S$S Mr. Hopkins's Eeplj/ to Dr. Wliewell's 



If not, by wliat process of exliaustion has he proved that the 

 alternation of bands of transparent and opake ice can arise 

 onlT/ from planes of separation ? Again, to explain the'forma- 

 tion of such planes, it is asserted, that, because there will be a 

 drag towards the middle, it will be in that direction that fila- 

 ments of the mass will slide past each other. But I maintain 

 that the fact of the drag being a maximum in that direction 

 is the very reason why the filaments in that same direction 

 will not slide past each other. A filament tends to slide past 

 a contiguous one, not because both are dragged, but because 

 they are dragged unequally ; whereas, in the direction in which 

 the drag is greatest, the filaments are e5'«a% dragged, simply 

 because the drag in that direction is a maximum. Dr. Whewell 

 appears in the first instance to have objected to Prof. Forbes's 

 mechanical theory of the laminar structure, asserting that the 

 planes of separation ought to be parallel to the sides of the 

 glacier^. This was not accurate, except in the particular case 

 of the absence of transversal pressure (Second Letter, art. 25, 

 p. 165), but was still the kind of approximation to the truth 

 which a first and general view of the problem was almost sure to 

 suggest. What may have been the "simple mechanical views" 

 which led him afterwards to adopt the reasoning of Professor 

 Forbes, I am unable to comprehend. It would have been 

 much more satisfactory if he had stated what those simple 

 views are; and it would have been better, if he would persuade 

 others to adopt them, that he should have given some proof 

 of the fallacy of my investigations on this point, since those 

 investigations (which must have been before him when he wrote 

 his second letter) lead to results so entirely at variance with 

 the conclusions he has adopted. This is what Dr. Whewell or 

 some other advocate of Prof. Forbes's theory must do, or that 

 theory, as affording a mechanical explanation of the handed 

 structure^ must necessarily fall. It cannot stand unless support- 

 ed by more conclusive reasoning than that on which alone it has 

 hitherto been made to rest. Such cases as that appealed to by 

 Prof. Forbes in the last Number of your Magazine (p. 206) may 

 afford analogies which may be delusive, or may be ultimately 

 useful in guiding us to the true mechanical solution of the 

 problem before us ; but such cases differ too widely from that 

 of a glacier to afford anything more than analogies, even if 

 the problem which each particular case presents were com- 

 pletely solved ; and assuredly, an appeal to such cases can 



tral part and the exterior ring being always opake. These successive rings 

 (with the exception of their circular form) exactly resembled the alternate 

 opake and transparent bands in glacial ice. [On this subject, see Mr. 

 Darwin's remarks quoted in p. 354 of the present number. — Edit.] 

 * See Professor Forbes's Seventh Letter on Glaciers. 



