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No. 1101:
Paconius's Folly

Today, a clever Roman engineer outsmarts himself. 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.

Historian Henry Petroski writes about failure. After all, success has far less to teach us than failure does. He goes to Vitruvius's 1900-year-old book on Roman engineering. Vitruvius tells about a proud engineer named Paconius. Paconius bid on the job of moving the pedestal for a statue of Apollo. The pedestal was a stone block, 12 feet by 8 feet by 6 feet -- about 50 tons of stone.

The Romans knew a lot about moving stone. To move a cylindrical column they'd drill axles into the ends, then tow it like a big roller. For a square column, they'd fit wooden wheels around the ends, then build a frame around the wheels and tow it like a cart.

But Paconius's pedestal was an unusual job and he had his own scheme for handling it. He built a great 15-foot-diameter horizontal wooden spool around it. Then he wrapped the spool with rope. The end of the rope came over the top of the spool and was attached to several yokes of oxen. As the oxen pulled the rope, the spool was to roll forward, playing out the rope.

It seemed to make sense. But try rolling a spool of thread forward by pulling on the thread. As long as the thread's wound in the middle of the spool, it works. But when the thread's off to one side, the spool slews away from the direction you're pulling. I'll read what Vitruvius says about Paconius's attempt:

As [the rope] uncoiled, it did indeed cause the wheels to turn, but it could not draw them in a line straight along the road. Hence it was necessary to draw the machine back again. Thus by this drawing to and fro, Paconius got into such financial embarrassment that he became insolvent.

So Paconius went bankrupt and Petroski looks for a moral in his story. Paconius did what any inventor must: he began by looking for an idea within his own head and he came up with a good one. But good designers also have to know how to attack their own idea. They invite others to attack it. They look for anything that could be wrong with it. Vitruvius says, "Paconius, with confident pride ... determined to make a machine of a [new] sort." The key word there is pride. It takes a dimension of humility to question our own ideas. Proud Paconius wasn't able to do that.

So he missed what a good critic might've seen. Petroski goes on to tell about modern designers who've brought far greater disaster on themselves in the same way.

But Paconius's story also harbors a second, and more subtle, message. Rome was conservative and so was Vitruvius. Rome capitalized on older inventions but added few of her own. Vitruvius portrays Paconius as foolish for wanting to "make a machine of a [new] sort." That lack of vision eventually caught up with Rome. She needed more Paconiuses -- more makers of ideas.

Good engineering is the flight of imagination within the constraints of reality. Engineering can't flourish without Paconius any more than it can flourish with people like Paconius alone.

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

(Theme music)

Petroski, H., Design Paradigms: Case Histories of Error and Judgment in Engineering. Cambridge: Cambridge University Press: 1994, Chapter 2.

Vitruvius, The Ten Books of Architecture (tr. by M.H. Morgan). New York: Dover Publications, Inc., 1960. See especially p. 289.