Zhang Heng (張衡)
|Field||Astronomy, mathematics, seismology, hydraulics, geography, ethnography, mechanical engineering, calendrical science, metaphysics, poetry|
|Known for||Seismometer, hydraulic-powered armillary sphere, pi calculation, shi, universe model, lunar eclipse and solar eclipse theory|
|Religious stance||Daoism, Chinese folk religion|
Zhang Heng (Traditional Chinese: 張衡; Simplified Chinese: 张衡; Hanyu Pinyin: Zhāng Héng; Wade-Giles: Chang Heng) (CE 78–139) was a Chinese polymath, being a astronomer, mathematician, inventor, geographer, cartographer, artist, poet, statesman, and literary scholar. He was from Nanyang, Henan, and lived during the Eastern Han Dynasty (25 – 220 C.E.).
Zhang made numerous contributions which gave lasting influences on the development of intellectual, technological, and literary traditions of China. Among his many accomplishments, Zhang is credited with inventing a seismometer. His device was able to determine the cardinal direction of earthquakes. Zhang Heng is also the first person to have applied hydraulic motive power to rotate an armillary sphere, an astronomical instrument representing the celestial sphere.
As an astronomer, Zhang cataloged 2500 stars, and recognized 124 distinct constellations, exceeding Hipparchus (c. 190 - c.120 B.C.E.) or Ptolemy (83-161 C.E.) who cataloged more than 1000. Zhang also invented the first odometer, often referenced as Archimedes (c. 287 - 212 B.C.E.). As a cartographer, Zhang was the first to make a mathematical grid reference.
Zhang was one of the best known writers in literature and poetry in the Eastern Han Dynasty. He wrote science fiction, cosmological, ethnographic literature, and started a new poetic style. He was also numbered as one of six best artists in the Eastern Han. In his poetry Zhang often expressed his moral criticism against decadent, lavish life styles of politicians of his time.
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Born in the town of Xi'e in Nanyang Commandery (located north of modern Nanyang City, Henan province), Zhang Heng came from a distinguished but not very affluent family.Rafe de Crespigny. 2007. A Biographical Dictionary of Later Han to the Three Kingdoms (23-220 C.E.). (Leiden: Koninklijke Brill. ISBN 9004156054), 1049.</ref> His grandfather, Zhang Kan, had been governor of a commandery, and one of the leaders who supported the restoration of the Han Dynasty by Emperor Guangwu (r. 25–57), following the death of the usurper Wang Mang and his short-lived Xin Dynasty (9–23 C.E.). At age ten, Zhang's father died, leaving him in the care of his mother and grandmother. An accomplished writer in his youth, Zhang left home in 95 to pursue his studies at universities in the ancient capitals of Chang'an and Luoyang. While traveling to Luoyang, Zhang passed by a hot spring near Mount Li and dedicated one of his earliest fu poems to it, the Wenquan (溫泉). After studying for some years at Luoyang's Imperial University (Taixue), he became well-versed in the classics, and befriended notable persons, such as the mathematician and calligrapher Cui Yuan (78 – 143), the official and philosophical commentator Ma Rong (79 – 166), and the philosopher Wang Fu (78 – 163). Government authorities offered Zhang appointments to several offices, including a position as one of the Three Excellencies, yet he acted modestly and turned down those positions. At age 23, he returned home with the title "Officer of Merit in Nanyang," serving as the master of documents under the administration of Governor Bao De (in office from 103–111). As he was charged with composing inscriptions and dirges for Bao De, he gained experience in writing official documents. As Officer of Merit in the commandery, he was also responsible for local appointments to office and recommendations to the capital of nominees for higher office. He spent much of his time composing rhapsodies on the capital cities. When Bao De was recalled to the capital in the year 111, to serve as a minister of finance, Zhang continued his literary work at home in Xi'e. Zhang Heng began his studies in astronomy at the age of 30, and began publishing his works in astronomy and mathematics.
In 112, Zhang was summoned to the court of Emperor An of Han (r.106 c. 125), who had heard of Zhang's expertise in mathematics. When he was nominated to serve at the capital, Zhang was escorted by carriage (a symbol of his official status) to Luoyang, where he became a court gentleman working for the Imperial Secretariat. He was promoted to Chief Astronomer for the Han court under Emperor An, serving his first term from 115 c. 120 and his second under the succeeding emperor from 126 - 132. As Chief Astronomer, Zhang was a subordinate of the Minister of Ceremonies, ranked just below the Three Excellencies. In addition to recording heavenly observations and portents, preparing the calendar, and reporting which days were auspicious or not, Zhang was also in charge of an advanced literacy test for all candidates of the Imperial Secretariat and Censorate (who were expected to know at least 9000 Chinese characters and all major writing styles). Under Emperor An, Zhang also served as Prefect of the Majors for Official Carriages under the Ministry of Guards, in charge of the reception of memorials (containing policy and administrative suggestions) submitted to the throne as well as nominees for official appointments.
When the government official Dan Song proposed the Chinese calendar should be reformed in 123 to adopt certain apocryphal teachings, Zhang opposed the idea. He considered the teachings to be of questionable stature and believed they could introduce errors. Others shared Zhang's opinion and the calendar was not altered, yet Zhang's proposal that apocryphal writings should be banned was rejected. The officials Liu Zhen and Liu Taotu, members of a committee to compile the dynastic history Dongguan Hanji (東觀漢記), sought permission from the court to consult Zhang Heng. However, Zhang was barred from assisting the committee due to his controversial views on apocrypha and his objection to the relegation of Emperor Gengshi's (r. 23–25) role in the restoration of the Han Dynasty as lesser than Emperor Guangwu's. Liu Zhen and Liu Taotu were Zhang's only historian allies at court, and after their deaths Zhang had no further opportunities for promotion to the prestigious post of court historian.
Despite this setback in his official career, Zhang was reappointed as Chief Astronomer in 126 after Emperor Shun of Han (r. 125–144) ascended to the throne. His intensive astronomical work was rewarded only with the rank and salary of 600 bushels, or shi, of grain (mostly commuted to coin cash or bolts of silk). To place this number in context, in a hierarchy of 20 official ranks, the lowest-paid official earned the rank and salary of 100 bushels and the highest-paid official earned 10,000 bushels during the Han. The 600-bushel rank was the lowest the emperor could directly appoint to a central government position; any official of lower status was overseen by central or provincial officials of high rank.
In 132, Zhang introduced an intricate seismometer to the court, which he claimed could detect the precise cardinal direction of a distant earthquake. On one occasion his device indicated that an earthquake had occurred in the northwest. As there was no perceivable tremor felt in the capital his political enemies were briefly able to relish the failure of his device, until a messenger arrived shortly afterwards to report that an earthquake had occurred about 400 km (248 mi) to 500 km (310 mi) northwest of Luoyang in Gansu province.
A year after Zhang presented his seismometer to the court, officials and candidates were asked to provide comments about a series of recent earthquakes which could be interpreted as signs of displeasure from Heaven. The ancient Chinese viewed natural calamities as cosmological punishments for misdeeds that were perpetrated by the Chinese ruler or his subordinates on earth. In Zhang's memorial discussing the reasons behind these natural disasters, he criticized the new recruitment system of Zuo Xiong which fixed the age of eligible candidates for the title "Filial and Incorrupt" at age 40. The new system also transferred the power of the candidates' assessment to the Three Excellencies rather than the Generals of the Household, who by tradition oversaw the affairs of court gentlemen. Although Zhang's memorial was rejected, his status was significantly elevated soon after to Palace Attendant, a position he used to influence the decisions of Emperor Shun. With this prestigious new position, Zhang earned a salary of 2000 bushels and had the right to escort the emperor.
As Palace Attendant to Emperor Shun, Zhang Heng attempted to convince him that the court eunuchs represented a threat to the imperial court. Zhang pointed to specific examples of past court intrigues involving eunuchs, and convinced Shun that he should assume greater authority and limit their influence. The eunuchs attempted to slander Zhang, who responded with a rhapsody called "Contemplating the Cosmos". Rafe de Crespigny states that Zhang's rhapsody used imagery similar to Qu Yuan's (340–278 B.C.E.) poem "Li Sao" and focused on whether or not good men should flee the corrupted world or remain virtuous within it.
Zhang retired from his position under Emperor Shun in 136, following which he was appointed Chancellor of Hejian (in modern Hebei). During his two years in office he worked to curtail the actions of the local king, Liu Zheng, and the powerful elite families to whom the king had granted special privileges. After arresting several lawbreakers, Zhang gained a reputation among the people of Hejian as a strict administrator. Zhang's writing at this time reflects his bitterness at being unable to effectively serve the emperor. Zhang retired from office in 138, and returned home to Nanyang. There he composed a rhapsody rejoicing over the opportunity his retirement gave him to read and to play his lute. A few months after his return home, Zhang was appointed to serve under the Imperial Secretariat and traveled back to the capital. He died there in 139, while still in office. By the time of his death, Zhang had composed thirty-two written works on literature, philosophy, science, and mysticism. He was buried in his hometown Xi'e, in Nanyang Commandery; his friend Cui Yuan composed the inscription for his tomb.
While working for the central court, Zhang Heng had access to a variety of written materials located in the Archives of the Eastern Pavilion. Zhang read many of the great works of history in his day and claimed he had found ten instances where the Records of the Grand Historian by Sima Qian (145–90 B.C.E.) and the Book of Han by Ban Gu (32 – 92 C.E.) differed from other ancient texts that were available to him. His account was preserved and recorded in the 5th century text of the Book of Later Han by Fan Ye (398–445). His rhapsodies and other literary works displayed a deep knowledge of classic texts, Chinese philosophy, and histories. He also compiled a commentary on the Taixuan (太玄, "Great Mystery") by the Daoist author Yang Xiong (53 B.C.E. – 18 C.E.).
Xiao Tong (501–531), a crown prince of the Liang Dynasty (502–557), immortalized several of Zhang's works in his anthology of literature, Wen xuan. Zhang's rhapsodies (fu 賦) include "Western Metropolis Rhapsody" (西京賦), "Eastern Metropolis Rhapsody" (東京賦), "Southern Capital Rhapsody" (南都賦), "Rhapsody on Contemplating the Mystery" (思玄賦), and "Rhapsody on Returning to the Fields" (歸田賦). The latter fuses Daoist ideas with Confucianism and was a precursor to later Chinese metaphysical nature poetry, according to Liu Wu-chi. 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. This set constitutes some of the earliest heptasyllabic shi Chinese poetry written. While still in Luoyang, Zhang became inspired to write his "Western Metropolis Rhapsody" and "Eastern Metropolis Rhapsody," which were based on the "Rhapsody on the Two Capitals" by the historian Ban Gu.
With his Response [to Criticism] of my Idleness (Yingxian), Zhang was an early writer and proponent of the Chinese literary genre shelun, or hypothetical discourse. Authors of this 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. He also used it as a means to criticize himself for failing to obtain high office, but coming to the conclusion that the true gentleman displays virtue instead of greed for power. In this work, Dominik Declercq asserts that the person urging Zhang to advance his career in a time of government corruption most likely represented the eunuchs or Empress Liang Na's (116–150) powerful relatives in the Liang clan. Declercq states that these two groups would have been "anxious to know whether this famous scholar could be lured over to their side," but Zhang flatly rejected such an alignment by declaring in this politically charged piece of literature that his gentlemanly quest for virtue trumped any desire of his for power.
For centuries the Chinese approximated pi as 3; Liu Xin (d. 23 C.E.) made the first known Chinese attempt at a more accurate calculation of 3.154, but there is no record detailing the method he used to obtain this figure.In his work around 130, Zhang Heng compared the celestial circle to the diameter of the earth, proportioning the former as 736 and the latter as 232, thus calculating pi as 3.1724. In Zhang's day, the ratio 4:3 was given for the area of a square to the area of its inscribed circle and the volume of a cube and volume of the inscribed sphere should also be 42:32. In formula, with D as diameter and V as volume, D3:V = 16:9 or V=D3; Zhang realized that the value for diameter in this formula was inaccurate, noting the discrepancy as the value taken for the ratio. Zhang then attempted to remedy this by amending the formula with an additional D3, hence V=D3 + D3 = D3. With the ratio of the volume of the cube to the inscribed sphere at 8:5, the implied ratio of the area of the square to the circle is √8:√5. From this formula, Zhang calculated pi as the square root of 10 (or approximately 3.162). In the 3rd century, Liu Hui made the calculation more accurate with his π algorithm, which allowed him to obtain the value 3.14159. Later, Zu Chongzhi (429–500) approximated pi as or 3.141592, the most accurate calculation for pi the ancient Chinese would achieve.
In his publication of 120 C.E.called The Spiritual Constitution of the Universe (靈憲, Ling Xian), Zhang Heng theorized that the universe was like an egg "as round as a crossbow pellet" with the stars on the shell and the Earth as the central yolk. This universe theory is congruent with the geocentric model as opposed to the heliocentric model. Although the ancient Warring States (403–221 B.C.E.) Chinese astronomers Shi Shen and Gan De had compiled the world's first star catalogue in the fourth century B.C.E., Zhang nonetheless catalogued 2500 stars which he placed in a "brightly shining" category (the Chinese estimated the total to be 14,000), and he recognized 124 constellations. In comparison, this star catalogue featured many more stars than the 850 documented by the Greek astronomer Hipparchus (c. 190–c.120 B.C.E.) in his catalogue, and more than Ptolemy (83–161 C.E.), who catalogued over 1000. Zhang supported the "radiating influence" theory to explain solar and lunar eclipses, a theory which was opposed by Wang Chong (27–97 C.E.).
The outflow clepsydra was a timekeeping device used in China as long ago as the Shang Dynasty (c. 1600–c. 1050 B.C.E.), and certainly by the Zhou Dynasty (1122–256 B.C.E.). The inflow clepsydra with an indicator rod on a float had been known in China since the beginning of the Han Dynasty in 202 B.C.E. and had replaced the outflow type. 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. Zhang Heng was the first to address this problem, indicated in his writings from 117, by adding an extra compensating tank between the reservoir and the inflow vessel.Cite error: Closing </ref> missing for <ref> tag Joseph Needham states that this was perhaps the ancestor of all clock jacks that would later sound the hours found in mechanical clocks by the eighth century, but he notes that these figures did not actually move like clock jack figurines or sound the hours. Many additional compensation tanks were added to later clepsydras in the tradition of Zhang Heng. In 610 the Sui Dynasty (581–618) engineers Geng Xun and Yuwen Kai crafted 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. Zhang mentioned a "jade dragon's neck," which in later times meant a siphon.
Zhang Heng is the first person known to have applied hydraulic motive power (i.e., by employing a waterwheel and clepsydra) to rotate an armillary sphere, an astronomical instrument representing the celestial sphere. The Greek astronomer Eratosthenes (276 - c. 194 B.C.E.) invented the first armillary sphere in 255 B.C.E. The Chinese armillary sphere was fully developed by 52 B.C.E., with the astronomer Geng Shouchang's addition of a permanently fixed equatorial ring. In 84 C.E. the astronomers Fu An and Jia Kui added the ecliptic ring, and finally Zhang Heng added the horizon and meridian rings. Zhang described this invention in his written work of 125, Apparatus for Rotating an Armillary Sphere by Clepsydra Water. The sphere itself was rotated by a turning waterwheel, which in turn was powered by the constant pressure head of water in the water clock tank. His water-powered armillary influenced the design of later Chinese water clocks and led to the discovery of the escapement mechanism by the eighth century.
Zhang Heng's water-powered armillary sphere had profound effects on Chinese astronomy and mechanical engineering in later generations. His model and its complex use of gears greatly influenced the water-powered instruments of later astronomers such as Yi Xing (683–727), Zhang Sixun (fl. tenth century), Su Song (1020–1101), Guo Shoujing (1231–1316), and many others. Water-powered armillary spheres in the tradition of Zhang Heng's were used in the eras of the Three Kingdoms (220–280) and Jin Dynasty (265–420), yet the design for it was temporarily out of use between 317 and 418, due to invasions of northern Xiongnu nomads. Zhang Heng's old instruments were recovered in 418, when Emperor Wu of Liu Song (r. 420–422) captured the ancient capital of Chang'an. Although still intact, the graduation marks and the representations of the stars, Moon, Sun, and planets were quite worn down by time and rust. In 436, the emperor ordered Qian Luozhi, the Secretary of the Bureau of Astronomy and Calendar, to recreate Zhang's device, which he managed to do successfully. Qian's water-powered celestial globe was still in use at the time of the Liang Dynasty (502–557), and successive models of water-powered armillary spheres were designed in subsequent dynasties.
In 132, Zhang Heng presented to the Han court what many historians consider to be his most impressive invention, the first seismometer. It was named Houfeng didong yi (候风地动仪, lit. instrument for measuring the seasonal winds and the movements of the Earth), and it was able to determine the exact direction (out of eight directions) of tremors and earthquakes. According to the Book of Later Han (compiled by Fan Ye in the fifth century), his copper urn-shaped device, with a swinging pendulum inside, was able to detect the direction of an earthquake hundreds of miles/kilometers away. This was essential for the Han government in sending quick aid and relief to regions devastated by this natural disaster.The device was considered important enough to be mentioned in the "Annals" chapter of the Book of Later Han, detailing the reign of Emperor Shun.
To indicate the direction of a distant earthquake, Zhang's device dropped a bronze ball from one of eight tubed projections shaped as dragon heads; the ball fell into the mouth of a corresponding metal object shaped as a toad, each representing a direction like the points on a compass rose. His device had eight mobile arms (for all eight directions) connected with cranks having catch mechanisms at the periphery. When tripped, a crank and right angle lever would raise a dragon head and release a ball which had been supported by the lower jaw of the dragon head. His device also included a vertical pin passing through a slot in the crank, a catch device, a pivot on a projection, a sling suspending the pendulum, an attachment for the sling, and a horizontal bar supporting the pendulum. Wang Zhenduo (王振铎) argued that the technology of the Eastern Han era was sophisticated enough to produce such a device, as evidenced by contemporary levers and cranks used in other devices such as crossbow triggers.
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. However, map-making in China had existed since at least the fourth century B.C.E. with the Qin state maps found in Gansu in 1986. 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. 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. Rafe de Crespigny asserts that it was Zhang who established the rectangular grid system in Chinese cartography. Robert Temple writes that Zhang not only presented a map to the emperor in 116 C.E., but his now lost works called Discourse on New Calculations and Bird's-Eye Map "laid the groundwork for the mathematical use of the grid with maps." Moreover, the Book of Later Han hints that Zhang was the first to make a mathematical grid reference, stating that he "cast a network of coordinates about heaven and earth, and reckoned on the basis of it." 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).
Zhang Heng is often credited with inventing the first odometer, an achievement also attributed to Archimedes (c. 287–212 BCE) and Heron of Alexandria (fl. CE 10–70). Similar devices were used by the Roman and Han-Chinese empires at about the same period. By the 3rd century, the Chinese had termed the device the ji li gu che, or "li-recording drum carriage" (the modern measurement of li = 500 m/1640 ft).
The South Pointing Chariot was another mechanical device credited to Zhang Heng. It was a non-magnetic compass vehicle in the form of a two-wheeled chariot. Differential gears driven by the chariot's wheels allowed a wooden figurine (in the shape of a Chinese state minister) to constantly point to the south, hence its name. The Song Shu (c. 500 CE) records that Zhang Heng re-invented it from a model used in the Zhou Dynasty era, but the violent collapse of the Han Dynasty unfortunately did not allow it to be preserved. Whether Zhang Heng invented it or not, Ma Jun (200–265) succeeded in creating the chariot in the following century.
Zhang Heng's mechanical inventions influenced later Chinese inventors such as Yi Xing, Zhang Sixun, Su Song, and Guo Shoujing. Su Song directly named Zhang's water-powered armillary sphere as the inspiration for his eleventh-century clock tower. The cosmic model of nine points of Heaven corresponding with nine regions of earth conceived in the work of the scholar-official Chen Hongmou (1696–1771) followed in the tradition of Zhang's book Spiritual Constitution of the Universe.  The seismologist John Milne, who created the modern seismograph in 1876 alongside Thomas Gray and James A. Ewing at the Imperial College of Engineering in Tokyo, commented in 1886 on Zhang Heng's contributions to seismology. The historian Joseph Needham emphasized his contributions to pre-modern Chinese technology, stating that Zhang was noted even in his day for being able to "make three wheels rotate as if they were one." More than one scholar has described Zhang as a polymath. However, some scholars also point out that Zhang's writing lacks concrete scientific theories.
Zhang's poetry was widely read during his life 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". The influential poet Tao Qian 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. Tao wrote that both Zhang Heng and Cai Yong "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".
Zhang was given great honors in life and in death. The philosopher and poet Fu Xuan (217–278) of the Wei and Jin dynasties once lamented in an essay 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?"
In honor of Zhang's achievements in science and technology, his friend Cui Ziyu (Cui Yuan) wrote a memorial inscription on his burial stele, which has been preserved in the Guwen yuan. Cui stated, "[Zhang Heng's] 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." The minor official Xiahou Zhan (243–291) of the Wei Dynasty made an inscription for his own commemorative stele to be placed at Zhang Heng's tomb. It read: "Ever since gentlemen have composed literary texts, none has been as skillful as the Master [Zhang Heng] in choosing his words well ... if only the dead could rise, oh I could then turn to him for a teacher!"
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