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No. 414:
South-Pointing Chariot

Today, an old technology meets a new one. 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.

The Chinese built a machine called a South-Pointing Chariot almost 2000 years ago. It was a small cart with a figure on top. As it moved down a twisting road, the figure always pointed south. When 19th-century historians first read about this odd machine, they assumed it used a compass. Actually, it didn't.

Hidden in the cart was an astonishing gear train. Its heart was a differential gear, like the one in your car. As the wheels turned, so did the figure on top. The gears kept the figure pointing in the same direction it'd been pointing when the trip began. The cart didn't use a compass at all. Its gears simply remembered which way the figure pointed at first. Those early Chinese had an amazing command of gearing to make such a thing.

Now that same principle is coming out of the Orient again. This time, the Honda company has announced a new navigation system for its cars. It'll display a map on a dashboard TV screen. The map is stored on a compact disc. You can replace it when you want to update the map. Your position and direction will show on the screen. But how do they manage that?

Three strategies can be used to keep track of a car's position. The most obvious is to use a compass. But compasses need all kinds of correction, and they're easily thrown off by any magnetic field. The best way to track position would be to read a radio signal the way an airplane does. But to do that you need a network of transmitting stations along the road. Someday we might use that kind of system, but not yet.

So car companies come back to what we call dead reckoning. That means keeping track of the distance your car travels and the turns it makes. We come back to the South-Pointing Chariot. Some companies want to tie into the differential -- to use the old Chinese technology directly. Honda changes that slightly.

The heart of their system is a helium jet flowing between two wires. When the car turns, the inertia of the jet carries it into one of the two wires and cools it. That sends an electric signal to a computer. The signal tells the computer how the direction of the car has changed.

That seems wonderfully high-tech, but the old South-Pointing Chariot rides within it. Like the South-Pointing Chariot, the Honda system has to be reset now and then -- the way you reset a clock. Its purpose might be similar, too. The old Chinese chariot was built to impress the people with the mystic powers of the emperor. Perhaps we see some faint echoes of that in the new Honda system, as well.

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

(Theme music)

Strandh, S., The History of the Machine. New York: Dorset Press, 1989, pp 43-45.

Judge, P.C., Computers to Help Drivers Find Their Way. New York Times, Wednesday, April 25, 1990.