Onagers in the Roman Legions

[DarkFrontier]

Hi all,

I'm doing some research for a novel, but I can't seem to find a definitive answer for this online, so I was hoping someone here could help me out. As I understand it, each century in a cohort was assigned a ballista, making for several dozen ballistae per cohort. However, I have read that each cohort was also assigned an onager, meaning each legion would also have a formidable amount of artillery firepower with it.

My question is this - were onagers built and assembled by carpenters / engineers as and when they were required - i.e. to support river crossings or when laying siege to a settlement and then dismantled, or were they a permanent fixture to a cohort? What I mean by the latter is - were they built and then kept in storage, being brought along when the legion began marching?

I am guessing it would be the former, as keeping onagers in storage would leave the wooden frames susceptible to rot and humidity damage, as well as risking the torsion ropes losing their elasticity and becoming frayed and slack. Not only that, with onagers being large and cumbersome, it would slow down the army's progress when on the march. Legions were reputed to be able to march 25 miles a day, it would be significantly harder if you were dragging 10 huge wooden catapults per legion along with you. Its my understanding that ballistae were primarily constructed of steel, meaning they were much easier to store without a great risk of damage.

Thanks!

[Titvs Statilivs Castvs]

Hi Paul, and welcome to RAT. You will get better information if you specify the time period you want to know about. :-)

Mr. Campbell is the expert here on artillery, he'll be able to help you when he sees this topic.

[D B Campbell]

Mr. Campbell is the expert here on artillery, he'll be able to help you when he sees this topic.

After an introduction like that, it would be churlish not to respond. :-)

As the OP mentions ballistas and onagers, I'm guessing we're in the second or third century AD. (Flippancy alert.) The ratios mentioned come from Vegetius, so we don't know if they're true or if he made them up. (End of flippancy.)

It's certainly difficult to imagine a legion dragging cart-loads of onagers on the march, but everything we know about Roman artillery (which, frankly, isn't that much) suggests that each legion was, indeed, responsible for its own catapults. Remember that artillery probably wasn't stored in a tensioned condition, so "rot and humidity damage" needn't have been a big problem, as there were specialists in charge of it, in any case.

I can't imagine anybody quibbling if your fictional legion has to start manufacturing onagers for a forthcoming siege. If you have any follow-up questions, I'm sure someone will pitch in to help.

[Flavivs Aetivs]

I heard that they only carried the important parts like Iron and Brass stuff with them, and left the wood to rot after they were finished with them. Is there any evidence for that?

Also, shouldn't it be Catapulta, not Ballista, as I understand the terms were mixed? And at that Vegetius says Scorpiones, does he not?

[D B Campbell]

I heard that they only carried the important parts like Iron and Brass stuff with them, and left the wood to rot after they were finished with them. Is there any evidence for that?

Nope. The onager is a bit of a grey area, but it wouldn't make sense to rip your ballistas apart, only to have to build new ones next time. Schramm's catapult shows that a wooden frame suffered no detrimental effects -- it stood in a draughty hallway at the Saalburg for decades, and periodic trials demonstrated that the horsehair springs were still serviceable years after they'd first been "sprung". The well-known front-plate from the Cremona catapult also shows that it had a long life (it records manufacture in AD 45 and was presumably destroyed in the battle of AD 69). These two examples are the Hellenistic-Early Roman catapult sometimes called the "scorpion", but I can't imagine that the situation was different regarding the OP's later Roman iron-framed arrow-shooting ballista.

Also, shouldn't it be Catapulta, not Ballista, as I understand the terms were mixed? And at that Vegetius says Scorpiones, does he not?

Ancient artillery terminology seems always to confuse. It's actually pretty straightforward (start here, for example).

[PhilusEstilius]

There has been evidence found by the late Raymond Selkirk and myself that the Romans did indeed carry ammunition with them when on the move, this is where we found an ornager stone ball at the side of an undiscovered Roman roadway in Northumberland.
This road goes off northwest from the Devils Causeway and runs along the east side of the Cheviots in the same direction and line as the Dere Street travels on its way to Scotland on the west side of these hills.
This particular road breaks away from the Devils Causeway a bit further south than the break away junction that the Berwick road takes from the Devils causeway as it carries on northeast to the coast at Howick.

[DarkFrontier]

Many thanks for your detailed replies, everyone! Indeed, I was referring to the 2nd/3rd century AD, basically the army at the height of the Imperial era. I've done a lot of research into infantry organisation and logistics, but artillery seems to be a grey area in both the era of Classical Antiquity and the Medieval era. The only place it starts to really become more detailed is the early Modern era, when brass and iron gunpowder cannons started to find their way into dedicated artillery units in the organisational structure of European armies.

When I was reading about the capabilities of both Roman and Medieval armies, I began to question if they could still maintain such capabilities while dragging fully-assembled cumbersome siege equipment with them; such as the Roman army's attested capability to march 25 miles per day. I also heard that sometimes, the Mongol armies invading Europe in the High Medieval era could march up to a staggering 65-70 miles per day, due to their armies being predominately cavalry based, though this would mean leaving Mangonels and Trebuchets straggling miles behind, assembling them only when they were needed in sieges.

When reading on the historical accuracy of Ridley Scott's Gladiator, I came across a point that mentioned the Romans would not have used onagers in pitched battles as the film depicts, as they were too cumbersome to assemble and transport, only being used in sieges or to support dangerous river crossings. I imagine it would be a similar story with Medieval trebuchets, although I can imagine that once they were built, they were transported along with the army and perhaps disassembled at the end of a conflict?

I suppose it wouldn't matter too much if said fictional cohorts began assembling their own onagers from stored wood and rope when they were needed, I'm just a sucker for accuracy I feel the little details in a story is what makes a story realistic and believable - thinking up an overall plot and story is the easy part, its the little details that make the world seem real and lived-in, rather than a world that is constantly referencing itself.

[ValentinianVictrix]

There a few accounts of Roman artillery being used on the battlefield. The ones that would more likely have been used would have been the cart mounted bolt throwers. I would think the stone throwers were used only in sieges.

[Koyuncu]

If I remember correctly, in the series of BBC's Building the Impossible : Roman War Machine, team build up scorpio used in famous siege of Masada. According to sources available about siege Romans built that machine in ten days but it is unclear if they are just brought together prefabricated parts or built if from scratch. In the documentary, Alan Wilkins another Roman artillery expert advise team about specifics.

Maybe this was the habit of Romans to build such machinery on field yet it seems this matter needs more information to be sure.

[Ildar]

I was referring to the 2nd/3rd century AD

We do not know what type of stone-throwers (one or two armed) was mainly in use by the Roman army in the 2nd – 3rd centuries C.E. It is quite possible that it used onagri, especially considering the witnesses of Tertullian and Vegetius.

but artillery seems to be a grey area in both the era of Classical Antiquity and the Medieval era

No more than any other aspects of the military matters of Antiquity and Medieval era.

though this would mean leaving Mangonels and Trebuchets straggling miles behind, assembling them only when they were needed in sieges

In the Medieval era, the siege and throwing engines were built on site from local wood; only their metal parts traveled with the troops. In Antiquity, the throwing engines, and sometimes even the siege engines, traveled with the troops in a disassembled state (they even had their designated place in the marching order of the Roman army).

I came across a point that mentioned the Romans would not have used onagers in pitched battles as the film depicts

Perhaps they did not use onagri, but we do have evidence on the use of the stone-throwers in the 2-3 centuries, for instance, in Arrian's "Acies contra Alanos” and in Cassius Dio's description of the battle of Issus between the armies of Pescennius Niger and Septimius Severus (I anticipate Duncan’s objections, but still think Dio referred to machines).

[DarkFrontier]

Many thanks guys, this has been very helpful!

[Macedon]

I just read a very interesting piece relevant to this thread. According to Michael Attaleiates (p.151, Bekker), Romanus IV Diogenes' siege machines at Manzicert were (re)assembled on the spot by very well organized wood pieces he had carried along in more than 1,000 wagons. Although in 1071, I doubt that the siege construction methods of the Romans would be that different to those of past eras.