Zhang Heng (c79 – 139CE)
I have never experienced an earthquake and have no desire to for that matter but it has always surprised me that seismology still appears to be a fairly inexact science. How many lives and how much property would be saved if we were given sufficient warning of an earth tremor?
Zhang Heng, a Chinese inventor, astronomer and polymath, applied his not inconsiderable grey cells to the problem of earthquake detection and in 132CE came up with a design for a seismograph, the granddaddy of all attempts to predict an impending earthquake. The starting point was his theory that the chief cause of an earthquake was wind and air. When air is agitated and trapped with no opportunity to escape “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”.
According to the 5th century Book of Later Han, his copper urn-shaped device was able to detect the direction of an earthquake hundreds of miles away, information essential for the Han dynasty to send aid and relief to the areas affected. So how did it work?
The urn was cast with eight dragons positioned around it to indicate the principal points of the compass. On the ground directly underneath each of the dragons’ heads was a copper toad, raising their heads and opening their mouths in anticipation. The interior of the urn was ingeniously constructed using mobile arms, cranks and catch mechanisms. When an earthquake occurred, the dragon facing that direction would open its mouth and the ball suspended within would drop into the waiting toad’s mouth, making it clear the direction of the tremor.
One day in 138CE the machine sprang into action, a ball dropping into the mouth of the toad representing the westerly direction. As anticipated, an earthquake had struck Longxi, what is now Western Ganxu Province, some one thousand kilometres away from the site of the seismograph. It took the Occidentals some 1,700 years to repeat the trick, John Milne inventing the horizontal pendulum seismograph in 1880.
Although the Greek, Erastothenes, had invented the first armillary sphere, an astronomical instrument representing the celestial sphere, the Chinese had developed it further by inserting firstly, a fixed equatorial ring and then an ecliptic ring and then, horizon and meridian rings. Heng’s contribution was the last addition but he hadn’t finished there. His masterstroke was to apply hydraulic motive power by using a waterwheel to rotate the sphere. In turn the waterwheel was powered by the constant water pressure in a water clock tank. So Heng became the first man to demonstrably use water in this way. It was revolutionary (in more senses than one) and went on to influence the water-powered instruments of later Chinese astronomers.
There is nothing new under the sun, it seems.