1846.] VELOCITY OF A GLACIER AT DIFFERENT DEPTHS. 175 



from the terminal face of the glacier, rendered it necessary to 

 consult the safety of the observer and the instrument ; and in 

 order to plant and maintain the wooden pins which marked 

 the points (1), (2), (3), it was necessary to commence by 

 laboriously removing the blocks and rubbish from the surface 

 of the glacier above, whose fall would at every instant have 

 threatened the safety and even the lives of my assistants. 

 Two men were laboriously employed for some hours at this 

 task. 



Circumstances prevented me from pursuing these observa- 

 tions for more than five days, which was to be regretted ; but, 

 in this time, ample evidence was obtained of the existence 

 and amount of the effect of friction in retarding the lower 

 ice. Less than 50 feet of thickness between Nos. (1) and (2) 

 corresponded to an apparent acceleration of nearly half the 

 motion at the lower point. The ratios of the motion at (1) and 

 (2) were, by three independent sets of observations, 

 1:1-41 ... 1:1-50 'V 1:1-49 



the acceleration of (3) upon (2) was (as anticipated) less con- 

 siderable, and also more difficult of correct estimation, owing to 

 the greater horizontal distance,* but the following results appear 

 to be worthy of confidence. 



?, a ft 



Motion from the 13th Aug., 11 A.M. to> Q7 . 1Q . 

 18th Aug., 3 P.M. .| 2 ' 8 



Eatios '// .... 1-00 2-46 1-62 

 Angle (p) made by the motion with 



the direction of X . . . 5-0 83 10-1 



The three points being approximately, 8, 54, and 143 feet above 

 the bed or floor of the glacier. 



These results have been computed by the following formulae, 

 which may be useful to those desirous of repeating the observa- 

 tions : 



Let X and x be the two trigonometrical stations from which 



* The horizontal distances of the points (1), (2), (3), from X were, at the com- 

 mencement of the observations, 95*79, 138'0, and 246'8 feet. 



