Professor Forbes''s Seventh Letter on Glaciers* 245 



Fig. 1. 



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and from the bottom upwards and forwards, as in Fig. 2. 

 You will find that I have endeavoured to explain this in the 

 last chapter of my book of Travels ; but not having it by me, 

 I cannot refer you to the particular passages. The point in 

 question is undoubtedly the least obvious and most difficult 

 part of the theory ; but as I have no doubt of its exactness, it 

 will have a proportionate weight in deciding in its favour the 

 opinion of persons accustomed to mechanical theories. It 

 would be difficult to bring it home to the apprehension of 

 ordinary readers ; and, for this reason, I have dwelt upon it, 

 perhaps, too shortly in the chapter alluded to. 



You will readily admit, that if I shall demonstrate separate 

 reasons for the existence of each of the structures figured 

 above, (the first a plan, the second a section), the result will 

 be the spoon-shaped structure which I have shewn to exist in 

 glaciers. 



(1.) The tearing asunder of the particles of the glacier, 

 owing to the friction of the sides is, nearly, but not quite, pa- 

 rallel to the sides ; for this reason, that the lines of greatest 

 strain are determined, not merely by the force of gravitation 

 which urges the particles forwards, but there is a drag to- 

 wards the centre of the stream, in consequence of the greater 

 velocity there. 



Let A B be the side of 

 the glacier, whilst the par- 

 ticle a moves to a', the cen- 

 tral particle h moves to h\ 

 b' which, owing to the cohe- 

 - /S sive bond between a and h 

 must produce a strain ob- 

 lique to the axis of the 

 glacier. 



Fig. 3. 



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