PROCEEDINGS OF GEOLOGICAL SOCIETIES. 



109 



the physical laboratory of this Institution by researches made by Professor 

 Tyndall, and not yet published. The difference between granite and water 

 arising from this cause becomes vastly augmented when it is considered 

 that the icebearing surfaces occupy an elevated position above the level of 

 the sea, consequently the mantle of aqueous vapour which their radiant 

 heat had to penetrate must have been much more attenuated than the 

 comparatively dense shell lying between them and the surface of the ocean. 

 Thus the obscure rays of heat streamed into space from the icebearing 

 surfaces with comparatively little interruption, whilst the radiation from 

 the sea was as effectually retarded as if the latter had been protected with 

 a thick envelope of non-conducting material. 



Whether we take into consideration, therefore, the conductivity of 

 water and granite, their specific heats, or, finally, the respective facilities 

 with which they can, under the cosmical conditions contemplated, throw 

 off their heat into space, we find everywhere a state of things tending 

 much more to the conservation of the heat of the water than to the reten- 

 tion of that of the land ; and this of course applies 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 hy 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 beautiful 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 vapor- 

 ous condition. But granite is a very complex substance, and fewer of the 

 heat-oscillations of its atoms are in unison with those of aqueous vapour ; 

 hence the heat-vibrations of granite disturb the molecules of aqueous va- 

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

 quently the granitic rays are less absorbed. 



Thus, whilst the ocean retained a temperature considerably higher than 

 at present, the icebearers had undergone a considerably greater refrigera- 

 tion. The evaporation from the ocean would therefore, at the period con- 

 templated, be greater than it is at present, whilst the capabilities of the 

 icebearers, as such, would not be perceptibly less. Moreover, it is evi- 

 dent that, during the whole of the cooling period, the ocean must have 

 been receiving heat from its floor, and thus have acted as a carrier of 

 warmth from the comparatively profound portions of the earth's crust to 

 the oceanic surface. It thus resembled a mass of w T ater contained in an 

 evaporating basin, placed over a very slow and gradually declining fire. 

 Under such conditions its cooling w r as protracted through a vast period, 

 allowing sufficient time, between a temperature inimical to animal life and 

 the commencement of the glacial epoch, to permit of the development and 

 decay of those forms of animal life which existed in the preglacial seas. 



Temp. P. 



Evaporation 

 per minute in 

 Calm. 



Evaporation 

 per minute in 

 Breeze. 



Evaporation 

 per minute in 

 High Wind. 





Grains. 



Grains. 



Grains. 



85° 



4-92 



6-49 



8-04 



75° 



365 



4-68 



572 



65° 



2-62 



337 



4-12 



55° 



1-90 



2-43 



2-98 



45° 



136 



1-75 



213 



35° 



•95 



122 



149 



