7. The first waves from a very distant earthquake come to us
directly through the Earth. The observed speeds of transmission
are the greater, in general, the more nearly the earthquake
origin is exactly on the opposite side of the Earth from the
observer; that is, the speeds of transmission are greater the
nearer the center of the Earth the waves pass. Now, we know
that the speeds are functions of the rigidity and density of
the materials traversed. The observed speeds require for their
explanation, so far as we can now see, that the rigidity of the
Earth"s central volume be much greater than that of steel, and
the rigidity of the Earth"s outer strata considerably less than
that of steel. Wiechert has shown that a core of radius 4,900
km. whose rigidity is somewhat greater than that of steel and
whose average density is 8.3, overlaid by an outer stony shell
of thickness 1,500 km. and average density 3.2, would satisfy
the observed facts as to the average density of the Earth, as
to the speeds of earthquake waves, as to the flattening of the
Earth,--assuming the concentric strata to be homogeneous in
themselves,--and as to the relative strengths of gravity at the
Poles and at the Equator. The dividing line, 1,500 km. below
the surface--1,600 km. would be just one fourth of the way from
the surface to the center--places a little over half the volume
in the outer shell and a little less than half in the core.
Wiechert did not mean that there must be a sudden change of
density at the depth of 1,500 km., with uniform density 8.3
below that surface and uniform density 3.2 above that surface.
The change of density is probably fairly continuous. It was
necessary in such a preliminary investigation to simplify the
assumptions. The observational data are not yet sufficiently
accurate to let us say what the law of increase in density and
rigidity is as we pass from the surface to the center.
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