5Q EARTHQUAKES. 



ous. This approaches more nearly to the actual motions 

 we feel as earthquakes. 



If we can imagine the ground shaken by an earth- 

 quake, made of a transparent material which transmitted 

 less light when compressed, and we could look down upon 

 a long extent of this at the time of an earthquake, we 

 should see a series of dark bands indicating strips of 

 country which were compressed. The distances between 

 these bands might be irregular. Keeping our attention 

 on one particular band, this would be seen to travel 

 forward in a direction from the source. If we kept our 

 eye on one particular point, it would appear to open and 

 shut, becoming light and dark alternately. 



As to the existence of these elastic waves in actual 

 earthquakes we have no direct experimental evidence. 

 The only kind of wave with which we are familiar is a true 

 surface undulation, which, although having the appearance 

 of a water-wave, may nevertheless represent a district of 

 compression. 



