Source : Webster's Revised Unabridged Dictionary (1913)

Seismograph \Seis"mo*graph\, n. [Gr. ??? an earthquake +
   -graph.] (Physics)
   An apparatus for registering the shocks and undulatory
   motions of earthquakes.

Source : WordNet®

seismograph
     n : a measuring instrument for detecting and measuring the
         intensity and direction and duration of movements of the
         ground (as an earthquake)