Professor Forbes' s Thirteenth Letter on Glaciers. 137 



** and that of plastic motion) will undoubtedly be determined 

 " by other means. The observations required are such as 

 " shall determine, as far as possible, the motions of the upper 

 " and lower surfaces of a glacier. We may never hope to 

 " have access to the bottom of a glacier in its deeper portions, 

 ** but at the extremities of glaciers the amount of sliding 

 " may easily be ascertained, as well as at many other points, 

 " probably, if sought for, along their flanks : fissures, also, of 

 " considerable depth are not unfrequently met with, in which 

 " the deviation from verticality, if it exist, might be easily 

 •• determined ; and though the evidence thus obtained might 

 " not aflford positive demonstration with respect to the deepest 

 *• portions of a glacier, still, should it all concur in shewing 

 " an approximate equality in the motions of the upper and 

 " lower surfaces, every candid and impartial mind must admit, 

 " I conceive, the sliding in preference to the viscous theory ; 

 " but if, on the contrary, it should be proved that the velocity 

 *' of the upper bears a large ratio to that of the lower surface, 

 " the claims of the latter theory must be at once admitted."* 

 It is very fortunate that independent observers on different 

 glaciers should have arrived, in ignorance of each others' re- 

 sults, at conclusions which permit only the alternative favour- 

 able to the viscous theory to be adopted ; the observations 

 having been made, too, one at ** the extremity," the other 

 " along the flanks," as suggested 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 con- 

 version of the Neve into the icy condition ; (3.) As to the ap- 

 parent 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 



* Mr Hopkins, ia London Philosophical Magazine, March 1845, p. 250. 



