1846.] INFLUENCE OF SEASONS ON VELOCITY ABLATION. 157 



of the glacier, by which we mean to express the combination of 

 circumstances determining its motion, varies from one season of 

 the year to another, owing not only to the general influence of 

 heat and cold, but also to the progressive communication of that 

 influence to portions of the glacier in successive stages of ele- 

 vation. Evidently the extremity nearest to the valley will 

 receive the earliest and most violent impression of solar heat, 

 whilst the middle and upper regions are involved in complete 

 winter. Partial dilatations must take place in spring, partial con- 

 densations in the decline of the year ; as is evident from the 

 consideration that temperatures inferior to freezing do not sen- 

 sibly affect the motion of the ice (see above, p. 138) which 

 higher temperatures do, consequently the influence of season 

 will be chiefly felt in those parts of the glacier where the tem- 

 perature of the air seldom falls in summer to 32, whilst the more 

 stable motion of the higher part acts as a drag or equalizer 

 upon the whole system. The condition of violent distension 

 produces crevasses, that of violent compression produces the 

 frontal dip of the veined structure, or that share of it which is 

 due to the relative motions in a vertical plane. The longitudi- 

 nal veins will result whether the axis of the glacier be distended 

 or compressed. Hence the reason why the frontal dip is diffi- 

 cultly seen in all the middle region of a glacier which, like the 

 Mer de Glace, is subject to much extension due to great and 

 increasing declivity, and to be well seen must be sought for in 

 the higher parts of the glacier, as above Trelaporte, at the foot 

 of the Couvercle,* and in glaciers subject to great compression, 

 as that of La Brenva, the glacier of the Rhone, the Aar, etc. 



Ablation of the Surface. One phenomenon is most satis- 

 factorily explained by the variations of velocity established and 

 illustrated in this paper. The collapsed state of the glacier after 

 the hot summer of 1842, and the absolute lowering of its surface 

 level by thirty feet in the space of a few months, had struck me 

 as requiring an energy altogether extraordinary in kind and 

 degree to restore next spring the level which had been lost, in 



* Travels, 2d edit. p. 167. 



