48o T. C. CHAMBERLIN 



The table would be additionally helpful if the central pressures, 

 the central densities, and the central temperatures could each be 

 given in terms equally trustworthy, but determinations of these 

 properties rest on a much less secure basis. The central pressures 

 can only be determined by assuming some law of downward increase 

 of internal density. The actual rate of such increase is uncertain, 

 beyond the fact that it must fall within certain rather broad limits 

 defined by precession and other astronomical phenomena whose 

 requirements are not precisely determinable. Laplace's theoretical 

 law of density is perhaps the most plausible and is the one commonly 

 used in preference to such others as have been proposed. Using 

 it, MacMillan finds the central pressure of the lo-mile body, when 

 assigned a mean density 3 . 30, to be only 11 . 8 lbs. per square inch, 

 i.e., less than the pressure of the earth's atmosphere. On the 

 other hand, that of the present earth is 22,500 tons per square inch, 

 or about 3,000,000 atmospheres. The results given by Laplace's 

 law are in general accord with those obtained earlier in this dis- 

 cussion from a comparison of the moon, Mars, Venus, and the 

 earth.^ However, reserve in placing implicit confidence in this law 

 is to be observed, for by carrying the series of determinations 

 upward from the earth on the same basis, MacMillan finds that at a 

 radius of about 5,000 miles the central density becomes infinite. 

 This seems to mean either that the law breaks down or else is 

 rendered inapplicable by some intercurrent factor whose nature is 

 as yet unknown. Dr. A. C. Lunn reached results of similar import 

 in his geophysical studies under the planetesimal hypothesis in 

 1909.^ The suggestive correlation of the densities of the whole 

 series of planets made by MacMillan in his paper "On Stellar 

 Evolution" deserves thoughtful consideration in this connection.^ 



It is clear, then, that until some elucidation is found for this 

 singular result so shortly reached after the dimensions of the 

 earth are passed, it is unsafe to build important conclusions upon 

 the law. 



I "The Order of Magnitude of the Shrinkage of the Earth Deduced from Mars, 

 Venus, and the Moon," Jour. GeoL, Vol. XXVIII (1920), pp. 1-17. 



' "The Tidal and Other Problems," Carnegie Publication No. 107 (1909), pp. 201-2. 

 3 Astrophys. Jour., Vol. XL VIII (July, 1918), pp. 36-40. 



