224' HELMHOLTZ 



Notwithstanding the powerful radiation of the sun the tem- 

 perature of the air above the snow-fields only rises to 5, 

 or at most 8; this, however, is sufficient to melt a tolerable 

 amount of the superficial layers of snow. But the warm 

 hours and days are too short to overpower the great masses 

 of snow which have fallen during colder times. Hence the 

 height of the snow-line does not depend merely on the tem- 

 perature of the mountain slope, but also essentially on the 

 amount of the yearly snow-fall. It is lower, for instance, 

 on the moist and warm south slope of the Himalayas than 

 on the far colder but also far drier north slope of the same 

 mountain. Corresponding to the moist climate of western 

 Europe, the snow-fall upon the Alps is very great, and hence 

 the number and extent of their glaciers are comparatively 

 considerable, so that few mountains of the earth can be 

 compared with them in this respect. Such a development 

 of the glacial world is, as far as we know, met with only on 

 the Himalayas, favoured by the greater height; in Green- 

 land and in Northern Norway, owing to the colder climate; 

 in a few islands in Iceland; and in New Zealand, from the 

 more abundant moisture. 



Places above the snow-line are thus characterised by the 

 fact that the snow which in the course of the year falls on 

 its surface does not quite melt away in summer, but remains 

 to some extent. This snow, which one summer has left, is 

 protected from the further action of the sun's heat by the 

 fresh quantities that fall upon it during the next autumn, 

 winter, and spring. Of this new snow also next summer 

 leaves some remains, and thus year by year fresh layers of 

 snow are accumulated one above the other. In those places 

 where such an accumulation of snow ends in a steep preci- 

 pice, and its inner structure is thereby exposed, the regularly 

 stratified yearly layers are easily recognised. 



But it is clear that this accumulation of layer upon layer 

 cannot go on indefinitely, for otherwise the height of the 

 snow peak would continually increase year by year. But 

 the more the snow is accumulated the steeper are the slopes, 

 and the greater the weight which presses upon the lower 

 and older layers and tries to displace them. Ultimately a 

 state must be reached in which the snow-slopes are too 



