The Interior of the Earth 



As soon as the ground shakes the pendulum 

 swings and traces on a sheet of paper fixed 

 underneath a line of varying amplitude in the 

 direction of the motion. 



These undulations come from afar. At times 

 they seem to emanate from a single centre, at 

 others to start from all points of a certain line ; 

 again the shocks originate in a more or less 

 extensive area, from which they are propagated, 

 gradually losing strength as they proceed. We 

 shall only consider the simpler case in which the 

 waves derive from a single centre. 



Suppose such a disturbance to take place in 

 a country where the internal structure of the 

 ground is as uniform as possible. At the first 

 shock all clocks stop, and record in this way 

 at every point of a vast territory the precise 

 instant when the ground began to shake. One 

 of all these clocks will be found to have stopped 

 first. The spot which it occupied is called the 

 epicentre of the earthquake, that is to say, the 

 original shock occurred underneath this point at 

 a distance which remains to be determined. 

 Now the records of the various clocks disclose 

 the points reached by the seismic wave one, two, 

 H 133 



