1898 - 99 .] Prof. Knott on Earthquake Vibrations. 583 
indirectly by its secondary effects in neighbouring regions, travel 
through the earth towards distant regions. Ko doubt also the 
quasi-elastic character of the larger vibrations referred to in my 
paper of 1888 will make the speed of propagation depend on the 
frequency. These considerations seem to me sufficient to account 
for the continuousness and the extension in time of the records at 
these distant localities. 
My original object in discussing the reflection and refraction of 
waves at the boundary of rock and water was to show that earth- 
quake vibrations, as then understood, could hardly be expected 
to retain their original characteristics after reflection and refrac- 
tion at several boundaries. The excessively ^complex character 
of the motion of a particle on the earth’s surface, when a seismic 
disturbance is passing, is demonstrated by all good seismograms, 
but is most completely demonstrated by Sekiya’s model, which was 
built up point by point by a laborious synthetic process from a 
seismogram giving the vertical and two horizontal components of 
motion (see Journal of the College of Science , Imperial University 
of Japan , vol. i. p. 361, 1881; also Nature , vol. 37, p. 297). 
In this particular instance, the complexity is largely conditioned 
by the character of the origin ; but even if we assume a compara- 
tively simple original form, reflections and refractions in the hetero- 
geneous crust of the earth must of necessity add complexity. 
But in Milne’s earthquake ‘ followers ’ we have the other aspect 
presented ; and, on the plausible assumption that we are dealing 
with reverberations or echoes, as they might be called, we are con- 
strained to postulate a comparative simplicity of structure through- 
out the greater part of the course of the waves. Where the solid 
nucleus of the earth passes into the somewhat plastic, and, as 
some believe, fluid magma on which the firm crust rests, the 
conditions may be favourable for reflection of a large part of the 
energy of the incident wave. It is probable that for some 
particular combinations of origin and observing locality the condi- 
tions for reflection and subsequent concentration of reflected rays 
are more favourable than for others, so that the repeating character 
of certain seismograms may be more marked than that of others. 
The elaborate and somewhat fascinating speculation of Bitter, 
that the earth has a gaseous nucleus highly heated and highly 
