Prof. Forbes's Tenth Letter on Glaciers. 159 



and indicated a method by which in future the superficial 

 loss might be accurately determined, namely, by driving a 

 horizontal hole into the wall of a vertical fissure, and by ob- 

 serving the depression of the surface relatively to it ; (Tra- 

 vels, p, 154, 1st edit. ; p. 155, 2d edit.) 



M. Martins has described the two methods which I used 

 for determining the total fall of the surface of the ice, and I 

 believe correctly. I have, in the very pages of my work to 

 which he refers, guarded against the supposition that these 

 could measure the superficial waste only. With respect to 

 i\iQ first method, I have said, p. 153, [2d Ed., p. 154] — " Now, 

 this depression is not necessarily the result of superficial waste 

 alone. I doubt whether it is even mainly due to that cause.^^ 

 Consequently there was here no pretension to an infallible 

 measurement of the ablation of the ice. Again, the second 

 process is described (p. 154,) [2d Ed., p. 155,] as *' a very sim- 

 ple method of measuring the absolute* depression of the sur- 

 face.'' There, also, the result is correctly limited and defined. 

 M. Martins says, — " En lisant I'ouvrage de M. Forbes xm lec- 

 teur pen attentif ne doutera pas un instant qu'apres avoir re- 

 jete comme inexactes la methode de M. Escher et la mienne, 

 Tauteur n'en ait employe, ou du moins n'en propose, une 

 troisieme, pure de tons les defauts qu'il reproche a celles de 

 ses predecesseurs." Now, I have shewn that I did propose 

 such a correct method, though I had not then put it in prac- 

 tice, and that I did not claim any exemption from error, or 

 rather from a complication of effects, for those which I did 

 employ. A few pages farther on, M. Martins admits as 

 much ; for he says, — " M. Forbes semble avoir compris lui- 

 meme que ses deux methodes ne lui donnent que la somme 

 des effets dus a Taff'aissement, ^ tassement, a la progression 

 eta la fusion du glacier, car il dit," &c. Thus it appears that 

 M. Martins first imagines what " un lecteur peu attentif '* 

 might gather from what I have said, or rather from what I have 

 left unsaid ; and then, after refuting the erroneous conclu- 

 sions of the inattentive reader, he concludes by shewing that 

 I had written so explicitly as to render a mistake impossible 



♦ Italics in the original. 



