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was improved, it was seen that this simple classification could 

 not be maintained. On the one hand it was found that the pre- 

 liminary tremors arrived at distant places at such times as 

 to indicate direct transmission through the body of the 

 Earth with a nearly constant velocity, whilst the larger waves 

 appeared to be transmitted over the surface of the Earth with 

 a smaller nearly constant velocity. Further, it was found that 

 both the preliminary tremors and the large waves were com- 

 posite. After the tremors have been going on during a few 

 minutes, a second series of tremors, showing certain charac- 

 teristic differences from the first, begin to be received, and the 

 result has been established that the movement is mainly longi- 

 tudinal in the first series, mainly transverse in the second. 

 Both series appear to travel through the body of the Earth with 

 nearly constant velocities. Again it has been found that the 

 large waves present a number of distinct phases, the most 

 important being an initial phase, in which the movement 'of 

 the ground is mainly horizontal and transverse to the direction 

 of propagation ; and a maximum phase, in which the horizontal 

 movement of the ground is mainly parallel to the direction of 

 propagation and is accompanied by considerable vertical move- 

 ment and a phase of subsidence. 



Concurrently with the accumulation of seismic records and 

 the classification of the types of movement which they disclose, 

 there has been a considerable development of the physico- 

 mathematical theory by means of which an account of such 

 movements can be rendered. The first step was the discovery 

 by Lord Rayleigh of a third type of waves. A disturbance 

 set up in a solid body spreads out in a composite wave, which 

 gradually resolves itself into two waves, one of compression, 

 the other of distortion, with a peculiar type of motion between 

 the two. When the front of a wave reaches a bounding surface 

 reflexion takes place ; the reflected waves are in general com- 

 posite at first and resolve themselves gradually into pairs of 

 waves of the two special types. The effect of a bounding 

 surface is therefore to produce changes which may disguise 

 the simplicity of the resolution into the two types ; the result 

 which Lord Rayleigh found was that disturbances emerging 

 at the surface give rise to a distinct class of waves, which travel 

 over the surface with a nearly constant velocity and never affect 

 appreciably the matter at any considerable depth beneath the 



