Today, we try to make sense of an old Roman power plant. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Anyone who's ever studied the history of technology has seen a drawing of 16 Roman water wheels, two abreast, arranged in stair-steps down a hillside. Historians have isolated that one mill like a sore. The common wisdom says that the Romans, who kept slaves, had no need of water power. This must be a lone oddity.

But the Romans were different people over many centuries and many lands they occupied. This mill was built just after 300 AD in a place called Barbegal near Arles in Southern France. In those days the emperor Constantine was in residence at Arles. Arles was far from Rome and it was also far from any reliable supply of barbarian tribes -- whom the Romans liked to enslave.

Power-generating water wheels had been around for about 600 years by then and they occasionally turn up in old writings and old ruins. But this Barbegal mill was a colossus among water mills.

An aqueduct brought water into a catch basin at the top of a small hill. From there it flowed through one pair of water wheels after another. The 16 mills ground enough grain to feed over 12,000 people -- roughly the entire population of 4th-century Arles.

The great stone foundations of the Barbegal mill still march down that hill. But they were covered by dirt, and not discovered until 1940. If Barbegal could stay hidden from history, what about smaller mills? So many mills, especially smaller ones, easily could've passed into dust.

We know water wheels were all over medieval Europe later on, when slaves weren't used at all. After 1066, William the Conqueror's Doomsday book listed nearly 6000 mills in England alone.

Trevor Hodge looks at that record and asks, "Where are all those English mills today?" Almost none of those 900-year-old mills have stood up. That means there had to be more of the really old Roman mills than we know about. And Barbegal was clearly a well-developed, well-understood technology.

Still, far fewer mills turn up in the Roman literature than in medieval documents. The Romans clearly didn't exploit mills fully and we wonder why. Hodge points to a matter beyond slavery. Horses hadn't yet been harnessed for hauling goods. It was still hard to move grain to and from central mills. A few big cities like Arles might've had mills. But it didn't yet pay to build grain mills where the population was sparse.

So the Barbegal mill is an important reminder that the offer of an effective new technology is not always easy to accept. The true usefulness of new technology depends on more factors than are ever obvious. By 1000 AD horses, stronger and faster than oxen, were now hauling goods. It was only then that the water wheel could complete its transformation of the human condition.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Hodge, T., A Roman Factory. Scientific American, November 1990, pp. 106-111.

Singer, C., Holmyard, E.J., Hall, A.R., Williams, T.I., A History of Technology, Vol. II. New York: Oxford University Press, 1956, Chapter 17, Power.