26 FOURTH LETTER ON GLACIERS. [1842. 



V. FOURTH LETTER on GLACIERS, addressed to 

 PROFESSOR JAMESON.* 



Change of Level and General Appearance of the Glacier in the month of September 

 Collapse of the Walls of the Crevasses Moulins Veined Structure Direction 

 of the Crevasses Both Veined Structure and Crevasses renewed from time to 

 time The Motion of a Glacier resembles that of a Viscid Fluid Objections to 

 the Dilatation Theory What the Cold of Winter may be supposed to accom- 

 plish. 



GENEVA, 5th October 1842. 



My Dear Sir Since my last letter from Zermatt, I have 

 had an opportunity of examining the glaciers on different sides 

 of Monte Rosa, particularly those of Lys and Macugnaga, and 

 those near the Valley of Saas ; and on my return to Chamouni 

 early in September, I devoted a day to each of the glaciers of 

 Trient and Argentiere, before resuming my station at the 

 Montanvert, where I remained until almost the last days of the 

 month. 



What I think it most interesting now to add, as supplemen- 

 tary to my former statements, is not a description of these 

 various glaciers, but with particular reference to the Mer de 

 Glace, to mention what the extended period of examination 

 which I have been able to give to it, has enabled me to con- 

 clude beyond what is contained in my previous letters, respecting 

 the Theory of glacier-movement generally. Having accurately 

 observed the condition and motions of this glacier throughout 

 by far the greater part of the season at which it, or indeed any 

 glacier is easily accessible, or sufficiently free from snow for 

 accurate observations, having also, especially during the month 

 of September, observed it under every circumstance of weather 

 and a great range of atmospheric temperature, I believe that I 

 have obtained the chief data necessary for basing a theory of 

 its motion, upon sound mechanical principles. The changes 

 which I have witnessed upon its surface, during the period of 



* Edinburgh New Philosophical Journal. Jan. 1843. 



