Professor Forbes' s Fourteenth Letter on Glaciers. 327 



ing the observations made by M. Coste, and states that the 

 commissioners have been able to verify their perfect ac- 

 curacy.* 



Fourteenth Letter on Glaciers. Addressed to Professor Jame- 

 son. By Professor J. D. Forbes. With a Plate. 



On the Variation of the Motion at different Seasons, 



Edinbuegh, \2th March 1847. 



My Dear Sir, — I am led to request you to reprint in the 

 Edinburgh Philosophical Journal, part of the 9th section of 

 my paper on the Viscous Theory of Glacier Motion from 

 the Philosophical Transactions for 1846, for the following 

 reason : — There has lately appeared in the Bibliotheque TJni- 

 verselle de Geneve^ a paper by M. Collomb, in which it is 

 maintained, on the authority of the experiments of M. Dollfuss 

 on the glacier of the Aar, that the velocity of progress of 

 that glacier is the same at all seasons and in all weathers ; 

 and that previous observers who arrived at different conclu- 

 sions on that glacier were in error. It may be so. The 

 glacier of the Aar is, as I have elsewhere shewn, f in very 

 peculiar conditions of mechanical constraint ; but it is im- 

 portant that it should be shewn by the testimony of by far 

 the completest series of observations which has yet been 

 published, and which extends over the entire year, that this 

 independence of the velocity upon external circumstances is 

 not at least the general rule. 



The passage in M. Collomb' s paper, to which I have al- 

 luded, may be thus translated : — " It results from the exa- 

 mination of the registers of M. Dollfuss, which contain 

 several thousand observations, that the glacier of the lower 

 Aar moves with perfect regularity ; — no sudden starts nor 

 pauses ; the movement of any particular point taken on the 

 surface of the glacier, whether in the centre or at the edges, 



* Supplement d la Bihlioth. Univ. de Geneve. No. 8, p. 437. 



t Ninth Letter on glaciers, Ed. Phil. Jour., vol. xxxviii., p. 332. I may 

 add, that M. Martins gives a similar account of the results of Mr DoUfuss's 

 experiments in the Comptes Bendus, 26th October 1846. 



