10 Prof. W. Thomson on the Secular Cooling of the Earth. 



tained constant ever after I answer the first objection by saying 

 that if experimenters will find the latent heat of fusion, and the 

 variations of conductivity and specific heat of the earth's crust 

 up to its melting-point, it will be easy to modify the solution 

 given above, so as to make it applicable to the case of a liquid 

 globe gradually solidifying from without inwards, in consequence 

 of heat conducted through the solid crust to a cold external 

 medium. In the mean time we can see that this modification 

 will not make any considerable change in the resulting tempe- 

 rature of any point in the crust, unless the latent heat parted 

 with on solidification proves, contrary to what we may expect 

 from analogy, to be considerable in comparison with the heat 

 that an equal mass of the solid yields in cooling from the tem- 

 perature of solidification to the superficial temperature. But, 

 what is more to the purpose, it is to be remarked that the objec- 

 tion, plausible as it appears, is altogether fallacious, and that the 

 problem solved above corresponds much more closely in all pro- 

 bability with the actual history of the earth, than does the modi- 

 fied problem suggested by the objection. The earth, although 

 once all melted, or melted all round its surface, did in all proba- 

 bility really become a solid at its melting temperature all through, 

 or all through the outer layer, which had been melted ; and not 

 until the solidification was thus complete, or nearly so, did the 

 surface begin to cool. That this is the true view can scarcely 

 be doubted when the following arguments are considered. 



20. In the first place, we shall assume that at one time the 

 earth consisted of a solid nucleus, covered all round with a very 

 deep ocean of melted rocks, and left to cool by radiation into 

 space. This is the condition that would supervene, on a cold 

 body much smaller than the present earth meeting a great 

 number of cool bodies still smaller than itself, and is therefore 

 in accordance with what we may regard as a probable hypothesis 

 regarding the earth's antecedents. It includes, as a particular 

 case, the commoner supposition, that the earth was once melted 

 throughout, a condition which might result from the collision 

 of two nearly equal masses. But the evidence which has con- 

 vinced most geologists that the earth had a fiery beginning, goes 

 but a very small depth below the surface, and affords us abso- 

 lutely no means of distinguishing between the actual phenomena, 

 and those which would have resulted from either an entire globe 

 of liquid rock, or a cool solid nucleus covered with liquid to any 

 depth exceeding 50 or 100 miles. Hence, irrespectively of any 

 hypothesis as to antecedents from which the earth's initial fiery 

 condition may have followed by natural causes, and simply 

 assuming, as rendered probable by geological evidence, that 

 there was at one time melted rock all over the surface, we need 



