THE CAUSES OF EARTHQUAKES 



how do we define " small " earthquakes ? A small earth- 

 quake is one such as we have described as taking place in 

 the valley of the Mississippi, and which, even thougn'it 

 may produce considerable disturbance in its neighbour- 

 hood, is not perceptible at any great distance away. A 

 great earthquake is one which sends its vibrations thou- 

 sands of miles. A very large earthquake, originating in 

 any part of the world, may be recorded in any other part 

 of the globe. Although only a few people in Great 

 Britain have been privileged to feel a home-made earth 

 tremor, every one of us is very many times a year moved 

 by earthquakes. We do not perceive them because the 

 back-and-forth motion of the ground is performed too 

 slowly, while if there is a movement of the ground the 

 undulations are so very flat that they cannot be per- 

 ceived. But at several places in England and at earth- 

 quake observatories (seismological stations) all over the 

 world, from Japan to Australia and from South 

 Africa to Greenland, instruments are set up which are 

 sensitive enough to record these tremors, though not 

 always to locate them. Sometimes when Professor 

 Behar in Germany, or Mr. Milne from his observa- 

 tory in the Isle of Wight, telegraphs to the news- 

 papers that signs of a great earthquake have appeared 

 on their instruments, the world hears no more of 

 these disturbances. They have occurred we are certain, 

 but the place where the great cataclysm which has thus 

 shaken the whole round world took place has been for- 

 tunately remote from inhabited portions of the earth, and 

 has very likely been beneath the waters of some ocean. 



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