( 136 ) 



Thirteenth Letter on Glaciers, Addressed to Professor Jame- 

 son. By Professor J. D. Forbes.* 



Acceleration of the Surface Motion confirmed — Nevj Deter- 

 minations of the Velocity of different Glaciers — Discovery 

 of a Knapsack buried ten years in the Ice — Manner of 

 Conversion of the Neve into Ice — Manner of ejection of 

 Stones from Glaciers, 



My dear Sir, — Since the completion of my Twelfth Let- 

 ter, I have observed, in the Comptes Kendus for 26th Octo- 

 ber, an account of a communication made to the Academy of 

 Sciences by M. Martins, of an experiment on the relative 

 motion of the surface and inferior part of a glacier, on which 

 I have also made the experiments detailed in my Eleventh 

 Letter (dated 16th September), and published on 1st Octo- 

 ber in The New' Edinburgh Philosophical Journal. My ex- 

 periments, it will be recollected, were made at three points 

 of the terminal face of the Glacier des Bois, and proved, as I 

 had long ago anticipated, that the superficial ice has by much 

 the most rapid motion. MM. Dollfus and Martins arrive at the 

 same result, establishing the desired identity with the motion 

 of rivers ; but their experiment being made, not on the termi- 

 nal face, but only on the steep lateral face of a glacier, is open 

 to the possible objections which I anticipated in my former 

 letter, and which I carefully endeavoured to avoid (see p. 418 

 of the Eleventh Letter, and fig. 2 of the accompanying Plate), 

 so that even the most scrupulous might be, if possible, satis- 

 fied. It is pleasing to me to find that the French observers 

 corroborate my result ; but I must remark, that they obtain- 

 ed it a fortnight later, and my publication preceded theirs 

 nearly a month. 



I shall not seem to insist too much upon the conclusive- 

 ness of this observation, when it is recollected that an oppo- 

 nent of the Viscous Theory has virtually staked the question 

 of the cause of glacier motion upon such an experiment (not- 

 withstanding that I think he has attached to it an undue im- 

 portance), in the following passage : — 



t< * * The claims of the two theories (the sliding theory 



♦ Read before the Royal Society of Edinburgh, 21st December 1846. 



