110 
THE GEOLOGIST. 
The rate of evaporation of 'water at diflPerent temperatures and under 
various circumstances was determined by Dal ton, 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 JN^orway, 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 Norw ay 
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 do\\ n tiie mountain slopes during 
the glacial epoch. 
But would not the increased or-eanic temperature tend to augment tlie 
m.ean 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 sliowed thai the limit of perpetual 
snow does not depend so much upon the mean temperature of tiie 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-hne 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 tlie 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 ftill 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 exem.pliiied 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 
niore than 2000 feet. The speaker had also seen in the Sor Fjord, under 
similar abnormal conditions, a mass of snow l3nng, 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. 
Height of Snow-line in feet. 
Coast. 
60° 
5,500 
62° 
5,200 
64° 
4,200 
66° 
3,700 
68° 
3,450 
70° 
3,350 
Interior. DiiFerence. 
4,450 
4,150 
3,650 
3,250 
3,000 
2,900 
1,050 
1,050 
550 
450 
450 
450 
