Mr. W. Hopkins on the Theory of the Motion of Glaciers. 225 



of ice, entirely new to us, on which certain peculiarities of the mo- 

 tions of glaciers depend. 



When we shall have obtained a Theory of the Motion of Glaciers 

 which shall command the general assent of philosophers, no qualify- 

 ing epithet will be required for the word theory ; it would indeed be 

 inappropriate, as seeming to indicate the continued recognition of 

 some rival theory. If, for instance, it should be hereafter admitted 

 that the sliding of a glacier over its bed and the property of rege- 

 lation in ice are equally necessary, and, when combined, perfectly 

 sufficient to account for the phenomena of glacial motion, there 

 would be a manifest impropriety, not to say injustice, in selecting 

 either of the terms sliding or regelation by which to designate this 

 combined theory. The author makes these remarks because he 

 believes that the preservation of the partial epithets above mentioned 

 has a tendency to prevent our regarding the whole subject in that 

 more general and collective aspect under which it is one of the prin- 

 cipal objects of this paper to present it. 



This object must necessarily give to the present paper something 

 of the character of a resume of what has hitherto been done, whether 

 it be our purpose to adopt or reject the conclusions of others. There 

 are periods in the history of almost every science when its sound 

 and healthy progress may almost as much demand the refutation of 

 that which is erroneous as the establishment of that which is true. 

 It is not intended, however, to enter into any review of the past 

 labours of glacialists with respect to exploded theories, but only to 

 notice those more recent researches and speculations which appear 

 either to demand refutation as erroneous, or to be admitted into any 

 well-founded theory as correct. 



With a view, in the first place, to remove the ambiguities which 

 have beset this subject from the want of explicit definitions, the 

 author enters into the following discussion and explanation of terms 

 employed to express properties of ice on which our theories of glacial 

 motion must essentially depend. 



1 . The external forms of all bodies in nature may be changed in a 

 greater or less degree, and without producing discontinuity in their 

 mass or destruction of their internal structure, by the action of any 

 external forces, the original or undisturbed form from which the 

 change of form is to be estimated being that which the body would 

 assume if acted on by no external forces whatever. This change of 

 form necessarily implies a change in the relative positions of the 

 component particles of the mass, or a certain greater or smaller 

 amount of molecular mobility, or power in the particles of moving 

 inter se. We may speak either of the general change of the form 

 of the whole body, or of that which takes place in each of its small 

 elementary portions ; it is, in fact, in this latter sense that we are 

 obliged to regard it in any accurate investigations, because the change 

 of form for different elements will usually be different. Change of 

 form in an element may or may not be accompanied by a change of 

 its volume. In the first case it leads to cubical extension or com- 

 pression ; in the latter, merely to extension or compression of the 



Phil. Mag. S. 4. Vol. 25. No. IC7. March 18G3. Q 



