Zhang Heng

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Zhang Heng (張衡)
Born	78 Nanyang, China
Died	139 Luoyang, China
Residence	Luoyang
Fields	Astronomy, Mathematics, Seismology, Hydraulics, Geography, Ethnography, Mechanical engineering, Calendrical science, Metaphysics, Poetry
Known for	Seismometer, Hydraulic-powered Armillary sphere, pi calculation, shi (poetry), Universe model, lunar eclipse and solar eclipse theory
Religious stance	Confucianism, Chinese folk religion

For other uses, see Zhang Heng (disambiguation).

This is a Chinese name; the family name is Zhang.

Zhang Heng (Chinese: 張衡; pinyin: Zhāng Héng; Wade-Giles: Chang Heng) (AD 78–139) was an astronomer, mathematician, inventor, geographer, cartographer, artist, poet, statesman, and literary scholar of the Eastern Han Dynasty in ancient China. He had extensive knowledge of mechanics and gears, applying this knowledge to several of his known inventions. According to historian Joseph Needham, Zhang Heng was noted in his day for being able to "make three wheels rotate as if they were one".^[1] He is best known for inventing the world's first water-powered armillary sphere to represent astronomical observation,^[2] improving the inflow clepsydra clock with an additional tank,^[3] and inventing the world's first seismometer device, which discerned the cardinal direction of earthquakes from incredibly far distances.^[2][4][5] In addition to writing an extensive star catalogue, Zhang also posited theories about the moon and its relationship to the sun, specifically the moon's sphericity, its illumination by reflecting sunlight, and solar and lunar eclipses. In the realm of early science, he is often compared by historians to his contemporaries in ancient Greece and ancient Rome, as well as later Chinese innovators such as Yi Xing and Su Song. His fu and shi poetry was also renowned and commented on by later Chinese poets. Furthermore, Zhang wrote the earliest known seven-syllabic shi poem.

[edit] Life

Born in what is now Nanyang, Henan, Zhang Heng was an accomplished writer at twelve. At sixteen, he left home to pursue his study in the capital cities. He spent at least ten years of his youth in literature studies and writing. He published several well recognized literary writings, and was admired for his straightforward style and natural poetic ability. Before his work in astronomy, mathematics, and technological invention, Zhang was a famed and notable literary author of poetry.

Zhang Heng began his studies in astronomy at the age of thirty, and began publishing his works in astronomy and mathematics. In 116, at the age of thirty-eight, Zhang became a government official for the court of Emperor An of Han. Zhang took on several positions since then, becoming the chief astrologer for the Han court. In his humble Confucian sensibility, Zhang respectfully declined many promotions that were offered to him that would have greatly advanced his career. Zhang eventually accepted the promoted position of President of the Imperial Chancellory. However, the 3rd century Chinese philosopher and poet Fu Xuan once lamented in an essay of his over the fact that Zhang Heng was never placed in the Ministry of Works. Writing highly of Zhang and the 3rd century mechanical engineer Ma Jun, Fu Xuan wrote:

...neither of them was ever an official of the Ministry of Works, and their ingenuity did not benefit the world. When (authorities) employ personnel with no regard to special talent, and having heard of genius neglect even to test it—is this not hateful and disastrous?^[6]

Zhang was given great honors in life and in death. In honor of his achievements in science and technology, Cui Ziyu wrote a memorial inscription on the burial stele of Zhang Heng that stated:

His mathematical computations exhausted (the riddles of) the heavens and the earth. His inventions were comparable even to those of the Author of Change. The excellence of his talent and the splendour of his art were one with those of the gods.^[7]

[edit] Literature and poetry

While working for the central court, Zhang Heng had access to a variety of written materials since he was allowed to peruse the Archives of the Eastern Pavilion.^[8] Zhang Heng read many of the great works of history in his day, as he even offered a bit of criticism for them by noting that he had found ten instances where the Records of the Grand Historian by Sima Qian (145–90 BC) and the Book of Han by Ban Gu (32–92 AD) differed from other ancient texts that were available to him.^[9] This account of Zhang's was preserved and recorded in the 5th century text of the Book of Later Han by Fan Ye (398–445).^[9]

Xiao Tong (501–531), a crown prince of the Liang Dynasty, immortalized several of Zhang's works in his anthology of literature, Wen xuan. Rhapsodies (fu 賦) include "Western Metropolis Rhapsody" 西京賦 (WX 2.36), "Eastern Metropolis Rhapsody" 東京賦 (WX 3.51), "Southern Capital Rhapsody" 南都賦 (WX 4.68), "Rhapsody on Contemplating the Mystery" 思玄賦 (WX 15.213), and "Rhapsody on Returning to the Fields" 歸田賦 (WX 15.222).^[10] The latter fu is an especially interesting piece, being very short compared with his other, mainly long epedeictic fu, and possessing a lyrical quality and refinement beyond its time. A set of four short lyric poems shi 詩, entitled "Lyric Poems on Four Sorrows" 四愁詩, is also included with Zhang's preface (WX 29.414). This set constitutes some of the earliest heptasyllabic shi Chinese poetry written. Zhang Heng's Four Stanzas of Sorrow is the first known seven-syllabic poem from China:

A Han terracotta figurine of a standing lady; in his poetry, Zhang Heng expressed his affinity for gracious and commendable women.

	In Taishan stays my dear sweetheart, But Liangfu keeps us long apart; Looking east, I find tears start. She gives me a sword to my delight; A jade I give her as requite. I'm at a loss as she is out of sight; Why should I trouble myself all night?
Zhang Heng[11]

In another poem of his called "Stabilizing the Passions"—preserved in a Tang Dynasty (618–907) encyclopedia but referred to earlier by Tao Qian (365–427) in praise of Zhang's lyrical minimalism—Zhang displays his admiration for an attractive and exemplary woman:

	Ah, the chaste beauty of this alluring woman! She shines with flowery charms and blooming face. She is unique among all her contemporaries. She is without a peer among her comrades.
Zhang Heng[12]

Western Han tomb painting of two men engrossed in conversation; Zhang's 'shelun' or 'hypothetical discourse' involved a written dialogue between imaginary or real personas in order to demonstrate how one could lead an exemplary life.

Zhang's long lyrical poems also revealed a great amount of information on urban layout and basic geography, with his rhapsody "Sir Based-On-Nothing" providing details on terrain, palaces, hunting parks, markets, and prominent buildings of Chang'an, the Western Han capital.^[10] In exemplifying his attention to detail, his rhapsody on Nanyang described gardens filled with spring garlic, summer bamboo shoots, autumn leeks, winter rape-turnips, perilla, evodia, and purple ginger.^[13] Zhang Heng's writing confirms the size of the imperial hunting park in the suburbs of Chang'an, as his estimate for the circumference of the park's encircling wall agrees with the historian Ban Gu's estimate of roughly 400 li (one li in Han times was equal to 415.8 m, or 1,364 ft, making the circumference of the park wall 166,320 m, or 545,600 ft).^[14] Along with Sima Xiangru (179–117 BC), Zhang listed a variety of different animals and hunted game inhabiting the park, which were divided in the northern and southern portions of the park according to where the animals had originally came from: northern or southern China.^[15] Somewhat similar to the description of Sima Xiangru, Zhang described the Western Han emperors and their entourage enjoying boat outings and sights of water plays, fishing, and archers shooting birds and other animals with stringed arrows from the tops of tall towers along Chang'an's Kunming Lake.^[16] The focus of Zhang's writing on specific places and their terrain, society, people, and their customs could also be seen as early attempts of ethnographic categorization.^[17] In his poem "Xijing fu", Zhang also shows that he was aware of a new foreign religion of Buddhism introduced via the Silk Road and the legend of the birth of Buddha with the vision of the white elephant bringing about conception.^[18]

With his Reply to Criticism, Zhang was an early writer and proponent of the Chinese literary genre 'shelun', or 'hypothetical discourse'. Authors of this literary genre created a written dialogue between themselves and an imaginary person (or a real person of their entourage or association); the latter poses questions to the author on how to lead a successful life.^[19]

Zhang wrote about the various love affairs of the emperor, not satisfied with the imperial harem, going out into the city incognito to seek out prostitutes and sing-song girls. This was seen as a general criticism of present Eastern Han emperors and their imperial favorites, guised in the criticism of earlier Western Han emperors.^[20] Besides criticizing the Western Han emperors for lavish decadence, Zhang Heng also pointed out that their behavior and ceremonies did not properly conform with the Chinese cyclical beliefs in yin and yang.^[21] In a poem criticizing the previous Western Han Dynasty, Zhang wrote:

	Those who won this territory were strong; Those who depended on it endured. When a stream is long, its water is not easily exhausted. When roots are deep, they do not rot easily. Therefore, as extravagance and ostentation were given free rein, The odour became pungent and increasingly fulsome
Zhang Heng[11]

[edit] Achievements in science and technology

[edit] Astronomy, mathematics, and calendrical studies

Printed star map of Su Song (1020–1101) showing the south polar projection.

With his astronomical finds and theories, Zhang was a pioneer of his day. In his publication of 120 AD called Ling Xian (靈憲, Mystical Laws, a summary of astronomical theories at the time), he approximated pi as 730/232 (or approx 3.1466), yet the later 3rd century Chinese mathematician Liu Hui would more accurately approximate pi. In one of his formulae for spherical volume calculation, he also used pi as the square root of 10 (or approx 3.162).^[22] Zhang Heng theorized that the universe was like an egg with the stars on the shell and the Earth as the yolk.^[5] This was similar to the Western geocentric model before the coming of Galileo Galilei. Although it was the ancient Warring States Chinese astronomers Shi Shen and Gan De who would compile the world's first star catalogue, Zhang nonetheless catalogued 2500 stars alone and recognized over 100 constellations. In relative comparison, this star catalogue featured many more positions for the stars than the Greek astronomer Hipparchus, who catalogued roughly 800 stars in his work of the 2nd century BC. In 123 Zhang Heng corrected the Chinese calendar to harmonize more accurately with the timing of the seasons, according to the finds of his astronomical observations. Zhang Heng also wrote of astronomical phenomena, such as reasoning for the phenomena of solar eclipse and lunar eclipse, Zhang supporting the 'radiating influence' theory opposed by Wang Chong (27–97). In the Ling Xian, he wrote:

The sun is like fire and the moon like water. The fire gives out light and the water reflects it. Thus the moon's brightness is produced from the radiance of the sun, and the moon's darkness is due to (the light of) the sun being obstructed. The side which faces the sun is fully lit, and the side which is away from it is dark. The planets (as well as the moon) have the nature of water and reflect light. The light pouring forth from the sun does not always reach the moon owing to the obstruction of the earth itself—this is called 'an-xu', a lunar eclipse. When (a similar effect) happens with a planet (we call it) an occultation; when the moon passes across (the sun's path) then there is a solar eclipse.^[23]

Writing more than a century before Zhang Heng, Jing Fang (78–37 BC) wrote about the spherical shape of the sun and moon while discussing eclipses:

The moon and the planets are Yin; they have shape but no light. This they receive only when the sun illuminates them. The former masters regarded the sun as round like a crossbow bullet, and they thought the moon had the nature of a mirror. Some of them recognized the moon as a ball too. Those parts of the moon which the sun illuminates took bright, those parts which it does not, remain dark.^[24]

The later scientist Shen Kuo (1031–1095) would also expand upon the reasoning of why the sun and moon were spherical.

[edit] Extra tank for inflow clepsydra

Diagram of Ctesibius' 3rd century BC clepsydra, similar to Zhang's statuettes guiding the indicator rod and pointing out graduations

The outflow clepsydra was an ancient timekeeping device used in China from as far back as the Shang Dynasty, and certainly by the Zhou Dynasty.^[25] The inflow clepsydra with an indicator rod on a float had been known in China since the beginning of the Han Dynasty in 202 BC and had replaced the outflow type.^[25] The Han Chinese noted the problem with the falling pressure head in the reservoir, which slowed the timekeeping of the device as the inflow vessel was filled.^[25] Indicated in his writing in 117, Zhang Heng was the first to address this problem by adding an extra compensating tank between the reservoir and the inflow vessel.^[3] Zhang also mounted two statuettes of a Chinese immortal and a heavenly guard on the top of the inflow clepsydra, the two of which would guide the indicator rod with their left hand and point out the graduations with their right.^[26] Joseph Needham states that this was perhaps the ancestor of all clock jacks that would later sound the hours in later hydraulic clocks, but he notes that these figures did not actually move like clock jacks or sound the hours.^[26] Many additional compensation tanks would be added to later clepsydras in the tradition of Zhang Heng, while in 610 the Sui Dynasty engineers Geng Xun and Yuwen Kai would craft an unequal-armed steelyard balance able to make seasonal adjustments in the pressure head of the compensating tank so that it could control the rate of water flow for different lengths of day and night during the year.^[27]

[edit] Water-powered armillary sphere

Zhang Heng is the first known person in history to apply hydraulic motive power (ie. by employing a clepsydra and waterwheel) to rotate the astronomical instrument of the armillary sphere.^[28][29] The Greek astronomer Eratosthenes invented the first armillary sphere in 255 BC, while the Chinese armillary sphere was developed in earnest by 52 BC, due to the adding of a permanently fixed equatorial ring by the astronomer Geng Shouchang.^[30] In 84 AD the astronomers Fu An and Jia Kui added the ecliptic ring, and finally Zhang Heng added the horizon and meridian rings to the armillary sphere by 125 AD.^[30] Zhang described this invention in his written work Lou Shui Zhuan Hun Tian Yi Chi (Apparatus for Rotating an Armillary Sphere by Clepsydra Water) of 125 AD. Besides being one of the most impressive inventions of his day, it would influence later Chinese horologists in making waterclocks that employed complex gears (leading to the discovery of the escapement mechanism by the 8th century). The historian Joseph Needham states:

The original diagram of Su Song's (1020–1101) clock tower, featuring an armillary sphere powered by a waterwheel, escapement mechanism, and chain drive.

What were the factors leading to the first escapement clock in China? The chief tradition leading to Yi Xing (725 AD) was of course the succession of 'pre-clocks' which had started with Zhang Heng about 125. Reason has been given for believing that these applied power to the slow turning movement of computational armillary spheres and celestial globes by means of a water-wheel using clepsydra drip, which intermittently exerted the force of a lug to act on the teeth of a wheel on a polar-axis shaft. Zhang Heng in his turn had composed this arrangement by uniting the armillary rings of his predecessors into the equatorial armillary sphere, and combining it with the principles of the water-mills and hydraulic trip-hammers which had become so widespread in Chinese culture in the previous century.^[31]

The Chinese tradition of hydraulic engineering was not initiated by Zhang in any sense, as it spanned back as far as the mid Zhou Dynasty (c. 6th century BC), with figures such as Sunshu Ao and Ximen Bao. In the contemporary age of Zhang Heng, there were those such as Du Shi (c. 31 AD), who was the first to apply the motive power of waterwheels to a mechanical device that operated bellows of the blast furnace to smelt cast iron. As noted above, Zhang Heng provided a valuable description of his water-powered armillary sphere in the Lou Shui Zhuan Hun Tian Yi Chi treatise of 125. In it, Zhang states:

The equatorial ring goes around the belly of the armillary sphere 91 and 5/19 (degrees) away from the pole. The circle of the ecliptic also goes round the belly of the instrument at an angle of 24 (degrees) with the equator. Thus at the summer solstice the ecliptic is 67 (degrees) and a fraction away from the pole, while at the winter solstice it is 115 (degrees) and a fraction away. Hence (the points) where the ecliptic and the equator intersect should give the north polar distances of the spring and autumn equinoxes. But now (it has been recorded that) the spring equinox is 90 and 1/4 (degrees) away from the pole, and the autumn equinox is 92 and 1/4 (degrees) away. The former figure is adopted only because it agrees with the (results obtained by the) method of measuring solstitial sun shadows as embodied in the Xia (dynasty) calendar.^[32]

In his treatise on the water-driven armillary sphere, Zhang Heng also provided valuable information on the Chinese clepsydra clock of his day, showing that the Chinese had invented the inflow clepsydra (as opposed to the earlier outflow clepsydra in late Zhou Dynasty China). In Zhang's description, he writes of a 'jade dragon's neck', which in later times meant a siphon,^[33] and wrote of the floats and indicator-rods of the inflow clepsydra:

Bronze Vessels are made and placed one above the other at different levels; they are filled with pure water. Each has at the bottom a small opening in the form of a 'jade dragon's neck'. The water dripping (from above) enters two inflow receivers (alternately), the left one being for the night and the right one for the day. On the covers of each (inflow receiver) there are small cast statuettes in gilt bronze; the left (night) one is an immortal and the right (day) one is a policeman. These figures guide the indicator-rod (lit. arrow) with their left hands, and indicate the graduations on it with their right hands, thus giving the time.^[33]

The water-powered armillary sphere of Zhang Heng would have profound effects upon Chinese astronomy and mechanical engineering in later generations. With Zhang Heng's complex use of gears and ingenious model, he would greatly influence later astronomers and engineers such as Yi Xing, Zhang Sixun, Su Song, Shen Kuo, Guo Shoujing, and many others. Zhang Heng's water-powered armillary sphere was continued in the eras of the Three Kingdoms (220–280) and Jin Dynasty (265–420), yet the design for it was temporarily out of use from the years 317 until 418, due to external invasions by northern Xiongnu nomads.^[34] The old instruments of Zhang Heng were recovered in 418 when Emperor Wu of Liu Song captured the ancient capital of Chang'an, and although still intact, the marks of graduation and the representations of the stars, moon, sun, and planets were quite worn down by time and rust.^[34] In the year 436, the emperor ordered Qian Luozhi, the Secretary of the Bureau of Astronomy and Calendar, to recreate Zhang's device, which was a success.^[34] Qian's water-powered celestial globe was still in use by the time of the Liang Dynasty, while continuing models of water-powered instruments in the tradition of Zhang Heng were designed.^[34]

A modern replica of Zhang Heng's seismometer, the Houfeng Didong Yi.

[edit] Zhang's seismometer

From the earliest times, the Chinese were concerned with the destructive force of earthquakes. It was recorded in Sima Qian's Shiji (Records of the Grand Historian) compiled by 91 BC that in 780 BC an earthquake had been devastative enough to divert the course of three different rivers.^[35] The ancient Chinese did not understand the cause of earthquakes as being the shift of tectonic plates in the earth's crust. Chinese of the ancient Zhou Dynasty (1046 BC–256 BC) explained the natural phenomenon in terms of disturbances with cosmic yin and yang, along with the heavens' displeasure with acts committed (or the common peoples' grievances ignored) by the current ruling dynasty.^[35] These theories were ultimately derived from the ancient text of the I Ching (Book of Changes), in its fifty-first hexagram zhen.^[36] There were other early theories about earthquakes by those in ancient Greece: Anaxagoras believed that they were caused by excess water near the surface crust of the earth bursting into the earth's hollows; Democritus believed that the saturation of the earth with water caused them; Anaximenes believed they were the result of massive pieces of the earth falling into the cavernous hollows due to drying; and Aristotle believed they were caused by instability of vapor (pneuma') caused by the drying of the moist earth by the sun's rays.^[36]

During the Han Dynasty, many learned scholars of the court gave credence to the 'oracles of the winds', including Zhang Heng.^[37] These oracles of the occult made observations of direction, force, and timing of the winds in order to speculate ideas about the operation of the cosmos and predictions of events on earth.^[38] This influenced the ideas of Zhang Heng about earthquakes; going against the grain of the early theories proposed by his fellow Chinese and contemporary Greeks, Zhang Heng believed earthquakes were caused by wind and air, writing:

The chief cause of earthquake is air, an element naturally swift and shifting from place to place. As long as it is not stirred, but lurks in a vacant space, it reposes innocently, giving no trouble to objects around it. But any cause coming upon it from without rouses it, or compresses it, and drives it into a narrow space...and when opportunity of escape is cut off, then 'With deep murmur of the Mountain it roars around the barriers', which after long battering it dislodges and tosses on high, growing more fierce the stronger the obstacle with which it has contended.^[39]

In 132, Zhang Heng presented to the Han court what many historians consider his most impressive invention. Zhang Heng invented the first seismometer, called Houfeng Didong Yi (lit. instrument for measuring the seasonal winds and the movements of the Earth), for determining the exact direction of tremors and earthquakes.^[29] According to the Book of Later Han (compiled by Fan Ye in the 5th century), his copper-made urn-shaped device, with swinging inverted pendulum inside, was able to detect the direction of an earthquake from hundreds of miles/kilometers away.^[40] This was essential for the Han government in sending quick aid and relief to regions devastated by this natural disaster.^[40][4] Later Chinese of subsequent periods were able to reinvent Zhang's seismograph, including the 6th century mathematician Xindu Fang of the Northern Qi Dynasty,^[41] and the astronomer and mathematician Lin Xiaogong of the Sui Dynasty (581–618).^[42] On June 13, 2005, modern Chinese seismologists announced that they had successfully created a replica of the instrument ^[43]

In the Book of Later Han there is a detailed description of the device in his biography, which states:

In the first year of the Yang-Chia reign period (132) Chang Heng also invented an 'earthquake weathercock' (i.e. a seismograph). It consisted of a vessel of fine cast bronze, resembling a wine-jar, and having a diameter of eight chhih (Note—8 chi is roughly 6 ft). It had a domed cover, and the outer surface was ornamented with antique seal-characters and designs of mountains, tortoises, birds, and animals. Inside there was a central column capable of lateral displacement along tracks in the eight directions, and so arranged (that it would operate) a closing and opening mechanism.^[44]

Outside the vessel there were eight dragon heads, each one holding a bronze ball in its mouth, while round the base there sat eight (corresponding) toads, with their mouths open, ready to receive any ball which the dragons might drop. The toothed machinery and ingenious constructions were all hidden inside the vessel, and the cover fitted down closely all round without any crevice. When an earthquake occurred the dragon mechanism of the vessel was caused to vibrate so that a ball was vomited out of a dragon-mouth and caught by the toad underneath. At the same instant a sharp sound was made which called the attention of the observers.^[44]

Now although the mechanism of one dragon was released, the seven (other) heads did not move, and by following the (azimuthal) direction (of the dragon which had been set in motion), one knew (the direction) from which the earthquake (shock) had come (lit. where the earthquake was). When this was verified by the facts there was (found) an almost miraculous agreement (i.e. between the observations made with the apparatus and the news of what had actually happened). Nothing like this had ever been heard of before since the earliest records of the Shu (Jing).^[44]

On one occasion one of the dragons let fall a ball from its mouth though no perceptible shock could be felt. All the scholars at the capital were astonished at this strange effect occurring without any evidence (of an earthquake to cause it). But several days later a messenger arrived bringing news of an earthquake in Long-Xi (Gansu) (Note: which was 400 miles away to the north-west). Upon this everyone admitted the mysterious power of the instrument. Thenceforward it became the duty of the officials of the Bureau of Astronomy and Calendar to record the directions from which earthquakes came.^[45]

Furthermore, Zhang Heng's seismograph device incorporated the following in order to function properly:^[46]

• Eight mobile arms radiating in as many directions and each connected with cranks which are provided with catch mechanisms at the periphery.
• Any one of the cranks which raises a dragon head and so releases a ball is thus at the same time caught and held, thus immobilising the instrument.
• A crank
• A right-angle lever for raising the dragon's head
• A vertical pin passing through a slot in the crank
• An arm of the pendulum
• The pendulum
• A catch
• A pivot on a projection
• A sling suspending the pendulum
• Attachment of the sling
• A horizontal bar supporting the pendulum
• The lower jaw of the dragon supporting the ball.

[edit] Cartography

The Wei (220–265) and Jin Dynasty (265–420) cartographer and official Pei Xiu (224–271) was the first in China to describe in full the geometric grid reference for maps that allowed for precise measurements using a graduated scale, as well as topographical elevation.^[47][48] However, map-making in China had existed since at least the 4th century BC with the Qin state maps found in Gansu in 1986.^[49] Pinpointed accuracy of the winding courses of rivers and familiarity with scaled distance had been known since the Qin and Han Dynasty, respectively, as evidenced by their existing maps, while the use of a rectangular grid had been known in China since the Han as well.^[50][51] Historian Howard Nelson states that, although the accounts of Zhang Heng's work in cartography are somewhat vague and sketchy, there is ample written evidence that Pei Xiu derived the use of the rectangular grid reference from the maps of Zhang Heng.^[52] Historian Florian C. Reiter notes that Zhang's narrative "Guitian fu" contains a phrase about applauding the maps and documents of Confucius of the Zhou Dynasty, which Reiter suggests places maps (tu) on a same level of importance with documents (shu).^[53]

[edit] Odometer and South Pointing Chariot

Odometer cart from a stone rubbing of an Eastern Han Dynasty tomb, c. 125.

Zhang Heng is often accredited with the invention of the first odometer (also pronounced hodometer) device in China, an achievement alongside earlier contemporaries Archimedes and Heron of Alexandria from the Hellenized West. An odometer is a device which mechanically or electronically measures the distance traveled by an operated vehicle (found standard in modern automobiles), and was a mechanical device used by the Roman and Han-Chinese empires in roughly the same time frame. By the 3rd century, the Chinese had termed the device as the 'ji li gu che', or 'li-recording drum carriage' (Note: the modern measurement of li = 500 m/1640 ft).^[54] Ancient Chinese texts tell of the mechanical carriage's functions, and as one li is traversed, a mechanical-driven wooden figure strikes a drum, and when ten li is traversed, another wooden figure would strike a gong or a bell with its mechanical-operated arm.^[54] However, there is evidence to suggest that the invention of the odometer was a gradual process in Han Dynasty China that centered around the huang men court people (ie. eunuchs, palace officials, attendants and familiars, actors, acrobats, etc.) that would follow the musical procession of the royal 'drum-chariot'.^[55] There is speculation that some time in the 1st century BC, during the Western Han Dynasty, the beating of drums and gongs were mechanically driven by working automatically off the rotation of the road-wheels.^[55] This might have actually been the design of one Loxia Hong (c. 110 BC), yet by at least 125 AD the mechanical odometer carriage was already known, depicted in a mural of the Xiao Tang Shan Tomb.^[55]

The South Pointing Chariot is another mechanical contraption accredited to Zhang Heng. It is a non-magnetic compass-vehicle shaped in the form of a wheeled chariot, and functioning off of differential gears. The mechanical device of the differential provides equal amount of torque to wheels rotating at different speeds. This device allowed a fixed wooden figurine (in the shape of a Chinese state minister) to constantly point in the southern direction, hence the name of the device. The Song Shu (c. 500 AD) records that Zhang Heng re-invented it from its earliest model found in the Zhou Dynasty period, but the violent collapse of the Han Dynasty unfortunately did not allow it to be preserved. Whether the ancient passage of this book on Zhang Heng is valid or not, Ma Jun (200–265) of the following century was successful in creating it.^[56]

[edit] Legacy

Zhang Heng's mechanical inventions influenced the likes of later Chinese inventors, such as Yi Xing, Zhang Sixun, Su Song, and Guo Shoujing. Su Song directly named the water-powered armillary sphere of Zhang Heng as the inspiration for his clock tower.^[57]

Zhang's poetry was widely read during and after his death. In addition to the compilation of Xiao Tong mentioned above, the Eastern Wu official Xue Zong (d. 237) wrote commentary on Zhang's poems "Dongjing fu" and "Xijing fu".^[58] The influential poet Tao Qian (365–427) wrote that he admired the poetry of Zhang Heng for its "curbing extravagant diction and aiming at simplicity," in regards to perceived tranquility and rectitude correlating with the simple but effective language of the poet.^[59] Tao wrote that both Zhang Heng and Cai Yong (132–192) "avoided inflated language, aiming chiefly at simplicity," and adding that their "compositions begin by giving free expression to their fancies but end on a note of quiet, serving admirably to restrain undisciplined and passionate nature."^[60]

Zhang Heng has also been given the honor of several things named after him, including a lunar crater,^[61] the asteroid 1802 Zhang Heng, and the mineral Zhanghengite.

[edit] See also

• List of Chinese people
• List of Chinese writers
• List of scientists
• List of astronomers
• List of mechanical engineers

[edit] Notes

This article contains Chinese text. Without proper rendering support, you may see question marks, boxes, or other symbols instead of Chinese characters.

[edit] References

[edit] External links

Wikimedia Commons has media related to:

Zhang Heng

• Zhang Heng at Chinaculture.org
• Zhang Heng at the University of Maine, USA
• Zhang Heng at the University of St Andrews, Scotland