108 



THE GEOLOGIST. 



that its effects were felt over the entire globe. 2nd, that it occurred at 

 a geologically recent period. 3rd, that it was preceded by a period of in- 

 definite duration, in which glacial action was either altogether wanting, or 

 was at least comparatively insignificant. 4th, that during its continuance 

 atmospheric precipitation was much greater, and the height of the snow- 

 line considerably less than at present. 5th, that it was followed by a 

 period extending to the present time, when glacial action became again 

 insignificant. 



All these conditions he believed to be the natural sequences of the gra- 

 dual secular cooling of the surface of our globe. The sole cause of the 

 phenomena of the glacial epoch tvas a higher temperature of the ocean than 

 that which obtains at present. 



He then examined the grounds upon which this hypothesis is based. 

 Numerous observations of the augmentation of temperature, at increasing 

 depths from the surface of the earth, no longer leave room for doubt that 

 the vast mass of materials constituting the interior of our globe is at the 

 present moment at a temperature far higher than that of the surface. If 

 this be so, the conclusion is almost inevitable, that at earlier periods of the 

 earth's history this high temperature must, at all events at depths com- 

 paratively little removed from the surface, have been still higher, and that 

 consequently the temperature of the surface itself must in former ages 

 have been much more influenced by the internal heat than is the case at 

 the present day. Tracing thus back the thermal history of our earth, it 

 is conceivable that the waters of the ocean once existed as aqueous vapour 

 in our atmosphere, — a condition which it is imagined obtains at the present 

 day in Jupiter, Yenus, and other planets, whose superior size or closer 

 proximity to the sun may be supposed to have retarded the refrigeration 

 of their surfaces. From the period, therefore, when the cooling of the 

 earth's crust permitted the ocean to assume the liquid condition, its waters 

 have gradually cooled from the boiling-point down to the present tempe- 

 rature, whilst the land has also undergone a similar process of refrigera- 

 tion. It was during the later stages of this cooling operation that the gla- 

 cial epoch occurred. For this assumption, however, it is necessary to 

 establish that the rate of cooling of the land and of the ocean surfaces was 

 unequal, otherwise the more rapid evaporation of the ocean due to increased 

 temperature would be more or less neutralized by the impaired efficiency 

 of the proportionately warm icebearers. The speaker then proceeded to 

 describe the results of his numerous experiments, which conclusively 

 proved that, under the conditions contemplated, the land would cool more 

 rapidly than the sea. This effect is brought about principally by two 

 causes, viz. the great specific heat of water compared with granite and 

 other rocks, and the comparative facility with w hich radiant heat escapes 

 from granite through moist air. The amounts of heat associated with 

 equal weights of water and granite are as 5 to 1 in favour of the former, 

 or, if equal volumes be taken, the water requires to lose twice as much 

 heat as the granite in order to cool through the same number of degrees : 

 but it is in regard to the escape of radiant heat from their surfaces that 

 the superior retention of warmth by the oceanic waters is most stronglv 

 marked. The readiness with which radiant heat escapes from equal sur- 

 faces of water and granite at the same temperature through perfectly dry 

 air is nearly equal ; but so soon as aqueous vapour is interposed in the path 

 of these rays, the conditions become wonderfully altered; the escape of 

 heat from both is interrupted, but its radiation from the water is retarded 

 in by far the greatest degree. This extraordinary intranscalency of aque- 

 ous vapour to rays issuing from water has just been conclusively proved in 



