Onager

The onager was a Roman stone-throwing engine with a single arm and a torsion power source. Its name refers to a wild ass: Ammianus Marcellinus compares the machine’s violent movement with a kick. In broad modern usage it can be called a catapult, although ancient terminology does not map neatly onto modern classifications.

Onager is a simplified version of a single-armed torsion engine for throwing rocks. The throwing arm is actuated by a torsion bar made of twisted animal sinew or hair. At the other end of the lever, a sling is attached, which increases the initial velocity of the projectile. The first mention of onager appears from the 4th century AD, but archaeological finds of individual metal parts allow us to attribute its appearance to the 3rd century. Description of the design of the engine was given by the late Roman officer and historian Ammianus Marcellinus (IV century AD).: "The scorpion, which is currently called onager, has this shape. Two logs are cut out of ordinary or stone oak and slightly rounded, so that they rise up on a hump; then they are fastened together like sawhorses and large holes are drilled on both sides. Strong ropes are passed through them, which give a clip to the car so that it does not disperse. In the middle of these ropes, a wooden pole like a drawbar rises obliquely. The ropes attached to it hold it so that it can climb up and down. Iron hooks are attached to its top, on which a hemp or iron sling is hung. Under this wooden structure, a thick bed is arranged, a pallet stuffed with crumbled straw, well reinforced and placed on a pile of turf or on a platform built of bricks. If this machine is placed directly on a stone wall, it will shake everything that is under it, not because of its weight, but because of a strong concussion. When it comes to combat, a round stone is placed in the sling, and four people on either side of the machine quickly rotate the navoi to which the ropes are attached and bend the rod back, bringing it almost to a horizontal position. The engine crew commander standing at the top of the car then knocks out the key that holds all the connections of the car with a strong blow of an iron hammer. Released by a quick push, the rod bends forward and, meeting a rebuff in the elastic mattress, throws out a stone that can crush anything that gets in its way. This machine is called the tormentum, because the tension is obtained by twisting (torquere), the scorpion, because it has a sting sticking up; modern times have also given it the name onagra, because wild donkeys, when pursued on the hunt, kick back and throw stones so that they pierce the chest of their pursuers or, having pierced the bones of the skull, smash the head.".

Ammianus Marcellinus describes stone-throwing engines in the defence of Amida in AD 359. Vegetius includes onagers in legionary equipment, but his late antique scheme should not be treated as a standard for every period of Roman history. A sling attachment alone does not establish an exclusively flat trajectory or rule out siege use; trajectory depends on the machine and its operation.

Related topics

• Throwing Machines
• Scorpio

Literature and sources

• Ammianus Marcellinus, XXIII.4: description of the onager