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The paper entitled "The Pont du Gard and the Aqueduct of Nimes" by N A F Smith, is published in the Transactions of the Newcomen Society, 1990-91 Vol 62.
The author sets the scene by quoting several well-known writers who have expressed their admiration of this structure, regarded by some as the most beautiful of all monuments of Roman Gaul:
"The aspect of this simple and noble work struck me all the more that is was in the midst of a desert where the silence and solitude increased its effect... I walked along the three stages of this superb construction, with a respect that made me almost shrink from treading on it. The echo of my footsteps under the immense arches made me think I could hear the strong voices of the men who had built it. I felt lost like an insect in the immensity of the work. " Jean-Jacques Rousseau."The preservation of the thing is so extraordinary; nothing has crumbled or collapsed; every feature remains, and the huge blocks of stone, of a brownish-yellow (as if they had been baked by the Provencal sun for eighteen centuries), pile themselves, without mortar or cement, as evenly as the day they were laid together. All this to carry a couple of springs to a little provincial city! " Henry James.
There are a number of puzzling questions relating to the aqueduct: why it was built, its date, its route, how it was surveyed, its strange curvature, the unusual third tier, and the water flow. The author discusses these questions, offering the reader some interesting theories, concluding with a description of the castellum divisorium, the delivery system in Nîmes.
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| The Pont du Gard © copyright |
On how the aqueduct was surveyed:
"One feature of the aqueduct is especially notable and that is the amazingly shallow gradient; in a total channel length of 49.5km the fall is a mere 17m so that the average gradient is 1 in 2,926 and over one section it is no more than 1 in 14,285.Gradients have always been a pre-occupation with writers on Roman aqueducts, one suspects because it is obviously so easy to measure what was built and compare it with the advice of the various authorities whose recommendations are: Vitruvius, not less than 1 in 4,800 or 1 in 200 depending on which manuscript is consulted; Pliny, who agrees with Vitruvius at 1 in 4,800; Faventinus, 1 in 40 to 1 in 67; and Palladius who agrees with Faventinus because he copied him.
What, by comparison was a critical surveying problem and one that engineering historians persistently overlook, is how a water source for city or town, garrison or villa, was located in the first place and its elevation above the delivery point established. After all, everything else depended on this initial measurement being accurate.
The water channel © copyright Unfortunately we do not really know how this was done. Most opinion selects the chorobates as the instrument - a long wooden device - as much as 6m long according to Vitruvius...Myself, I am inclined to think that more consideration should be given to the possibility that the Romans used the A-frame level for water channels, setting the instrument with its apex uppermost so that the plumb-line intersected with a mark of scale on the bar..."
Having discussed how the aqueduct might have been surveyed, and the evidence that this most challenging task might have gone wrong, the author concludes that it could well have been a surveying error that lay behind the unusual third tier of the bridge, the range of small arches carrying the water channel itself.
"The spans of these small arches vary subtly because the engineer cleverly adapted them to match the spacing of the big arches below. Indeed, a number of writers have been greatly impressed by the aesthetic fashion in which the third tier was integrated into the design without considering that the whole exercise was, very possibly, an unwanted and unexpected nuisance. In short, perhaps the third level was an afterthought, so to speak, a remedial modification to overcome what we were considering just now, a surveying error."
Left: the Pont du Gard as built. Right: a conjectural two-tier bridge. © copyright
The complete text of this paper can be purchased on line from our archive.
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