Proceedings of Societies. 357 



heit) — appears so excessive as to be unlikely ; I have therefore 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 niy guide, Auguste Balmat, some years since, on the 

 same glacier, at the same season. 



With respect to the ice of the glacier of Blatiere, which is above tho 

 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 para- 

 dox of water remaining uncongealed in the fissures of the ice at this sea- 

 son, 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 conduction, but much more rapidly by the per- 

 colation 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 re- 

 tardation 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 heat has been withdrawn.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 Blatiere 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 ; whilst the latter, at its 

 lower extremity, and in contact with the ground, is scarcely moving 

 at all. 



There is nothing of which we know less than the cause of this seemingly 

 capricious advance and retreat of the extremities of glaciers at the same 

 time, and under, seemingly, the same circumstances. 



In the present case, I will only mention as & possible explanation, that 

 the glacier of Blatiere probably possesses a continuous slope, from 

 its middle and higher region down to its lower extremity. But the 

 Bossons, after its steep descent from Mont Blanc, proceeds along way on 

 a comparatively level embankment, which at an early period it cast up of 

 its own debris, and in which it has dug itself a hollow bed in which it 

 nestles. The angular slope of the bottom in contact with the soil is very 

 probably much less than in the case of the glacier of Blatiere. Now, 

 when winter has dried up the percolating water, the viscosity of the mass 

 may be insufficient to drag it over the less slope although it carries it over 

 the greater. That the motion of the ice close to the ground should be 

 nearly nothing, whilst the more superficial part of the glacier over-rides 

 it by its plasticity, is as a separate fact quite in accordance both with 

 theory and previous observation. 



But as the snout, or lower end of the glacier of Bossons, is almost sta- 

 tionary, whilst the middle region is moving at the rate of a foot a day, 

 Mr Blackwell very pertinently asks, " What becomes, then, of the ice 

 continually descending from above ? Does it not go to thicken the whole 

 mas, accumulating behind the more rigid portion below, as water behind 

 a dam?" I answer, undoubtedly ; and he will find this explanation given 

 ten years ago in my Travels in the Alps, (2d edit., p. 386.) Speaking 

 of the superficial waste of the glaciers in summer and autumn, and the 

 manner in which it is repaired before the ensuing spring, I there observed, 

 " The main cause of the restoration of the surface is the diminished 



