INTERPRETATION OF SEISMOGRAMS 53 
trate beneath the crust. What we do know is that there is 
a shell of radiation spreading from the focus, within which 
there is disturbance and beyond which there is none. 
In this connexion it is worth while to remember that the 
long waves in a seismogram suggest an importance out of all 
proportion to their physical effect. For example in the 
Galitzin Seismograph (primary period 24°) we should have to 
divide the apparent amplitude of a vibration 20° period by 
about 8 in order to compare with the apparent amplitude of a 
vibration of 1° period, and if further we remember that to 
compare the accelerating effects we should have to divide 
again by 400, we find that the long waves dwindle very much 
in their physical importance. 
This entirely agrees with Wiechert’s remark that the long- 
wave phase, interesting as it is, is a residual phenomenon, 
Neverthless the elucidation of the Long-wave phase and the 
Coda is highly important on account of the information it 
promises to afford as to the crust of the Earth, and here it 
seems probable that seismic dispersion may play a very im- 
portant part. 
We shall next suppose that the times of incidence of P, S, 
and L have been determined at the station for a well-defined 
earthquake, and that similar determinations have been made 
at a number of stations distributed over the earth. Further, 
we shall suppose that by one or other of the methods to be 
described in the next chapter, the position of the focus and 
the time of occurrence has been ascertained. We are then in 
a position to set out ona diagram the time taken for P, S, and 
L to travel from the focus as a function of Mand. The 
curve so obtained may be called a time curve (Laufzeit kurve). 
For theoretical purposes it is, however, convenient to correct 
the curve to what we should have got had % been o,:and we 
then obtain a curve expressed by T=/(4). The general 
character of the mean results so obtained by Zoppritz and 
Geiger from several well-defined earthquakes (Gott. Nach., 
1907) are shown in Plate 5, and the values obtained by inter- 
polation are given in the table, p. 54. 
