Sunday, October 5, 2008

South Pointing Chariot

The South Pointing Chariot is widely regarded as one of the most complex geared mechanism of the ancient , and was continually used throughout the medieval period as well. It was supposedly invented sometime around 2600 BC in China by the Yellow Emperor Huang Di, yet the first valid historical version was created by Ma Jun of Cao Wei during the Three Kingdoms. The chariot is a two-wheeled vehicle, upon which is a pointing figure connected to the wheels by means of gearing. Through careful selection of wheel size, track and gear ratios, the figure atop the chariot will always point in the same direction, hence acting as a non-magnetic compass vehicle. Throughout history, many Chinese historical texts have mentioned the South Pointing Chariot, while some described in full detail the inner components and workings of the device.

Legend and history


Legend


Legend has it that Huang Di, credited as being the founder of the Chinese nation, lived in a magnificent palace in the Kunlun Mountains.

There was also at this time another tribal leader, Chi You, who was skilled at making weapons and waging war. He attacked the tribe of Yan Di, driving them into the lands of Huang Di. Huang Di was angered by this and went to war with Yan Di, initially suffering several defeats. At some stage in the fighting, Chi You conjured up a thick fog to confound Huang Di's men, however the South Pointing Chariot was used to find their way, and they were ultimately victorious.

History


Despite legend, it was recorded in the ''Sanguo Zhi'' that the 3rd century mechanical engineer Ma Jun from the Kingdom of Wei was the inventor of the South Pointing Chariot . After being mocked by Permanent Counsellor Caotang Long and the Cavalry General Qin Lang that he could not reproduce what they deemed a non-historical and nonsensical pursuit, Ma Jun retorted "Empty arguments with words cannot compare with a test which will show practical results". After inventing the device and proving those who were doubtful wrong, he was praised by many, including his contemporary Fu Xuan, a noted poet of his age.

After Ma Jun, the South Pointing Chariot was re-invented by Zu Chongzhi , after the details of its instructions had been lost temporarily in China. During the Tang Dynasty and Song Dynasty the South Pointing Chariot was combined with another mechanical wheeled vehicle, the distance-measuring odometer.

Historical texts for the South Pointing Chariot



Earliest sources


The South Pointing Chariot, a differential mechanical-geared wheeled vehicle used to discern the southern cardinal direction , was given a brief description by Ma's contemporary Fu Xuan. The contemporary 3rd century source of the ''Weil├╝e'', written by Yuan Huan also described the South Pointing Chariot of Ma Jun.





cquote|These vehicles, constructed as they had been by barbarian 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 south-pointing carriage ought to be constructed. So towards the close of the Sheng-Ming reign period the emperor , during the premiership of the Prince of Qi, commissioned 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 , 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'' , but the book was lost by his time. This was followed up by several more chariot devices built in 666 AD as well.





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:



cquote|The body of the south-pointing carriage was 11.15 ft. , 9.5 ft. wide, and 10.9 ft. deep. The carriage wheels were 5.7 ft. in diameter, the carriage pole 10.5 ft. long, and the carriage body in two stories, upper and lower. In the middle was placed a partition. Above there stood a figure of a ''xian'' holding a rod, on the left and right were tortoises and s, one each on either side, and four figures of boys each holding a tassel. In the upper story there were at the four corners trip-mechanisms, and also 13 horizontal wheels, each 1.85 ft. in diameter, 5.55 ft. in circumference, with 32 teeth at intervals of 1.8 inches apart. A central shaft, mounted on the partition, pierced downwards. Joseph Williamson used a differential for correcting the equation of time for a clock that displayed both mean and solar time. Even then, the differential was not fully appreciated in Europe until James White emphasized its importance and provided details for it in his ''Century of Inventions'' .

How it works


The South Pointing Chariot is a mechanical compass that transports a direction, given by the pointer, along the path it travels. The differential in the gear system integrates the difference in wheel rotation between the two wheels and thus detects the rotation of the base of the chariot. The mechanism compensates this rotation by rotating the pointer in the opposite direction.

Mathematically the device approximates parallel transport along the path it travels. In the Euclidean plane, the device performs parallel transport. On a curved surface it only approximates parallel transport. In the limit where the distance between the wheels tends to zero, the approximation becomes exact.

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.

Timeline



The South Pointing Chariot has been invented and reinvented at many times throughout Chinese history. Below is a partial timeline of the major events;



Where they can be seen



While none of the historic South Pointing Chariots remain, full sized replicas can be found.

The History Museum in Beijing, China holds a replica based on the mechanism of Yen Su .
The National Palace Museum in Taipei, Taiwan holds a replica based on the Lanchester mechanism of 1932.

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