EARTHQUAKE MOTION DISCUSSED THEORETICALLY. 4.5 



first necessary to understand what is meant by the term 

 elastic. In popular language the term elastic is confined 

 to substances like india-rubber, and but seldom to rock- 

 like materials, through which earthquake waves are 

 propagated. India-rubber is called elastic because after 

 we remove a compressive force it has a tendency to spring 

 back to its original shape. The elastic force of the india- 

 rubber is in this case the force which causes it to resist a 

 change of form. Now, a piece of rock may, up to a 

 certain point, like the india-rubber, be compressed, and 

 when the compressing force is removed it will also tend to 

 resume its original form. However, as the rock offers 

 more resistance to the compressing force than the india- 

 rubber offers, we say that it is the more elastic. It may 

 be here observed that a substance like granite offers great 

 resistance, not only to compression or a change of volume, 

 but also to a change of form or shape ; whereas a sub- 

 stance like air, which is also elastic, only offers resistance 

 to compression, but not to a change of shape. 



With these ideas before us we will now proceed to 

 consider how, after a body has been suddenly compressed 

 or distorted, this disturbance is propagated through the 

 mass. For the elastic body let us take a long spiral 

 spring hung from the ceiling of a room and kept slightly 

 stretched by a weight. If we give this weight an up- 

 ward tap from below, say with a hammer, we shall observe 

 a pulse-like wave which runs up the spring until it reaches 

 the ceiling of the room. Here it will, so to speak, re- 

 bound, like a billiard ball from the end of a table, and 

 run towards the weight from which it started. Whilst 

 this is going on we may also observe that the weight is 

 moving up and down. 



Here, then, we have two distinct things to observe — 

 one being the transmission of motion up to the ceiling. 



