Professor Forbes's Eleventh Letter on Glaciers. 419 



wanting, and there, consequently, the veined structure is al- 

 ways feebly developed. 



I succeeded, however (though not without difficulty), in 

 establishing points of observation in the terminal face of the 

 Glacier des Bois, at Chamouni, whose relative position will be 

 understood from the sketch of a front view of the glacier, 

 fig. 3, and from the vertical section parallel to the length of 

 the glacier in fig. 4. It will be seen by fig. 3, that whilst the 

 lateral parts of the glacier were fissured and scaly, in conse- 

 quence of the action described in the last paragraph, the cen- 

 tral mass was exceedingly compact and uniform. The ter- 

 minal face of compact ice was inclined, at the point selected 

 for experiment, at an angle of about 40° to the horizon, and 

 the three stations (1.), (2.), (3.), were selected one above the 

 other, in a vertical plane passing through this face in a di- 

 rection which w^as judged to be nearly that of the progressive 

 motion of the ice. Any variation in the motion of these three 

 points could only be imputed to the effect of the friction of 

 the soil, for by the progress of the glacier each would pass in 

 succession vertically over the same spot. The position was 

 also unexceptionable in this respect, that the glacier is here 

 subject to no extraordinary constraint. The sides are free, 

 and the base rests on a bed of rubbish and debris nearly flat, 

 therefore offering no fixed barrier to the forward motion of 

 the ice ; the retardation, if it exist, can only, therefore, be 

 due to the ligitimate effect of friction. The glacier, too, is 

 here seen from top to bottom, for the contact with the soil is 

 only concealed by the trifling mound of rubbish not many 

 feet in height, shewn at M in fig. 4, which it presses before 

 it ; the gravel between M and X being flat, and untouched 

 by the glacier for many years. The lowest mark (1.) was esti- 

 mated at not less than 4 feet, and not exceeding 12 feet from 

 the real base or soil of the glacier. The mark (2.) was 46 feet 

 vertically above (1.), and No. (3.) was 89 feet vertically above 

 No. (2.) From the analogy of the lateral friction of glaciers, 

 and from the phenomena of rivers, it was anticipated that 

 the retardation of (1.) upon (2.), and of (2.) upon (3.), would be 

 sensible, but that the former would be greatest, which the 

 results confirm. 



