STRUCTURE OF THE EARTH — HODGSON 



299 



an S wave, and so on. The number of such possible complex wave 

 paths is very great, and the times, once computed, may, as we have 

 said, be checked on seismograms. Some of the possible paths are 

 shown in figure 10. 



So, step by step, by cut and try, by modification of theory and data, 

 we have the means of probing the earth not only in the crust but in 

 the mantle down to a depth of about 1,800 miles. But we stand at 

 the barrier — the margin of a core of radius 2,200 miles. What can 

 we learn about the core? 



We can learn something of the nature of the material in the crust 

 and mantle by knowing the velocity with which earthquake waves 

 traverse them. We know from gravity measurements the total mass 



FiGtJBB 11. — Path of PKP-TB.y, sometimes designated PtPtP or PcPcP. 



of the earth. The mantle and crust do not nearly make up their 

 share of mass per volume (density). The core must be very heavy; 

 the density must be great. We can deduce the approximate average 

 density of the core. If we had a time- distance curve for the core 

 and knew the true velocity just inside its boundary, we could apply 

 the Herglotz-Wiechert method again to an earth stripped to the 

 core and find a velocity-depth curve for those greater depths. So 

 far no means has been found to do this. We know the total time for 

 a wave such as that shown in figure 11 to reach the surface. We 

 can compute the time required for the sections outside the core, and 

 we can find the end points of these branches at the core boundary. 

 We know thus the time required for a wave to traverse the core 



