Seismoscope - Zhang Heng
The Leonardo da Vinci of ancient China was Zhang Heng (79-139AD) an inventor, astronomer, engineer, scientist, scholar and artist. His most
intriguing design, created almost 2000 years ago was the design of the first earthquake sensing device invented. This ancient, art-science,
sculptural instrument named Houfeng didong yi could remotely detect earthquakes hundreds of kilometres away. Known as a Seismoscope, this inertia
sensing instrument was the very beginning for the science's in the field of seismology.
Zhang Heng believed that earthquakes were caused by wind and air;
"The chief cause of earthquake is air, an element naturally swift and shifting from place to place. As long as it is not stirred, but lurks in a
vacant space, it reposes innocently, giving no trouble to objects around it. But any cause coming upon it from without rouses it, or compresses it,
and drives it into a narrow space ... and when opportunity of escape is cut off, then 'With deep murmur of the Mountain it roars around the barriers', which after long battering it dislodges and tosses on high, growing more fierce the stronger the obstacle with which it has contended."
To indicate the direction of a distant earthquake, Zhang's device dropped a bronze ball from one of eight tubed projections shaped as dragon heads;
the ball fell into the mouth of a corresponding metal object shaped as a toad, each representing direction in which the seismic wave was travelling.
His device had eight mobile arms (eight directions) connected with cranks having catch mechanisms at the periphery. When tripped, a crank and right
angle lever would raise a dragon head and release a ball supported by the lower jaw of the dragon head. The seismoscope device also included a
vertical pin passing through a slot in the crank, a catch device, a pivot on a projection, a sling suspending the pendulum, an attachment for the
sling, and a horizontal bar supporting the pendulum.
There are no clear historical documents and physical remains of Zhang Heng's seismoscope device. Several reconstruction designs have been attempted
to replicate Heng's original seismoscope which today is still a famous but mysterious instrument. Many scholars tried to reconstruct Zhang Heng's
seismoscope in the past 150 years. In the early stage, some exterior appearances of Zhang's seismoscope were proposed (Imamura 1942; Milne 1883;
Wang 1963; Bolt 1978). Later, the interior mechanical structures with various operating functions were presented (Wang 1936; Feng et al.
2006 a, b; Sleeswyk and Sivin 1983; Imamura 1939; Wang 1963; Lee 1994; Imamura 1942; Yan and Hsiao 2007). It is believed that the design principle
of Zhang's seismoscope and early modern seismograph are based on the principle of inertia.