PROFESSOR W. THOMSON ON THE SECULAR COOLING OF THE EARTH. 165 



instant and maintained constant ever after. I answer the first objection by say- 

 ing, 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 

 meantime, we can see that this modification will not make any considerable 

 change in the resulting temperature 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 temperature of solidification to the super- 

 ficial temperature. But, what is more to the purpose, it is to be remarked that 

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

 problem solved above corresponds much more closely, in all probability, with 

 the actual history of the earth, than does the modified problem suggested by 

 the objection. The earth, although once all melted, or melted all round its 

 surface, did, in all probability, 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 in- 

 cludes, 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 convinced most geologists 

 that the earth had a fiery beginning, goes but a very small depth below the sur- 

 face, and affords us absolutely 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, w^e need not 

 assume the depth of this lava ocean to have been more than 50 or 100 miles ; 

 although we need not exclude the supposition of any greater depth, or of an 

 entire globe of liquid. 



PART I. VOL. XXIII. 2 z 



