Chang Heng (78–139) was the leading scientist of the Later (or Eastern) Han Dynasty of first-and second-century China. He was a scholar in many areas, among them astronomy, cartography, mathematics, philosophy, and literature. Also known as Zhang Heng and Pingzhi, Heng is credited with creating the first seismograph to record earthquakes, devised an armillary—or celestial globe—to track the movement of planets and stars, proposed the concept of the lunar eclipse, developed longitude and latitude grids for maps, invented the odometer, and wrote love poems and other literary works.
Chang Heng was born in the Chinese Year of the Horse 2776, or 78 A.D., in the Xie county of Nayang, north of what is today Nanyang County in Henan Province of China. He was born during the Han Dynasty, a golden age of 400 years of enlightenment, scientific discovery, and domestic prosperity. Heng grew up amid literature and learning, becoming a good writer by age twelve. With a desire to travel around China and learn new things, at age 16 he left home to pursue his passion for knowledge. Heng visited the Han capital city of Chang'an where he learned history, current events, and culture, then ventured to the East Han capital of Luoyang and that region's school of higher learning, Taixue. At Taixu Heng spent several years honing his skills in writing and the study of literature.
In 111 the Chinese government asked for Heng's services, so he became an official assigned as the court historian. First serving as a junior officer then as a senior, Heng spent his longest term, 14 years, working under scholar Taishiling. Taishiling oversaw the observation of astronomical phenomena, the compilation of calendars, weather prediction, and meteorological occurrences. Heng studied the astronomical calendar, managed official documents, and became a national historian. Some biographers claim Heng was known for his moral attitude, never taking bribes, and cleaning up government corruption when he encountered it.
In time, Heng became the chief astronomer of the Imperial Chancellery for Astronomical and Calendrical Science and a chief minister under Emperor An'ti. Possessed of the rare talent of interdisciplinary interests, Heng embodied ancient China's reputation for scientific discovery and observation. Although he pursued knowledge in many fields, he is most remembered for devising the first seismograph, which he did in 132 A.D.
Invented the Seismograph: The Dragons
and the Toads
Known for excellent record keeping, the Chinese kept accurate records of not only celestial events, but of earthquakes as well. At the time, people believed that seismic events were supernatural in origin: signs from heaven from angry gods designed to punished those below. Heng discounted these superstitions because he had been making careful observations of the symptoms of earthquakes and believed he could explain them by scientific means.
Because news about earthquakes occurring in distant parts of the country took too long to reach the Court, Heng desired a device capable of indicating tremors, their distance from the seismic event, and the event's location. To do so would not only save lives, but raise Heng's status in the Han Court.
Although Heng's original seismograph did not survive time, its description did, and several modern scholars have tried to recreate the device. Called the houfeng didong yi, or "instrument for inquiring into the wind and the shaking of the earth," the first seismograph was a cast bronze kettle with a domed lid. The diameter of the device was eight chhih—1.8 meters or six feet.
The design of Heng's seismograph was indicative of Chinese artistry. The kettle sported eight dragonheads arranged in eight directions around the outside rim. Each head held a ball in its mouth. At the base of the vessel were eight corresponding toads with their empty mouths open. The "toothed machinery and ingenious constructions" inside the device were hidden. Actually it was an inverted pendulum that reacted to the slightest tremor of the earth. The pendulum swung, tapping a mechanism that ejected one of the balls from the corresponding dragonhead. The ball would fall into the waiting toad's mouth sitting directly under it and make a loud clang. Whichever toad had the ball indicated the direction of the earthquake's location.
Earned Reputation for Accurate
Heng's reputation was put into question one day in February 138 when a ball fell from a dragon's mouth indicating an earthquake had occurred. No one felt any shocks or tremors and questioned the results of the device. A few days later, a messenger arrived at court announcing that Longxi county in western Gansu Province, 400 miles away, had been struck by an earthquake. The sensitivity and accuracy of Heng's invention solidified his position as chief scientist.
Heng's "scientifically designed" instrument should more accurately be called a seismoscope, since it really measured the direction of ground motion without noting time or amplitude. Nevertheless, it worked on the principle of inertia: the tremor shook the device, causing a displacement between the mass and the kettle. This movement caused the ball to fall out of the dragon's mouth. Heng was able to design the seismoscope to pick up the signals of actual earthquakes and reject false signals.
Heng's machine was in use for four centuries in the form of "earthquake weathercocks." Despite the genius and practicality of the machine, however, exploration in this area of science ended when the Mongols overran China in the 13th century. Some later Chinese historians had doubted that such a device was even possible. Only 1,400 years later in France in 1703, did De la Hautefeuille invent the first modern seismograph.
Created Second Seismograph and
In competition with another noted scientist of his time, Li Pao, Heng devised another type of seismograph to prove his theory, which was contradictory to Li's. Heng's new machine consisted of a bronze dragon embracing a water driven cylinder covered with porous porcelain plates. In the dragon's mouth was balanced a glass bulb filled with red ink. Many of these machines were placed throughout China. If earthquakes were caused internally in the Earth, as Heng believed, the tremor would dislodge all the ink at the same time. Li believed that earthquakes were caused by meteorites hitting the earth, therefore dislodging only the ink bulb nearest the impact. The bulb would break and ink would flow down the rotating cylinder in a spiral pattern. Unfortunately there is no record of who won the competition.
Heng believed that the Earth was round. In his writing titled Hun-i chu, he describes the world: "The sky is like a hen's egg, and is as round as a crossbow pellet; the Earth is like the yolk of the egg, lying alone at the center. The sky is large and the Earth small." To further study his interpretation of the universe, Heng constructed the first rotating celestial globe, or armillary.
The Chinese introduced the first permanently mounted equatorial armillary ring in 52 B.C. Successive astronomers adding rings. In 125 Heng added a ring for the meridian and one for the horizon. He built a wooden sphere at first, then a bronze version nearly five meters in circumference, affixed stars to the device, and made it rotate by water pressure. The water clock, or clepsydra, regulated the rotation of the device so it turned in real time—one circuit in one day, one complete rotation in one year. This allowed people to observe the movements of the sun, moon, and stars as they interrelated. In his book The Chart and Interpretation of Armillary Sphere, Heng calculated the year as 365 and a quarter degrees. Heng was the first to add such features as the north and south poles, equator, and elliptics.
Heng's definitive astronomical text is Lin Xian, which describes celestial phenomena. In this work he proposes that the moon reflects sunlight rather than being lit on its own. He also sets forth the theory that the moon could be eclipsed by the shadow of the Earth, gives reasons for the shortening and lengthening of the days throughout the year, and argues that the universe is infinite in time and space. He also calculates the angular diameter of the sun and moon as 29'24" or 1/736 of the celestial globe (the true average angular diameter is 31'59"26). He charted 124 constellations consisting of 2,500 stars he observed while in Luoyang, 320 of which had names. He also explains the optical illusion that makes the sun look bigger at morning and evening and smaller at noon.
A Man of Many Disciplines
Heng was an engineer, meteorologist, geologist, and philosopher. He invented the odometer, or "mileage cart," which carried a figure that struck a drum as each li—or 0.5 km—distance was traversed. He is also remembered as being one of the four great painters of his time. A visionary mathematician, Heng computed the value of pi as the square root of 10, or approximately 3.162, not far off from 3.14. He constructed a sundial to measure the position of the sun. In 123 he improved the calendar to coordinate it with the seasons. He invented a compass vehicle in which a wooden figure inside a carriage would always point in the southerly direction due to the specialized gear system. He even created a wooden flying bird.
Many of his efforts focused on geography. Heng invented quantitative cartography, applying a grid system to maps, from which positions, distance, and itineraries could be calculated. His book, Discourse on New Calculations, established the basis for the mathematical use of the grid with maps, and he presented one of his maps to the Chinese emperor in 116. He perfected the science of latitude and longitude, and his grids were said to form a "net over the Earth."
Composed Poetry and Literary Works
The Han Dynasty produced narrative poets who described the grandeur of China's imperial court, the region's prosperity, and the lives of the elite. In this climate, Heng wrote more than 20 literary works. His poetry includes verses on such topics as leisure, academics, politics, love, and erotica. In his "Four Stanzas of Sorrow," he earliest known seven-syllabic Chinese poem, Heng writes, "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?"
Heng's other notable poems include "Bones of Zhuang Zi," "Going Back to the Field," "Two Capitals," "A Song of Simultaneous Sounds," and "Thinking about Mysteries," the last in which he describes his astral travels among the solar system. In the prose poem "The Fu" Heng presents a critique of the former emperors of the Han Dynasty.
Heng died in the Year of the Tiger 2837, or 139 A.D. in China. Many of the accomplishments of early Chinese scientists have been lost or forgotten, a symptom of the social structure in which academics sought patrons to sponsor their work. Knowledge was localized and spread slowly. Due to his vast number of achievements, however, Heng's work has survived. In the summer of 1980 the People's Republic of China presented an exhibit of science and industrial products in San Francisco, California, that included a model of Heng's ancient seismograph. And more recently, as China announced in 2003 that it planned to build a space telescope fashioned after NASA's Hubble telescope, rumors indicated that the instrument—which was planned for operation in 2008—would be named in honor of Heng. The ancient Chinese inventor has also been honored in space; Heng has a lunar crater on the back side of the moon named after him.
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