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ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 9 



of the various rates of travel on the subways concerned; for some 

 subway platforms are deeper than others and some are more difficult 

 of access. If you took off from the tabulated values the distances 

 traveled forward in going down the steps on entry and in ascending 

 on exit you would have new distance values in each case. If you 

 deducted the time spent in entering and leaving you would have new 

 time values in each case. Using these corrections to make adjust- 

 ments to the original timetables, you would have, in effect, stripped 

 off New York to the subway level. At least you would have developed 

 a subway-level table from a surface-level table. 



After a procedure somewhat analogous, we may assume we now 

 have a new and hypothetically perfected time-distance graph for an 

 earth stripped of its crust. We know that by applying the refraction 

 method we can obtain the time velocity just at the bottom of the 



FiGDEB 6. — Diagram illustrating the method of deducing the angle of emergence, e, of a 

 ray at any chosen cplcentral distance, EA. 



crust and in the outer edge of the mantle. We assume that, through- 

 out any part of a concentric spherical shell defined by any given 

 radial distance from the earth's center, the conditions are the same — 

 the true velocities are uniform over the whole shell. Thus, the 

 velocity deduced just below the shell is taken as uniform over the 

 entire earth. 



You will remember that it has been stated that we have recently 

 learned that this is not so. The true velocity at the outer edge 

 of the mantle under Timiskaming is greater than that for the outer 

 edge of the mantle in Japan. Very well ! we shall have to continue 

 this process of stripping the earth and adjusting our time-distance 

 curve to a still greater depth until our premised conditions are 

 satisfied. It will be sufficient if it is here made plain : That we can 

 use seismological methods to get a time-distance curve for an earth 



