SLIGHT EFFECTS OF SUNSHINE ON SNOWS. 291 



of the Avhole atmosphere, while the existence of con- 

 tinuous day prevents the refrigeration which would 

 otherwise occur by night, and thus, when the southern 

 wind blows softly, the great disruption of the northern 

 ice is brought about. 



We ought here to make a short digression from 

 our narrative to say a few words upon the scientific 

 aspects of snow in its effects upon climate. Rains, 

 as we know, produce no permanent effects upon 

 temperature; in cold countries especially, their effects 

 soon pass away, as the water runs off and in no 

 case, however heavy may be the rainfall, do they 

 produce any permanent rise in the general level of 

 the waters. But the effect of snow on the other hand, 

 is constantly cumulative, so long as it continues to 

 fall. A. very heavy and long continued snow-fall may 

 in this way therefore accumulate in such masses that 

 the warmth of the whole of the following summer 

 may be unable to melt it. This occurs upon high 

 mountains all over the world, where a regular line of 

 perpetual snow becomes more or less constant. In 

 the tropics for instance, this may, as we have shown, 

 be roughly located at an elevation of about 16,000 feet 

 above sea level. 



When this takes place, as Mr. Wallace has pointed 

 out, cold is rendered permanent, no amount of sun- 

 heat prevailing to warm the air, or earth, much above 

 freezing-point. 



" The quantity of heat (he goes on to explain) required to 

 melt ice or snow is very great. To melt I cubic foot of ice, 

 as much heat is required as would heat a cubic foot of water 

 from the freezing point to 176 Fahr., and to melt a layer of 

 ice only \\ inches thick, requires as much heat as would 



