of the Glacial Epoch. 335 



Whether we take into consideration, therefore, the diffusion of 

 heat through water and granite, the quality possessed by the 

 ocean of abstracting heat, through the agency of dislocations, 

 &c, from depths still more profound than its own floor, or finally 

 the respective facilities with which, under the cosmical condi- 

 tions contemplated, water and granite throw off their heat into 

 space, we find everywhere a state of things tending not only to 

 the greater conservation of the heat of the water, but also to a 

 less rapid increase of temperature from the surface downwards, 

 than is the case with the solid crust of the globe; and this ap- 

 plies also, mutatis mutandis, to the retention of that heat which is 

 received from solar radiation. The luminous heat-rays of the 

 sun pass freely through aqueous vapour, and are absorbed by 

 both granitic and oceanic surfaces ; but once absorbed, these rays 

 issue forth again as obscure heat of two different qualities, or 

 rates of vibration. To use Tyndall's explanation of the pheno- 

 menon, the vibrations of the liquid water-molecules are of 

 such rapidity as can be best taken up and absorbed by the same 

 molecules in the vaporous condition. But granite is a very com- 

 plex substance, and fewer of the heat-oscillations of its atoms are 

 in unison with those of aqueous vapour ; hence the heat-vibra- 

 tions of granite disturb the molecules of aqueous vapour in their 

 passage through the atmosphere in a less degree, and conse- 

 quently the granite rays are less absorbed. 



Thus the chief process by which the ocean lost heat was eva- 

 poration ; for this, as is well known, proceeds at a surface of 

 water until the superambient air is saturated with vapour. 

 The latter, by virtue of diffusion, rises into the presence of the 

 dry-air condenser, there to yield up its latent heat and be con- 

 verted into rain or snow, according to the temperature of the 

 medium in which the radiating process takes place. Hence the 

 one heat-dissipating process of which the water is most capable, 

 is that by which the ice-bearers are provided with their snowy 

 burthen. 



Objections. — To the hypothesis which I have endeavoured to 

 develope in the preceding pages, I am aware that, in addition to 

 those already alluded to, several objections may be urged which 

 appear at first sight very formidable, but which, I think, lose 

 this character, to a great extent at least, when more closely in- 

 vestigated. 



1. Perhaps the most palpable of these objections is one which 

 may be taken to my statement that a more copious atmospheric 

 precipitation would cause a greater accumulation of snow upon 

 elevated portions of land, and thus depress the snow-line. It 

 may be urged that although such an increased atmospheric pre- 

 cipitation would have the effect of depositing more snow in 



