The Theory of Glacier motion. 125 



and, with the assistance of his guide, Auguste Balmat, 

 satisfied himself, by marking rocks, etc., that glaciers move 

 with considerable velocity even in winter {E, P.J.y XXXVI. 

 J217, 218). A few months later, on visiting the South of 

 Italy, he was able to compare the motion of glaciers with 

 that of a lava stream. He pointed out that in two respects 

 the comparison fails: (i) In respect of the great liquidity 

 of the lava near its source ; (2) From its unequal rate of 

 •consolidation a crust is soon formed more massive than 

 the subjacent mass, and the fluidity is thus not distributed 

 through the mass, " but a tolerably perfect fluid struggles 

 with the increasing load of its ponderous crust, which it 

 tears and rends by the mighty energy of hydrostatic 

 pressure," and thus the crust gets broken, and the whole 

 becomes more like a torrent loaded with blocks of ice than 

 the regulated progress of a glacier, with a graduated 

 retardation towards the sides. In other respects the 

 ■analogy is more complete, as he shews in considerable 

 •detail. In all these comparisons we must remember that the 

 analogy is not quite complete, or the results would probably 

 be identical ; but, as Forbes remarks, " ice passes from a 

 brittle solid into limpid fluid by heat, while lava passes like 

 sealing wax through every intermediate degree of viscidity." 

 In the experiments he made in 1844 o" the Mer de 

 •Glace, which were conducted with great care and minutely 

 measured, he showed by the convexity and regularity of 

 the curves made by the moving points that the movement 

 of the ice is molecular, and, as he says, " proving a regular 

 plastic action of gravity or other propelling force, acting 

 from point to point on the mass of the glacier" {Phil. Trans. 



1845). 



In July, 1844, Forbes applied his methods of observation, 

 which had proved to him the molecular movement of 

 the greater glaciers, to the smaller ones reposing in the 

 •cavities of high mountains or on the cols, which De 



