50 EARTHQUAKES. 



velocity will come about at the ends of each semi-oscilla- 

 tion ; and if v be the maximum velocity of the particle, 

 and a the amplitude or half semi-oscillation, then the 



maximum acceleration equals — . 



a 



Later on it will be shown, as the result of experiment, 

 that certain of the more important earth oscillations in 

 an earthcjuake are not simple harmonic motion. Never- 

 theless the above remarks will be of assistance in show- 

 ing how the velocity and other elements connected with 

 the motion of an earth particle, which are required by the 

 practical seismologist, may be calculated, irrespective of 

 assumptions as to the nature of the motion. 



Propagation of a Disturbance. — "We may next con- 

 sider the manner in which a disturbance, in which there 

 are both vibrations of compression and of distortion, is 

 propagated. The first or normal set of vibrations are pro- 

 pagated in a manner similar to that in which sound 

 vibrations are propagated. From a centre of disturbance 

 these movements approach an observer at a distant 

 station, so to speak, end on. The other vibrations have 

 a direction of motion similar to that which we believe to 

 exist in a ray of light. These would approach the ob- 

 server broadside on. 



If the disturbance passed through a formation like a 

 series of perfectly laminated slates, each of these two sets 

 of vibration might be subdivided, and we should then 

 obtain what Mallet has termed ordinary and extraordinary 

 normal and transverse vibrations. 



In consequence of the difference in the elastic forces 

 on which the propagation of these two kinds of vibration 

 depends, the normal vibrations are transmitted faster than 

 the transversal ones — that is to say, if an earthquake 

 originated from a blow, the first thing that would be 



