^36 Mr. Hopkins's jR^p/j/ to Dr. Whewell's 



the discontinuity characterizing the second class of problems 

 exists, and equally absurd to designate as solid a mass in which 

 on account of the small cohesion between its particles, such 

 discontinuity could not be produced by forces acting continu- 

 ously upon it, or, if produced by impulsive action, could only 

 be momentarily maintained. I am not asserting that we have 

 thus the means of drawing an accurate line of demarcation 

 between that which is solid and that which is semifluid ; nor 

 am I asserting that there are no cases with respect to which 

 there might not be doubt as to which of the above classes they 

 must be referred. Those points are immaterial with reference 

 to any practical application of these considerations to the main 

 subject before us. I only wish to indicate a class of cases in 

 which the term solid ought to be used in contradistinction to 

 semifluid; and (what is of much more importance) to point 

 out the existence of the two classes of problems above defined, 

 and to remind those interested in the subject, that solutions for 

 one class cannot be applicable to the problems of the other. 

 The case of a glacier is manifestly that of a discontinuous 

 mass, for it is full of dislocations ; the case of a semifluid sli- 

 ding down a sloping canal (the problem proposed by Dr. 

 Whewell as best calculated to elucidate the motion of a gla- 

 cier) belongs to the other class ; and, therefore, I maintain 

 that if the complete solution of this problem were attainable, 

 it could only afford an imperfect and, possibly, a delusive type 

 of glacial motion. The investigations which have for their 

 object the determination of the internal pressures and tensions 

 of a glacier, and the laws according to which the breaches of 

 continuity will take place, setting out with certain well-esta- 

 blished data respecting the motion, are likely to prove far more 

 useful in explaining the real mechanism of the motion than 

 any attempt at the mathematical solution of the problem under 

 its most general form. Such investigations I have given in 

 my Second Lettei-, Phil. Mag., February. The gentleman to 

 whom I am now replying is one of the few interested in the 

 subject who may be expected to grapple with its mathematical 

 difficulties. I invite his criticism on the solutions I have 

 offered. 



Dr. Whewell has pointed out the phaenomena which would 

 be presented by ajlexible glacier, in contradistinction to those 

 which would characterize a viscous or plastic one. He states 

 that, in the first case, " the straight lines originally trans- 

 verse will become curves, with the convexity downwards. 

 The whole mass will be in a state of tension, produced by this 

 distortion; but the distortion will be limited, and the down- 

 ward projection of the curved lines also limited, by the flexure 



