[00:01] The Trevi Fountain is one of Rome’s most 
spectacular sights. Across a pale green pool,
[00:07] between two tritons wrestling with 
winged horses, from beneath the feet
[00:11] of the mighty sea-god Oceanus, a silver 
cascade rushes over steps of stone.
[00:17] Impressive though all this is, the most 
remarkable part lies behind the riot of statues,
[00:22] where the water that feeds the fountain flows, 
as it has for more than two millennia, through
[00:27] the concrete channel of a Roman aqueduct.
Greek engineers began building aqueducts
[00:33] as early as the sixth century BC. A stone-lined 
channel carried spring water to Archaic Athens,
[00:39] and Samos was served by an aqueduct that plunged 
through a tunnel two-thirds of a mile (1 km) long.
[00:45] Even more impressive systems appeared during 
the Hellenistic era, when the acropolis of
[00:49] Pergamon was supplied with water flowing 
under pressure through huge lead pipes.
[00:55] The Roman aqueducts differed from their 
Greek predecessors in their use of arches
[00:58] and hydraulic concrete. But it was sheer number 
and scale that truly set them apart. Hundreds of
[01:05] aqueducts were constructed across the Roman world, 
some well over 50 miles (80 km) long and capable
[01:12] of delivering millions of gallons each day.
Contrary to what you might assume, the majority of
[01:15] Roman aqueducts were not built to supply drinking 
water. Most Roman cities existed for centuries
[01:22] before they constructed their first aqueduct, and 
had established networks of wells and cisterns.
[01:28] More often than not, aqueducts were luxuries, 
designed to supply bath complexes, ornate
[01:33] fountains, and the houses of the elite.
The time-consuming and extremely expensive
[01:39] process of building an aqueduct began 
with locating a usable water source.
[01:44] Lakes were almost never chosen – stagnant water 
was regarded as unhealthy – and rivers were only
[01:49] tapped in exceptional cases, since they carried 
sediment and fluctuated seasonally. Usually,
[01:55] the source was a hillside spring.
A Roman aqueduct was an artificial river,
[02:00] flowing downhill from source to city. The channel 
gradient had to be both gentle and consistent. If
[02:08] it was too steep, the mortar lining would 
begin to erode; if it was too gentle,
[02:12] water would stagnate. Most Roman aqueducts descend 
only five or ten feet every mile (1.5 – 3 m / km),
[02:21] and some have slopes as gradual as 1 in 
20,000 – that is, a few inches per mile.
[02:27] To maintain such miniscule gradients, 
Roman engineers relied on the dioptra
[02:32] and the chorobates. The dioptra – an ancestor of 
the modern theodolite – was a sighting platform
[02:38] used to measure the relative position and height 
of distant points. The chorobates, a long table
[02:45] with a central channel, was a water level. 
With competent use of these instruments and
[02:50] adequate stocks of manpower and money, an 
aqueduct could be built almost anywhere.
[02:56] For most of their length, Roman 
aqueducts ran underground,
[02:59] following the contours of the landscape as 
they slowly descended from their sources.
[03:04] Although the water flowing down them was rarely 
more than knee-deep, their channels were made
[03:09] tall enough for maintenance workers to walk along 
without stooping. To minimize leakage, the masonry
[03:15] walls were coated with waterproof cement.
When an aqueduct had to cross a valley,
[03:20] its gradient was maintained by elevating 
the channel on rows of masonry arches.
[03:25] Until the reign of Augustus, these arcades 
were normally built with blocks of local stone.
[03:30] Later, they tended to be brick-faced concrete. 
Exceptionally deep depressions might be bridged
[03:36] with two or even three tiers of arches.
The most spectacular example is undoubtedly
[03:42] the Pont du Gard, just outside Nimes. No less 
than 160 feet (50 m) high, it consists of huge
[03:51] blocks of limestone laid without mortar, which 
support a channel so carefully graded that its
[03:56] level descends less than an inch from end to end.
When an aqueduct crossed a valley too deep for a
[04:03] bridge, Roman engineers built an inverted siphon 
– a pipe running at ground level from a header
[04:09] tank on one side of a valley to a receiving tank 
on the other side. As long as the receiving tank
[04:15] was at least slightly lower than the header, the 
water in the pipe would rise to its own level,
[04:19] flowing up the slope and out of the valley.
The counterparts of the siphons and bridges
[04:25] that allowed aqueducts to traverse valleys were 
the tunnels that carried them through hills.
[04:31] Unless the cutting was exceptionally deep, the 
usual construction method involved excavating
[04:35] a series of shafts and boring in both directions 
from the bottom. This didn’t always go as planned:
[04:41] an inscription from North Africa records how 
two work gangs, tunneling from either side
[04:46] of a mountain, became disoriented and 
began digging in opposite directions.
[04:52] When – having hewn through hills, vaulted 
valleys, and marched majestically o’er
[04:57] the plains – an aqueduct finally 
reached the city it was to supply,
[05:01] its terminus was often marked by a spectacular 
fountain. Most of its water, however,
[05:07] was channeled into the distribution tanks that 
the Romans called castella. These fed batteries of
[05:13] pipes, which in turn led to smaller distribution 
tanks. Pompeii had 12 of these; Rome had 247.
[05:22] In the northwestern provinces, water pipes were 
often made of tree trunks joined with iron bands.
[05:29] In the eastern Mediterranean, they might consist 
of long lines of hollowed stone blocks. Most
[05:34] Roman pipes, however, were made of terracotta or 
lead. Although they knew that lead caused health
[05:40] problems, the Romans persisted in making pipes 
from it, simply because lead was cheap, easy to
[05:46] work, and didn’t rust. The Romans were only saved 
from lead poisoning by the swiftness with which
[05:52] water flowed through the pipes, and by the calcium 
deposits that tended to coat their insides.
[05:58] Although most cities with aqueducts drew some 
of their drinking water from wells or cisterns,
[06:03] aqueduct water was – at least in the city 
of Rome itself – regarded as healthier
[06:07] and better-tasting. Most households 
accessed aqueduct water by drawing it
[06:12] from a public fountain or basin (or by paying a 
water-carrier to fetch it for them). By one count,
[06:15] Rome had 1,352 fountains. In Pompeii, there 
was one for about every 160 inhabitants.
[06:24] Baths were almost equally common. In 
the city of Rome alone, aside from
[06:28] the colossal imperial thermae, there were more 
than 850 neighborhood baths by late antiquity.
[06:34] The largest complexes used so much water 
that they needed dedicated aqueducts.
[06:40] The Baths of Caracalla, for example, 
were fed by a spur of the Aqua Marcia,
[06:45] and featured a reservoir with 32 chambers and a 
capacity of more than 2,000,000 gallons (8,000,000
[06:51] L). The outflow of wastewater from these baths 
was copious that it was used to power watermills.
[06:58] Private connections to aqueduct 
water were relatively rare.
[07:02] In Rome, the process for installing a 
tap involved appealing to the emperor,
[07:07] bringing the emperor’s authorization to the 
water commissioner, and finally receiving
[07:11] a calix, a bronze nozzle stamped with the 
owner’s name. The grant was not permanent:
[07:17] as soon as the recipient died or 
sold his home, his calix was removed.
[07:23] Although some private connections were granted 
to the owners of industrial facilities,
[07:27] most belonged to members of the elite, who used 
the water to supply the gardens, fountains,
[07:32] and private baths of their mansions. The situation 
seems to have been broadly similar in Pompeii,
[07:37] where only 10% of households had access to piped 
water, but that 10% used it so extravagantly
[07:43] that one house had no fewer than 33 faucets.
Maintaining the aqueducts was a constant struggle.
[07:51] In the city of Rome, a permanent staff of 700 
installed new pipes, braced collapsed arches,
[07:57] and kept the channels clear. Mud and 
stones had to be removed from the
[08:01] settling tanks – depressions in the 
channel designed to catch suspended
[08:05] sediment and debris – and mineral deposits 
were periodically scraped from the walls.
[08:10] Although Rome’s aqueducts seem to have been 
fairly well-maintained until late antiquity,
[08:15] not all cities were so scrupulous, and some 
aqueducts became completely clogged with debris.
[08:22] Speaking of clogs, this video is 
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[08:36] When they weren’t clogged, the aqueducts were 
awesome manifestations of the Roman knack
[08:41] for practical engineering on a monumental scale. 
The aqueduct that served Carthage ran 55 miles
[08:48] (90 km) from a sacred spring to the 
cavernous cisterns of the city’s great baths.
[08:55] The aqueduct that Augustus built along 
the Bay of Naples was even longer,
[08:59] and featured at least a dozen branches 
supplying the naval station at Misenum,
[09:03] the elaborate seaside villas at Baiae, and 
the doomed cities of Pompeii and Herculaneum.
[09:09] The aqueduct of Constantinople, whose channels 
had a combined length of over 300 miles
[09:14] (500 km), filled a vast series of 
artificial lakes and covered reservoirs.
[09:21] Most impressive of all were 
the eleven aqueducts of Rome,
[09:24] which may have collectively carried as much 
as a million cubic meters of water each day.
[09:30] Tapping springs and streams in the surrounding 
hills, and carried over suburban villas
[09:34] and market gardens on miles-long arcades, Rome’s 
aqueducts entered the city proper in a spectacular
[09:40] web of pipes, conduits, and distribution tanks.
Since only a few of the aqueducts were high enough
[09:47] to supply all 14 of the city’s regions, most 
had a fairly localized distribution network.
[09:54] The whole system, however, was interconnected, so 
that if one aqueduct were shut down for repairs,
[10:00] another could be diverted to replace it. This 
feature was not always appreciated, since Rome’s
[10:06] aqueducts ranged in quality from the Aqua Marcia 
– fed by the emerald pools of a delicious mountain
[10:12] spring – to the Aqua Alsietina, whose water was 
so muddy that it was considered undrinkable.
[10:19] But thanks to hundreds of millions of sestertii in 
funding, endless maintenance work, and the basic
[10:23] quality of their construction, the aqueducts 
continued to flow long after the emperors were
[10:28] gone. A few, as we’ve seen, are still flowing 
today, another living legacy of ancient Rome.
[10:36] 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.