244 Professor Forbes's Seventh Letter on Glaciers. 



But, even setting aside this important consideration, the 

 principle of the variation of velocity being chiefly confined to 

 the neighbourhood of the sides and bottom, and the compara- 

 tively quiescent and passive state of the central and super- 

 ficial part, seem sufficient to explain the facts within the rea- 

 sonable limits of error. The depth of 140 feet appears, from 

 M. Agassiz's own observations, not to exceed one sixth, at 

 most, of the depth of the glacier of the Aar in that part. 

 Now, let ABC, &c., represent points in the vertical section 

 of the glacier ; then, from all that we know of the superficial 



motion of glaciers, or of the parallel case of rivers whose 

 velocity has been ascertained at diff'erent depths, the veloci- 

 ties will vary in some such manner as A a, B 6, C c, &c., — the 

 variation being scarcely sensible at first, and very rapid at 

 the bottom, where the velocity may even be zero, if the curve 

 be prolonged to the point h. But, supposing G to be the 

 bottom of the glacier, it will be seen how insignificant may be 

 expected to be the variation of velocity between A, the sur- 

 face, and B, one-sixth of the depth, during the short period of 

 a few weeks, or even months. I have the honour to be, &c. 



Seventh Letter on Glaciers, — On the Veined Structure of the 

 Ice. Addressed to the Rev. Dr Whewell, Master of 

 Trinity College, Cambridge. By Professor Forbes. 



Salerno, May 18. 1844. 

 * * * ***** 



You object that the shells produced by the rupture of the 

 parts of the ice caused by excessive friction, should be all 

 parallel to the sides and bottom of the trough of the glacier, 

 instead of being inclined from the sides inwards and forwards 

 towards the centre, as in Fig. 1, 



