406 Prof. Forbes's Reply to Mr. Hopkins 



and the like, against which the author is nominally contending, 

 being plentifully scattered over the whole of his later writings. 

 Yet throughout, my work is quoted only as a repertory of facts, 

 or as containing deductions fit only to be refuted. 



2. Mr. Hopkins takes his stand on ground where no pru- 

 dent theorist will follow him. Truly the problem of glacier 

 motion is far from even approximate solution, if we are to wait 

 until mathematicians shall have agreed upon the manner of 

 acting of complicated forces upon all parts of a yielding mass, 

 neither rigid nor absolutely fluid. Every one knows that such 

 problems are beyond the compass of exact mathematics; but 

 as in all such cases the great leading truths of mechanics 

 enable us to point out with certainty whether phaenomena of 

 a palpable kind are at variance with them within reasonable 

 limits of error or uncertainty. And still more, if, leaving con- 

 jecture and speculation, a theorist appeals to facts, and shows 

 that viscous or plastic bodies do actually move in such and 

 such a way, that they are torn asunder, or tend to slide over 

 their own particles in certain directions; and then he draws 

 an exact parallel in the case of the great problem in question, 

 — such a theorist is entitled to claim for his speculations the 

 character of a substantive discovery, although it must remain 

 for a later stage of improvement to reduce the proximate laws 

 inferred from general mechanical reasoning substantiated by 

 careful experiments, to the accuracy which the later epochs 

 of the exact sciences alone present. 



I formally decline to rest the mechanical theory of glaciers 

 upon such ingenious but tottering fabrics of argument as 

 deductions from hypotheses respecting the constitution of 

 matter and the effects of force on its integrant parts, which 

 are as seductive by their apparent simplicity as they are known 

 by experience to be inconclusive and erroneous when carried 

 out into their legitimate consequences. That Mr. Hopkins 

 may not think that in so expressing myself I mean to speak 

 disparagingly of his talents, I would refer to the innume- 

 rable controversies of even the last forty or fifty years, and 

 amongst mathematicians of the highest name, upon these same 

 questions, of the mutual actions of bodies in certain states of 

 aggregation (not rigid), the effects of distension and compres- 

 sion upon simple rods, and of the mutual attractions of fluids 

 and solids ; problems in themselves simple compared to the 

 unravelling of the whole forces which a plastic body descend- 

 ing a trough exerts on its own molecules, and which yet were 

 found sufficiently intricate, not merely to baffle mathemati- 

 cians, for that is not the point, but to lead mathematicians of 

 equal and superlative merit to results opposed to one another, 



