From words machines shall be made

Sources for the onager

In the current issue of Ancient Warfare (XVII.2) an article by Wolfgang Wilsch and Boris Dreyer explores the reconstruction and testing of the Roman onager (“wild ass”), a stone-throwing engine used in the field and in defence (and besieging) of walls. Included was an examination of the passage describing onagers in Ammianus Marcellinus (Res Gestae 23.4.4-6). There are other sources, too, which we should explore. They are all we have when it comes to reconstructing these ancient siege engines.

The first-century BC architect Marcus Vitruvius Pollio (ca. 80 BC-ca. AD 15) served as an artilleryman, possibly under Julius Caesar. His De architectura is the only architectural treatise to survive from antiquity, and its tenth book includes passages on siege machines. It is clear, therefore, that siege machines were considered the realm of mathematicians and architects – something Trajan’s architect Apollodorus of Damascus continued with his own Poliorcetica (Siege Engines) treatise. Vitruvius does not, however, describe an onager as we understand them. Instead, he describes the catapult and the ballista – the latter of which by the fourth century was understood as the onager (although the engine's designs were different). Vegetius (Epitoma 4.22) provides the rough distinction that ballistae shot arrows, but onagers threw stones. In the Roman Republic and early Empire, however, the catapult would have been understood to launch arrows and the ballista to have thrown stones. When this change in terminology came about is unclear, but it was firmly in place by the fourth century.

At 10.10.1, Vitruvius tells us that “I will set forth the construction of scorpions and ballistae” – using the term scorpion to stand in for all designs of catapults. Arrow-launching catapults are described first, and in 10.11 Vitruvius moves on to ballistae. He tells us that some are operated by levers and windlasses, some by wheels “yet all ballistae are constructed with a view to the proposed amount of the weight of the stone which such a machine is to let fly (mittere)” (10.1.1). Therefore, he tells us, the design of such machines needed to be left to the professionals. The ideas behind the later onager are the same. The amount of torsion was corresponded to the different weights of the stones they would launch (Wilsch and Dreyer include the range of weights their reconstructed machine could launch).

For the sinews used in the construction of these machines, Vitruvius recommends women’s hair or the sinews of animals. More sinews were needed for heavier stones and longer catapult bolts. He then goes into specifics of the weight of the stone in relation to the aperture opening required for that weight stone. This implies that the munition for each device was quite specific in its shape and weight. In section 10.12, Vitruvius explains how sinews were stretched to the correct tension – they would be “tuned to the right note by a musical ear” (10.12.2). He then goes on to discuss sieges and the uses of engines in them. 

Roman legionaries are seen working with a bolt-throwing engine on Trajan
A Roman Imperial scorpion - bolt thrower - as depicted on Trajan's Column

Various machines are shown on Trajan’s column, though an onager is not seen, but there are other forms of arrow-launching catapult. Trajan’s architect Apollodorus, however, does mention a “one-armed stone-thrower” (Poliorcetica 18) which fits the description of the (later) onager. Arrian’s Tactica (44.1-2) mentions a variety of machines of war and his Ektaxis kat’ Alanon (19) mentions machines (mechanai) posted on the wings of his formation in AD 136/7.

The next author who deals with onagers is Vegetius. His Epitoma Rei Militaris (also known as the De Re Militari) was written in the fourth or fifth century AD. Closer to 383 is more likely (he refers to Gratian as dead), but any date up to 450 has been argued. In any case, the Epitoma was written after Ammianus had served (up to AD 363), but the treatise was probably written before Ammianus wrote his book in the 390s.

Vegetius includes several references to machines of war, including onagers, in 2.10, 25; 3.14, 3.24; 4.22. He tells us that it was the duty of the praefectus castrorum to look after the “rams, onagers, ballistae and all the other engines of war.” He adds that each legion had ten onagers, one for each cohort, “drawn ready armed on carriages by oxen.” At 3.14 and 25, he tells of carriage-ballistae being deployed behind the legions but does not specifically mention onagers there. In the defence of cities, ballistae and onagers are also used (4.8) and Vegetius advises on keeping a supply of sinews ready for all torsion-engines (4.9); women’s hair, he tells us works just as well as that from horse tails and manes (this point was probably taken from Vitruvius although the sense of the line is different there). 4.22 differentiates between ballistae, onagers, catapults and scorpions – telling us that these used to be called manuballistae or hand-ballistae. This also seems to have been the distinction Vitruvius drew (10.10.1, 3; Caesar Gallic War 7.25.2). Vegetius summarises the onager: it “throws various weights in proportion to the thickness of the sinews and the size of the stones. The larger the machine, the bigger the stones it hurls like a thunderbolt” (4.22).

What's in a name?

There may be a clue about the date of the change in meaning of the scorpion or ballista came, and it comes from a very unusual source. The late second and early third century Christian apologist, Tertullian, wrote a treatise called Scorpiace “Antidote to the scorpion’s sting” in the early third century AD. This was a time of Christian persecution under Septimius Severus. Tertullian offers an antidote to the following scorpion's sting: the idea among some (heretical) Christians that they should find a way within the wording of Christian teachings to compromise their beliefs and sacrifice to the emperor as was demanded. This thinking was the scorpion’s sting for Tertullian, one which would prove fatal to Christian believers if no antidote could be found. And, like the diminutive scorpion, Tertullian argues, though such ideas may have seemed small and not dangerous, they could nonetheless prove deadly. In his introductory paragraph (1.1.1-2), Tertullian compares the actual scorpion with the military engine named after them. It is clear that, for him, writing in the early third century, the scorpion was still an arrow-launcher with a sting in its tail. That is, it had not yet become the onager as it was for Ammianus.

At 23.4.4 Ammianus describes the ‘scorpion’ or ‘wild ass’, stating that the former was its old name, but it is now known (in the late fourth century) as an onager. He then describes its construction and gives an etymology (23.4.7):

“the machine is called tormentum as all the released tension is caused by twisting (torquetur); and scorpion, because it has an upraised sting; modern times have given it the new name onager, because when wild asses are pursued by hunters, by kicking they hurl back stones to a distance, either crushing the breasts of their pursuers, or breaking the bones of their skulls and shattering them.”

Tertullian, however, describes the poisoned veinlet of the scorpion’s tail, that “rising up with a bow-like bound, draws tight a barbed sting at the end, after the manner of an engine for shooting missiles. From which circumstance they also call after the scorpion, the warlike implement which, by its being drawn back, gives an impetus to the arrows.”

Between AD 203-210 when Tertullian wrote, and Ammianus and Vegetius writing in the fourth century, then, the ideas and terminology about stone-throwers and arrow-launchers changed. What is more, the explanation of the “scorpion” in both Ammianus and Vegetius does not give as succinct a reasoning as Tertullian does.

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