South-pointing chariot

The south-pointing chariot (or carriage) was an ancient Chinese two-wheeled vehicle that carried a movable pointer that indicated south, no matter how the chariot turned. Usually, the pointer took the form of a doll or figure with an outstretched arm. The chariot was supposedly used as a non-magnetic compass for navigation and may also have had other purposes.

The ancient Chinese invented a mobile-like armored cart in the 5th century BC called the Dongwu Che (Chinese: 洞屋车). It was used for the purpose of protecting warriors on the battlefield, as a kind of mobile protective cart with a shed-like roof. It would be rolled up to city walls to provide protection for sappers digging underneath to weaken its foundation. The early Chinese war wagon became the basis of ancient Chinese south-pointing chariots.

There are legends of earlier south-pointing chariots, but the first reliably documented one was created by the Chinese mechanical engineer Ma Jun (c. 200 – 265) of Cao Wei during the Three Kingdoms. No examples still exist, but many extant ancient Chinese texts mention them, mentioning they were used intermittently until about 1300. Some of the texts include information about their inner components and workings.

There were probably several types of south-pointing chariots that worked differently. In most or all of them, the rotating road wheels mechanically operated a geared mechanism to keep the pointer aimed correctly. The pointer was aimed southward by hand at the start of a journey. Subsequently, whenever the chariot turned, the mechanism rotated the pointer relative to the body of the chariot to counteract the turn. This kept the pointer aiming in a constant direction, equal to the starting position. Thus the mechanism did a kind of directional dead reckoning, which is inherently prone to cumulative errors and uncertainties. Some chariots' mechanisms may have had differential gears.