, Shake, Swing. Roll. Shake expresses a quicker, more sudden, and less uniform motion than the others: as, to shake a tree or a carpet; his knees shook. Rock expresses the slow and regular motion to and fro of a body supported below—as a cradle upon rockers, or a rocking stone—or at the sides. Swing expresses the regular and generally slow motion to and fro, or around and around, of a body supported or held at one end, generally above: as, the swinging of a pendulum, a censer, a sword. Roll is sometimes used of an irregular motion to and fro, suggesting the rolling over of a round log: as, a rolling walk; the rolling of a ship in the trough of the sea. The figurative uses of these words are akin to their literal meanings: a ship rocks when the wind is steady on the aft quarter: it swings about its anchor with the change of the tide; it shakes with each blow from a heavy wave.