MECHANICS OE GLACIERS. 71 



found. To say that ice is transparent is to say that luminous 

 radiant energy can freely traverse it. It does not do so however 

 equally at all times. Obviously, more luminous energy enters the 

 glacier from the sun by day than by night — more during the more 

 numerous hours of daylight, the higher ascension of the sun, and 

 the greater freedom from the diffusive action of the snow at the sur- 

 face in summer than in winter. A certain amount of the luminous 

 portion of a beam of solar radiation is absorbed by even clean ice, 

 more especially by granular ice, otherwise the beautiful silvery blue 

 colour which is perceived in the ice overhead when one eDters an 

 ice-cavern would be wanting. But it is chiefly by opaque and semi- 

 opaque bodies within the glacier (stones, earth, organic germs, &c.) 

 that the luminous solar radiation which enters the ice is absorbed ; 

 and the radiation thus absorbed is, in accordance with the law of 

 conservation of energy, converted into heat, just in the same way 

 as it is in an ordinary greenhouse. Heat thus developed within the 

 mass of the glacier, during the hours of daylight, and most so 

 during the summer, must promote the differential movement of the 

 glacier. Of course, if the glacier is clean enough and thin enough, 

 some of the luminous energy may penetrate to the rocks beneath ; 

 but the transformation of energy would in that case be the same, 

 the heat developed promoting the descent of the glacier as a whole, 

 instead of its differential motion. 



Discussion. 



Mr. Callaed stated that in 1878 he had had the opportunity of 

 studying the base of the Rhone glacier, which glacier has been 

 slowly receding for a length of time. Beneath the glacier, as seen in 

 the terminal ice-cave, the ground was not ploughed but only some- 

 what smoothed. 



Prof. Seelet pointed out that before we could accept the author's 

 views it would be necessary for him to explain the origin of the 

 quantity of detritus carried out from the end of the glacier and the 

 smoothed surfaces on the bottom and sides of the glacier-bed. He 

 thought that the effect of the continual melting and freezing of 

 water at the bottom of the glacier on the rock masses below had 

 been overlooked by the author, and that these effects must be very 

 striking. The late Mr. Clifton Ward had shown that some lake- 

 basins occur at the point where tributary glaciers join the principal 

 ones. 



Mr. Blaneoed referred to the a priori argument, based on the 

 fact that rock-bound lake-basins abound in districts which have been 

 glaciated, and are rare in other regions. He pointed out that the 

 author seemed to have lost sight of the fact that the erosion was 

 performed not by ice but by stones &c. held in the ice. 



Lieut.-Col. H. H. Godwin -Austen said that the great glaciers of 

 the Himalayas, which are so much larger than those of the Alps, 

 are advancing, breaking away the ground along their sides, and 

 pushing forward their older moraines. It must also be remem- 

 bered that glaciers of the great thickness of the older glaciers in 

 Switzerland must have produced very different effects on rocks of 



