50 
MR. GEORGE W. WALKER ON THE PROBLEM OF 
on the seismogram, are now totally separate phenomena for a deep focus. Fig. 5 shows 
that PS camiot occur imtil A is 149 degrees, and beyond this there are two PS's for any 
given A. There is no PS 2 until A exceeds 180 degrees, and we do not pursue it. The 
times of PS are included in fig. 1. Fig. 4 explains the position as regards SP . . . SP x . 
We cannot get an SP wave at all until A — 11 degrees, and thereafter it may continue 
to be shown up to 180 degrees, the point at which reflexion takes place approaching the 
