70 



NINTH LETTER ON GLACIERS. 



[1845. 



viscous theory ; and the glacier of the Aar, in this respect, a 

 more instructive example than the Her de Glace of Chamouni. 

 I have shown in one of the passages of my work just cited, 

 that the velocities of the different portions of the glacier depend, 

 among other things, on their inclination or slope ; and hence, I 

 should have inferred, that in a glacier which did not slope faster 

 and faster towards its lower end, till it becomes almost precipi- 

 tous, there would be accumulating resistance due to the friction 

 of the ice on the bed of a long, nearly uniform, gently sloping 

 valley, such as that which contains the glacier of the lower Aar, 

 which must magnify the tendency which the ice has to be 

 squeezed forwards and upwards against the mass immediately 

 in advance of it, which produces the frontal dip of the ribboned 

 structure or slaty cleavage of the ice, in the way that I have 

 explained in my Seventh Letter.* In a glacier then, whose 

 slope is nearly constant and small, I should expect a condensa- 

 tion of the ice longitudinally, and a swelling of the surface de- 

 pending upon the motion of the plastic ice in the direction of 

 least resistance. Now this is exactly what we have in the 

 results of measurement. If the annexed figure represent the 



/ 



Kg. 17. 



plan of the glacier, and the ice be divided into imaginary com- 

 partments by vertical sections ; since, whilst AB moves 177 

 feet, CD moves but 141 feet, there is a condensation of the 

 mass of ice ABC D, from back to front, of no less than 36 feet 

 in that time, and so for the successive slices EF, etc. How 

 then, is this shrinking to be accounted for ? Not by the mere 



* [See page 59.] 



