328 Professor Forbes's Fourteenth Letter on Glaciers. 



is very slow, uniform, and independent of atmospheric in- 

 fluences. [He goes on to say, that, a^ it is fastest in the 

 centre of the glacier, and almost nothing at the edge], Fine 

 weather or rain, dryness of the air or humidity, cold or heat, 

 day or night, or different seasons, have no influence upon the 

 velocity of any particular point of the surface of the glacier. 



" The observers of previous years believed, on the other 

 hand, that the general march of great glaciers was in inti- 

 mate connection with the accompanying state of the atmo- 

 sphere. They believed, for instance, that the movement was 

 retarded in dry and cold weather, and accelerated by mois- 

 ture and rain. The observations of M. Dollfuss, which will, 

 without doubt, be published one day in detail, have proved 

 the contrary, and have shewn that a glacier^ like a semifluid 

 body, urged by a mechanical force, moves on without being in- 

 fluenced by the state of the surrounding mediumy^ 



Now, Sir, the last passage, the italics of which are in the 

 original, conveys to us, in the first place, the pleasing infor- 

 mation that M. Collomb, and probably M. DoUfuss also, 

 have accepted the viscous theory of glacier motion. The 

 word " semifluid'* applied to a glacier, is now, notwithstand- 

 ing its seeming harshness, an adopted word. But I must 

 enter an earnest protest against the supposed discovery of 

 the uniformity of the motion and its entire independence of 

 atmospheric circumstances, being assumed to add any proba- 

 bility to the viscous theory, as the phraseology of the preced- 

 ing extract seems to infer. On the contrary, if there be any 

 glacier which does not present the law of variable velocity, 

 which I established, for the first time, on the Mer de Glace 

 of Chamouni in 1842, and which has since been found on the 

 glacier of Bossons, on the glacier of Grindelwald, and was 

 supposed to have been found on the lower glacier of the Aar, 

 such a glacier affbrds a proof the less in favour of viscous or 

 semifluid theory. 



* Bibliotheque Vhiverselle, publi6e 15 Decembre 1846, p. 212, note. The pas- 

 sage in italics stands thus in the original, — *' Un glacier comme un corps semi- 

 fiuide pousse par une force mecaniqut, marche en avant sans se laisser influencer 

 par Vital du milieu ambiant." 



