NO. 6 AGE OF TPIE EARTH — BECKER 19 



mately the density of iron) surrounded by a shell of much smaller density 

 (near 3) will satisfy the observations on precession, ratio of surface 

 density to mean density and the ellipticity of sea level. 



Considering the materials of which the earth is composed and the high 

 pressures which must have existed at some distance from the surface at 

 any stage of the earth's growth, it seems clear that very high temperatures 

 must have prevailed within its mass, while for the reasons stated above, 

 tidal instability at any epoch since the ocean came into existence is inad- 

 missible. Hence the hypothesis of a constant initial temperature will not 

 satisfy the conditions. 



The question thus arises whether the initial temperature may be sup- 

 posed to have been graduated in such a manner as to satisfy known con- 

 ditions. I believe that this question may be answered affirmatively. 

 Kelvin himself contemplated a very different distribution of temperature 

 from the uniformity assumed in his equations. The earth, he said, " did 

 in all probability become solid at its melting temperature all through or 

 all through the outer layer"; " convective equilibrium of temperature 

 must have been approximately fulfilled until solidification commenced " ; 

 and " the temperature of solidification will, at gi^eat depths, because of the, 

 great pressure there, be higher than at the surface if the fluid contracts 

 .... in becoming solid." 



Modified Problem. 

 If the initial temperature at the consistentior status increased with 

 distance from the surface, it was probably according to some complex 

 law intimately related to that of convective equilibrium, but the thick- 

 ness of the shell which has been sensibly affected by cooling is very small. 

 At a distance of 80 miles below the surface the temperature is probably 

 now very near 99 per cent, of what it was at the consistentior status. 

 Hence if a layer double this thickness is considered, the conditions which 

 prevailed in the remainder of the earth are of no consequence. The 

 inner part, with a radius of say 3840 miles, may have been originally at 

 the temperature of ice or of the electric arc ; it may conduct heat as well as 

 silver or as ill as magnesia; in any case the influence on the outer surface 

 would be insensible even after scores of millions of years. N"ow, though 

 the temperature at the consistentior status did vary with distance from 

 the surface according to a highly complex law, it is altogether probable 

 that for so short a distance as 2 per cent, of the radius this law may be 

 adequately represented by a straight line. This would be comparable 

 with the superficial portion of Mr. Barus's curve, which is nearly recti- 

 linear. Mr. Barus's experiments indicated that the relation between pres- 

 sure and the melting point of diabase is a linear one, so that, had he 

 confined his conclusions to a shell whose thickness was small relatively to 



