The Rev. H. Mosely on the Descent of Glaciers. 77 



proportion than that sustained by the lead upon the board, and 

 variations of temperature far less, without passing the limits within 

 which a probability is created by the experiment that the descent of 

 the glacier is due to the same cause as that of the lead. 



In the act of descending on the board, the slab of ice of which we 

 have spoken could not but be thrown into a state of extension in 

 some parts and of compression in another. The conditions of the 

 descent being in other respects given, the amount of this extension or 

 compression might be at any point determined. If at any point the 

 extension exceeded the tenacity of the ice, the slab would there sepa- 

 rate across its length ; and if at any point the compression exceeded 

 the resistance to crushing, it would there crush. 



Supposing it to be thinner at the sides than in the middle, the sur- 

 face-motion of the middle would be faster than that of the sides, and 

 from this differential motion would result cracks oblique to the axis of 

 the slab, the explanation of which, as they exist in glaciers, is one of 

 the most successful attempts yet made at the solution of the me- 

 chanical problem of srMcier-motion. These conditions of the descent 

 of the slab, when referred to a glacier, explain the formation of trans- 

 verse and lateral crevasses, and the fact of a glacier crushing itself 

 through a gorge. 



The Mer de Glace moves faster by day than by night*. Its mean 

 daily motion is twice as great during the six summer as during the six 

 winter months f . It moves fastest in the hottest months, and in those 

 months varies its motion the most, because in them the variations of 

 temperature are the greatest. It moves most slowly in the coldest 

 months, and in those varies its motion the least, because in those 

 months the variations of temperature are the least. These differences 

 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 J." 

 " 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 on 

 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 



* Forbes, ' Occasional Papers/ p. 12. 



f Ibid. p. 129. Tyndall, ■ Glaciers of the Alps,' p. 294. 



X Forbes, ' Travels in the Alps,' p. 148. § Ibid. 



