Modern seismographs are extremely sensitive instruments. And although technology is on our side, the principle of operation of modern devices does not differ much from the oldest device, which was created almost 2 thousand years ago, before people understood what an earthquake is.
Jang Heng lived in China during the Han Empire, and history remembers him as a scholar who excelled in many fields. He worked on calculating Pi, mapped the sky, was a civil servant and inventor. He improved the accuracy of the clepsydra, a water clock that measured time with a flowing liquid, and created the first armillary sphere, an astronomical instrument. But most of all, Jang Hyun is famous for the invention of the world's first seismoscope.
He presented the device to the imperial court in the capital Luoyang in 132 BC, 7 years before his death. Although there are no surviving illustrations of its seismoscope, historical descriptions say it was a large bronze instrument, shaped like an urn or vase. On the vase were 8 dragons, each with a bronze ball clamped in its jaws. Directly below these mythical creatures were 8 bronze toads, whose mouths were open to catch the ball.
Ancient texts are more vague when it comes to the inner contents of a seismoscope. In general, it is believed that a pendulum hung inside the hollow body of the seismoscope, and linkages connected to each of the dragons surrounded it. The shock wave from the earthquake should have caused the pendulum to swing, activating one of the mechanisms inside. The corresponding dragon would drop its ball to the toad, informing the courtyard not only that the earthquake had occurred, but also where the earthquake came from.
Jang Hyun's invention was initially met with skepticism, but, as the story goes, a few years later, the ball finally fell. A few days later, a messenger arrived to report a serious earthquake that had occurred thousands of kilometers from Luoyang, exactly in the direction the dragon was looking.
Jan named his seismoscope Hufen Didun Yi, which means "an instrument for measuring seasonal winds and the movement of the Earth." He believed that tremors were caused by winds and changes in air pressure.
Attempts to reinvent the device in the 19th and 20th centuries were unsuccessful. It remained unclear how the pendulum in the ancient instrument was sensitive enough to detect earthquakes hundreds of kilometers away. Moreover, how could vibrations of the earth's crust set in motion only one mechanism and not touch others?
In 2005, a group of seismologists and archaeologists from the Chinese Academy of Sciences announced that they had succeeded in creating a working copy of Jang's invention. In their design, the pendulum itself did not interact with the lever mechanisms. Instead, he hovered over a ball on a thin pedestal. When the pendulum swung, it pushed the central ball down one of the 8 channels. The ball touched the trigger system, which "animated" the dragon's mouth.
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Although Jan's device is almost two thousand years old, the principle of its operation is still used today. A popular form of modern seismograph uses the same properties of inertia - a static base and a pendulum that move independently of each other when the ground shakes. Only today the pendulum is a magnet, and the induced current is analogous to a falling ball. Digitizing signals allows seismographs to document the intensity of earthquakes and their duration.