The ballista was a military machine designed to throw projectiles of
various types. In appearance the ballista resembled an enormous
crossbow and, like the crossbow, its propelling force was provided by
tightly twisted sinews of rope formed of horse-hair or raw hide.
An artillery piece used by the Romans to provide destructive fire against fortified places and large bodies of enemy troops. Until the end of the 1st century AD the term ballista referred to a weapon which projected heavy stones and which was only occasionally adapted for firing bolts. By the 4th century however, artillery terminology had changed and ballista was used solely to denote a weapon firing bolts.
Its propulsive power usually was generated by the torsion of ropes on a windlass. When the windlass was released, the projectile was thrown. The ballista was somewhat similar to a catapult, and later, smaller models were like crossbows. The bow-string of the ballista was drawn back by windlass until the required tension was obtained, whereupon a bolt was placed in a narrow channel leading to an aperture at the front of the machine. Once the target had been selected the bow was released by a trigger and the bolt sped on its flight, often to a range of 300 meters.
Some ballistas used by the Romans as heavy artillery weighed four tons and could hurl a 60-pound (27 kg) rock a quarter of a mile (0.4 km). Lighter ballistas discharged smaller missiles, such as flaming arrows.
In its mobile form the ballista was mounted on a cart drawn by mules and the combination was termed a carro-ballista. Ballistas are depicted on Trajan's Column which shows both mobile and stationary versions, the latter mounted on a platform of logs.