INTERPRETATION OF SEISMOGRAMS 51 
interpretation of the first and second phases is that of the 
origin of the irregular movements that follow on P and S. 
These may in some measure arise from subsidiary shocks either 
at the primary focus or at other points, and I have pointed 
out that in a uniform earth we have a diffraction effect due to 
the surface. This in itself is, however, insufficient, and the 
facts obtain an obvious explanation in the multiple diffraction 
of the primary disturbance that must go on in the hetero- 
geneous mass of rock that constitutes the earth’s crust. There 
will thus be not only one principal, but also many subsidiary 
brachistochronic paths from the focus to the station. 
The suggestion that dispersion analogous to optical dis- 
persion may be called in to explain the asserted oscillatory 
movement in the first and second phases may be dismissed as 
not required, since heavily damped seismographs show that 
there is no general oscillation to explain, but only a highly 
irregular succession of impulses. The influence of dispersion 
is shown in the rounding of: turning-points, so that it is only 
a slightly modifying influence and not a determining cause. 
This argument is not affected by the minute vibrations of 
period about I second that sometimes appear after P on both 
Wiechert’s and Galitzin’s instruments. They are only shown 
when the earthquake is very great or the station sufficiently 
near the focus, and are thus accidental and not essential. 
Wiechert’s suggestion (see Wiechert and Zoppritz ‘‘ Ueber Erd- 
beben Wellen Gott. Nach.,” 1907) that they represent a natural 
vibration of a layer of rock seems to be the only explanation 
available. 
We have next to consider the long waves. We have 
already remarked that they are found by measurement to travel 
round the earth’s surface with a general speed agreeing closely 
with that of Rayleigh waves. But the long wave phase is a 
complex phenomenon, and the fact that the waves are strongly 
periodic (mainly 12-second and 20-second periods) presents 
considerable difficulty when we remember that the primary 
disturbance is an impulse. 
With regard to the long wave phase, it has been asserted 
that the first portion consists of waves in which the displace- 
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