MR, BLACKWELL ON THE GLACIERS IN WINTER. 273 



observation to show the difference between the middle and lower 

 glaciers : 



From December 28 to January 11 14 days. 



Middle glacier (somewhat above where it is usually crossed). 

 Centre, 14 ft. 7 in. (fourteen feet seven inches). 

 Side, 11 ft. 6 in. (eleven feet six inches). 

 Lower glacier during the same period. 



Ridge, 1 ft. 7 in. (one foot seven inches). 

 Interior of vault, ft. 2 in. (two inches). 



Observations on Mr. Blackwelfs Letter, by Professor Forbes* 



" The cold described ( 25 to 30 of Reaumur 241 to 35^ 

 of Fahrenheit) appears so excessive as to be unlikely ; I have there- 

 fore written to enquire if the thermometer could be depended on. 



" It is highly satisfactory that the superficial velocity of the glacier 

 of Bossons about a foot in twenty-four hours coincides closely with 

 the measurements of my guide, Auguste Balmat, some years since, 

 on the same glacier, at the same season. 



" With respect to the ice of the glacier of Blaitiere,' which is above 

 the level of trees probably at least 7000 feet above the sea being 

 still in motion, it merely confirms the deductions long ago made by 

 me as to the continuity of glacier motion even in winter. And as 

 to the apparent paradox of water remaining uncongealed in the fissures 

 of the ice at this season, though I have nowhere affirmed the presence 

 of liquid water to be a sine qua non to the plastic motion of glaciers, 

 it would be difficult to assert positively that it is everywhere frozen in 

 the heart of a glacier even in the depth of winter. Heat, we know, 

 penetrates a glacier (up to 32 and no further), not only by conduc- 

 tion, but much more rapidly by the percolation of water ; but cold 

 penetrates solely by conduction, and that according to the same law 

 as in solid earth, though it may be more rapidly. Now, it is known 

 that at a depth of 24 or 25 feet in the ground, the greatest summer 

 heat has only arrived at Christmas. A similar retardation in the" effects 

 of cold must occur in glaciers. Not a particle of water detained in the 

 capillary fissures can be solidified until its latent heat has been withdrawn. 



" The contrast the writer draws between the glaciers of Blaitiere 

 and Bossons, the latter of which is some thousand feet lower in point 

 of level, is curious and instructive. The former, he says, appears the 

 more active, and is pushing forwards its moraine y-j- whilst the latter, 



* [Proc. Roy. Soc. Edin., as above.] 



f [Even this observation is not without some ambiguity. For no direct measures 

 of the motion of the ice seem to have been made, and the appearance of pushing the 

 moraine might be due to movements which bad taken place some time previously.] 



T 



