ON SEISMOLOGICAL INVESTIGATION. 229 
travelled more quickly than 1, whilst the latter have travelled more 
slowly than 1’, it is difficult to recognise the latter as echoes of the former. 
Beyond 5/ the vibrations suddenly become small, but they apparently 
show such a marked repetition in form and uniformity in their time of 
recurrence that these characteristics can hardly be the result of accident. 
To facilitate comparison these have been enlarged, and are here reproduced, 
the later group being placed beneath those which arrived earlier. (Fig. 4.) 
The triangularly-headed echo 2’ is not unlike 2 ; its spherically formed 
successor 3 is repeated in 3’ ; and so we may continue through the series 
until we reach the gourd formed 9 and 10 reflected in corresponding shape 
by 9/ and 10’. 
The time intervals between these corresponding groups are from twenty- 
Fig. 5. 
eight to thirty-one minutes. We here appear to be dealing with a 
series of vibrational groups each of which took almost exactly half an 
hour to travel to and fro between two reflecting surfaces or districts. If 
the waves were compressional in character the distance between these 
surfaces would be about 8,000 kms., but if they travelled with the velocity 
Fia. 4. 
of the waves of shock this distance would be reduced to something under 
3,000 kms. From their period and amplitude it is probable that the 
distance lies between these values. 
The main point at issue, and the one to be answered before we enter 
into further speculations, is whether seismograms showing this musical-like 
repetition can be interpreted in the manner here suggested. The con- 
eluding vibrations of an earthquake have usually been regarded as a 
disorderly mob of pulsatory movements resulting from spasmodic impulses 
which gradually grew feebler as the activity at a seismic centre became 
exhausted. The question before us is whether an earthquake dies by a 
process analogous to repeated and irregular settlements of disjointed 
materials, or whether it is simply a blow or blows which come to an end 
