228 



SCIENCE 



[N. S. Vol. XXVII. No. 



earth ■would be unstable and this instability 

 would only just disappear at 24 million years. 

 I am obliged to conclude that if an earth 

 could cool in this way — if the crust could be 

 prevented from breaking — the 24-million-year 

 earth would only just have reached the " con- 

 sistentior status " or the epoch of solidity. 



The real earth, however, has been in a con- 

 dition of tidal stability at least since the 

 beginning of the Cambrian. For the strata 

 are full of ripple marks, sands and pebbles 

 rearranged by tidal currents, beach footprints 

 and similar evidence of tides. Now oceanic 

 tides would not exist upon a tidally unstable 

 earth and therefore the consistentior status 

 occurred long ago. It was the remoteness of 

 this epoch which Kelvin attempted to cal- 

 culate. 



King gives data for only one earth which is 

 satisfactory from this point of view. It had 

 an initial temperature of 1,230° C. and reached 

 a surface gradient of 1° F. in 50.6 feet in 

 10 million years. It was solid almost from 

 the beginning. But apart from the excessive 

 brevity of the age, it seems to me that this 

 earth must likewise be rejected. The tempera- 

 ture was insufficient to melt even diabase a few 

 miles below the surface, much less andesites 

 and rhyolites, while there is a mass of well- 

 known evidence that the earth has been fluid 

 at least to depths of many miles from its 

 growing surface. This is shown by the gen- 

 eral dependence of gravity on latitude, the 

 nearly spheroidal shape of the earth, the ob- 

 lateness of the interior layei-s of equal density 

 and the fact demonstrated by Kelvin,* Roche" 

 and Wiechert" that a nucleus of constant high 

 density (approximately the density of iron) 

 surrounded by a shell of much smaller density 

 (near 3), will satisfy the observations on pre- 

 cession, ratio of surface density to mean den- 

 sity and the elliptieity of sea level. 



Considering the materials of which the 

 earth is composed and the high pressures 

 which must have existed at some distance 



• " Natural Philosophy," Pt. II., p. 420. This 

 article also appeared in the first edition of the 

 "Natural Philosophy," 1867. 



° MdTO. Acad. Montpellier, 1882. 



" Gottingen Naohrichten, 1897, p. 221. 



from the surface at any stage of the earth's 

 growth, it seems clear that very high tempera- 

 tures must have prevailed within its mass, 

 while for the reasons stated above tidal insta- 

 bility at any epoch since the ocean came into 

 existence, is inadmissible. Hence the hypoth- 

 esis of a constant initial temperature will not 

 satisfy the conditions. 



The question thus arises whether the initial 

 temperature may be supposed to have been 

 graduated in such a manner as to satisfy 

 known conditions. I believe that this ques- 

 tion may be answered affirmatively. Our 

 great master in geophysics himself contem- 

 plated a very different distribution of tem- 

 perature from the uniformity assumed in his 

 equations. The earth, he said, " did in all 

 probability become solid at its melting tem- 

 perature all through or all through the outer 

 layer " ; " convective equilibrium of tempera- 

 ture must have been approximately fulfilled 

 until solidification commenced " and " the 

 temperature of solidification will, at great 

 depths, because of the great pressure there, 

 be higher than at the surface if the fluid con- 

 tracts ... in becoming solid." 



If the initial temperature at the consisten- 

 tior status increased with distance from the 

 surface, it was probably according to some 

 complex law, intimately related to that of 

 convective equilibrium, but the thickness 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 re- 

 mainder of the earth are of no consequence. 

 The inner part, with a radius of say 3,840 

 miles, may have been originally at the tem- 

 perature of ice or of the electric arc; it may 

 conduct heat as well as silver or as iU as 

 magnesia; in any case the influence on the 

 outer surface would be insensible even after 

 scores of millions of years. Now, though the 

 temperature at the consistentior status did 

 vary with distance from the surface according 

 to a highly complex law, it is altogether prob- 

 able that for so short a distance as 2 per cent. 



