A.] FORBES AND RENDU. 535 



and was, I believe, due to M. Agassiz having trusted these im- 

 portant measurements to his guides, without having given them 

 any previous training. So little can even the measures of untu- 

 tored peasants be relied on. 



Rfindu's Estimates. From M. Eendu's work, it would appear 

 that one solitary measurement of any description made by himself 

 upon a glacier is recorded. These are his words : ' J'avais fixo 

 en 1838 la position de deux blocs de rochers qui etaient a la sur- 

 face du Glacier des Bois ; une amide apres je suis alle* mesurer 

 le chemin qu'ils avaient parcouru ; 1'uii avait avance* d'environ 

 qiuitre cent pieds, et 1'autre avait disparu/ &C. 1 Will it be 

 maintained that this rough estimate of ' about 400 feet ' is to 

 be the geometrical basis of the whole theory of glaciers ? 



Rendu's other estimates repose, like those of most of the earlier 

 writers, on the reports of the guides, and he does not hesitate 

 to let us know in many places the uncertainty which he attached 

 to them. He describes them as ' des donndes un peu vagues,' 

 ' pas toujours pre'cise'ment d'accord,' ' manquent de precision 

 de mesure et de quantiteV 2 Indeed, with reference to the 

 reported observation of an annual velocity of 40 feet per annum 

 near the side of the Glacier des Bois, near Montanvert, there 

 was enough on the face of the statement to inspire doubt. In 

 the first place, it was six times less than the velocity of a block, 

 described as ' sur le bord/ which he had quoted (at p. 85), on 

 the same authority, at 242 feet. In the next place, it was the 

 result of the recollected (not recorded) position of the block, one, 

 two, three, four, and five years previously. No one who knows 

 the ideas of the peasants of Ghamouni on such subjects twenty 

 years ago can give credit to such an alleged precision of memory, 

 and I am confident that in this Professor Tyndall will agree 

 with me. Lastly, it implied, if true, an amount of plasticity in 

 the ice which ought to have excited an amount of scientific scep- 

 ticism which could only be overcome by direct and authentic 

 measurements. No such were produced. 



Now, on this last point Professor Tyndall quotes Rendu's faci- 

 lity of ln.-li'f in admitting that the central velocity might be at 

 the Montanvert ten times that of the lateral velocity of the ice, 

 in favourable contrast to my obvious doubt of the fact expressed 

 by the mark of admiration (!) which I attached to the citation 



at my surest ion, and were chiefly pursued by methods specifically indicated 

 by me f : 'hods similar to those which 1 myself nut in practice. 



It is also well ki no such ohservat ions had heeii undertaken by M. 



Agassiz until after the per od -f my visit to the glacier of the Aar. I had 

 publicly urged the necessity for suck observations being undertaken, iu the 



'< Review for April 1812. 

 1 Thiorit, p 86. Ibid. pp. Ul, U5. 



