EARTHQUAKES 



209 



not tin- movement of objects on the surface, which may be much 

 greater. A sudden shock with an amplitude of 5 or 6 millimeters 

 is sufficient to shatter a 

 chimney. 



Destructive effects. 

 The disastrous effects of 

 earthquake shocks result 

 from (i) the suddenness 

 and strength of rather 

 small vibrations of earth- 

 matter, and from (2) the 

 freedom of motion of the 

 bodies affected. The deep- 

 er rocks probably transmit 



seismic vibrations without Fig ' 2 8 ' Track of electric railway, between 



South San Francisco and San Bruno Point, 

 appreciable disruptive (Photo, by Moran.) 

 effect; but bodies at the 



surface are broken, overturned, and displaced. The tap of a ham- 

 mer sends an almost imperceptible vibration along the floor; but 

 this vibration would throw a glass ball beneath which it runs con- 

 siderably above the floor. Similarly the minute seismic vibrations 



Fig. 209 



i\i-\vave on the coast of Ceylon. (Sieberg.) 



