52 MODERN SEISMOLOGY 
ment is entirely horizontal and at right angles to the direction 
of propagation, and that there follows the maximum move- 
ment in which there is horizontal movement in the direction 
of propagation along with vertical motion. This is only very 
roughly true. The seismogram reproduced, Plate 11, is a case 
in which the first portion of the long wave phase gives horizon- 
tal motion in the direction of propagation, while in the follow- 
ing maximal phase the horizontal motion is at right angles to 
the direction of propagation. What shall we say of cases 
where horizontal motion transverse to the direction of propa- 
gation is associated with pronounced vertical motion, or 
where horizontal motion in the direction of propagation occurs 
with little or no vertical motion? 
No combination of transverse waves of purely horizontal 
displacement (velocity V,) and of Rayleigh waves (velocity 
‘92V,) will explain these facts, which, it appears to me, can only 
be met by supposing that the long wave phase is complicated by 
effects arising from reflexion backwards and forwards between 
the Earth’s surface and a layer of discontinuity at some depth. 
Wiechert (“ Ueber Erdbebenwellen,” l.c.) introduced the 
hypothesis of such a crust resting on a sheet of plastic material 
(magma). So far as such a crust provides by its natural 
vibration a means of explaining the dominant period of the 
long waves (say 20 seconds) we may agree; although the 
argument that the thickness of the layer is half the wave 
length of the dominant waves, and thus about 35 km., hardly 
applies to Rayleigh waves; 40 km., however, as the half wave 
length of purely transversal waves travelling across the layer 
would give the 20 seconds period, and also about 12 seconds 
for longitudinal waves travelling across the layer. But the as- 
sumption ofa plastic sheet, which would hardly be accepted 
on astromonical grounds, would not serve to contain the long 
waves within the layer without at the same time confining the 
first and second phase movements, which we have to admit: 
penetrate the whole Earth. 
At present we know nothing as to whether these long 
waves diminish in amplitude as the depth increases, nor does 
it appear to me necessary to suppose that they do not pene- 
