First Seismometer--An Earthquake Detector

##########Chinese astronomer, a brilliant mathematician and a scientist in the Han Dynasty Chan Heng(AD 78-139) invented an earth quake detector in 132 AD.

Necessity behind invention:##########The ancient Chinese believed seismic events, with their death and destruction, were important signs from heaven. So it was important to the Han Court be alerted to earthquakes occurring anywhere in the kingdom. A correct call wins one immortal fame, as in Chan Heng’s case, while disgrace and punishment awaits a wrong call.

Description:##########Chan Heng’s original seismometer was lost. Luckily, its description survived. It was a cast bronze vessel with a domed lid, resembling a wine jar. The working innards, of "toothed machinery and ingenious constructions," were hidden. The surface of the vessel was decorated with motifs of mountains, tortoises, bi

rds, animals and antique writing. Around the vessel were eight dragons(representing eight directions) whose mouths held a carefully balanced ball. Round the base of the vessel sat eight corresponding metal toads with their mouths open, looking upwards.When the first tremors of an earthquake made the device vibrate, the balls fell into the frogs' mouths, making a noise to warn the upcoming danger.The direction of the earthquake is indicated by the position of the toad which swallowed the ball. This instrument was most remarkable.

History says:##########On one occasion one of the dragons let fall a ball from its mouth though no perceptible shock could be felt. All the scholars at the capital were astonished at this strange effect occurring without any evidence of an earthquake to cause it. But several days later a messenger arrived bringing news of an earthquake in Lung-Hsi (400 miles away). Upon this everyone admitted the mysterious power of the instrument.
Principle behind:##########The workings of Chan Heng’s seismometer were never revealed. Most experts agree however, that it worked on the principle of inertia. A mass is suspended. An earthquake shakes the vessel, causing a slight displacement between the mass and the vessel. This movement is transmitted via levers and gears to push out a ball.
Amusements behind the design:##########In the design of any instrument, the goal is to make the instrument sensitive to the desired signal while simultaneously rejecting false signals. For a pendulum, this optimal point is at the resonance. Model seismometer designed based on the description of Chan Heng is intended as a demonstrator. To test its efficiency a man shakes the table to simulate an earthquake. So our seismometer’s pendulum is "tuned" to this input(a false signal). The crust of the earth absorbs the high frequency content of a quake. The signal from a distant earthquake is in the sub-audio range. So to detect actual earthquakes the pendulum would need to be several feet in length.How can Chan Hengs seismometer detect an eartquake that occured 400 miles away!!!!.
##########Indeed, we read that Chan Heng’s seismometer was described as having a diameter of "eight chhih"! (A little more than 6 ft.) His device appears to be -- incredibly -- "scientifically designed!" In 132 A.D., did he "really know" what we know today? Or did he hit on it by pure luck? Earthquakes occur so infrequently, how does one study it? Consider also the expense of making bronze castings the size of a horse in that age.
##########Another question you may want to ponder is why did Chang Heng take such pains to decorate his instrument with celestial motifs. What was the real purpose of these decorations? Do you think this helps the instrument to function better? Or did he hope this would make his device more believable?Why he selected bronze? If Chan Heng is reading this blog please give us the answer Sir.........