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Transcript
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The Trevi Fountain is one of Rome’s most
spectacular sights. Across a pale green pool,
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between two tritons wrestling with
winged horses, from beneath the feet
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of the mighty sea-god Oceanus, a silver
cascade rushes over steps of stone.
0:17
Impressive though all this is, the most
remarkable part lies behind the riot of statues,
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where the water that feeds the fountain flows,
as it has for more than two millennia, through
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the concrete channel of a Roman aqueduct.
Greek engineers began building aqueducts
0:33
as early as the sixth century BC. A stone-lined
channel carried spring water to Archaic Athens,
0:39
and Samos was served by an aqueduct that plunged
through a tunnel two-thirds of a mile (1 km) long.
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0:45
Even more impressive systems appeared during
the Hellenistic era, when the acropolis of
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Pergamon was supplied with water flowing
under pressure through huge lead pipes.
0:55
The Roman aqueducts differed from their
Greek predecessors in their use of arches
0:58
and hydraulic concrete. But it was sheer number
and scale that truly set them apart. Hundreds of
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aqueducts were constructed across the Roman world,
some well over 50 miles (80 km) long and capable
1:12
of delivering millions of gallons each day.
Contrary to what you might assume, the majority of
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Roman aqueducts were not built to supply drinking
water. Most Roman cities existed for centuries
1:22
before they constructed their first aqueduct, and
had established networks of wells and cisterns.
1:28
More often than not, aqueducts were luxuries,
designed to supply bath complexes, ornate
1:33
fountains, and the houses of the elite.
The time-consuming and extremely expensive
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1:39
process of building an aqueduct began
with locating a usable water source.
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Lakes were almost never chosen – stagnant water
was regarded as unhealthy – and rivers were only
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tapped in exceptional cases, since they carried
sediment and fluctuated seasonally. Usually,
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the source was a hillside spring.
A Roman aqueduct was an artificial river,
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flowing downhill from source to city. The channel
gradient had to be both gentle and consistent. If
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it was too steep, the mortar lining would
begin to erode; if it was too gentle,
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water would stagnate. Most Roman aqueducts descend
only five or ten feet every mile (1.5 – 3 m / km),
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and some have slopes as gradual as 1 in
20,000 – that is, a few inches per mile.
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To maintain such miniscule gradients,
Roman engineers relied on the dioptra
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and the chorobates. The dioptra – an ancestor of
the modern theodolite – was a sighting platform
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used to measure the relative position and height
of distant points. The chorobates, a long table
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with a central channel, was a water level.
With competent use of these instruments and
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adequate stocks of manpower and money, an
aqueduct could be built almost anywhere.
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For most of their length, Roman
aqueducts ran underground,
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following the contours of the landscape as
they slowly descended from their sources.
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Although the water flowing down them was rarely
more than knee-deep, their channels were made
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tall enough for maintenance workers to walk along
without stooping. To minimize leakage, the masonry
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walls were coated with waterproof cement.
When an aqueduct had to cross a valley,
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its gradient was maintained by elevating
the channel on rows of masonry arches.
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Until the reign of Augustus, these arcades
were normally built with blocks of local stone.
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Later, they tended to be brick-faced concrete.
Exceptionally deep depressions might be bridged
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with two or even three tiers of arches.
The most spectacular example is undoubtedly
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the Pont du Gard, just outside Nimes. No less
than 160 feet (50 m) high, it consists of huge
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blocks of limestone laid without mortar, which
support a channel so carefully graded that its
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level descends less than an inch from end to end.
When an aqueduct crossed a valley too deep for a
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bridge, Roman engineers built an inverted siphon
– a pipe running at ground level from a header
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tank on one side of a valley to a receiving tank
on the other side. As long as the receiving tank
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was at least slightly lower than the header, the
water in the pipe would rise to its own level,
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flowing up the slope and out of the valley.
The counterparts of the siphons and bridges
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that allowed aqueducts to traverse valleys were
the tunnels that carried them through hills.
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Unless the cutting was exceptionally deep, the
usual construction method involved excavating
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a series of shafts and boring in both directions
from the bottom. This didn’t always go as planned:
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an inscription from North Africa records how
two work gangs, tunneling from either side
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of a mountain, became disoriented and
began digging in opposite directions.
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When – having hewn through hills, vaulted
valleys, and marched majestically o’er
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the plains – an aqueduct finally
reached the city it was to supply,
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its terminus was often marked by a spectacular
fountain. Most of its water, however,
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was channeled into the distribution tanks that
the Romans called castella. These fed batteries of
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pipes, which in turn led to smaller distribution
tanks. Pompeii had 12 of these; Rome had 247.
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In the northwestern provinces, water pipes were
often made of tree trunks joined with iron bands.
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In the eastern Mediterranean, they might consist
of long lines of hollowed stone blocks. Most
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Roman pipes, however, were made of terracotta or
lead. Although they knew that lead caused health
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problems, the Romans persisted in making pipes
from it, simply because lead was cheap, easy to
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work, and didn’t rust. The Romans were only saved
from lead poisoning by the swiftness with which
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water flowed through the pipes, and by the calcium
deposits that tended to coat their insides.
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Although most cities with aqueducts drew some
of their drinking water from wells or cisterns,
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aqueduct water was – at least in the city
of Rome itself – regarded as healthier
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and better-tasting. Most households
accessed aqueduct water by drawing it
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from a public fountain or basin (or by paying a
water-carrier to fetch it for them). By one count,
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Rome had 1,352 fountains. In Pompeii, there
was one for about every 160 inhabitants.
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Baths were almost equally common. In
the city of Rome alone, aside from
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the colossal imperial thermae, there were more
than 850 neighborhood baths by late antiquity.
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The largest complexes used so much water
that they needed dedicated aqueducts.
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The Baths of Caracalla, for example,
were fed by a spur of the Aqua Marcia,
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and featured a reservoir with 32 chambers and a
capacity of more than 2,000,000 gallons (8,000,000
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L). The outflow of wastewater from these baths
was copious that it was used to power watermills.
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Private connections to aqueduct
water were relatively rare.
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In Rome, the process for installing a
tap involved appealing to the emperor,
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bringing the emperor’s authorization to the
water commissioner, and finally receiving
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a calix, a bronze nozzle stamped with the
owner’s name. The grant was not permanent:
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as soon as the recipient died or
sold his home, his calix was removed.
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Although some private connections were granted
to the owners of industrial facilities,
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most belonged to members of the elite, who used
the water to supply the gardens, fountains,
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and private baths of their mansions. The situation
seems to have been broadly similar in Pompeii,
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where only 10% of households had access to piped
water, but that 10% used it so extravagantly
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that one house had no fewer than 33 faucets.
Maintaining the aqueducts was a constant struggle.
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In the city of Rome, a permanent staff of 700
installed new pipes, braced collapsed arches,
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and kept the channels clear. Mud and
stones had to be removed from the
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settling tanks – depressions in the
channel designed to catch suspended
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sediment and debris – and mineral deposits
were periodically scraped from the walls.
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Although Rome’s aqueducts seem to have been
fairly well-maintained until late antiquity,
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not all cities were so scrupulous, and some
aqueducts became completely clogged with debris.
8:22
Speaking of clogs, this video is
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8:36
When they weren’t clogged, the aqueducts were
awesome manifestations of the Roman knack
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for practical engineering on a monumental scale.
The aqueduct that served Carthage ran 55 miles
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(90 km) from a sacred spring to the
cavernous cisterns of the city’s great baths.
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The aqueduct that Augustus built along
the Bay of Naples was even longer,
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and featured at least a dozen branches
supplying the naval station at Misenum,
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the elaborate seaside villas at Baiae, and
the doomed cities of Pompeii and Herculaneum.
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The aqueduct of Constantinople, whose channels
had a combined length of over 300 miles
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(500 km), filled a vast series of
artificial lakes and covered reservoirs.
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Most impressive of all were
the eleven aqueducts of Rome,
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which may have collectively carried as much
as a million cubic meters of water each day.
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Tapping springs and streams in the surrounding
hills, and carried over suburban villas
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and market gardens on miles-long arcades, Rome’s
aqueducts entered the city proper in a spectacular
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web of pipes, conduits, and distribution tanks.
Since only a few of the aqueducts were high enough
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to supply all 14 of the city’s regions, most
had a fairly localized distribution network.
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The whole system, however, was interconnected, so
that if one aqueduct were shut down for repairs,
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another could be diverted to replace it. This
feature was not always appreciated, since Rome’s
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aqueducts ranged in quality from the Aqua Marcia
– fed by the emerald pools of a delicious mountain
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spring – to the Aqua Alsietina, whose water was
so muddy that it was considered undrinkable.
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But thanks to hundreds of millions of sestertii in
funding, endless maintenance work, and the basic
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quality of their construction, the aqueducts
continued to flow long after the emperors were
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gone. A few, as we’ve seen, are still flowing
today, another living legacy of ancient Rome.
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If you enjoyed this video, please consider
supporting toldinstone on Patreon. You might also
10:42
enjoy my book, Naked Statues, Fat Gladiators,
and War Elephants. Thanks for watching.
— end of transcript —
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