Oxybeles

The Oxybeles (Οξυβόλος, literally "bolt shooter") was a weapon used by the Ancient Greeks starting in 375 B.C.E. The weapon was basically an oversized gastraphetes, a composite bow placed on a stand with a stock and a trigger. It was supplanted by the scientifically engineered ballista. The difference between the two is the use of torsion power.

Reference

http://members.lycos.nl/onager/history.html