The ballista stored its punch in twisted ropes, not a bow
A crossbow gets its power from a bent bow. The ancient ballista worked differently: two separate arms slotted into tightly twisted bundles of rope made from animal sinew. Cranking the string back twisted them further, and these torsion springs snapped forward fast enough to throw bolts accurately over 460 metres, sometimes picking off individual soldiers.
Its ancestors were Greek. The gastraphetes, or belly bow, was a handheld crossbow spanned by bracing one end on the ground and pushing a slider with the stomach, a ratchet stopping it from firing mid-load. Its larger cousin, the oxybeles, sat on a tripod and used a winch. Both relied on bows, which spring back relatively slowly and pass limited energy to light projectiles. Torsion bundles relax much faster, so the same effort could launch lighter missiles faster and farther.
The earliest ballista is thought to have been built for Dionysius of Syracuse around 400 BC. It really flourished under Philip II of Macedon, who kept engineers on staff to design siege weapons, and his son Alexander, whose engineers refined the coiled rope springs. Crews carried only the metal fittings and cut local timber for the frame when they arrived. Machines were mounted in armoured siege towers or at the edge of a battlefield, and they could switch between stone balls and bolts as needed.
After Rome absorbed the Greek city-states in 146 BC, it took over the technology along with much else, including captured machines like one from Pergamon later carved among trophy weapons. Roman ballistae had wooden frames bound with iron plates, a slider for the ammunition, and winches and a claw to haul the string back. The caps holding the sinew bundles could be adjusted with pins, letting operators balance the two arms and compensate for changing weather.
Improvements kept coming. A universal joint, reportedly invented for the purpose, let crews change aim without taking the stand apart, and smaller portable versions known as scorpions were in use by the Second Punic War. The name comes from the Greek verb meaning to throw.
Source: Ballista
