200 GEOPHYSICAL THEORY UNDER THE PLANETESIMAL HYPOTHESIS. 



Table 7. 



The zone of rising temperatures vanishes at epoch 29.7 T, correspond- 

 ing to the maximum surface gradient of 1° in about 180 meters, about one- 

 sixth of that obtained from observations at the present time. The great- 

 est total rise of temperature occurs at depth about 480 miles, where it is 

 2,200°, with initial temperature 2,340°. 



As compared with the former case sketched above, the results of the 

 present assumptions, though giving considerably larger primitive temper- 

 atures, show a close qualitative similarity in the thermal changes. But the 

 zone of rising temperatures is at first somewhat deeper, the temperature- 

 increments within it are greater, and the period of its existence longer. This 

 is to be ascribed largely to the influence of the second component of the 

 series, whose coefficient is here large, while in the former case it is relatively 

 trivial. In both cases the conductivity has been treated as constant. It has 

 often been supposed, however, that it would probably increase considerably 

 with increase of density. The effect of this would be to facilitate the trans- 

 fer of heat from the central parts to the outer zone, presumably increasing 

 the total temperature-increments there, perhaps shortening the time dur- 

 ing which the temperature rises. It would be likely, also, to increase the 

 value of the maximum surface-gradient, but whether this could be brought 

 up to the present observed value through any reasonable assumption of 

 this character remains to be determined. 



The magnitude of the thermal changes in the zone of rising tempera- 

 tures, resulting from the arbitrary special conditions developed above, seems 

 to make it probable that the existence of this zone should be considered an 

 essential feature of the thermal process under the planetesimal hypothesis. 



