Onager (siege weapon)
The onager was a post-classical Roman siege engine, which derived its name from the kicking action of the machine, similar to that of an onager (wild ass).The onager consisted of a frame placed on the ground to which a vertical frame of solid timber was rigidly fixed at its front end; through the vertical frame ran an axle, which had a single stout spoke. On the extremity of the spoke was a cup to receive the projectile.
In action the spoke was forced down, against the tension of twisted ropes or other springss, by a windlass, and then suddenly released. The spoke thus kicked the crosspiece of the vertical frame, and the projectile at its extreme end was shot forward.