PEOCEEDINGS OF GEOLOaiCAL SOCIETIES. 
109 
the physical laboratory of this lustitution by researches made by Professor 
Tyndall, and not yet ])ublished. 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 tlio 
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 \^ ith 
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 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 TyndalFs 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 woidd 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 water contained in an 
evaporating basin, placed over a very slow and gradually declining fire. 
Under such conditions its cooling M as protracted through a vast period, 
allowing sufiicient 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. F. 
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° 
3-65 
4-68 
5-72 
65° 
2-62 
3-37 
4-12 
55° 
1-90 
2-43 
2-98 
45° 
1-36 
1-75 
213 
35" 
•95 
1-22 
1-49 
