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tionaiy. This is doubtless a clue to the assertions ol" some authors, 

 ' that the glacier is stationary in winter ;' — they only looked at the 

 end. What becomes, then, of the ice continually descending from 

 above ? Does it not go to thicken the whole mass, accumulating 

 behind the more rigid portion below, as water behind a dam ? I 

 have no space to add more at present, but will write again if I have 

 your approval of my proceedings. Meanwhile I have fixed (yester- 

 day) an intermediate station, for the purpose of determining where 

 this comparative immobility begins. I have noted my observations, 

 and kept a register of weather, &c. I give one observation to show 

 the difference between the middle and lower glaciers :— 



From December 28 to January 11 — 14 days. 



Middle glacier (fomewhat above ivhere it is usualhj crossed). 

 {Centre, 1-1 ft. 7 in. (fourteen feet, .seven inches). 

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

 Lower glacier during the name period. 

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

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



Observations on Mr BlackwelV s Letter by Professor Forbes. 



The cold described ( — 25" to — 30° of Reaumur— 24-1^" 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 Bahnat, 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 aflSrmed 

 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 win- 

 ter. Heat, we know, penetrates a glacier (up to 32" and no fur- 

 ther), not only by conduction, but much more rapidly by the perco- 

 lation of water ; but cold penetrates solely by conduction, and that 

 according to the same law as in solid earth, though it may be more 



