The onager was a Roman

Roman Empire

The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

 siege engine

Siege engine

A siege engine is a device that is designed to break or circumvent city walls and other fortifications in siege warfare. Some have been operated close to the fortifications, while others have been used to attack from a distance. From antiquity, siege engines were constructed largely of wood and...

, which derived its name from the kicking action of the machine, similar to that of an onager

Onager

The Onager is a large member of the genus Equus of the family Equidae native to the deserts of Syria, Iran, Pakistan, India, Israel and Tibet...

 (wild ass), it was created as a simpler, cheaper version of the ballista

Ballista

The ballista , plural ballistae, was an ancient missile weapon which launched a large projectile at a distant target....

. The Onager is a type of catapult

Catapult

A catapult is a device used to throw or hurl a projectile a great distance without the aid of explosive devices—particularly various types of ancient and medieval siege engines. Although the catapult has been used since ancient times, it has proven to be one of the most effective mechanisms during...

 that uses torsional

Torsion (mechanics)

In solid mechanics, torsion is the twisting of an object due to an applied torque. In sections perpendicular to the torque axis, the resultant shear stress in this section is perpendicular to the radius....

 pressure, generally from twisted rope, to store energy for the shot. The onager consisted of a large frame placed on the ground to whose front end a vertical frame of solid timber

Timber

Timber may refer to:* Timber, a term common in the United Kingdom and Australia for wood materials * Timber, Oregon, an unincorporated community in the U.S...

 was rigidly fixed; through the vertical frame ran an axle

Axle

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

, which had a single stout spoke

Spoke

A spoke is one of some number of rods radiating from the center of a wheel , connecting the hub with the round traction surface....

. On the extremity of the spoke was a sling

Sling (weapon)

A sling is a projectile weapon typically used to throw a blunt projectile such as a stone or lead "sling-bullet". It is also known as the shepherd's sling....

 used to launch a projectile. Some onagers, as depicted in ancient drawings used a bowl-shaped bucket instead of a sling to fire the projectile.
In action the spoke or arm was forced down, against the tension of twisted rope

Rope

A rope is a length of fibres, twisted or braided together to improve strength for pulling and connecting. It has tensile strength but is too flexible to provide compressive strength...

s or other spring

Spring (device)

A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

s, by a windlass

Windlass

The windlass is an apparatus for moving heavy weights. Typically, a windlass consists of a horizontal cylinder , which is rotated by the turn of a crank or belt...

, and then suddenly released. As the weighted sling swung outwards through centrifugal acceleration, one end would release, as with a staff-sling, and the projectile would be hurled forward. The arm would then be caught by a padded beam or bed, whence it could be winched back again.

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

In the Middle Ages

Middle Ages

The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

 (recorded from around 1200) a less powerful version of the onager was used that held the projectile in a fixed bowl instead of a sling. This was so that many small projectiles could be thrown, as opposed to one larger one. This engine was sometimes called the mangonel

Mangonel

A mangonel was a type of catapult or siege engine used in the medieval period to throw projectiles at a castle's walls. The exact meaning of the term is debatable, and several possibilities have been suggested. Mangonel may also be indirectly referring to the 'mangon' a French hard stone found in...

, although the same name may have been used for a variety of siege engines.

External links

• Onager Physics An analysis of the Onager
• Onager Animation Animated Onager
• Caerphilly Castle's onager firing
• Onager video: http://www.youtube.com/watch?v=1BPa_nOiK4A
• http://members.multimania.nl/onager/onager.html