EARTHQUAKES 



283 



Earthquake Waves. — When the earth is shaken by an earthquake 

 two sets of vibrations are started, one which follows the surface of 

 the earth and another which passes through it, the former traveling 

 more slowly than the latter, which passes through the 8000 miles 

 of the diameter of the earth in from 20 to 22 minutes. Earthquake 

 instruments (seismographs, p. 287) situated on the side of the earth 

 opposite an earthquake shock show three series of vibrations : (1) pre- 

 liminary tremblings, 20 to 22 minutes after the shock, followed by 

 (2) the strong vibrations of the principal shock, and finally by (3) a 



Vibrations not 

 far distant 



Vibrations at 

 epicentrum 



Fig. 281. — Diagram showing the path of earthquake waves and the vibrations 

 which they produce. (After Sieberg.) 



series of feeble vibrations (Fig. 281). Some of the waves are (1) 

 compressive or longitudinal and have the same nature as the vibra- 

 tions which travel through a liquid, and some are (2) transverse and 

 vibrate at right angles to the direction of the transmission of the 

 shock. Such waves as the latter can be propagated only in a solid. 

 The velocity of the earthquake waves which pass through the earth 

 is uniformly aboat 375 miles a minute, on the assumption that this 

 movement is along a straight line. This indicates a rigidity of the 

 earth's interior of one and a half times that of steel. The velocity of 

 the surface waves varies with the rock through which they pass, and 

 other conditions ; that of the earthquake at Naples in 1857 being nine 

 or 10 miles a minute, while in Germany in 1874 tne rate was 2 8 miles 



