The ballista is a siege weapon, invented by the ancient Greeks, and quickly adopted by the roman army. The ballista resembles a large crossbow, mounted upon a large-based monopod stand, allowing the weapon to be aimed without needing to be picked up. The Roman version was intended as an infantry support weapon, the various versions having a span of four to eight feet across, enabling it to be carried around by its operators, and used in rough terrain. The ballista was the largest siege weapon to see regular use in the roman invasion of Britain
Unlike a crossbow, the tension in a ballista is stored not it the arms, which are wide and rigid, but in large bundles of rope or sinew at the front of the machine. It was drawn using a system of levers that pulled a trolley (that held the bolt holder) back over a ratchet, and a trigger that detatched the bolt holder from the trolley. The ballista fires a five foot wooden bolt, with a steel tip, capable of penetrating almost anything the army would have to face on the battlefield. The ballista could even be used in a seige, firing bolts through the walls and roofs of hill forts. Skeletons of ballista casualties have been found, with ballista tips embedded between vertebrae, and square holes in their skull where a ballista bolt had gone straight through.
Modern replicas of the ballista have been created using the same materials and techniques available to the Romans, and skilled operators have shown the machine to be capable of firing three or more bolts per minute, over a range of five hundred meters or more. The Romans also used a repeating ballista, which automatically loaded and fired the weapon, the operators only having to turn the handle. Replicas of this repeating ballista can fire twelve or so shots per minute.
Replica ballistas feature in an episode of What did the Romans do for us?, a BBC documentary. It's worth seeing just for Adam Hart-Davis and a bunch of roman reenactors running around like little boys with a new toy...
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