110 



THE GEOLOGIST. 



The rate of evaporation of water at different temperatures and under 

 various circumstances was determined by Dalton, whose results are em- 

 bodied in the foregoing table. The evaporation took place in each case 

 from a circular surface six inches in diameter. 



We have no sufficient data to calculate the present mean temperature of 

 the ocean, but in lat. 69° 40' off the coast of Norway, at noon on a remark- 

 ably hot summer's day, Professor Forbes found the temperature to be 

 46*5° Fahr. The assumption of 40° Fahr. as the mean temperature off the 

 coast of Norway, would therefore probably be in excess of the truth. Now, 

 taking the mean of Dalton's results obtained at 35° and 45°, and comparing 

 it with the mean of his results at 55° and 65°, it will be seen that an in- 

 crease of 20° in the temperature of the ocean off the coast of Norway 

 would double the evaporation from a given surface. Such an increased 

 evaporation, accompanied as it necessarily must be by a corresponding 

 precipitation, would suffice to supply the higher portions of the land with 

 that gigantic ice-burden which groaned down the mountain slopes during 

 the glacial epoch. 



But would not the increased oceanic temperature tend to augment the 

 mean temperature of the atmosphere even at considerable elevations, and 

 thus raise the snow-line and reduce the area of perpetual snow ? In an- 

 swering this question, the speaker showed that the limit of perpetual 

 snow does not depend so much upon the mean temperature of the atmo- 

 sphere at that particular elevation, as upon the amount of snow accumulating 

 during the cold season. Under the equator, the mean temperature of the 

 snow-line is 35° Fahr. ; in the Alps and Pyrenees, about 45° ; and in lat. 

 68°, in Norway, it is only 21°. Thus the mean temperature of the snow-line 

 rises as we approach the equator, which means that the snow-line itself 

 descends below its normal height, owing principally to augmented oceanic 

 evaporation accompanied by increased atmospheric precipitation. The 

 deluges of rain which fall within the tropics far surpass the rainfall in the 

 temperate and frigid zones, and doubtless the fall of snow upon intertro- 

 pical mountains is proportionately great. The important influence which 

 the amount of precipitation exercises upon the lower limit of perpetual 

 snow is beautifully exemplified at the fine waterfall of Tysse Strenger, 

 near the head of the Hardanger Fjord, and was first noticed by Mr. M. 

 Williams. The spray from this fall, being frozen in winter, covers the 

 valley for nearly half a mile with a stratum of snow and ice, so thick as to 

 defy the solar rays of summer to melt it ; thus lowering the snow-line by 

 more than 2000 feet. The speaker had also seen in the Sor Fjord, under 

 similar abnormal conditions, a mass of snow lying, in the month of August 

 last, within 10 feet of the level of the sea, although the normal snow-line 

 is there at least 4500 feet above the sea-level. That the height of the 

 snow-line is essentially dependent upon the amount of precipitation, and 



Latitude. 



G0° 

 62° 

 64° 

 66° 

 68° 

 70° 



Height of Snow-liue in feet. 



Coast. 



5,500 

 5,200 

 4,200 

 3,700 

 3,150 

 3,350 



Interior. 



4,450 

 4,150 

 3,650 

 3,250 

 3,000 

 2,000 



Difference. 



1,050 

 1,050 

 550 

 450 

 450 

 450 



