VI. On the Motioji of Glaciers. By William Hopkins, M.A. and F.R.S., 



Fellotv of the Cambridge Philosophical Society, of the Geological Society, and 

 of the Royal Astronoynical Society. 



[Read May 1, 1843.] 



SECTION I. 

 On the Present State of Theories of Glacial Motion. 



De Saussure appears to have been the first to examine with accuracy, and to describe in detail, 

 the various phenomena which the Alpine glaciers present to us. The phenomena connected with the 

 motion of glaciers, constituting the class with which alone we are concerned in the present communi- 

 cation, engaged their share of his attention, though his observations did not aim at that degree of 

 exactness with which observers of the present day are conducting their researches. Nor did he 

 fail to speculate on the causes of glacial movements. He considered glaciers to slide along the sur- 

 faces over which they move, the motion being due to the inclination of those surfaces to the horizon, 

 and the action of gravity on the moving mass ; and though he was not the first who adopted this 

 theory of glacial movement, it is now usually associated with his name, from his having been the 

 first to acquire any exact knowledge of such movements, or to form, perhaps, any very definite 

 conception of the mechanical causes to which they might be referrible. From his time to a recent 

 period the subject seems to have excited little comparative interest ; but within the last few years 

 glacial phenomena have been investigated with great care, and attention has been again directed 

 to them, not only as forming an interesting branch of physical enquiry, but also as pregnant with 

 geological inferences of the first importance. We are especially indebted to M. Agassiz for his 

 active researches among the Alpine glaciers. The influence of his name has awakened an interest 

 in them which might otherwise have long slumbered ; and whether some of the opinions he has 

 promulgated respecting the motion of glaciers be ultimately established or refuted, geology must 

 continue equally indebted to him for the manner in which he has directed our attention to the im- 

 portance of the subject in its geological bearings. 



One of the consequences of these renewed researches has been to cast great doubt on the ade- 

 quacy of De Saussure's theory to account for the motion of glaciers. The inclination of the surface 

 over which some of the Alpine glaciers move is found to be so small as to render it apparently 

 inconceivable that such glaciers should not only descend, but overcome powerful obstacles to their 

 descent, if there be no other moving force than that of gravity. The mean inclination of the 

 surface of the Aar glacier is stated not to exceed 3° (and that of its bed must be still less), an incli- 

 nation much smaller than that at which a very smooth hard body will descend down an equally 

 smooth and hard plane*. Nor is the difficulty diminished by the consideration of the great 



• The following results are given by Professor Whewell in liis 

 Mechanics of Engineering, on llie autliority of Morin. If S 



be the angle of tlie plane down which sliding will just take place, 

 and ju the coeilicient of friction, we liave for 



Hard Limestone on Hard Limestone. 



Brass on Brass 



Brass on Iron 



Cast Iron on Cast Iron 



Cast Iron on Cast Iron, greased 



Brass on Iron, greased 



