South-pointing chariot

It would serve to be rolled up to city fortifications to provide protection for sappers digging underneath to weaken a wall's foundation.

[1][2] 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.

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 country to be traversed was a boundless plain, in which people lost their bearings as to east and west, so (the Duke) caused this vehicle to be made in order that the ambassadors should be able to distinguish north and south.

The Gui Gu Zi book says that the people of the State of Zheng, when collecting jade, always carried with them a 'south-pointer', and by means of this were never in doubt (as to their position).

[5] 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.

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.

[8] 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.

[4] 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.

[9] 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.

In the 5th year of the Tian-Sheng reign period of the emperor Renzong (1027), Yan Su, a Divisional Director in the Ministry of Works, made a south-pointing carriage.

When it has turned round 12 teeth, the carriage moves due west, but still the wooden figure stands crosswise and points south.

(On vertical axles) reaching to the top (of the compartment) left and right, were two small horizontal wheels which could rise and fall, having an iron weight (attached to) each.

[13] 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.

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.

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.

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.

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.

Considerable scepticism is therefore warranted as to whether this type of south-pointing chariot, using a differential gear for the whole time, was used in practice to navigate over long distances.

If turns were brief and rare, this would have greatly reduced the pointing errors, since they would have accumulated only during the short periods when the doll and differential were connected.

If the real purposes of the chariot and the accounts of it were amusement and impressing visiting foreigners, rather than actual long-distance navigation, then its inaccuracy might not have been important.

Considering that a large mechanical wagon or chariot would be obligated to travel on roads, the destination in question would typically not be in an unknown direction.

Some of the ancient descriptions suggest that some south-pointing chariots could move in only three ways: straight ahead, or turning left or right with a fixed radius of curvature.

The small gear wheels were raised and lowered by an arrangement of weights, pulleys and cords which were attached to the pole to which the horses that pulled the chariot were harnessed.

The radius of the curve around which the rotating wheel moved would have equalled the track-width of the chariot, and the gears turning the doll would have been chosen accordingly.

The Song Shi description of the gears in Yan Su's chariot, and the numbers of teeth on them, suggests that it worked this way, without a third road wheel.

The History Museum[clarification needed] in Beijing, China, holds a replica based on the mechanism of Yen Su (1027).

Referred to as the "southern pointing man", two replicas can also be seen (and physically experimented with) at the Ontario Science Centre in Toronto, Canada.