The present paper examines the problem of the earth's density variation and includes a quantitative discussion of the errors likely to be involved. Figures are given for the density distribution of the earth's outer mantle, and it is shown that all these are probably accurate within about 0.05 gm/cm.3 On the figures presented, the density in the earth's mantle ranges from 3.32 gm/cm.3 at the base of the crustal layers to 5.68 gm/cm.3 at the base of the mantle. The mean density of the central core is shown to be 10.7 gm/cm.3 within an error of order 0.1 gm/cm.3 Application of equations derived by Birch from Murnaghan's theory of finite strain indicates agreement with the density figures found, but it is to be noted that the rigidity of the lower portion of the earth's mantle appears to increase more slowly with increase of depth than would be consistent with the equations of Birch. Values which are expected to need very little future amendment are also given for the pressure and gravitational attraction within the earth's mantle.

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