How did the Chinese Detect Earthquakes Two Thousand Years Ago
In the Han Dynasty (206 BC - 220 AD) of China, earthquakes took place quite frequently. According to historical records, there were 26 large earthquakes between the year 92 and 125. In order to better detect earthquakes, Zhang Heng, a great scientist in ancient China, invented the world’s first seismograph in 132 AD.
According to historical records, Zhang’s seismograph was made of copper in the shape of a wine-jar. Inside the device was the core mechanism part. Eight Chinese Long (dragon) surrounded it in the eight directions: east, south, west, north, southeast, northeast, southwest and northwest. Eight toads acted as the support of the device. There was a copper ball in every of the eight Chinese Long’s mouth. If an earthquake took place somewhere, the Chinese Long in that direction would open its mouth and drop the ball, which would fall into the mouth of the toad under it, making a loud noise to notify people.
After the making of Zhang’s seismograph, there was no earthquake for two years. In the year 134, the mechanism of the seismograph suddenly launched and a ball was dropped into the toad’s mouth. However, the people there could not feel or find anything signifying an earthquake, so they began to doubt that the machine is not precise. Several days later, a messenger from a thousand miles away arrived, reporting that there was indeed an earthquake at that time. Thus, successfully detecting the earthquake happening a thousand miles away, Zhang’s seismograph proved its accuracy and sensitivity.
After the time of Zhang Heng, a few scholars wrote about the mechanism and application of the seismograph, together with its schematic diagram. However, their writings were all lost after the Tang Dynasty (618 - 907). Today’s researchers have to speculate about the mechanism. Among the many hypotheses about Zhang’s seismograph, two are the most popular. One was raised by Wang Zhenduo in 1951, the other by Feng Rui in 2005. [reference (in Chinese)]
Why can Zhang’s seismograph detect earthquakes? Modern science has explanations. The earth has two kinds of vibration, seismic wave and the non-seismic wave. The non-seismic waves are usually longitudinal while the seismic waves are usually full of transversal energy. Some devices, such as a pendant lamp, have little reaction to longitudinal waves but are very sensitive to transversal waves. So, observing the pendant lamp is a very handy and reliable way to detect earthquakes. According to researchers, this probably is the basic mechanism used in Zhang’s seismograph.
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Thank you, Ulaf. I’ll do my best.