One of the most impressive Roman constructions you will ever see is the aqueduct of Segovia, Spain.
Aqueduct of Segovia (file photo)
It still brings good water down from the mountains fifteen kilometers away. For most of that distance the closed canal rides on a low wall of Roman concrete. Then for the last kilometer, to cross the valley where Segovia lies, it is supported by great pillars and arches of stacked stones—granite blocks so perfectly hewn that no mortar was needed to hold them in place. By the time the canal crosses the main square of town it is more than 100 feet above the streets and, if you are seeing it for the first time, it takes your breath away.
Aqueduct of Segovia (file photo)
The canal is rectangular and its inner walls are smoothed with fine morter. The typical Roman inclination for a water course like that was a mere one per mil—considered enough to make up for the slowing down from friction as the water flowed through.
When was the aqueduct built? No one knows exactly—facts like that often go under in a land so old and where so much has happened. But recently the archaeologist Geza Alfoldy, a specialist in epigraphy, may have found out. How? Originally there were Latin inscriptions on the aqueduct. They were formed with bronze letters and held in place by drilling holes into the blocks. The holes are still there. For Alfoldy, those holes are as good as the letters; by comparing other monuments with known inscriptions he has decided which letter needed which holes. The Latin writing was the same on both sides of the aqueduct, with slight variation, so Alfoldy used the one side to confirm his reading of the other. He proposed the following:
By order of the Emperor Trajan Nerva, Germanic Caesar Augustus, Pontifex Maximus, Twice Tribune, Twice Consul, Father of the Country, Publius Mumius Mumianis and Publius Fabius Taurus, Segovian Municipal Flavian Duumviri, rebuilt this aqueduct.
If Alfoldy is right, this means, judging by the titles given to the Emperor Trajan, that the inscription was posted on the aqueduct in 98 AD.
Why did the Romans build aqueducts over valleys? It is sometimes said that had they known the principle that water meets its own level they would not have needed to build them. They would merely have brought their canals to the brink of the valley and then, instead of constructing a complex and expensive aqueduct, led the water through a simple pipeline down to the bottom of the valley and back up the other side until it was on a level with the canal, and so could continue its flow.
But this is to insult the Romans. They knew the principle very well (the siphon) and made good use of it in countless places to cross valleys and rivers. A famous example was the siphon channel at Lugdunum, capital of Gaul (near Lyons, France). The pipes were sometimes made of lead, as at Caesaraugustus (Zaragoza, Spain), where one 32 cm. in diameter crossed the wide Ebro River before reaching the city. Most of these were long ago melted down for other purposes. But others were clay and even stone, as at Sexi, Spain, and have survived.
Ceramic pipes found at Ephesus
and stone pipes used for the siphon at Sexi, Spain
(photos and illustration from the catalog of an exhibition at the Museo Arqueológico Nacional, Madrid, in March-July 2002: Artifex: Ingeniería Romana en España by Ignacio González Tascón
The plan of a Roman siphon, here leveled off at the bottom of the valley
Then why was the aqueduct of Segovia necessary? Couldn’t a siphon have been used?
Perhaps an aqueduct was considered easier to maintain than a siphon channel, perhaps the terrain was not suitable. You may be quite sure the engineers did not build the aqueduct out of ignorance. They will have studied other bridges and aqueducts of all kinds before deciding on this amazing monument. Perhaps its very monumentality was the determining factor.
What are those holes in the old stones? See How Did They Lift Those Stones?
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Great Names in History 
Does nobody know how to read here? It clearly says in 98AD the Roman Emperor ordered that the Aqueducts be “rebuilt”. So we have no records or by whom or when or even an idea of why the Aqueducts were build yet we come to the definite conclusion the Romans built them yet the only possible historical clue left says the Roman rebuilt or repaired them. It is called quackademic.
goedjn: if you are going to be a didactic and a hater make sure you are 100% correct. The Romans and the Greeks before them, used inverted siphons. An inverted siphon is still very much the same thing it was back then and calling them simply siphons is perfectly correct, even in “modern English.” Furthermore, any conduit flowing full is under pressure and the pressure in the valley of a Greek or Roman siphon could be considerable which is in no way “far more tolerant of leaks.”
@Tod: at least in modern English, “siphon” is the wrong word. A siphon works to pull water over a high barrier from a lower reservoir to an even lower outlet, using atmospheric pressure to force water over the hump. (which is why there’s a 30′ or so limit) this is just using the principle that the reservoir is higher than ANY point in the sealed pipe, and the intervening ups and downs don’t matter. The entire system is under positive pressure, relative to the outside, and it’s far more tolerant of leaks.
good stuff
joyofreadinglinson: Thanks. I enjoyed my visit at your site. Our ideas of history are similar: it is about personalities–or can be. I just deleted a silly comment that said I HATE ART AND HISTORY. The fact is, I do too as it is usually presented.
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Hey are you a historian? The second blog that I see of yours has tightened your grip on me. History never seemed so interesting until now. I am revitalizing my historical readings after a long long time.
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digby: A good question–I don’t know. Let me investigate.
why does the aqueduct take the course it does from the city walls accross the valley. the designers didnt take the most direct path and have to incorporate a sharp bend ?
Thanks, Todd. At first I got scared away by those formulas but now, braver, I see what you mean. The stone pipe near Granada was certainly tight enough to take the pressure of that siphon but just how high the the water had to be raised I can’t say. Maybe the Roman engineers thought the valley at Segovia exceeded the limit for clay pipes.
I may be mixing things up here but I thought there was a limit to the height that a siphon could lift a water column. A quick search on the brain (aka the web) reveals a formula:
“the height of the column of water is given by the equation:
h=P/(density*g)
where P is the pressure (standard atmosphere pressure is about 1.01*10^5 Pa)
density is the density of the liquid (in this case water= 1000 kg/m^3)
and
g is the local gravity acceleration (it varies, but it is about 9.8 m/s/s).”
from http://tinyurl.com/6hojwx
Even if this formula does not apply in this case one could imagine that the pressure within the system could exceed the ability of the parts to withstand it, therefor a siphon would have some limit.
Danu: I started writing about Augustus and his reforms once and the more I wrote the deeper the trouble I felt I was getting into. He brought back the old censor of morals; he made people go back to the temples to worship gods they probably took with a grain of salt; he insisted that everyone wear the toga; he gave long propaganda speeches before every play and circus performance. All the writers had to (or felt they had to) take a party line. After our War II and the “great” Fascist dictatorships, these measures are downright repellent to us. But you are right–that constant propaganda was important to Romanness. Caesar did things only to impress, such as build that bridge across the Rhine in ten days and then destroy it ten days later while the Germans watched in disbelief. Of course that constant drumming on her greatness worked and made people respect Rome and feel proud to be part of the empire. Those engineers and governors knew the aqueduct wowed.
98 was the time of the first war in Dacia (on the second one Trajan would defeat the Dacians and transform their land into a roman province, Dacia Felix) Amazing stuff! and being what they were I cannot put aside the variant that those spectacular aqueduct, apart their functional role, had also the one, quite important, to prove the Roman Greatness, to be there as a proof of this greatness… Political propaganda sounds horible now but the Romans knew one or two things about it…
Madame Monet: I just looked up Morlaix and see that they speak of a viaduct, not an aqueduct. It doesn’t look very old and is not very pretty either. I couldn’t find its construction date.
This Roman aquaduct looks very much like what I saw over the valley in Morlaix, France. Do you know if that was built by the Romans, too?
Best regards,
Madame Monet
Yes, Erika, for a long time I was one of the ignorant who believed the Romans could just have strung a pipeline down in the valley and brought it back up the other side. No aqueduct necessary. No one could ever tell me better. Then I saw the whole siphon thing explained. The aqueduct to Cadiz was the longest in Spain and used the siphon several times to negotiate valleys on the way to the city.
Very impressive structure, and it’s still in working condition? wow Monumentality might have been one reason they built them. But there must have been other reasons too. Very interesting and convincing theory by Alfoldy.