286 



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 solidi- 

 fied until its latent heat has been withdrawn.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 ; 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 the 

 seemingly capricious advance and retreat of the extremities of 

 glaciers at the same time and under, seemingly, the same circum- 

 stances. 



In the present case, I will only mention as a possible explana- 

 tion, that the glacier of Blaitiere probably possesses a continuous 

 slope, from its middle and higher region down to its lower extre- 

 mity. But the Bossons, after its steep descent from Mont Blanc, 

 proceeds a long 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 Blaitiere. 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 

 stationary, 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 mass, accumulating behind the more rigid por- 

 tion 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 re- 



