110 [May 22, 



with a four-foot aperture ; and therefore it is no wonder if I find 

 nights of the requisite degree of tranquillity somewhat more rare. 

 Yet I find my own physical strength insufficient to allow me to use 

 up half the quantity of available sky, and my next want will probably 

 be some efficient and energetic assistance in the duty of observing. 



To General Sabine, I remain, &c., 



President of the Royal Society, fyc. WM. LASSELL. 



II. " On the Theory of the Motion of Glaciers." By WILLIAM 

 HOPKINS, Esq., F.U.S. Eeceived April 14, 1862. 



(Abstract.) 



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 in- 

 vestigation with those of direct observation. Each of these views 

 has been regarded, improperly, in the author's opinion, 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 funda- 

 mental property on which the views advocated by this theory de- 

 pended. Again, the Regelation 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 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 



