172 ELEVENTH LETTER ON GLACISES. [1846. 



As we do not know correctly the slope of the bottom or 

 bed of the glacier, it is impossible to estimate how much of the 

 subsidence is owing to the declivity. It is probable, however, 

 that the greater part of it may be thus accounted for. The 

 amount of geometrical depression agrees well with that ascer- 

 tained by me in 1842, at the Angle, which is in a position in- 

 termediate between the stations U and Q, but nearest to Q. 

 During the height of summer, i. e., from the 26th June to the 

 28th July, the daily depression was 4*1 inches.* 



Relative Velocity of the Surface and Bottom of a Glacier. 



The influence of the sides or walls of a glacier in retarding 

 its motion laterally, was demonstrated by my first observations 

 in June 1842 ; and the same cause might well be presumed to 

 influence the motion of the ice in a vertical plane. That the 

 superficial ice should overflow that which presses on the bottom, 

 seems a simple corollary, from the fact that the centre of a 

 glacier flows past its sides. Even admitting the , irregularities 

 of the bottom to be less than those of the lateral expansions 

 and contractions of the valley, the enormous pressure on the bed 

 must generate a friction proportionally great. Some persons 

 have, however, found so much difficulty in conceiving the fact 

 of varying velocity in a vertical plane (notwithstanding the 

 evident analogy of a river), that I was glad to take an unex- 

 ceptionable opportunity of demonstrating it. 



I have already shown, at the close of my Sixth Letter, that 

 the effect of friction in retarding the rate of motion must be 

 most sensible nearly in contact with the soil ; and that when 



* As some numerical or typographical errors have slipped into the Table of 

 Depressions of the Level of the Ice at the Angle in 1842 (Travels, p. 154, 2d Edit.) 

 I take this opportunity of correcting them, after a careful comparison with my 

 note-books. The observed depression from June 26 to June 30 ought to be 1 ft. 

 4'5 in., instead of 1 ft. 9'0 in. ; and the daily depression should be the following : 

 1842, June 26 June 30 . . . 4-1 inches. 

 June 30 July 28 . . . 4'09 

 July 28 Aug. 9 ... 3'92 

 Aug. 9 Sept. 17 ..- . . ; . 3-06 



