Prof. Tyndall on Ice and Glaciers. 95" 



it, at least to the thickness of one year's fall, or by being repeated 

 in two or more years consolidates it more effectually. Thus 

 M, Ehe de Beaumont most ingeniously accounts for the alleged 

 non-existence of glaciers between the tropics by the fact that the 

 seasons have no considerable variations of temperature, and the 

 thaw and frost do not separately penetrate far enough to con- 

 vert the snow into ice." (Travels, p. 31.) 



This passage naturally rose to my mind when I read the fol- 

 lowing in the thirteenth letter upon glaciers :— " I am satisfied, 

 then (and it is only after long doubt that I venture this con- 

 fident expression), that the conversion of snow into ice is due to 

 the effects of pressure upon the loose and porous structure of 

 the former." This is the only formal retractation which the 

 letter contains; and I think I was justified in regarding it as an 

 abandonment of the view expressed in the passage first quoted, 

 and not as an abandonment of the theory of the veined structure. 



The only mention of " infiltrated water " which the section 

 referred to by Prof. Thomson contains, is in its last paragraph, 

 where it thus occurs :— " We are therefore relieved from the dif- 

 ficulty of accounting for the cold which would be necessary to 

 freeze the infiltrated water which was [I] at one time believed 

 necessary to explain the conversion of the neve into 

 PROPER ICE :" — not a word about the veined structure. 



In no part of this letter, that I can find, does Prof. Forbes 

 state that he has abandoned his first theory of the structure ; 

 and one sentence alone could, by implication, be construed into 

 such an abandonment. Referring to a particular observation 

 made on the Talefre glacier, he concludes " that the conversion 

 into ice is simultaneous and, in this case, identical with the forma- 

 tion of the blue bands." I am quite willing to accept whatever 

 interpretation Prof. Forbes chooses to attach to this sentence ; 

 but in justification of myself I would ask, is it likely that a 

 theory of such importance, and on which so much labour had 

 been expended, was meant to be broken down and rebuilt by a 

 sentence of this kind ? I would here remark that this thirteenth 

 letter can only be aj)preciated by those who have made themselves 

 perfectly conversant witli what Prof. Forbes had before written. 

 In his book, in his previous letters, in his papers in the Phi- 

 losophical Transactions, in his controversial discussions, the 

 same view is constantly advocated. — The blue veins are "un- 

 doubtedly infiltrated crevices," the crevices being produced by 

 " differential inotion." " Mr. Hopkins," writes Prof. Forbes in 

 1815, in reference to an experimental proof, "denies that the 

 ribboned (veined) structure is produced by differential motion, 

 ..... No person who has seen the model made, or even been told 

 how it was made, and inspects the ribboned structure upon its sur- 



