of the Glacial Epoch. 329 



In calm. In breeze. In high wind. Mean. 



2*26 grains. 2*90 grains. 3*55 grains. 2*90 grains. 



These absolute numbers were obtained with dry air, and there- 

 fore they cannot be taken as representing the actual evaporation 

 from a surface of water like that of the ocean, which is in con- 

 tact with air of ever- varying hygrometric qualities. All I con- 

 tend for is, that Dalton's numbers represent, under similar con- 

 ditions in the two cases, therelative 3 but not the absolute evapo- 

 ration from a given oceanic surface ; and this being granted, it 

 follows that an elevation of 20° F. above the present temperature 

 of the ocean bathing the shores of Norway would double the eva- 

 poration from a given surface. Such an increased evaporation, 

 accompanied as it necessarily must be by a corresponding preci- 

 pitation, would suffice to fill the fjords and cover the western 

 coast of that country with ice, provided that the ice-bearers were 

 in a sufficiently effective condition. But would not the increased 

 oceanic temperature tend to augment the mean temperature of 

 the atmosphere even at considerable elevations, and thus to raise 

 the snow-line and reduce the area of perpetual snow ? 



The reply to this question is contained in the second of the 

 above propositions. That the limit of perpetual snow does not 

 entirely depend upon the mean temperature of the atmosphere 

 at that particular elevation is conclusively proved by the very 

 different mean temperature of the snow- line in various localities. 

 Thus, under the equator it is about 35°; in the Alps and Pyre- 

 nees 25°; and, according to Von Buch, in lat. 68° in Norway, it 

 is only 21°*. These numbers are very instructive: why does 

 the mean temperature of the snow-line rise as we approach the 

 equator ? The answer to this question has been already given by 

 Mr. Hopkins in his admirable memoir on the Influence of the 

 Earth's Secular Heat upon Climatef . He considers that the low 

 snow-line in the tropics is due to a more equable temperature and 

 greater atmospheric humidity. The deluges of rain which fall 

 within the tropics far surpass the precipitation in the temperate 

 and frigid zones ; and doubtless the fall of snow upon most inter- 

 tropical mountains is proportionately great. The important influ- 

 ence which the amount of precipitation alone exercises upon the 

 lower limit of perpetual snow is strikingly exemplified at the fine 

 waterfall of Tysse Stronger near the head of the Hardanger Fjord, 

 and was first noticed by Mr. M. Williams J. 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 



* Humboldt's Cosmos, vol. i. p. 9. Forbes's ' Norway,' p. 205. 

 t Geological Society's Journal, vol. viii. p. 78. 

 X Through Norway with a Knapsack. 



