Molae Ianiculenses
About
Trajan opened the Aqua Traiana in 109 CE to serve both drinking fountains and industry, channeling fresh water down the steep slope of the Janiculum hill above Trastevere. Archaeologists excavating under the American Academy in Rome in the 1990s found a multi-wheeled flour-milling complex built astride that channel, but the mills themselves date to the late 3rd century CE, roughly a century and a half after Trajan died in 117 CE. The aqueduct he built kept powering Rome's grain mills for centuries afterward regardless: the mills became so vital to feeding the city that the Ostrogoths deliberately cut the Aqua Traiana in 537 CE hoping to starve Rome out, forcing the general Belisarius to improvise floating mills anchored in the Tiber instead. On 11 August 117 CE the aqueduct's water was already running past this exact hillside; the wheels that would one day turn on it were still a century and a half away.
What happened here
The wheel turns all day and the miller's household lives to its noise. Water is brought in along a channel, dropped onto the wheel from above where the fall allows, and the gearing turns a horizontal drive into a vertical one for the millstones. One mill can grind for a small town. Where the flow is strong, mills are stacked in a cascade, each using the water the one above has finished with, and a site like that can feed a city. Grain arrives by cart and leaves as flour, with the miller taking a share rather than a fee. Stones are dressed regularly, and a mill also saws stone and works trip-hammers. Floods, drought and the millpond silting are the recurring troubles.
Sources
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Roman Maps shows the empire frozen at 11 August 117 CE, the day Trajan died — its moment of greatest territorial extent. Explore the full map →