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. The most notable use of the Oxybeles was under Alexanders rule. he introduced its use into almost every battle, and showed its wonderfull benefits.

Reference

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