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, showing 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
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.Needham, Volume 4, Part 2, 40. 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. The Jin dynasty (266–420) era text of the Shu Zheng Ji (Records of Military Expeditions), written by Guo Yuansheng, recorded that south-pointing chariots were often stored in the northern gatehouse of the Government Workshops (Shang Fang) of the capital city. However, the later written Song Shu (Book of Song) (6th century) recorded the south-pointing chariot's design and use in further detail, as well as creating the background legend of the device's (supposed) use long before Ma's time, in the Western Zhou dynasty (1050–771 BC). The book also provided a description of the south-pointing chariot's re-invention and use in times after Ma Jun and the Three Kingdoms. The 6th-century text, translated by the British scientist and historian Joseph Needham, reads as follows (the south-pointing chariot is referred to as the south-pointing carriage):
The last sentence of the passage is of great interest for navigation at sea, since the magnetic compass used for seafaring navigation was not used until the time of Shen Kuo (1031–1095). Although the Song Shu text describes earlier precedents of the south-pointing chariot before the time of Ma Jun, this is not entirely credible, as there are no pre-Han or Han dynasty era texts that describe the device.Needham, Volume 4, Part 2, 287–288. 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.Needham, Volume 4, Part 2, 288 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.
Japan
The invention of the south-pointing chariot also made its way to Japan by the 7th century. The Nihon Shoki (The Chronicles of Japan) of 720 described the earlier Chinese Buddhist monks Zhi Yu and Zhi You constructing several south-pointing Chariots for Emperor Tenji of Japan in 658.Needham, Volume 4, Part 2, 289. This was followed up by several more chariot devices built in 666 as well.
Song Shi
The south-pointing chariot was also combined with the earlier Han dynasty era invention of the odometer, a mechanical device used to measure distance traveled, and found in all modern automobiles. It was mentioned in the Song dynasty (960–1279) historical text of the Song Shi (compiled in 1345) that the engineers Yan Su (in 1027) and Wu Deren (in 1107) both created south-pointing chariots, which it details as follows.Needham, Volume 4, Part 2, 291. (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:
Chariots with differential gears
Background and explanation
The English engineer George Lanchester proposed that some south-pointing chariots employed differential gears.Joseph Needham (1986). Science and Civilization in China: Volume 4, Part 2, page 296. Taipei: Caves Books, Ltd. A differential is an assembly of gears, nowadays used in almost all automobiles except some electric and hybrid-electric versions, which has three shafts linking it to the external world. They are conveniently labelled A, B, and C. The gears cause the rotation speed of Shaft A to be proportional to the sum of the rotation speeds of Shafts B and C. There are no other limitations on the rotation speeds of the shafts.
In an automobile, Shaft A is connected to the engine (through the transmission), and Shafts B and C are connected to two road wheels, one on each side of the vehicle. When the vehicle turns, the wheel going around the outside of the turning curve has to roll further and rotate faster than the wheel on the inside. The differential permits this to happen while both wheels are being driven by the engine. If the sum of the speeds of the wheels is constant, the speed of the engine does not change.
In a south-pointing chariot, according to the hypothesis, Shaft B was connected to one road wheel and Shaft C was connected through a direction-reversing gear to the other road wheel. This made Shaft A rotate at a speed that was proportional to the difference between the rotation speeds of the two wheels. The pointing doll was connected (possibly through intermediate gears) to Shaft A. When the chariot moved in a straight line, the two wheels turned at equal speeds, and the doll did not rotate. When the chariot turned, the wheels rotated at different speeds (for the same reason as in an automobile), so the differential caused the doll to rotate, compensating for the turning of the chariot.
The hypothesis that there were south-pointing chariots with differential gears originated in the 20th century. People who were familiar with modern (e.g. automotive) uses of differentials interpreted some of the ancient Chinese descriptions in ways that agreed with their own ideas. Essentially, they re-invented the south-pointing chariot, as it had previously been re-invented several times in antiquity. Working chariots that use differentials have been constructed in recent decades. Whether any such chariots existed previously is not known with certainty.
Although the Antikythera mechanism is believed to have used differential gears, the first true differential gear definitely known to have been used was by Joseph Williamson in 1720.Needham, Volume 4, Part 2, 298. He used a differential to display on separate fixed dials driven by the clock's pendulum mean and solar time. Before that, clockmakers had displayed mean and solar time on a single dial with hands which followed mean time but with a separate ring driven by a kidney cam which moved forwards and backwards to follow the equation of time.
Geometrical properties If the south-pointing chariot were built perfectly accurately, using a differential gear, and if it travelled on an Earth that was perfectly smooth, it would have interesting properties. It would be a mechanical compass that transports a direction, given by the pointer, along the path it travels. Mathematically the device performs parallel transport along the path it travels.
The chariot can be used to detect straight lines or geodesics. A path on a surface the chariot travels along is a geodesic if and only if the pointer does not rotate with respect to the base of the chariot.
Because of the curvature of the Earth's surface (due to it being curved around as a globe), the chariot would generally not continue to point due south as it moves. For example, if the chariot moves along a geodesic (as approximated by any great circle) the pointer should instead stay at a fixed angle to the path. Also, if two chariots travel by different routes between the same starting and finishing points, their pointers, which were aimed in the same direction at the start, usually do not point in the same direction at the finish. Likewise, if a chariot goes around a closed loop, starting and finishing at the same point on the Earth's surface, its pointer generally does not aim in the same direction at the finish as it did at the start. The difference is the holonomy of the path, and is proportional to the enclosed area. If the journeys are short compared with the radius of the Earth, these discrepancies are small and may have no practical importance. Nevertheless, they show that this type of chariot, based on differential gears, would be an imperfect compass even if constructed exactly and used in ideal conditions.
Lack of precision, and implications Real machines are never built perfectly accurately. Simple geometry shows that if the chariot's mechanism is based on a differential gear and if, for example, the width of the track of the chariot (the separation between its wheels) is three metres, and if the wheels are intended to be identical but actually differ in diameter by one part in a thousand, then if the chariot travels one kilometre in a straight line, the "south-pointing" figure will rotate nearly twenty degrees. If it initially points exactly to the south, at the end of the one-kilometre trip it will point almost to the south-southeast or south-southwest, depending on which wheel is the larger. If the chariot travels nine kilometres, the figure will end up pointing almost due north. Obviously, this would make it useless as a south-pointing compass. To be a useful navigational tool, the figure would have to rotate no more than a couple of degrees over a journey of a hundred kilometres, but this would require the chariot's wheels to be equal in diameter to within one part in a million. Even if the process of manufacturing the wheels were capable of this precision (which would not be possible with ancient Chinese methods), it is doubtful that the equality of the wheels could be maintained for long as they are subjected to the wear and tear of travelling across open country. Irregularity of the ground would add further errors to the device's functioning.