According to two authors of the later Roman Empire who wrote on military affairs, the onager derived its name from the kicking action of the machine that threw stones into the air, as did the hooves of the wild ass, the onager, which was native to the eastern part of the empire.
Design
The onager consisted of a large frame placed on the ground to whose front end a vertical frame of solid timber was rigidly fixed. A vertical spoke that passed through a rope bundle fastened to the frame had a sling attached which contained a projectile. To fire it, the spoke or arm was forced down, against the tension of twisted ropes or other springs, by a windlass, and then suddenly released. As the sling swung outwards, 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, when 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 large stones that could be covered with a combustible substance and set alight.
History
The onager was used from the 4th century AD until the 6th century AD. The late-fourth century author Ammianus Marcellinus describes 'onager' as a neologism for scorpions and relates various incidents in which the engines fire both rocks and arrow-shaped missiles. According to Ammianus, the onager was a single-armed torsion engine unlike the twin-armed ballista before it. It needed eight men just to wind down the arm and could not be placed on fortifications because of its great recoil. It had very low mobility and was difficult to aim. In 378, the onager was used against the Goths at Adrianople and although it did not cause any casualties, its large stone projectile was incredibly frightening to the Goths. The late-fourth or early-fifth century military writer Vegetius stipulates that a legion ought tofieldten onagers, one for each cohort. These he says should be transported fully assembled on ox carts to ensure readiness in case of sudden attack, in which case the onagers could be used for defence immediately. For Vegetius, the onagers were stone throwing machines. In the late 6th century the Avars brought the Chinese traction trebuchet, otherwise known as the mangonel, to the Mediterranean, where it soon replaced the slower and more complex torsion powered engines.
