Sunday 16 February 2020

Did Roman engineers stand under bridges?

Did Roman engineers or architects have to stand underneath their bridges, to prove that they were properly built? This story sounds weird, and it’s totally implausible. (Bear in mind that the Romans built bridges to go over water.)
Hmmm, thinks: if I were a Roman engineer, where would be the best place for me to stand under this bridge so it can be tested? (The Ponte di Tiberio, Rimini, dating to the principates of Augustus and Tiberius, early 1st cent. CE)
If you haven’t heard the story before, I’ll grant that it is niche. But like so many myths about antiquity, it does pop up all over the place. Here’s Nassim Taleb in a 2012 book:
First, never get on a plane if the pilot is not on board. ...

The first heuristic addresses the asymmetry in rewards and punishment, or transfer of fragility between individuals. Ralph Nader has a simple rule: people voting for war need to have at least one descendant (child or grandchild) exposed to combat. For the Romans, engineers needed to spend some time under the bridge they built -- something that should be required of financial engineers today. The English went further and had the families of the engineers spend time with them under the bridge after it was built.
-- Taleb, Antifragile (2012), chap. 23
We shouldn’t expect Mr Taleb to be very accurate about the Romans, mind. In 2017 he infamously had an online shouting match with Professor Mary Beard, the eminent Roman historian, in which he insisted tenaciously (and falsely) that there was racial purity within each province in the Roman empire.
I’d better grant that later in the same chapter Taleb adds a couple more snippets about the Romans which are at least partially accurate. (1) Roman soldiers had to swear a military oath on joining the army (apparently Taleb believes other armies don’t do that); (2) there existed an extremely rare military punishment called decimatio, the random execution of 1 in every 10 or every 100 soldiers (Taleb comments that ‘putting more than 10 per cent to death would lead to weakening of the army’ -- apparently a 10% casualty rate wouldn’t do that). It’s clear he gets his ancient history mainly from popular culture.
Taleb didn’t invent this story, but I do wonder if it’s because of his book that the notion entered popular culture.
Predictable aswer alert! (QI, ‘Keys’, 2013)
Stephen Fry. In Roman times, they’d get the constructor of the arch to stand right under the arch when the support scaffolding was taken away, just to show that he had faith enough in his own, er ...

Tim Minchin. Well, it’s natural selection of arch-builders, isn’t it. Is that guy any good? Well he’s still here!

Isy Suttie. I like that idea of getting people to test things. It’s like going to a barbecue and getting someone to try the sausage.
-- QI, series 11 episode 8 ‘Keys’ (first broadcast 25 October 2013)
Notice how Stephen Fry spots the problem with having this as a story about bridges: he makes it about arches instead. There’s no basis for that either, just some imaginative rewriting, to try to get the story to make some kind of sense.

It wasn’t QI or Taleb that invented the story. The oldest version I’ve found is a signature line used in a USENET post back in 2004:
"When Roman engineers built a bridge, they had to stand under it while the first legion marched across. If programmers today worked under similar ground rules, they might well find themselves getting much more interested in Ada!"
-- Robert Dewar
-- Preben Randhol, post to comp.lang.ada, 11 Feb. 2004 (alternate link)
Robert Dewar was a computer scientist who ran a company involved with the Ada programming language. Is he the ultimate origin of the myth? Who knows.

But I will say this: to me, it sounds awfully like the kind of thing you might hear from a tour guide.

I have a sneaking suspicion that the story really originates with the Pons Fabricius, in central Rome. It’s about 10 minutes’ walk from the Forum and from the Circus Maximus, and just around the corner from the Theatre of Marcellus. It was built in 62 BCE, and it’s still in use for pedestrians and cyclists to cross between the east bank and the Isola Tiberina.
The Pons Fabricius, a.k.a. Ponte Fabricio
Not that Fabricius had to stand underneath his bridge while the legions marched across! No no, I have in mind something much more mundane. It’s a simple misinterpretation. You see, there are inscriptions on the side of the bridge recording who built it and who restored it. Things like

L(ucius) Fabricius G(ai) f(ilius), cur(ator) viar(um) faci<e>ndum, c<u>ravit

Lucius Fabricius, son of Gaius, curator in charge of making roads, supervised (the building of the bridge)
But in separate places, the following gets tacked on:

idemque probavit

and the same man (i.e. Fabricius) approved it
(Don’t mind the spellings, that’s just what Latin looked like when Caesar was in his 30s.)
The thing is, the word probavit is ambiguous. Probare can mean ‘approve’, but it can also mean ‘test, demonstrate’. The same ambiguity can be seen in two English words derived from probare: ‘approve’ and ‘prove’, with ‘prove’ in the sense of test (as in, ‘the exception proves the rule’).

The modern Italian derivative, ha provato, is more specific. It’s almost always going to mean ‘he tried, he demonstrated, he tested’. So if someone like a tour guide were explaining or describing the inscriptions, I’m imagining they might well give the the meaning as ‘test’.

And just to show how plausible this is, here’s an ancient history website set up by an Italian family that reports the inscription exactly like that.
A latin inscription above the arch, on both sides of the bridge reminds us that it was built by Fabricius curator viarum (warden of roads) and that "idemque probavit" - he personally tested it.
-- ‘Ancient Roman Bridges’,, May 2006
I suggest we’ve got three stages in the development of the myth:
  1. The original: ‘Fabricius supervised the bridge, and the same man (idem) approved it (probavit).’
  2. An intermediate version, like on the MariaMilani site, with the mistranslations: ‘Fabricius built the bridge and personally (mistranslation of idem) tested it (mistranslation of probavit).’
  3. The misinterpretation of the mistranslation: ‘Fabricius built the bridge and personally tested it by standing underneath it.’
When tourists in Rome want to walk across a real, ancient, Roman bridge, they’re going to be crossing the Pons Fabricius. It’s nice and central, ten minutes’ walk from the Forum, as I said. So this is a misinterpretation that stands a good chance of going viral.

Of such things are myths made. Even ones as small as this.


  1. It is highly inaccurate to say that Prof Taleb was angry because Mary Beard said it was racial purity. It was because she treated all Meds as Africans which he found to be racist.

  2. I had not heard this one before! It strongly reminds me of the popular myth that (supposedly) spiral staircases in medieval castles were always built clockwise so that right-handed defenders at the top of the staircase would have more room to draw and swing their swords. They're both myths about architecture that have both probably been promoted primarily by tour guides and, in both cases, if you really think about them, they're both really quite silly.

    As far as medieval staircases are concerned, as I talk about in this article from December of last year, counter-clockwise staircases are actually fairly common in medieval castles. There's also the fact that a person would have to be practically insane to try to defend a staircase in a castle anyway, because fighting on a staircase automatically gives the attackers coming up the stairs a huge advantage because they can take swipes at the defenders' unprotected legs while protecting their own heads with their shields.

    Also, medieval castle stairwells are usually extremely confined spaces with extremely steep, narrow steps, which would provide a huge disadvantage to anyone trying to fight in one of them and shows that they clearly weren't designed to be fought in. Finally, if the enemies are already inside the castle and coming up the stairs to the keep, there's really no hope left for the defenders because the castle has already failed and they've already lost.

    A lot of the same logical problems apply here to the idea of Roman architects being forced to stand under bridges. Only an extraordinarily poorly designed bridge would collapse the first time it is crossed; unless the architect is totally incompetent, the bridge is far more likely to collapse years later after it has suffered substantial wear.

    1. There's also, of course, the obvious problem you mentioned that Roman bridges are usually built over water.

    2. I did puzzle over how anyone thought that idea would work out. I haven't spotted anyone other than Stephen Fry thinking about that problem!

  3. I liked the Ada reference. Did you know that classicists make the best programmers? Well, my evidence base is shallow, to be sure. Back in the late ‘eighties, my engineering company had difficulty recruiting software engineers from among science graduates and so conducted an experiment: they recruited arts graduates and put them on a three-month induction programme during which they had to learn programming. The graduates showing the most aptitude were classicists and musicians. Sadly, very few of them stayed on.

  4. I'm uncomfortable with your comment about "the exception proving the rule" (in the sense of testing the rule). My understanding is that this English phrase actually derives from an ancient legal maxim ("exceptio probat [or (con)firmat] regulam in casibus non exceptis")—see the last section of this Straight Dope column which dates it back to one of Cicero's defences (though of course I do understand that Cecil Adams is not, actually, a canonical reference).

  5. As a point of information, I first heard the story about Roman arches (not bridges) in 1974 in Colorado Springs, CO in the USA. John Silber, later president of Boston University, gave an address entitled "The Tremble Factor". The title of the talk alluded to fear as a motivation for doing work well, and was based on the story of the Roman engineers. Silber claimed it was a true story but provided no references that I remember. The talk was awful and upsetting to nearly everyone, so no one asked for a citation!

    1. Oh that is interesting, thank you. Maybe with a bit more digging around one of us can turn up some written source: I hadn't looked as far back as the 1970s. Until you told me this, I hadn't dreamed that the story might be anything like so long-lived!

    2. Heard the anecdote even earlier in my childhood and my Dad was an architect.

  6. Regardless of the truthiness of Nassim Taleb's historical research, the principle that justifies his idea (re: the relationship between the verification of bridges/financial systems and their designers/architects) is better outlined in the entirety of his book "Skin in the Game" moreso than in "Antifragile".