EARTHQUAKES—BULLEN 325 
The behavior of P and S waves in the mantle reveals further that 
there is some measure of inhomogeneity in the mantle below the crust. 
In the outermost 700 miles of the subcrust the velocity gradients are 
too great to be compatible with a uniform chemical and physical state. 
The precise location of these changes is complicated by the fact that 
the steep velocity gradients cause the travel-time curves to bend back 
on themselves, and this makes for difficulty in identifying the cor- 
responding pulses on seismograms. The evidence shows that these 
changes of property may set in at a level as high as a hundred miles 
below the surface, and that they are complete at a depth of 700 miles 
or less. The changes may be due to variation in chemical composition 
or to a physical change of state brought about by the considerable 
pressure. 
From a depth of 700 miles to within 100 miles or so of the core, 
the P and S velocity gradients are very steady, and it is likely that 
the chemical composition is nearly uniform in this part of the earth. 
In the lowest 100 miles of the mantle the velocity gradients fall to 
nearly zero and suggest some departure from uniformity. 
Prior to 1936, the central core was thought to consist of nearly 
homogeneous molten iron and nickel. Then in 1936, a Danish seis- 
mologist, Miss I. Lehmann, looked more closely at records of two New 
Zealand earthquakes in the “shadow zone.” It was already known 
that the shadow zone is not a complete shadow—that there are rela- 
tively small observations of P waves in the zone. Before 1936, these 
observations had been attributed to various diffraction effects—to 
deviations from the ray theory in waves refracted by the core bound- 
ary. Similar diffraction effects are well known in the transmission 
of light. Miss Lehmann proposed, as an alternative explanation, that 
some of the waves observed in the shadow zone are really associated 
with rays that have passed into an inner core, inside the central core, 
the P velocity in the inner core being great enough to bend sharply 
upward certain of the rays which penetrate into it. 
Gutenberg and Richter showed that the travel-time data available 
by 1938 fitted Miss Lehmann’s hypothesis, and in 1939 the existence 
of the inner core came to be well established when Jeffreys showed 
that the competing hypothesis of diffraction could not explain the 
size of some of the observed waves in the shadow zone. Small though 
the waves were, some of them were too large to be accounted for by 
diffraction. Recently, Burke-Gaffney and I have been able, from a 
study of several hydrogen-bomb explosions of 1954, to show the 
separate existence both of Miss Lehmann’s waves, and of diffracted 
waves, on the same seismograms; this adds a further link to the chain 
of evidence for the existence of the central core. 
Thus the earth is further divided into what, for want of a better 
name, is called the outer core extending between depths of 1,800 and 
