The Theory of Glacier motion. iii 



friction, its chief motion being in its surface layers, so is a 

 glacier. No doubt the subjacent streams tend to lubricate 

 the ice mass in a measure, doing it more in the day than at 

 night, in summer than in winter, there being then more 

 water available from the melting of its surface. They would 

 do so more if they occupied the whole of its foundation 

 instead of only a part, but this is largely counterbalanced by 

 the tremendous weight of the mass. 



It is time that we should now turn to Forbes's own view, 

 namely, the theory that ice, notwithstanding its apparent 

 rigidity, is really of a plastic nature, and that a large part 

 of its motion is due to this quality. 



The first person who suggested this was apparently 

 Bordier, who, in 1773, published a work, entitled, Voyage 

 pittoresque aux glaciers de la Savoie^ in which he compared 

 ice to soft wax, flexible and ductile to a certain point, 

 and he attributed to it sufficient ductility to enable it 

 to move down from high ground to low (see Tyndall, 

 Glaciers^ 133-4)- He was followed many years after by 

 Captain Basil Hall, who, in his work called " Patchwork," 

 describes the glaciers of Miage. He argues that when the 

 successive layers of snow, often several hundred feet thick, 

 are half melted by the sun, and by the innumerable torrents 

 which are poured upon them from every side, to say nothing 

 of the heavy rains of summer, they form a mass, not liquid, 

 indeed, but such as has a tendency to move down the highly 

 inclined faces on which it lies {op. cit. I., 104). Later, in 

 the same work, he compares a glacier with a lava stream, 

 and says, they are more or less frozen rivers ; they both 

 obey the law of gravitation with great reluctance, being 

 eventually so sluggish that, although they both move along 

 the bottoms of valleys with a force well nigh irresistible, 

 their motion is sometimes scarcely perceptible (2^., III., 118). 

 Canon Rendu in his Theorie des Glaciers de la Savoie^ 

 published at Chambery, in 1 840, was the next to adopt this 



