160 Prof. Forbes' s Tenth Letter on Glaciers. 



unless by overlooking entire sentences.* Finally, he says, 

 that a reviewer of my Travels has misunderstood the pas- 

 sages in question. This is surely no ground for animadver- 

 sion against me. At all events, the very distinguished 

 author of the article in the review in question is abundantly 

 qualified to defend himself. 



There are other points, both of fact and of reasoning, in M. 

 Martins' article with which I do not agree ; but, as I do not 

 write in a controversial spirit, I think it needless to discuss 

 them, since those who are qualified to form a judgment will 

 do so without difficulty and without my assistance. I have 

 merely wished to remove from the mind of M. Martins and 

 his readers the idea that I had intended the slightest dis- 

 courtesy to him personally, or the depreciation of an obser- 

 vation to which it appears that he now attaches a peculiar 

 importance. I remain, my dear sir, yours very truly, 



James D. Forbes. 



Professor Jameson. 



* The following is the entire passage which has occasioned M. Mar- 

 tins' unexpected remonstrance. After quoting the superficial depres- 

 sion as observed and measured by myself, I say, '^ Now this depres- 

 sion is not necessarily the result of superficial waste alone. I doubt 

 whether it is even mainly due to that cause, and not to a subsidence of 

 the entire mass of the ice, which visibly collapses as the warm season 

 advances. Such a collapse may be due to several circumstances : 1. The 

 undermining of the glacier by the excavating action of the water streams 

 which flow beneath it in summer ; 2. The fusion of the ice in contact 

 with the soil, due to the earth's heat ; 3. The lower extremity of the 

 glacier moving faster than its higher portions, and thus extenuating the 

 mass, a cause which acts with energy at those seasons when the differ- 

 ence of motions of the two parts is a maximum. The superficial waste 

 is not so easily measured as at first sight it might appear to be. M. 

 Escher de la Linth measured it in 1841, on the glacier of Aletsch, by 

 the exposure of stakes inserted to a certain depth in the ice, — as the ice 

 melted, the stakes were exposed. M. Martins measured it by the geo- 

 metrical depression of the surface. The last method we have seen mea- 

 sures several effects instead of one ; the former may lead to the most 

 inaccurate results. When the stakes have been exposed to a certain 

 depth, the apparent result is actually inverted — the hole is deepenedj' — 

 Travels in the Alps, Ist Ed., p. 153. 



M. Martins is never again mentioned nor alluded to. 



