Zhang Heng (78-139), Chinese scientist, examining his seismoscope. Zhang was born in Nanjing, China. As an astronomer he saw that the source of the Moon's light was sunlight and as a mathematician he calculated pi as 3.146. However, it was in the field of geophysics that Zhang made his biggest contribution. In 132 he invented the first seismoscope for measuring earthquakes (seen here). The instrument was two metres in diameter, with metal balls balanced in each of the eight dragon's mouths. Strong seismic tremors caused a ball to be released from the mouth of a dragon into the mouth of the toad below. The position of the ball falling indicated the direction of the shock wave.