Mr. Hopkins's Remarks on Prof. Forbes's Reply. 593 



subglacial streams, or hanging cavernous above them, must 

 be in this brittle, loose and dissolving condition, and hence 

 must crumble away under pressure from above with great ra- 

 pidity, while the crystalline particles thus pulverized (so to 

 speak) must be washed away and melted in the subglacial 

 water, and thus a source of very rapid descent added to all 

 others. 



Dublin, April 25, 1846. RoBERT MaLLET, 



LXXXV. Remarks on Professor Forbes's Reply. By W. 

 Hopkins, i^sg'., M.A.^ F.R.S., S^c. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 

 T MUST request of you permission to offer a few remarks 

 -*- on Prof. Forbes's reply to my letters on glaciers. 



The Professor says that 1 have misrepresented his theory. 

 The first proof he offers of it is, that I have dismissed it some- 

 what too summarily in my second memoir on glacial motion. 

 This may imply an inadequate appreciation of the theory, in 

 the estimation of its author, but ought not assuredly to be 

 considered as a misrepresentation of it. 



Again, the Professor cites, as another instance of misrepre- 

 sentation, my observations respecting the inadequacy of his 

 theory, even if his mechanical reasoning were admitted, to 

 account for the observed convexity of lines of fissure. His 

 curves of structure have been uniformly represented as elon- 

 gated loops, of which the convexity is turned towards the 

 lower end of the glacier; and he has said (I have quoted the 

 words in my last letter) that "crevasses will naturally occur, 

 crossing the structure at right angles,^' conceiving that he has 

 previously demonstrated that the directions of the loops will 

 be those of maximum tension*. In my remarks on these 

 statements, I asserted that according to this theory the incli- 

 nation of the crevasses to the axis of the glacier must be less 

 for points nearer than for those more remote from the axis, as 

 represented in figure 1 in my Third Letter, and copied by 

 Prof. Forbes in his reply. That such is the case is too ob- 

 vious to require a moment's consideration, and yet the Pro- 

 fessor exclaims. How could I suppose him so dull as not to be 

 aware that, in the centre of the glacier, the crevasses and the 

 loops must be parallel'^ My reply is, that I had his own ex- 

 plicit declaration for it, that the loops and crevasses were 



* There is a manifest oversight in the note at the bottom of page 378 

 of the Travels, where it is stated that the structural bands me perpendicu- 

 lar to the lines of greatest tension. 



Phil. Mag. S. 3. No. 1 76. Suppl. Vol. 26. 2 R 



