Sir George Darwin, John Milne 179 



countries, a large number of suitable stations at which earth 

 tremors were accurately observed. The records of the obser- 

 vations, interpreted partly by Milne himself and partly by 

 other seismologists, proved to be of the highest interest. 

 The waves propagated through the earth from the centre 

 of a large disturbance are found to be noticeable with 

 delicate instruments all over the world. We now know that 

 the general movement spreads out from the centre of a 

 disturbance in three distinct waves, each propagated with its 

 own peculiar velocity. The first is a longitudinal wave, which 

 passes through the earth like a sound wave does through air. 

 The second is a transverse wave, arriving somewhat later; 

 both these waves reach us by transmission across the body 

 of the earth. A third set of waves, which in the records 

 appears as an oscillation of larger amplitude and longer 

 period than the rest, spreads over the surface of the earth 

 with a velocity of about 3-5 kilometres per second. The 

 interval between the arrival of these three types of waves 

 serves to indicate the distance of the centre of the dis- 

 turbance, and Prince Galitzin has shown how the direction 

 of the first impulse gives us the direction in which that 

 centre lies. Hence it is now possible to locate a distant 

 earthquake by means of observations taken at any one 

 place where it is still able to affect the delicate instruments 

 which, by a self-registering arrangement, are always ready 

 to record the waves. 



The scientific interest of the subject lies in the information 

 it is likely to yield on the internal constitution of the earth ; 

 for some of the waves that reach us, if the centre of dis- 

 turbance be far away, have passed through deep regions, 

 approaching in some cases the actual centre of the earth. 

 The manner in which their path bends round owing to 

 changes in the elastic properties of the earth at different 

 depths is indicated by the direction and magnitude of the 

 oscillation which the wave impresses on our instruments. 

 It is difficult to interpret completely the observed effect, 

 but the investigation has already advanced sufficiently to 

 show that important results may still be expected from that 

 study of earth tremors which Milne initiated. 



The survey of the history of British physical science has 



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