The historical evolution of seismographs

The oldest instrument used to detect the occurrence of an earthquake was probably constructed in China during the second century AD and is attributed to the philosopher Chian-hen. This instrument consisted in a bronze figure of a dragon with eight heads in whose mouths there were eight balls. Inside the figure there was some kind of pendular device that pushed the balls and made them fall when it was shaken by an earthquake. The figure was oriented in the geographic directions so that, upon the arrival of seismic waves, the corresponding ball will fall and show the occurrence and orientation of a shock. In Europe, the first instrument was a mercury seismoscope designed by De Haute-Feuille in 1703, consisting in a vessel with mercury connected by eight channels to eight cavities. Earthquakes will make the mercury flow into one or several of the cavities, indicating their orientations and sizes (quantities of mercury spilled). It is not certain that the instrument was actually built, although we have a description of it, but similar instruments were built in 1784 by Cavalli and in 1818 by Cacciatori (Ferrari, 1992). Vertical and horizontal pendulums started to be used around 1750. These instruments have an alarm to indicate the occurrence of an earthquake or a stylus attached to the mass that left a mark on sand or smoked plate of glass in which case they are called seismoscopes.