Onager
The Onager was a Roman siege engine that used torsion power to hurl rocks and missiles against enemy troops and fortifications. It had a padded buffer in front of its throwing arm to absorb some of the impact and was normally placed on a padded platform because the recoil from it firing would crush anything beneath it including a stone wall if it was placed upon one. It gained its name from the Roman word for a wild ass that kicks and bucks when someone tries to ride it, just as this siege engine would buck when fired due to recoil. The far end of the throwing arm would normally have a hook from which would be attached a sling in which the missile would be placed as contrary to popular belief a sling is much more efficient at throwing a missile than a bucket or spoon as seen in the traditional view of a catapult.
How to cite this article:
Dugdale-Pointon, TDP. (19 April 2004), Onager, http://www.historyofwar.org/articles/weapons_onager.html