MODERN SEISMOLOGY — HODGSON 



359 



in crossing a discontinuity. They are reflected on reaching the sur- 

 face or at the inner side of the great discontinuity at 2,900 km 

 depth. Seismologists name the various phases appearing on their 

 records according to the paths they have probably taken. For ex- 

 ample ScPc PcS represents a wave which began as a transverse 

 vibration, traversed the discontinuity at the core (c) ; went on as 

 a P wave but was totally reflected at the inner face of the 2,900 km 

 discontinuity; proceeded as P; again traversed the core and com- 

 pleted its journey to the seismograph as a transverse wave. Figure 

 2 shows some of the waves going directly through the core, re- 

 fracted but not reflected. These are designated the P' waves. 



CnMTITIC lATEH 



/ BASALTIC U7ER 



DLTHABASIC ROCKS 



FiGuitE 4. — Diagram of the paths of propagation of the longitudinal waves 

 through the upper layers of the earth (after Gutenberg, see footnote 13) 



Where reflection takes place, bars above the letters bracket each 

 leg of the path, as indicated in the extended symbol above. 



Near the surface we have a multiplicity of refractions. Figure 4 

 shows some of the paths which have been suggested as possible. 



It will be seen that we have a very large number of wave arrivals 

 on our seismogram ; at least that we may expect many. As a matter 

 of fact, some arrive with such small energy content that they register 

 but faintly. This, nevertheless, affords a further check on the theory, 

 as that theory attempts to predict which waves should register thus. 

 The proposed structure may thus be checked in many ways by means 

 of longitudinal and transverse internal or body waves of earthquakes. 

 We have not mentioned that there are also two types, at least, of 

 surface waves. These also serve to throw light on the study of the 



