Tlie Theory of Glacier motion. 12J 



with water, rendering the whole an imbibed mass, like a 

 sponge, and consequently depends not only on the tem- 

 perature of any period, but upon the wetness of the surface, 

 whether derived from mild rain, thawing snow, or any 

 meteorological accident {id?). 



He thus sums up the various facts presented by 

 crevasses in support of his plastic theory : "The general 

 convexity of the crevasses upwards, notwithstanding the 

 excess of motion in the centre ; the general verticality of 

 the crevasses, notwithstanding the retardation of the 

 bottom ; the perfect state of the crevasses every spring 

 succeeding their visible collapse in autumn ; the ascertained 

 velocity of different parts of the glacier, and the diversity 

 of the annual changes which their velocities present ; the 

 seemingly opposed facts showing the glacier to be subjected 

 to powerful tension, producing crevasses, and yet to be 

 under a compression which produces in some places the 

 frontal dip ; and finally, the renewal of the level of the ice 

 during winter, which has been lost partly by superficial 

 melting, but as much or more so by the attenuation and 

 collapse of the glacier during summer" (P////. Trans., 1846). 



The experiments of M. Agassiz's staff on the glacier of 

 the Aar continued, and it was with some natural exultation 

 that Forbes declared, as these experiments became more 

 precise, so did their results accord more and more with his. 

 Inter alia, they established, he says, that "the movement of 

 the centre of that glacier was to that of a point five metres 

 from the edge as 14 to i. Such is the effect of plasticity. 

 Thirteen-fourteenths of the motion of the glacier of the 

 Aar are due to the sliding of the ice over its own surface, 

 and one-fourteenth only to its motion over the soil" (^Ed: 

 Phil. Jour. XXXVIII. 3 39). Shortly after the publication of 

 these words M. Agassiz apparently abandoned the dilatation 

 theory, of which he had been so long the champion. We 

 find this change of view reported in the Bibliotheque 



