The ballista was a type of torsion powered artillery that bore a significant resemblance to a giant crossbow. Although in modern usage the name tends to refer to a bolt or arrow firing weapon (although given the size the ammunition in question is probably better thought of as a spear or javelin with flights on it) the Romans - from who the word is derived - considered a ballista a stone throwing device - arrow like things were fired from catapultae not ballistae1 - a neat reversal of the modern understanding. Unlike a crossbow, a substantial part of a ballista's power tended to come not from the bow itself but from bundles of rope mounted on it, in which much of the torsion was generated.
As pieces of pre-gunpowder artillery go, this was a fairly complicated one, requiring sound materials (human hair was prized for the torsion bundles), careful engineering and, it is said, a good ear for pitch to determine how much strain the bow was under. These engineering requirements caused the supply of ballistae to falter somewhat in the dark ages and middle ages and by the time they could be maunfactured reliably once more, they had been more or less displaced by cannon.
Compared to counterbalance engines, ballistae could be built smaller and (far more) mobile - to the extent that in the late Roman Empire a cart mounted version (known as the carroballista) was developed. Ironically, this fired bolts rather than stones. Less extreme versions were those used as field artillery by the Roman legions and mounted on fortifications and warships by most classical powers with the ability to construct them.
Game and Story Use
- Somewhat overused in many fantasy settings - and almost always of the bolt firing variety.