420 Professor Forbes' s Eleventh Letter on Glaciers, 



The progress of each point, as well in direction as in 

 amount, was rigorously determined by a trigonometrical 

 process, reference being had to two fixed stations, one of 

 which, X, seen in fig. 4 was in the original plane of the 

 points observed, the other was 75525 feet distant to the 

 right hand of fig. 3. The choice of stations was limited by 

 the peculiar local circumstances, and was not otherwise the 

 most desirable. The continual fall of blocks which bounded 

 with great velocity 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 ne- 

 cessary 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 em- 

 ployed 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 , 1 : 1-49 

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

 considerable, and also more difficult of correct estimation, 

 owing to the greater horizontal distance,* but the following 

 results appear to be worthy of confidence. 



(1.) (2.) (3.) 



Ft. Ft. Ft. 

 Motion from 13th Aug., 11 a.m., to 



18th Aug., 3 P.M., . . . 2-87 4-18 4-66 



Ratios, I'OO 1-46 1-62 



Angle {<p) made by the motion with 



the direction of X, . . . 5°.0 8°-3 10°-1 



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

 commencement of the observations, 9579, 138-0, and 246*8 feet. 



