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AI^NUAL REPORT SMITHSONIAN INSTITUTION, 19 39 



exists a "blind zone" as shown in figure 9. No single part of the 

 earth structure defined by seismology is quite so certain as the 

 presence of the core. The blind zone is very clearly demarcated. 

 It is possible to read off directly from a velocity-depth curve, such 

 as that of figure 8, the depth to the core. The fact that the blind 

 zone begins abruptly shows the surface of discontinuity to be 

 sharply defined. 



If a mathematician be given the dimensions of the earth, the 

 structure of the crust, the variation of velocity in the mantle, and 



Herd 



Stismomttir- 

 S^a^ion 



Longi^udinalwell^ -■ — --Transversalwelle 



FiGURH 10. — Some typical complex earthquake rays from the focus (Herd). The subscript 

 4 Indicates the passage of the so-called fourth surface of discontinuity within the earth 

 (Unstetlgkeitsflilche 4), which marks the boundary of the core. 



the depth to the core, he can compute the time it should take various 

 waves, reflected within the earth, to reach the surface, and the 

 seismograms may be searched to see if they confirm the work which 

 gave us the velocity-depth curve — not only the reflected P waves, 

 but the S waves which we have nearly forgotten; for, as we got 

 a velocity-depth curve for the P waves, so we can get one for the 

 S waves. Furthermore, when an elastic wave either of F or ^iS type 

 is reflected or refracted it generates other waves of both types. Thus 

 we may have a P wave reflected at the under side of the surface as 



