South-Pointing Chariot: no matter the direction carriage moves, or how many times it turns, the arrow remains pointing south (or whatever direction it was placed initially). This amazing mechanism utilizes a differential gear train, the famous device used in cars to allow wheels to rotate at different rates when cornering. Differential gear trains have the following property, the rotation rate of one shaft is equal to the average rotations rates of the other two shafts. Each wheel of this device