thumb|Exhibit in the [[Science Museum (London)|Science Museum in London, England. This conjectural model chariot incorporates a differential gear.]]
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 (). 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 ( – 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.
Historical texts
Earliest sources
thumb|upright|An image of a south-pointing chariot from [[Sancai Tuhui (first published 1609)]]
The south-pointing chariot, a mechanical-geared, wheeled vehicle used to discern the southern cardinal direction (without magnetics), was given a brief description by Ma's contemporary Fu Xuan. The contemporary 3rd century source of the Weilüe, written by the East Han dynasty politician Yuan Huan also described the south-pointing chariot of belonging to the Chinese mechanical engineer and politician Ma Jun.
In the State of Wei, (in the San Guo period) Gaotong Long and Qin Lang were both famous scholars; they disputed about the south-pointing carriage before the court, saying that there was no such thing, and that the story was nonsense. But during the Qing-long reign period (233–237) the emperor Ming Di commissioned the scholar Ma Jun to construct one, and he duly succeeded. This again was lost during the troubles attending the establishment of the Jin dynasty.
Later on, Shi Hu (emperor of the Jie Later Zhao dynasty) had one made by Xie Fei, and again Linghu Sheng made one for Yao Xing (emperor of the Later Qin dynasty). The latter was obtained by emperor An Di of the Jin in the 13th year of the Yi-xi reign-period (417), and it finally came into the hands of emperor Wu Di of the Liu Song dynasty when he took over the administration of Chang'an. Its appearance and construction was like that of a drum-carriage (odometer). A wooden figure of a man was placed at the top, with its arm raised and pointing to the south, (and the mechanism was arranged in such a way that) although the carriage turned round and round, the pointer-arm still indicated the south. In State processions, the south-pointing carriage led the way, accompanied by the imperial guard.
These vehicles, constructed as they had been by barbarian (Qiang) workmen, did not function particularly well. Though called south-pointing carriages, they very often did not point true, and had to negotiate curves step by step, with the help of someone inside to adjust the machinery. The ingenious man from Fanyang, Zi Zu Chongzhi frequently said, therefore, that a new (and properly automatic) south-pointing carriage ought to be constructed. So towards the close of the Sheng-Ming reign period (477–479) the emperor Shun Di, during the premiership of the Prince of Qi, commissioned (Zi Zu Chongzhi) to make one, and when it was completed it was tested by Wang Seng-qian, military governor of Tanyang, and Liu Hsiu, president of the Board of Censors. The workmanship was excellent, and although the carriage was twisted and turned in a hundred directions, the hand never failed to point to the south. Under the Jin, moreover, there had also been a south-pointing ship. In fact, the first known source to describe stories of its legendary use during the Zhou period was the Gu Jin Zhu book of Cui Bao (c. 300), written soon after the Three Kingdoms era. Cui Bao also wrote that the intricate details of construction for the device were once written in the Shang Fang Gu Shi (Traditions of the Imperial Workshops), but the book was lost by his time. This was followed up by several more chariot devices built in 666 as well. (In Needham's translation, inches and feet (ft) are used as units of distance. 1 inch is 25.4 millimetres. 1 ft is 12 inches or 304.8 mm.)
After this initial description of Yan Su's device, the text continues to describe the work of Wu Deren, who crafted a wheeled device that would combine the odometer and south-pointing chariot: