The History
In 132 AD, Chinese scientist and inventor named Zhang Heng had invented a simple system that used the complex laws of physics; the worlds first seismograph. A seismograph is a machine used to detect, or 'feel', tremors from hundreds of miles today. Today's new and advanced seismographs do the same exact thing, except today's are more sensitive and can feel earthquakes or tremors from thousands of miles away. Before he knew they knew Heng would soon change the world, they rejected his ideas. Soon enough though, one of the balls dropped and the people nearby felt nothing. However, a few days later a runner arrived from a village 400 miles away confirming an earthquakes happening. After that, the emperor had several of Zhang Heng's inventions in the palace.
How it Works
The seismograph traditionally is said to look similar to a beer barrel in its basic shape, with eight dragons on the sides on the barrels with eight pure bronze balls in their mouths, which would drop into a frogs mouth if a tremor was sensed. The ball comes out of the dragons mouth when a tremor disturbs the ball inside, which rolls in the direction of the tremor, hitting one of eight sticks which then hits the ball inside the dragons mouth, making it roll out and into the frogs mouth. The entire system is said to have been made out of pure bronze. (the picture below shows a similar system, there were a few different/ improvised systems but all used the same basic laws and had the same result)
What is amazing for this system though is that it wouldn't just react to any tremor, but it would react to an earthquake tremor, or horizontal tremor, as proven in the video in the link above. Even though the system is sensitive enough to detect an earthquake from hundreds of miles away, it is strong enough to determine if an earthquake was faulty or not. Even the design with the dragons and frogs were there to symbolize the natural balance.
What is amazing for this system though is that it wouldn't just react to any tremor, but it would react to an earthquake tremor, or horizontal tremor, as proven in the video in the link above. Even though the system is sensitive enough to detect an earthquake from hundreds of miles away, it is strong enough to determine if an earthquake was faulty or not. Even the design with the dragons and frogs were there to symbolize the natural balance.