158 Prof. Forbes' s Tenth Letter on Glaciers. 



to the previous pages, and to the plate, it will be found that 

 I"" 540 was the geometrical depression of the surface of the 

 glacier in 41 days ; and that what is called the mean daily fu- 

 sion is the 41st part of this, viz., 37 millimetres nearly, which 

 includes the various sources of error specified in my Travels 

 as cited by M. Martins. 



The primary object of M. Martins' experiments was to 

 shew that stones might seem to be ejected from the ice in 

 consequence of the mere fusion of the mass surrounding 

 them ; and he found (as was to be expected) that a stone, 

 placed in a vertical hole in the ice constantly fell to a lower 

 level, instead of being expelled upwards. This depression he 

 attributes to " son affaissement dans le trou"* — a very pro- 

 bable explanation, since small stones are well known to pro- 

 duce vertical holes in the ice, and as we have it on M. Mar- 

 tins' authority that the holes contained much water. Hence 

 the attempt subsequently/ madef by M. Martins to deduce the 

 ablation of the surface from the diminishing depth of the 

 holes is subject to the same error which I have noticed in M. 

 Escher's experiments J and in my own,§ where sticks were 

 used to keep the holes open. 



2dly, M. Martins proceeds to shew (Bulletin, p. 232 ; Bi- 

 bliotheque Universelle, p. 333) that my own experiments 

 were subject to errors of the same kind with those of my pre- 

 decessors, in the estimation of the superficial fusion of the 

 glacier ; a point upon which I never insinuated a doubt ; on 

 the contrary, I shewed that the effect was in either case a 

 compound one. I analysed the different concurring causes, 



* Annales des. Sciences Geologiques, par M. Riviere, 1842 ; p, 9, 

 iiote, 



t I say subsequently, because in the original paper the superficial 

 fusion is in every case deduced from the sum of all the depressions. In 

 order to render the discordance of the results in the first and second 

 memoirs less conspicuous, M. Martins has included in his last calculations 

 the depth of freshly fallen snow melted, and counted it as so much solid 

 ice ! Any one who has seen how the first sunshine or rain carries off a 

 few inches of fresh snow can judge of the admissibility of such a pro- 

 ceeding. 



I Travels, p. 153, [2d Ed., p. 154.] § Ibid, p. 154, [2d Ed., p. 155.] 



