328 



Prof. Frankland on the Physical Cause 



All these conditions would be the natural sequences of a 

 gradual cooling of the ocean from a higher down to its present 

 temperature. The sole cause of the phenomena of the glacial 

 epoch ivas a higher temperature of the ocean than that which 

 obtains at present. 



This hypothesis rests chiefly upon the two following proposi- 

 tions : — 



1st That a higher oceanic temperature would give rise to an 

 increased evaporation, and consequently to an augmented atmo- 

 spheric precipitation. 



2nd. That this increased atmospheric precipitation would 

 augment the average depth of permanent snow upon the ice- 

 bearers, and would, within certain limits, depress the snow-line. 



Let us now examine these propositions in detail. As a gene- 

 ral proposition, the first is an indisputable truth, and it is there- 

 fore only necessary to inquire into the extent to which oceanic 

 evaporation would be thus affected. The rate of evaporation of 

 water at different temperatures and under various circumstances 

 was determined by Dalton, whose results are embodied in the 

 following Table. The evaporation took place in each case from 

 a circular surface 6 inches in diameter ; — 





Evaporation 



Evaporation 



Evaporation 



Temp. F. 



per minute 



per minute 



per minute in 





in calm. 



in breeze. 



high wind. 





Bjs.^ 



grs. 



grs. 



85 



4-92 



6-49 



8-04 



75 



3-65 



4-68 



5-72 



65 



2-62 



3-37 



4-12 



55 



1-90 



243 



2-98 



45 



1-36 



175 



213 



35 



-95 



1-22 



1-49 



We have no sufficient data for calculating the present mean 

 temperature of the ocean ; but in lat. 69° 40', off the coast of 

 Norway, at noon on a remarkably hot summers day, Forbes 

 found the surface temperature to be 46°*5 F. The assumption 

 of 40° F. as the mean temperature off the coast of Norway will 

 therefore probably be in excess of the truth. Now, taking the 

 mean of Dalton's results obtained at 35° and 45°, the evapora- 

 tion in each minute from a circular surface 6 inches in diameter 

 would be at 40° :— 



In high wind. 



Mean. 



1*81 grain. 



1*48 grain, 



In calm. In breeze. 



1-15 grain. T48 grain. 



And further, the evaporation from a similar surface at 60°, 

 according to the mean of the results obtained by Dalton at 55 ~ 

 and 65°, would be as follows : — 



