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  • Cool Retrotech

    Posted by David Foster on April 24th, 2009 (All posts by )

    At this blog we have lots of smart Chicago Boyz and Chicago Grrlz and Readerz…but is anyone here as smart as a certain Chinese official from 1000 BC…or maybe even earlier?

    Imagine that you are the official in charge of caravans and messengers. Some of these travelers need to cross an unmarked plain, which is subject to sandstorms, thick cloud layers, and heavy fogs…and they have frequently been getting lost. You need something that will aways point south. Problem: the magnetic compass has not yet been invented.

    Can you think of a way–without using magnetic principles or other technology that wasn’t likely to be available in 1000 BC…to solve this problem? Think about it for a few minutes before reading further.

    The south-pointing chariot was based on differential gearing. This mechanically compared the rotation of the two wheels of the chariot. If the left wheel was turning faster than the right wheel, then the chariot must have been turning right. If the right wheel was turning faster than the left, the it had to be turning left. If the wheels were turning at exactly the same speed, then the chariot was going straight. The output of the differential drove a pointer, which in principle would always point in the same direction, regardless of the twists and turns of the chariot’s actual path.

    It probably wasn’t really usable for practical navigation…slight differences in the diameters of the wheels would lead to errors which would accumulate over time. And even if the wheel sizes matched perfectly, and the gearing had been done with extreme precision, there would have surely been at least some slippage between the wheels and the ground. I doubt that it really maintained accuracy for more than a mile or so…but what an amazing sight it must have been, especially when seen for the first time, in a pre-compass, pre-sextant, pre-GPS world. And what a brilliant achievement!