Dragons for earthquake detection
Luoyang (China), c. 132 132. The cartographer, mathematician, astronomer, painter and writer Zhang Heng (78-139) presented in the imperial court of the Han del Este dynasty a device for the recognition of earthquakes 1,700 years before the invention of modern seismographers (developed in Japan by Milne, Ewing and Gray in the 1880s).
Zhang's seismograph was a kind of large brass jar, about two meters in diameter. They carried eight dragons with them, pointing to the cardinal points, and each dragon carried a copper ball in his mouth. On the ground, under the eight dragons, there were so many other frogs also located on cardinal points, with the mouth completely open. When an earthquake occurred, the pitcher's hole into a vibration amplifier and moved a pendulum. The mechanism containing the pendulum was set in motion, causing one of the dragons to unscrew the ball and get into the frog's mouth. The noise of the impact of the metals warned the neighbours that the earth had shattered and the direction in which the devoured frog earthquake had occurred.
The seismic detector Zhang Hengen cannot be technically called a seismograph or a seismometer, since it does not measure the intensity of the earthquake, but we should call it a seismoscope, as it only indicated that the seismic movement occurred.
But it seems that the contraption of the Chinese scientist and artist was very accurate. Once, in year 143, the seismoscope declared an earthquake with the ball released by the Western dragon. But in Luoyang's court there wasn't the slightest tremor. Had the invention of the wise man failed, among other things, to give a correct explanation of the moon eclipses and to accommodate the timetable for the seasons?
A few days later a messenger arrived at the palace, which reported an earthquake in the province of Gansu, about 600 kilometres west of Luoyang.
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