The first seismograph (dì dǒng yí 地动仪) of the world was invented by Zhang Heng (zhāng héng 张衡), a famous scientist in the Eastern Han Dynasty (dōng hàn 东汉, 25-220). Zhang Heng (78-139) was from Nanyang (nán yáng 南阳) in Henan Province (hé nán shěng 河南省). He studied diligently, and was especially keen on astronomy, calendars and mathematics. As a whole, Zhang Heng can be regarded as an erudite and talented scientist.

Earthquakes of ancient times
From the earliest times, the Chinese were concerned with the destructive force of earthquakes. It was recorded in Sima Qian's (sīmǎ qiān 司马迁) Records of the Grand Historian(shǐ jì 史记) of 91 BC that in 780 BC an earthquake had been powerful enough to divert the courses of three rivers. The ancient Chinese did not understand that earthquakes were caused by the shifting of tectonic plates in the Earth's crust; instead, the people of the ancient Zhou Dynasty (zhōu cháo 周朝) explained them as disturbances with cosmic yin and yang (yīn yáng 阴阳), along with the heavens' displeasure with acts committed (or the common peoples' grievances ignored) by the current ruling dynasty. These theories were ultimately derived from the ancient text of the Yijing (yì jīng 易经, Book of Changes), in its fifty-first hexagram.

During the Han Dynasty (hàn cháo 汉朝), many learned scholars—including Zhang Heng—believed in the "oracles of the winds". These oracles of the occult observed the direction, force, and timing of the winds, to speculate about the operation of the cosmos and to predict events on Earth. These ideas influenced Zhang Heng's views on the cause of earthquakes. Against the grain of earlier theories proposed by his fellow Chinese and contemporary Greeks, Zhang Heng believed that earthquakes were caused by wind and air.

Structure of the Seismometer
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 (hòu fēng dì dòng yí 候风地动仪, instrument for measuring the seasonal winds and the movements of the Earth). The description we have, from the History of the Later Han Dynasty (hòu hàn shū 后汉书), says that it was a large bronze vessel, about 2 meters in diameter and looked like a big cup with a lid. The surface of the instrument was cast with eight dragons, whose bodies were downward vertically and heads pointed to eight directions (east, south, west, north, southeast, northeast, southwest, and northwest), and each dragon had a copper ball in the mouth. On the ground vertically below the mouths of dragons, there were eight copper toads raising their heads and opening their mouths. The inner side of the seismograph was elaborately constructed. When an earthquake occurring at that direction, to which the dragon pointed would open its mouth, and the ball would fall into the toad’s mouth, automatically indicating the earthquake occurring at that direction.
One day in 138 AD, the dragon pointing to the west expelled its ball. As expected, an earthquake had occurred on that day in Longxi  (lǒng xī 陇西, in present-day Gansu Province) four hundred miles away from Luoyang (luò yáng 洛阳). It was the first time that mankind had used the instrument to sense an earthquake. This was essential for the Han government in sending quick aid and relief to regions devastated by this natural disaster.  
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Apart from the Seismograph, Zhang Heng also invented the first water-driven armillary sphere (shuǐ yùn hún tiān yí 水运浑天仪) in the world to measure the position of celestial bodies. With so many contributions, Zhang Heng was highly esteemed by most people. A crater on the moon was named after him.

张衡和地动仪
      世界第一架地震仪是由中国东汉时著名的科学家张衡发明的。张衡（78-139），东汉建初三年（公元78年）生；永和四年（公元139年）卒。字平子，南阳西鄂（今河南南阳市石桥镇）人，汉族。他是我国东汉时期伟大的天文学家、数学家、发明家、地理学家、制图学家、诗人、汉朝官员，为我国天文学、机械技术、地震学的发展做出了不可磨灭的贡献；在数学、地理、绘画和文学等方面，张衡也表现出了非凡的才能和广博的学识。
      张衡的一个杰出贡献的科学领域是地震学。他的代表作就是震烁古今的候风地动仪的发明。现在中国所见到的地动仪，并不是张衡发明的地动仪，而是后人复原的。张衡发明的地动仪早就毁于战火了。候风地动仪发明于阳嘉元年(公元132年)，全部用精铜铸成，外形像一个带盖的大茶杯。仪器表面铸有八条垂直向下的龙，龙头分别对准东、南、西、北、东南、东北、西南、西北八个方向，每条龙的嘴里都含有一个铜球。在正对龙嘴的地上，蹲着八个仰头、张嘴的铜蟾蜍（chán chú）。地动仪的内部结构非常精细巧妙，当某个方向发生地震时仪器上对着那个方向的龙嘴就会张开，铜球就会掉进铜蟾蜍的嘴里，自动报告发生地震的方向。公元138年的一天，地动仪西边的龙嘴吐出了铜球。果然，远在千里之外的陇西（今甘肃省）在这一天发生了地震。这是人类第一次用仪器报地震。                                                                                  
      这台仪器不仅博得当时人们的叹服，就是在今天的科学家看来也无不赞叹。世界上地震频繁，但真正能用仪器来观测地震，在国外那是19世纪以后的事。候风地动仪乃是世界上的地震仪之祖。虽然它的功能尚只限于测知震中的大概方位，但它却超越了世界科技的发展约1800年。
      张衡还制造出了世界上第一架测量天体位置的水运浑天仪，为古代科学发展做出了突出贡献，并被人们所崇敬。联合国天文组织曾将太阳系中的1802号小行星命名为“张衡星”。人们还将月球背面的一环形山命名为“张衡环形山”。