Wednesday, October 18, 2017

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An Ancient China's Seismograph

Back in AD 132, in the ancient capital of Jing Shi ( present day Luoyang in Henan Province) of Eastern Han Dynasty, news broke out that an official by the name of Zhang Heng ( 张衡 ) invented a mechanical machine that was capable of predicting the time and location of earthquakes. Many people did not believe him at that time.

Zhang Heng ( 张衡 ) was born in AD 78 and passed away in AD 139. He was an outstanding scientist in ancient China, well versed in subject matters such as mathematics, astrology and earthquakes. What Zhang invented was what would have known as a seismograph in modern day terminology. It was the first ever equipment invented to predict earthquakes.

The original equipment was lost. However, according to records, the ancient seismograph was made of high quality bronze and shaped like a rounded wine jar with a height of about 5.5 feet and a diameter of about 6 feet in length. On the outside, the seismograph was decorated with eight dragons, each facing one of the eight directions. At the ground in each of the direction was a bronze toad structure that opened its mouth wide to face the dragon.

Zhang Heng's Seismograph
Zhang Heng's Seismograph

The mouth of each dragon contained a metallic ball which would be triggered to drop into the mouth of the toad underneath whenever an earthquake occurred in one of the direction from the seismograph. The mechanics of the seismograph was embedded inside the structure and operated on the theory of inertia. A pendulum was hung inside the jar and when movement of the ground displaced the pendulum and the jar, the ball triggered to be drop via levers and gears. The sound created from the drop of the metal ball would alert the attendant that an earthquake had occurred.

On one particular day, the mouth of the west facing dragon opened to drop its ball into the toad underneath, signaling that an earthquake had occurred to the west. However, at that time in Jing Shi, people did not believe the prediction of the machine as they did not feel any movements from the ground. A few days later, to the amazement of the residents, a messenger arrived on a horseback to inform the capital that a huge earthquake had occurred hundreds of miles to the west (in southwestern part of present day Gansu Province). Later research had shown that this particular earthquake had occurred on December 13 in AD 134, a magnitude 7 earthquake with epiceneter at Tianshui about 400 miles away from Luoyang.

The modern day seismograph was invented only in 1880, about 1,700 years later, and was molded from the concept of Zhang’s invention.


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