2 Sg Me ee ee ted Wa 
ALTITUDES Of SNOW-FIELDS AND GLACIERS, 135 
of all the ‘causes tending to its soeut al etion: over those 
which ‘tend to its waste or fusion, is indeed a very com- 
plicated fact, and cannot be taken as the simple expression 
of any one of the elements of the climate. The snow 
line is far from having invariably a mean temperature of 
32°, as was at one time supposed. Under the equator, it 
“is about 35°; in the Alps and Pyrenees about 25°; and 
in latitude 68° in No orway it is (according t to Van Buch), 
‘only 21°. Yet there are regions, both in the extremity of 
‘Siberia and in Arctic’ America, ‘of which the mean tem- 
‘perature is below zero of Fahrenheit | (as, eg., Melville 
Island), And it is quite established, on the concurrent 
authority of those best: acquainted with those regions, that 
nowhere tn the northern hemisphere. does the snow-line attain 
the low level of the sea, The explanation is to be sought 
principally in the intensity of the summer heat during 
the period of perpetual day, which effectually thaws the 
soil, though only to a trifling depth, and raises upon its 
surface @ certain amount of brief vegetation, suitable for 
the support of Arctic animals. 
“Another cause affecting exceedingly the. level of the 
snow-line is the amount of snow which falls. The interior 
-of continents being far drier than the coasts, the snow to 
be melted is a comparatively slight covering. The snow- 
line on the north side of the Himalaya is “at least 3000 
feet higher than towards the burning plains of, Hindostan. 
This is chiefly due to the excessive dryness of the climate 
of Thibet. In like manner five times less rain falls on the 
coast of the Baltic than at Bergen., All this confirms the 
excellent generalisation of Von Bach that 7 is the tempera- 
ture of the summer months which determines the plain of per- 
pétwal snow. It is thus easy to understand why the méan 
temperature of the snow-line diminishes towards the pole, 
because for a given mean temperature of the whole year, 
the summer is far hotter in proportion, Also, places at 
which the temperature of the summer is low are those 
which have a ‘moderated or coast climate; but there also 
the fall of rain’ and snow is most abundant, whilst in exces- 
