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XXXI. On the Theory of the Motion of Glaciers. 
^3/ William Hopkins, Esq., St. Peter's College, Cambridge, M.A., F.E.S. &c. 
Eeceived April 14, — Bead May 22, 1862. 
Almost all the numerous discussions which have taken place during the last twenty 
years respecting our theories of glacial motion have had for their object the assertion 
of some particular -view, rather than the establishment of a complete and sufficient 
theory founded on well-defined hypotheses and unequivocal definitions, together with a 
careful comparison of the results of accurate theoretical investigation with those of 
direct observation. Each of these views has been regarded, in my own opinion impro- 
perly, as a Theory of Glacial Motion. The Expansion Theory ignored the Sliding Theory, 
though they were capable of being combined ; the latter theory was equally ignored by 
the Viscous Theory, in which, moreover, instead of the definitions of terms being clear and 
determinate, no definition of viscosity was ever given, though that term designated the 
fundamental property on which the views advocated by this theory depended. Again, 
the Eegelation Theory is not properly a theory of the motion of glaciers, but a beautiful 
demonstration of a property of ice, entirely new to us, on which certain peculiarities of 
the motions of glaciers depend. When we shall have obtained a Theory of the Motion of 
Glaciers which shall command the general assent of philosophers, no qualifying 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 regela- 
tion 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. I make these remarks because I believe that the preservation of the 
partial epithets above mentioned has a tendency to prevent our regarding the whole sub- 
ject in that more general and collective aspect under which it is one of the principal 
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. I shall not, however, 
enter into any review of the past labours of glacialists with respect to exploded theories, 
but shall only notice those more recent researches and speculations which appear to 
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