Editing Roman siege engines (section)

===Onager===
{{Main|Onager (weapon)}}

[[Image:Roman Onager.jpg|300px|right|thumb|Sketch of an Onager, from ''Antique technology'' by Diels]]

The ''onager'' was a post-classical Roman siege engine, which derived its name from the kicking action of the machine, similar to that of an [[onager]] (wild ass). It is a type of [[catapult]] that uses [[torsion (mechanics)|torsional]] pressure, generally from twisted rope, to store energy for the shot.

The onager consisted of a frame placed on the ground to whose front end a vertical frame of solid timber was rigidly fixed; through the vertical frame ran an [[axle]], which had a single stout [[spoke]]. On the extremity of the spoke was a [[sling (weapon)|sling]] used to launch a projectile.

In action the spoke was forced down, against the tension of twisted [[rope]]s or other [[spring (device)|spring]]s, by a [[windlass]], and then suddenly released. The spoke thus kicked the crosspiece of the vertical frame, and the projectile at its extreme end was shot forward.

The onagers of the Roman Empire were mainly used for besieging forts or settlements.  They would often be loaded with large stones or rocks that could be covered with a flammable substance and set alight.

In the [[Middle Ages]] (recorded from around 1200 A.D.) a less powerful version of the onager was used that employed a fixed bowl rather than a sling, so that many small projectiles could be thrown, as opposed to a single large one. This engine was sometimes called the ''[[mangonel]]'', although the same name may have been used for a variety of siege engines.