Today, we go back 2200 years to North Africa. 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.

Alexander the Great fell ill and died in 323 BC, after 12 years of conquest -- but not ordinary conquest. He'd worked very hard at mixing the cultures of East and West -- at stimulating trade, cultural exchange, and intermarriage. His empire fell apart after his death, but the effects of the mixing remained.

He'd founded some fifteen cities named Alexandria, but the one just west of Cairo emerged as the center of Mediterranean culture. A new dialect of Greek called the Koinia became the common language of the Eastern Mediterranean basin. And Alexandria drew talent like a magnet. Euclid, Archimedes, and the astronomer Ptolemy all worked there. Alexandria remained the intellectual center of the world for three centuries -- until Rome took it over after the death of Cleopatra in 30 BC.

The much-praised technology of the Romans was built on the inventions of the great Alexandrian engineers. Plumbing, gearing, and water wheels, for example, all came from Alexandria. These engineers were less famous than Euclid and Archimedes. They had names like Ktsebios, Heron, Vitruvius, and Philon.

The most remarkable Alexandrian invention was feedback control. A feedback device automatically corrects the way a machine functions. Present-day feedback devices include thermostats, speed controllers, and pressure regulators. The Alexandrian engineers invented all sorts of float valves and other liquid-level regulators. The most important machine that used these gadgets was the water clock. The Alexandrian water clock was the basic time-keeper for about 1500 years -- until the mechanical clock replaced it in the 14th century.

Alexandria was free-wheeling, open, polycultural, and wonderfully inventive. After the authoritarian Romans took it over, invention dried up in Alexandria. It's interesting that the development of feedback also stopped dead-cold in its tracks and didn't resurface until a new craving for freedom swept Europe in the middle of the18th century; but that's another story.

Alexandria re-emphasizes a familiar message -- one that good educators understand. The people who really invent things are people who expose themselves to a lot of different influences and can let their minds run free.

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

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Clough, S., A History of the Western World, Vol. I, Ancient Times to 1725. Boston: D. C. Heath, 1964. Mayr, O., The Origins of Feedback Control. Cambridge, MA: MIT Press, 1970. 29

George, B., Reaping the Wind, American Heritage of Invention & Technology, Vol. 8, No. 3, Winter 1993, pp. 8-14.

Kealey, E., Harvesting the Air: Windmill Pioneers in Twelfth-Century England. Berkeley: University of California Press, 1987, Chapter 7.

This episode has been substantially revised as Episode 1422.