As the firing arm was released, the sling-like cradle would unhook, firing the projectile. This arm was inserted, twisted, and propelled rocks or shot through torsion. First mentioned by Philon of Byzantium in his treatise on ‘Mechanics’, the onager made use of a sling-like cradle attached to a firing arm. Inaccurate but powerful, it was an effective siege machine against buildings and fortifications, but was of far less use on the battlefield unless firing into massed infantry ranks. One of the simplest forms of one-armed catapult, the onager relied on twisted skeins of animal sinew which provided the power to hurl either rocks or shot.
