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Aqueducts
part II
PART III - what can be seen today PART IV - from ancient Rome to the contemporary age


HOW ROMAN AQUEDUCTS WORKED

Aqueducts collected water from several natural springs, located far away from the city (the furthest source was that of the Anio Novus, 59 miles or 87 km east of Rome).
A "water" was chosen according to many factors: the position of its springs, the purity of its water, its taste, its temperature, and sometimes even its alleged medical properties due to the mineral salts it contained.
No source of energy moved the water towards the city but gravity, i.e. the aqueduct acted as a continuous slope, all the way from the springs to the final output. To obtain this result, each of them was drawn so that every point of their long course was slightly lower than the previous segment, and slightly higher than the following one. For this reason water had to be drawn from springs located in hilly areas, above Rome's position, particularly from the city's eastern surroundings, and every single point of the aqueduct's route had to be carefully planned, according to the ground features it encountered.
diagram of a dioptra:
A - sights
B - screw for adjusting the angle
C - screw for adjusting the direction
Roman architects were very skillful in this kind of work, for which they used sophisticated tools: besides the ordinary level, similar to the one used today by carpenters, they used devices such as the chorobates, a sort of bench with weighted strings on its sides for measuring the ground's angle on a system of notches, and a short channel in the center, likely for testing the direction of the water flow. The dioptra was a different kind of level: rested on the ground, and finely adjusted by tilting and rotating the top part by means of precision screws, it could assess the angle of a stretch of aqueduct by looking through pivoting sights.

diagram of a chorobates:
A - sights
B - weighted strings and notches
C - central channel
Before being collected into the duct, the water passed through one or more pools, called piscinae limariae, where the speed of the flow would slow down, so that mud and other particles could settle.

springs aqueduct

Similar pools or basins were also set along the course of most aqueducts, to remove any further impurity.

Off the urban area most of the aqueduct's length ran underground: the adequate level to mantain a sloping course was reached by digging vertical pits, and then the tunnel, or specus, was opened through the rock.
In most cases the walls of the specus were lined with a waterproof facing, made of concrete (mortar mixed with small fragments of broken tiles and amphors).


the remains of the specus of the Aqua Alexandrina
The average size of its section was about 1 m wide by 2 m in height (3 ft. by 6 ft.) , but these dimensions could considerably vary according to the expected water delivery of each single stretch.

The vertical pits were left open, so to be used as service passages for the maintainance of the aqueduct: Rome's water is very rich of calcium salts, especially the one drawn from the springs located east of the city, and huge quantities of deposits had to be frequently removed to prevent the ducts from being clotted.
Bleeds were also present along the specus, so that in the case of an overflow its walls would not be damaged.

Along the outer course of the aqueduct, every 240 feet (70 metres) a large stone marked the presence of the underground channel, and a safety distance of 5 feet (1.45 metres) from the course had to be kept free, to avoid damages or pollution.
In fact, all aqueducts were public properties, owned by the government for the benefit of the citizens. Damaging the aqueducts or polluting the water was severely sanctioned, as well as drawing water for private mansions or grounds by illegally connecting a pipe to a public duct. Private branches did exist, but they were only allowed to use the water surplus, and a fee had to be payed for this facility.


Due to the ground's natural features, some parts of the duct had to follow the surface (see picture on the right), along a trench whose sides were timbered.
Stone slates covered the specus from being exposed to the direct sunlight, and from dirt, leaves, etc.; they were either flat, or rounded, or formed an angle. Usually a low wall with some kind of cover protected the aqueduct's exposed parts, with a safety distance from the "water" lengthened to 15 feet (4.40 m).

When the duct reached a steep slope or a canyon, one solution was to build a bridge, or a viaduct, to cross the gap and reach the other side at a slightly lower level: here the duct's course would return underground.

Another way by which such natural features could be crossed was the "inverted siphon", a technique based on a simple physical principle.
Just before the slope, the water was collected into a cistern, from which a pipe carried it to the bottom of the hollow by gravity, and then up again into a second cistern, thanks to the pressure generated along the first slope.
A small viaduct was often built on the bottom of the hollow to reduce its maximum height, thus to minimize the water pressure needed to climb the opposite side.

The siphon was not often used for roman aqueducts, since the pipes available by those times, made of lead or earthenware, could not be soldered steadily enough to hold the rather strong pressure generated by the slope, causing a substantial loss of water and requiring frequent repairs.
The architects, instead, in most cases preferred to lengthen the course of the aqueduct, sometimes quite considerably (as in the case of the Aqua Virgo, shown in the map on the right), so to follow the ground's natural features and constantly meet a regular slope. This explains why most aqueducts were much longer than the direct distance between their source and their urban output.


Where the ground turned flat, approaching the city, the flow was made possible by building the famous series of archways, some of which almost reached 30 m. (90 ft.) in height.
They crossed the countryside for miles, keeping the water level high enough to reach the urban area.
In fact, it was along these impressive structures that most aqueducts entered Rome. The higher the water travelled, the greater was the number of districts it could run to.
Along these viaducts, doors located in the upper part, where the water channel ran, allowed the same maintainance work needed by the underground tunnels.

Having to exploit as much as possible the natural height of the grounds they crossed, several aqueducts came to Rome following an almost identical course; therefore, two or even three "waters" could share long stretches of the same viaduct, flowing in separate channels at different levels (see the cross section on the right), according to the height each of them had respectively reached.

The main urban outputs were located on the city's highest spots. In particular, most aqueducts approached the city's boundary from the south-east, in a place known as the Spes Vetus ("old hope"), after an ancient Temple of Hope that once stood there. The water then entered Rome from the nearby Esquiline Hill, from where most other parts of the city could be served.

In some cases, "richer" aqueducts helped others to mantain a sufficient water volume for the districts they supplied: for instance, the Aqua Claudia poured about 1/8 of its total flow into the A.Iulia and A.Tepula.
Not all aqueducts entered Rome above a viaduct: the earliest one, Aqua Appia, ran almost completely underground, as well as the ones coming from the north-west, Aqua Alsietina and Aqua Traiana, which supplied the 8th regio, Trans Tiberim (i.e. Trastevere district), from the top of the Janiculum Hill.
ducts of the Aqua Marcia, Tepula and Iulia by Porta Maggiore (left) and Porta Tiburtina

In such cases, within the urban area the lapides perterebrati were used: special hollow bricks which fit one into the other to make a waterproof channel.
Many of these bricks have been found in archaeological excavations, allowing the identification of several underground ducts mentioned by literary sources.



the naumachia, from Pirro Ligorio's
map of ancient Rome (1561)

diagram of the lapides perterebrati,
used for urban underground channels


The Aqua Alsietina, the earliest of the two western aqueducts (2 BC), drew water from a small lake north of Rome called Lacus Alsietinus (now Lago di Martignano). This water was not suitable for drinking: emperor Octavianus Augustus used it only to fill his naumachia in Trastevere, where he and the public enjoyed naval battles, and for the irrigation of Caesar's Horti (gardens), in the same district. This gives an idea of how much water ancient Rome had at its disposal.

The main output of an aqueduct came in the shape of a castellum ("castle"), a structure of variable size with one or more chambers similar to the piscinae limariae, where the water flow slowed down and the last impurities settled. The water was then poured outside through a number of chalice-like nozzles.
Most castelli looked like simple prysms, but a few of them came in the shape of fountains or nymphaeums, decorated with statues, reliefs or mosaics. Wardens patrolled them, to avoid any tampering of the ducts or pollution of the water.

a "castle", highlighted in yellow, pouring the
A.Iulia or A.Tepula next to Diocletian's Baths
(from E.Du Perac's map of ancient Rome, 1574)

From the main castellum, other parts of the city were reached by means of smaller branches of the aqueduct, either along an elevated viaduct or an underground channel, always following a sloping route, as the main one. These ones too sometimes had further branches, and ended with smaller "castles", or directly served public baths, fountains, etc.: ancient Rome was actually crossed by a complicated network of water ducts.


diagram of a standard output:
A - main duct       B - "castle"       C - secondary branch       D - nozzle



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