9. Irregularities in the direction of the plumb-line and in the
force of gravity as observed widely and accurately over the
Earth"s surface indicate that the surface strata are very
irregular as to density. To harmonize the observed facts
Hayford has shown the need of assuming that the heterogeneous
conditions extend down to a depth of 122 km. from the surface.
Below that level the Earth"s concentric strata seem to be of
approximately uniform densities.
