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The
dragons and frogs system
A Chinese scholar, Chang Heng, invented the seismoscope in AD
132. It had a very interesting appearance – a cylindrical shape with eight
dragonheads around its upper circumference. Each of the dragons was equipped
with a ball in its mouth. The ball was not decorative but functional. The lower circumference had eight frogs and each frog was positioned
directly under a dragonhead. At the occurrence of an earthquake the device would record
the event using this procedure- one ball would drop from a dragon’s mouth
and be trapped in the frog’s mouth directly underneath. This action would
be accompanied by a sound. Interesting though the device was, it was
restricted to showing that an earthquake had occurred. Later developments
brought forth devices that used water filled bowls and mercury filled cups.
But they too merely recorded the occurrence.
The
seismometer – a forerunner of the seismograph
The year 1855 saw an Italian scientist, Luigi Palmieri, devise a
seismometer which recorded the time of occurrence, the relative intensity of
the oscillation and even the duration of time for which the oscillations
occurred. His device had a clock that stopped when ground motion was
registered by the device. Thus the exact time of occurrence of the quake
would be registered.
Improvements on this device
led to the use of pendulums in seismometer. The year 1840 saw a
pendulum-based seismometer installed in Scotland. And the very next year, an
improved version with an inverted pendulum was installed.
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A severe earthquake in
Japan in 1880 prompted some British scientists Sir
James Alfred Ewing, Thomas Gray, and John
Milne to intensify their study of earthquakes. As a result
several devices were tried out, they laid the foundation for the seismograph
used today. Among them was the horizontal pendulum seismograph attributed to
John Milne. This device found widespread use and was later improved upon.
The Press-Ewing seismograph that is used throughout the world uses a
modified Milne type pendulum.
Other
uses of seismographs
Seismographs are also used to detect microseisms – small
oscillations of the ground, which may be of long duration. These microseisms
do not result in earthquakes. Seismographs can also be used for detecting
remote nuclear underground tests. A number of
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The seismograph |
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seismographs are spread in a particular
area to obtain as accurate readings as possible.
The detecting ability of
seismographs is also used for oil and gas exploration. Detailed geographical
studies and astronomical studies have also been made possible. The
seismograph is also used to measure the intensity of ground shocks that are
caused intentionally like blasts conducted in mines, quarries, and civil
work projects. Small portable
seismometers known as geophones are used as a safety device in mines. Their
recordings can warn miners of potential dangers like sudden rock blasts as a
result of earlier dynamite explosions. Seismologic measurements are taken at
various points in the event of an earthquake for the purpose of identifying
its epicenter. Accurate readings help people prepare for any resultant
hazards.
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