INTERPRETATION OF SEISMOGRAMS 61 
diode lon Vi Vz co 
km. km. km./sec. km./sec. Bere 
fo) o 7 fae7) 4°01 0°273 
1,000 100 7°60 4°24 0272 
I,500 200 8°0r 4°47 0°272 
1,800 300 8°42 4°70 0274 
2,200 400 8°83 4°93 0°272 
2,500 500 9°23 5°15 0°274 
2,800 600 9°62 5°37 0°273 
3,200 700 10°00 5°59 0°272 
3,500 800 10°37 5°80 ae 
3,700 goo 10°73 6°00 Bons 
4,000 1,000 II°07 6°21 0270 
4,300 1,100 TI"43 6°41 0269 
4,500 1,200 II‘75 6°60 ara 
4,800 1,300 T2'08 6°80 
5,000 I,400 12°40 6°87 
5,300 I,500 et 2.72 6°87 
13,000 3,300 12°78 6°87 
leads to singularities in the time curve. In particular Wiechert 
shows that if there is a sudden increase of velocity, there will 
be a corresponding point on the time curve at which the slope 
changes suddenly. It would then really consist of two portions 
cutting at a definite angle and there would bea certain range 
within which the seismograms would show two sharp impulses. 
If on the other hand there is a sudden reduction of the velocity 
there will be a gap in the time curve corresponding to a range 
of distance not reached by the waves. 
In this way Wiechert in a recent investigation (Inter. Seis. 
Assoc. Manchester, 1911) concludes that there are such surfaces 
of discontinuity situated at depths of 1200, 1650, and 2450 km. ; 
but I am not aware that any numbers have been published 
showing what change this makes in the table of velocities 
derived from his former investigation. He further concludes 
that for depths greater than 3000 km. the velocities diminish 
gradually (see Geiger and Gutenberg, Gott. Nach., 1912). 
Interesting as Wiechert’s results are, they must be regarded 
as indicating the manner in which Seismology may be expected 
to throw light on the nature of the interior of Earth, rather 
than as results of great accuracy. Very slight changes in the 
slope of the time curve would lead to very considerable changes 
in the inferences ; and in this respect it appears to me that we 
still require an analytical method which depends on the original 
