108 
TUE 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 was a higher tempei^ature of the ocean than 
that which ohtains 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 tlie 
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, Venus, 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 which radiant heat escapes 
from granite through moist air. The amounts of heat associated witli 
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 strongly 
marked. The readiness with which radiant heat escapes from equal sur- 
f\ices 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 intranscaleucy of aque- 
ous vapour to rays issuing from water has just been conclusively proved in 
