THEORY OF SEISMIC PROSPECTING 37 
a 
xe 
= — \ 
a-3s oe \ 
I re \fe 
. — : 
EEN 
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/ 
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| 
! 
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FIG. 2 
are parabolas. When # is greater than a the iso-time curves have two 
branches and are hyperbolas. 
As is well known (see Osgood and Grausteins’ Plane and Solid 
Analytical Geometry, 1927, p. 144) the foci can be constructed by 
281 
