Page "Dead reckoning" Paragraph 16

The south-pointing chariot was an ancient Chinese device consisting of a two-wheeled horse-drawn vehicle which carried a pointer that was intended always to aim to the south, no matter how the chariot turned.

The chariot predated the navigational use of the magnetic compass, and could not detect the direction that was south.

Instead it used a kind of directional dead reckoning: at the start of a journey, the pointer was aimed southward by hand, using local knowledge or astronomical observations e. g. of the Pole Star.

Then, as it travelled, a mechanism possibly containing differential gears used the different rotational speeds of the two wheels to turn the pointer relative to the body of the chariot by the angle of turns made ( subject to available mechanical accuracy ), keeping the pointer aiming in its original direction, to the south.

Errors, as always with dead reckoning, would accumulate as distance travelled increased.