104 Kev. C. Trotter. Physical Properties of Ice [Jan. 29, 



VI. Temperature of Glaciers, and significance of the " Bergschrund. ,y 



Throughout this discussion it has been assumed that the tempera- 

 ture of the great mass of a glacier does not differ much from 0° C. 

 It seems desirable to examine the reasons for this assumption, to 

 consider whether it is liable to any important exceptions, and if so to 

 deduce their consequences. I do not know that there is in what 

 follows much that is substantially novel, or that is seriously contro- 

 verted, but it may be worth while to formulate in a systematic shape 

 what has hitherto been contained in a number of isolated remarks or 

 suggestions scattered through glacier literature. 



It is obvious that no part of a glacier can be at a higher tempera- 

 ture than 0° C, for the simple reason that ice or snow cannot exist 

 at a higher temperature. 



The snow which is the ultimate source of the glacier ice falls at 

 temperatures whicli may vary from 0° C. downwards. Snow weather 

 is, however, not usually very cold weather, and it is probable that 

 falls of snow at a very low temperature are exceptional. 



The surface of a glacier may sometimes become very cold by 

 radiation, but ice is a very bad conductor of heat, and dry snow is still 

 worse. There is no reason to suppose that the cold of a summer 

 night can penetrate many inches into the glacier, and it is improbable 

 that the cold of winter can be felt at more than a few feet below the 

 surface, since the whole glacier is in winter covered with snow. On 

 the other hand, on a warm summer day a very large amount of heat 

 is absorbed in the upper layers of a glacier, and is carried into the 

 lower layers by the water which sinks into the mass. It is clear that 

 so long as water at 0° C. sinks through snow or neve at 0° C, it will 

 descend unchanged, neither giving up nor receiving heat. When it 

 meets with snow or neve at a lower temperature, it will be partially 

 frozen, and its latent heat of freezing will warm the snow or neve up 

 to 0° C. Forbes found (" Eleventh Letter on Glaciers," Edinburgh, 

 "New Philosophical Journal," October, 1846, reprinted in "Occa- 

 sional Papers," pp. 169, 170) that the mean daily waste of the 

 surface of the Mer de Glace opposite the Montanvert during hot 

 weather in July and August, 1846, was 3*62 inches per day, while 

 higher up between 1' Angle and Trelaporte it was 2*73 inches. Now as 

 the latent heat of fusion of ice is about 80 calories, and the specific 

 heat of ice about *5, the heat required to melt a cubic inch of ice 

 would raise about 160 cubic inches through 1° C. No doubt in the 

 upper regions of neve the heat absorbed is a good deal less ; the air 

 is colder, and a smaller proportion of the solar radiation enters, and 

 is absorbed by, the glacier. On the other hand, in these regions the 

 whole of the wate T * produced by melting must sink into the glacier, 



