612 HISTORY OF GEOLOGY. 



such an increase may arise from this ; that the 

 earth, at some former period, passed (by the motion 

 of the solar system in the universe,) through a 

 portion of space which was warmer than the space 

 in which it now revolves (by reason, it may be, of 

 the heat of other stars to which it was then nearer). 

 He supposes that, since such a period, the surface 

 has cooled down by the influence of the surround- 

 ing circumstances ; while the interior, for a certain 

 unknown depth, retains the trace of the former 

 elevation of temperature. But this assumption is 

 not likely to expel the belief in the terrestrial origin 

 of the subterraneous heat. For the supposition of 

 such an inequality in the temperature of the dif- 

 ferent regions in which the solar system is placed 

 at different times, is altogether arbitrary; and, if 

 pushed to the amount to which it must be carried, 

 in order to account for the phenomenon, is highly 

 improbable 7 . The doctrine of central heat, on the 

 other hand, (which need not be conceived as imply- 

 ing the universal fluidity of the mass,) is not only 



7 For this hypothesis would make it necessary to suppose 

 that the earth has, at some former period, derived from some 

 other star or stars more heat than she now derives from the sun. 

 But this would imply, as highly probable, that at some period 

 some other star or stars must have produced also a mechanical 

 effect upon the solar system, greater than the effect of the sun. 

 Now such a past operation of forces, fitted to obliterate all order 

 and symmetry, is quite inconsistent with the simple, regular, 

 and symmetrical relation which the whole solar system, as far 

 as Uranus, bears to the present central body. 



