A.] . FORBES AND RENDU. 537 



Even bad the guides of M. Eendu, favoured by some excep- 

 tional circumstances, commenced observations on a block so ex- 

 travagantly retarded by the glacier-wall, it is physically certain 

 that before two seasons, much more before five, it would have 

 been stranded on the fixed moraine, conformably to the rule 

 in such cases. 1 



But I have not quite done with the quotation from Renclu 

 (A, page 15). Professor Tyudall says, at page 306 of his 

 work, that I came tardily in 1845 to admit a differential 

 motion of a glacier even greater than Rendu admitted in 1811. 

 The distinction in the two instances lies in this, that what 

 Eendu admitted as applicable to the Mer de Glace at Montan- 

 on very indifferent evidence, I admitted in a different case 

 or cases, and on satisfactory geometrical proof. It is certainly 

 the tirst time that it has been urged against me that I was not a 

 thoroughgoing believer in my own theory. Most persons pro- 

 s' thought that I was too facile in accepting evidence in 

 ir of it. The fact undoubtedly is, as Professor Tyndall says, 

 that in 1844 and 1845 I was perfectly satisfied, that in im- 

 mense glacier streams of small slope and low velocity, like 

 those of Aletsch and the lower Aar, the central velocity might 

 diminish towards the side in almost any given ratio. The evi- 

 dence on which I accepted this astonishing proof of plasticity 

 was in the first case (Aletsch) my own observations with the 

 theodolite, in the second (Aar) similar measures by M. Agassi/' 

 surveyors. P>ut I did not do so without calling attention (which 

 Professor Tyndall omits to state) to the fundamental diffe- 

 rence bet wen these glaciers and the Mer de Glace, which pre- 

 ta us from expecting so striking a confirmation in the 

 latter case. ' A glacier/ I then stated, ' like the Mer de Glace 

 of Chamouni has so considerable a velocity (on an avern. 



three times that of the glacier of the Aar), that the ice 

 i- impetuously borne along and torn from the sides at tin 

 pense of innunn-rabk' lacerations and crevasses. So that, in the 

 whole extent of the middle and lower regions of that glacier, in 

 no place do the ice and ground meet without the former 1 icing 



6 or less fUsim-d by rents. But the contrary is the ca 

 the L - icrs, which move on small slopes, and with smaller 



I am sorry to have been obliged to dilate so much upon the 



1 This happened afu-r twelve months lo the Hloek D 7, which I com- 

 menced observing in Sej under th.-sr riiTuinMaiin s, ami nr:r 

 -ame locality. Sec my Travel*, second edition, chap, vii., and < 



* At , 73. 



