Unlike the trebuchet, the ballista is a so-called “spring” gear. Its operation is based on various mechanisms using the action of two levers on torsion springs, consisting of several bundles of twisted fibers. Early versions threw heavy arrows or spherical projectiles, like stones of different sizes.
In the same principle as before, two men use a winch to which the rod is attached by a rope. Once brought back to the horizontal position, the loader places the projectile into the spoon and the shooter removes the iron clip releasing the rod. The latter, attached to two springs made of wood and nerve, is driven towards the front of the machine and is locked in a vertical position by a cross-member.
To adjust the range of the pull, it was enough to increase the thickness of the crossbar with skins or furs. The more the rod was vertical, the farther the projectile went.
This machine could also be used by both sides. Unlike the trebuchet that was in a curtain and had to send its ball over the wall (except if a gigantic platform exists in height), the ballista took place on a platform at the level of the walkway.
Originally, the word “catapult” refers to an arrow-throwing machine, whereas the term “balista” refers to a machine that throws stones, but the meaning of the two terms was reversed from the 4th century onwards era, hence sometimes a certain confusion in the terms. The fact that the word “catapult” (which has given a verb: “catapulter”) has become a generic term, which at certain epochs indifferently designates all the siege engines of the trebuchet ballista, obscures terminology.