Prof Forbes's Tenth Letter on Glaciers* 157 



ti€ pressure, which might arise from water being sealed up 

 in cavities between the glacier and its bed, it seems unneces- 

 sary to take any account, until the possibility or existence of 

 such water-tight cavities shall have been proved. 



* I have observed with regret that M. Martins, in a paper 

 published in the Bulletin de la Societe Geologique de France 

 (Seance 20 Janvier 1845), and in the Bibliotheque Univer- 

 selle de Geneve, considers that I have not sufficiently ac- 

 knowledged his prior observations on the superficial fusion 

 of the ice of glaciers, and that I have attributed to them errors 

 from which they were free. I notice this paper rather for 

 the purpose of expressing my regret that I should involun- 

 tarily have given offence to M. Martins than because there 

 appears to have been anything in my statements requiring 

 correction ; for his own paper furnishes a reply to his alle- 

 gations. Nor should I now have mentioned the subject, 

 but that, having been called upon to revise a new edition of 

 my Travels, without having it in my power to consult the 

 Bulletin (which had not reached this country, and of which 

 no copy has been communicated to me by M. Martins), I 

 might appear to pass over in silence the appeal thus made to 

 me. 



1*^, I am charged with having stated that the method by 

 which MM. Martins and Bravais measured the superficial 

 fusion of the Blau Gletscher in 1841, did not determine 

 that quantity correctly, but " measures several effects in- 

 stead of one." To this statement I adhere. M. Martins' 

 original Memoirt contains the following conclusion : — '' Pen- 

 dant I'ete de J841, savoir du 26 Juillet, au 4 [5 ?] Septembre 

 la fusion diurne moyenne a ete de 37 millimetres, et la sur- 

 face du glacier s'est abaissee pendant la meme periode de 

 1™ 540 en estimant a 55 millimetres la fusion des demi jour- 

 nees tres-chaudes du 7 et du 8 Aout." Now, by a reference 



* The latter portion of this letter was written in July. 



t Annales des Sciences Geologiques, par M. Riviere, 1842 ; p. 11 of 

 the separate copy sent to me by M. Martins, which I did not receive 

 until long after my return from Switzerland in 1842, when my own ex- 

 periments were made. 



