Information about Zhang Heng

Zhang Heng (張衡)

Born	78 Nanyang, China
Died	139 Luoyang, China
Residence	Luoyang
Field	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
Religion	Confucianism

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, 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 (neng ling san lun du zhuan ye)".^[1] He is best known for his invention of the world's first water-powered armillary sphere to represent astronomical observation,^[1] and the world's first seismometer device, which discerned the cardinal direction of earthquakes from incredibly far distances.^[1][3][4] In the realm of early science, he is often compared by historians to his contemporaries in ancient Greece and ancient Rome.

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. In a poem criticizing the previous Western Han Dynasty, Zhang Heng once 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 reign, The odour became pungent and increasingly fulsome
Zhang Heng[5]

Before his work in astronomy, mathematics, and technological invention, Zhang was a famed and notable literary author of poetry. Xiao Tong (501–531) 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).^[6] The latter fu is an especially interesting piece, being very short compared to 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:

	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[5]

His 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.^[6] He also 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.^[7] The focus of Zhang's writing on specific places, its terrain, society, people, and their customs could also be seen as early attempts of ethnographic categorization.^[8] With his Reply to Criticism, Zhang Heng was also an early writer and proponent of the Chinese literary genre 'shelun', or 'hypothetical discourse', where the author creates a written dialogue between himself and an imaginary person (or they may be a real person of his entourage or association), and where the other poses questions to the author on how to lead a successful life.^[9]

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:

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

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Achievements in science and technology

Astronomy, mathematics, and calendrical studies

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).^[10] Zhang Heng theorized that the universe was like an egg with the stars on the shell and the Earth as the yolk.^[4] 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. Like his contemporary, the philosopher Wang Chong, Zhang Heng also wrote of astronomical phenomena, such as the solar eclipse and lunar eclipse. In the Ling Xian, he wrote:

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Writing more than a century before Zhang Heng, Jing Fang of the 1st century BC wrote about the spherical shape of the sun and moon while discussing eclipses:

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

The water-powered armillary sphere

Zhang Heng is the first known person in history to apply hydraulic-run motive power (ie. by employing a clepsydra and waterwheel) to rotate the astronomical insturment of the armillary sphere.^[11][12] 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.^[13] 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.^[13] 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, and 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:

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Indeed, 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. Still, Zhang's water-powered armillary sphere was a marvel of his day. As noted before, 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:

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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,^[14] and wrote of the floats and indicator-rods of the inflow clepsydra:

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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.^[15] 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.^[15] 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.^[15] 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.^[15]

Zhang's seismograph

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.^[12] According to the Book of Later Han, 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.^[12] This was essential for the Han government in sending quick aid and relief to regions devastated by this natural disaster.^[15][3] Later Chinese of subsequent periods were able to reinvent Zhang's seismograph, including the 6th century mathematician Xindu Fang of the Northern Qi Dynasty,^[17] and the astronomer and mathematician Lin Xiao-gong of the Sui Dynasty (581–618).^[18] On June 13, 2005, modern Chinese seismologists announced that they had successfully created a replica of the instrument ^[19]

Odometer and South Pointing Chariot

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).^[20] 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.^[20] 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'.^[21] 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.^[21] 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.^[21]

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 AD) of the following century was successful in creating it.^[22]

A closer look at Zhang's seismograph

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) that in 780 BC an earthquake had been devastative enough to divert the course of three different rivers.^[23] 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 (1050 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.^[23] These theories were ultimately derived from the ancient text of the I Ching (Book of Changes), in its fifty-first hexagram zhen.^[25] There were other early theories about earthquakes by those in ancient Greece; Anaxagoras believed that earthquakes derived from excess water towards the surface crust of the earth bursting below into the hollows of the earth; Democritus believed it was because of the saturation of the earth with water; Anaximenes believed it was because of massive parts of the earth falling into the cavernous hollows due to drying; Aristotle believed it was caused by instability of vapor (pneuma) caused by the drying of the moist earth by the warmth of sun rays.^[25]

Going against the grain of the early theories proposed by his fellow Chinese and contemporary Greeks, Zhang Heng believed earthquakes to be caused by air, writing:

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In any case, Zhang was responsible for detecting the cardinal direction of this natural phenomenon. In the Hou Han Shu (Book of Later Han, compiled by Fan Ye in the 5th century), there is a detailed description of the device in his biography, which states:

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Furthermore, Zhang Heng's seismograph device incorporated the following in order to function properly:^[26]

• 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.

Named in his honour

• lunar crater Chang Heng
• asteroid 1802 Zhang Heng
• mineral Zhanghengite

He also features as a Great Engineer in the PC game Civilization IV.

See also

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

Notes

References

External links

• A Biography of Zhang Heng
• Science and its Times on Zhang Heng
• Asianresearch.org on Zhang Heng
• Zhang Heng at the University of Maine