Quoting from [cindycindy]:
According to the History of Later Han Dynasty (25-220 AD), Zhang Heng's seismograph was an urn-like instrument with a central pendulum. An earth tremor would cause the pendulum to loose balance and activate a set of levers inside.

       No doubt this Ancient Seismometer is a great invention by the Chinese mathematician ahead of his time. We are not actually going backwards in technology or stagnate in our scientific discoveries, as implied by some postings above.


In fact the measurement of Earth's tremors, vibrations and velocity are very much in the advanced stage as compared to the Han Dynasty seismograph.

Richter scale was introduced as the measure for the magnitude of the highest peak in the earth's movement and it is in the Logarithmic scale to base 10.


For an earthquake with a magnitude of 2.5, nobody can feel it, almost like a big truck passing through the neighbourhood main road. But a magnitude of 7 is like the explosion of 6 million tons of TNT on the ground.


How does the scientist measure these vibrations if they do not have the Seismic Instruments? Companies make these instruments for the Oil and Gas Industries, in the exploration, the underground layers of fossilized hydrocarbons.


The early 1900s, had a simple Seismic instrument like a steel ball attached to a string which is tied on top of a support. Below the ball, there is a needle and pen to trace movements of the ball on piece of paper. The entire pendulum is enclosed in a glass case, where the wind will not affect its movement.


However, the time of the movements, should they occur is not recorded, and it has to take many pages of the recorder paper before detecting an earthquake. The earlier devices are fully mechanical, with no batteries or electricity supplied. Until the age of the scientific measurement and controls, of the 20th century, we see a surge of companies researching into this Geophysical measuring instruments. Then the 3 axis vibration monitor, measuring X, Y and Z directions. Somehow short of the Dragon - Frog Seismograph, with eight axis. They simply increase the number of sensors to cover the 8 directions. Another method is to have the instruments at two locations, by criss crossing the directions of the signals, they can trace the origin of the earth movement. Not only the direction, but also the distance.


Dicky