Professor Forbes' s Eleventh Letter on Glaciers, 417 



seven-tenths were produced by ablation, three-tenths by sub- 

 sidence. These relations, together with those opposite the 

 old station Q, are shewn in one view in the following table of 



:Mean Results. 



Station U. 

 Station Q. 



Slope of 

 Surface. 



2°£ 



Daily Pro- 

 gression. 



Inches. 



18-7* 

 21-2 



Daily 

 Ablation. 



Daily 

 Subsidence 



Inch. 

 3-62 



2-73 



Inch. 



1-63 

 0-97 



Geometr. 

 Depression 



Inch. 

 6-25 



3-70 



Proportion due to 

 Ablation. Subsid* 



•74 



•31 



•26 



The last two columns shew the effects of the ablation and 

 subsidence in hundredth parts of the whole depression. 



As we do not know coiTectly 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 pai*t of it may be thus accounted for. The 

 amount of geometrical depression agrees well with that as- 

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

 intermediate 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 41 inches. t 



Relative Velocity of the Surface and Bottom of a Glacier, 

 The influence of the sides or walls of a glacier in retard- 

 ing its motion laterally, was demonstrated by my first obser- 

 vations in June 1842 ; and the same cause might well be pre- 

 sumed to influence the motion of the ice in a vertical plane. 

 That the superficial ice should overflow that which presses on 



* Taken from the observation of the neighbouring mark D*2. 

 t 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 compari- 

 son 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. 90 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. 9 392 ... 



Aug. 9— Sept. 17 306 ... 



