188 THIRTEENTH LETTER ON GLACIERS. [1846. 



theory to be adopted ; the observations having been made, too, 

 one at " the extremity," the other " along the flanks," as sug- 

 gested in this passage. 



I shall now proceed to conclude the account of the most 

 material observations which I had an opportunity of making last 

 summer ; and those which remain refer chiefly to three points : 

 (1.) The rate of motion of glaciers generally, and especially 

 during the summer of 1846 ; and, (2.) To the conversion of the 

 Neve into the icy condition ; (3.) As to the apparent ejection of 

 stones upon the surface. 



(1.) RATES OF MOTION. 

 A. At points previously observed. 



It is very interesting to compare the ANNUAL motion of 

 glaciers in different years, and also the comparative motions of 

 a given point, at the same season, in different years. I have 

 obtained several such results on the Mer de Glace of Chamouni, 

 all of which tend to show a marked increase of the rates of mo- 

 tion during the two years which have elapsed since my last 

 measures were made. 



Thus, at station C, the Pierre Platte, opposite to the pro- 

 montory of Tacul,* from the 19th August 1844, to 21st July 

 1846 (701 days), the advance was 622 feet, or 10'65 inches 

 daily. 



The following little Table shews the relation of this to 

 former years : 



The mean velocity was, therefore, about a fourth part 

 greater in 1845-6 than in 1843-4. 



The following Table shews the daily motion, in inches, of 



* See Travels in the Alps, 2d Edition, pp. 92 and 135, and Map annexed, 

 f [The numbers for 1842-3 and 1843-4 were inadvertently transposed in the 

 former impression.] 



