The Great Scientist Zhang Heng
2 min readZhang Heng（78-139）was a multitalented scientist and also a minister in the government.His most important invention was the seismograph，used to detect earthquakes at a great distance.In 132，Zhang invented the instrument for measuring earthquakes.Zhang’s device，which he called Houfeng Didong Yi，was made of copper.It was in the shape of an egg with eight dragon heads around the top，each with a copper ball in its mouth，and a pendulum in the center.Around the bottom were eight frogs，each directly under a dragon head.
When an earthquake occurred，a ball fell out of a dragon’s mouth into a frog’s mouth，making a noise.The seismograph detected an earthquake in February of 138 and Zhang reported this fact to the Emperor despite no other evidence of the earthquake being felt in the capital，Luo Yang.He was even able to indicate that the earthquake was to the west of the capital.He achieved fame when reports of an earthquake more than a thousand li northwest reached the capital several days later.
The Armillary Sphere to map the heavens might have appeared by the 4th century BC，but it was Zhang Heng who made bold innovations to the instrument by designing and making the water-driven armillary sphere，which can icompletely demonstrate the theory of sphere heavens.4 The major part was a spherical model，representing the celestial globe.Inside the globe there was a metal axis going through the center，with the sphere and the axis intersecting at two pins:the north and south celestial points.The direction of the globe represents the direction of the earth’s axial rotation.To enable the armillary sphere to rotate，Zhang employed the gear system，linking the celestial globe and the kettle clepsydra. Powered by the water leaking out of the kettle，the gear drove the celestial globe to rotate around the axis，one day per circuit，in synchronization with the sun.In this way，the celestial changes were presented accurately.By observing the instrument，one can know when a star rises in the east and set in the west.With this instrument Zhang was able to make more accurate star maps than earlier Chinese astronomers.
