432 Dr. Whewell's Remarks on Mr. Hopkins's Reply. 



In my remarks, I described four steps which a theorist 

 might make, in passing gradually from the doctrine of a solid 

 to that of a plastic glacier. The first step is, to suppose the 

 solid glacier divided into longitudinal strips by arbitrary sepa- 

 rations ; the second is, to allow a possible separation by sli- 

 ding at all points of the mass ; the third is, to allow that the 

 flanks of the glacier are plastic ; the fourth is, to allow the 

 glacier to have a secular plasticity. Each of these supposi- 

 tions has been made, in order, by Mr. Hopkins ; having been 

 previously excluded by him : and in this manner he has 

 brought his solid theory to agree quite, or nearly, with the 

 plastic theory. In making these changes of his hypothesis, 

 I conceived that he had been influenced by a growing percep- 

 tion of the evidence of Prof. Forbes's theory ; and I thought 

 that in his mode of introducing his hypothesis, he was likely, 

 however unintentionally, to mislead his readers, as to the ob- 

 ligations which the theory of glaciers owes to Prof. Forbes. 

 It appeared to me, therefore, that justice required some one to 

 warn the readers of the Philosophical Magazine of this danger. 



Having done this, I shall not enter further into the subject 

 in your pages. Mr. Hopkins invites me to discuss his ma- 

 thematical proofs on this subject. 8uch a discussion would 

 have too much the character of a personal controversy between 

 us. And I must add, that I do not see any probability that 

 such a discussion would either throw light on the subject, in- 

 terest the public, or bring us to agreement. Mathematicians 

 will judge for themselves of Mr. Hopkins's demonstrations. 

 They will also judge for themselves whether, when a plastic 

 mass is drawn by a force in any direction, it appears by simple 

 mechanical considerations, that the tendency of the parts to 

 separate by sliding will be in the direction of the force, and 

 the tendency to separate by rupture will be perpendicular to 

 the direction of the force. They will further judge, whether 

 such a motion of a plastic mass, as Prof. Forbes supposes in 

 a glacier, will produce separations of the mass in directions, 

 such as those of the structural bands which Prof. Forbes dis- 

 covered. 



I will make one remark. 1 had objected to the term secular 

 plasticity, because the plasticity of glaciers produces a conspi- 

 cuous effect in a few days. Mr. Hopkins says, that "no such 

 application of the term was contemplated." I reply that such 

 an application of the term describing the plasticity of glaciers 

 ought to have been contemplated, after Prof Forbes's decisive 

 observations showing that the effect is conspicuous in a kw 

 days. 



1 will only add, with reference to my supposing that Prof. 



