Source: CCTV.com

08-07-2006 13:55

Zhang Heng's seismograph was invented during the Eastern Han Dynasty (AD 23 – AD 221). Zhang Heng was a great scientist in ancient China. He had several major achievements not only in seismology but also in astronomy, law, and geology among others. He employed many superb craftsmen and made the world's first seismograph to sense earthquakes in 132 A.D. This invention reflects the development of Chinese science and technology at that time.

According to historical records, Zhang Heng's seismograph was cast in bronze. It looks like a huge wine goblet, 2.66 meters across, with a cover. It was carved with seal scripts, mountains, turtles, birds and beasts. Its exterior was inlaid with eight dragons that had a bronze ball in each of their mouths. On its base are eight toads with their mouths open wide, face to face with the dragons' so that they can catch the balls falling from the dragons' mouths. How can the seismograph sense earthquakes and process signals?

Zhang Shaoquan is a Chinese seismological expert. He tells us that the seismograph basically consists of two parts. One is an upside-down pendulum. It forms an unstable system and can sense quakes.

Zhang Shaoquan, Research Professor (Institute of Geophysics China Earthquake Administration)

This is a water bottle. It's placed on my desk upside down. If the ground and my desk are still, it keeps still. But once my desk moves slightly, it falls down in the opposite direction of the desk's movement due to inertia.

In addition to the upside-down pendulum, Zhang Heng's seismograph has eight groups of levers, which are connected to the pendulum and distributed around it. They're the eight directions of the pendulum. Once the pendulum inclines, they can magnify and record the quake waves.

Zhang Shaoquan: If the ground shakes even slightly, the pendulum will fall toward a direction. The eight groups of levers are linked to a fulcrum. Each group consists of two levers. One lever is connected to the pendulum, and the other is linked to the upper jaw of a dragon, whose lower jaw is fixed. When the pendulum tilts toward a group of levers, the lever linked to it presses the other lever and forces the latter to pull up the dragon's upper jaw, and the copper ball in its mouth falls into the mouth of the toad below.

The process of the seismograph sensing an earthquake is as follows: when the seismograph senses earthquake waves, its pendulum falls down; through the action of the eight groups of lever mechanics, the copper ball corresponding to the direction of the earthquake center falls down from its dragon's mouth. This can help people identify the basic orientation of the earthquake's epicenter.

In the China Earthquake Popular Science and Education Museum located at the foot of the Fragrant Hills, Beijing, we can see a modern seismograph. What relationship does it have with Zhang Heng's? They both have an earthquake wave receiver, magnifier and recorder and can sense ground tremors and analyze the situation of epicenters according to ground tremors. There are usually two types of ground tremors. One is a high-frequency or fast tremor, and the other is a slow or low-frequency one. Prof. Zhang demonstrates them to us.

Zhang Shaoquan: For instance, when I stamp my feet, we can see high-frequency tremor waves on the screen. When I just quiver slightly, we can see low-frequency tremor waves on the screen. An actual ground tremor is a mixture of high-frequency and low-frequency waves and has broad-spectrum waves. So the modern seismograph gives us far more information than the ancient one. Yet, it has the same principle as the latter and is built on the basis of the latter.

Zhang Heng's seismograph is the first times humans used scientific methods to understand earthquakes. We can say that he was a pioneer of seismological studies and a great seismologist.

 

Editor:Ge Ting