175 



are more remarkable at lower stations on a glacier than at higher, " be- 

 cause the lower are exposed to more violent alternations of heat and 

 cold than the higher : this (says Forbes) we shall find to be general." 



It moves fastest on the hottest days. " This I apprehend (says 

 Forbes) to be clearly made out from my experiments, that thaw- 

 ing weather and a wet state of the ice conduce to its advancement, 

 and that cold, whether sudden or prolonged, checks its progress *." 

 " The striking variations in September, especially at the lower sta- 

 tions, which were frequently observed, prove the connexion of tem- 

 perature with velocity to a demonstration-]"." 



It is, however, impossible to do justice to the positive character of 

 the evidence on which this conclusion has been founded by Professor 

 Forbes without reference to those diagrams, by means of which he 

 has compared the mean rates of the daily motions of glaciers and the 

 corresponding mean temperatures. This comparison is founded ou 

 observations made by himself and Aug. Balmat, as to the motion of 

 the " Mer de Glace," at fourteen different stations in three different 

 years, and on observations on the mean temperature of the atmo- 

 sphere made at the same times at the Great St. Bernard and at 

 Geneva. It results from it that no change in the mean temperature 

 of the atmosphere is unaccompanied by a corresponding change in. 

 the mean motion of the glacier. 



The glacier moves with different velocities at different depths, the 

 surface- motion being faster (probably two or three times) than that of 

 the deepest part. The motions at different depths cannot but be re- 

 lated to one another : so that as the influence of variations of tempe- 

 rature is felt on the surface, it cannot but be felt throughout the glacier. 



If every change of solar heat is associated with a corresponding 

 change of glacier-motion, it seems to follow that the two are either 

 dependent upon some common cause, or that the one set of changes 

 is caused by the other ; and the former of these conclusions being 

 inadmissible, we are forced on the latter. It is not necessary to show 

 how it is that changes of external temperature penetrate glaciers. Of 

 the power of the sun upon them there are, however, evidences in the 

 ablation of surface constantly going on and in the preservation of the 

 ice which is covered by the stones of a moraine, which sometimes 

 forms an icy ridge from 50 to 80 feet high, and some hundred feet in 

 width. 



* Forbes, ' Travels in the Alps,' p. 148. f Ibid. 



