44 EARTHQUAKES. 



To obtain a true idea of earthquake motion is a matter 

 of cardinal importance, as it forms the key-stone of many 

 investigations. 



If we know the nature of the motion produced by an 

 earthquake, we are aided in tracking it to its origin, and 

 in reasoning as to how it was produced. If our knowledge 

 of the nature of the motion of an earthquake is incorrect, 

 it will be impossible for ns intelligently to construct 

 buildings to withstand the effects of these disturbances. 

 We have thus to consider, in this portion of seismology, a 

 point of great scientific importance, and shall deal with it 

 at some length. 



Nature of Elastic Waves and Vibrations. — When it is 

 stated that an earthquake consists of elastic waves of 

 compression and distortion, the student of physics has 

 a clear idea of what is meant and a knowledge of the 

 mechanical laws which govern such disturbances. The 

 ordinary reader, however, and the majority of the in- 

 habitants of earthquake countries, who of all people have 

 the greatest interest in this matter, may not have so clear 

 a conception, and it will, therefore, not be out of place to 

 give some general explanation on this point. 



The ordinary idea of a wave is that it is a disturbance 

 similar to that which we often see in water. Waves like 

 these must not, however, be confounded with elastic waves. 

 A disturbance produced in water, say, for instance, by 

 dropping a stone into a pond, is propagated outwards by the 

 action of gravity. First, a ridge of water is raised up by 

 the stone passing beneath the surface. As this ridge falls 

 towards its normal position in virtue of its weight, it 

 raises a second ridge. This second ridge raises a third 

 ridge, and so on. The water moves vertically up and down, 

 whilst the wave itself is propagated horizontally. 



To understand what is meant by elastic waves, it is 



