2 Mr. Hopkins on the Motion of Glaciers. 



ferred to, I have endeavoured to avoid, as far as possible, 

 mere controversy and criticism, as inconsistent with the legi- 

 timate objects of a scientific memoir. I would still rather have 

 preserved the same line in the further discussion of the sub- 

 ject, but when Prof. Forbes, at ihe conclusion of his Eighth 

 Letter on Glaciers, intimates that I have adopted views op- 

 posed to his, without having carefully studied his writings ; 

 and that, if I had so studied them, I should not have advanced 

 objections against the viscous theory " very easily refuted," 

 I feel myself called upon to show that I have not opposed his 

 theory without a careful study of it, and to endeavour to show 

 that my objections to it are not of easy refutation. For these 

 purposes I shall in this communication give an explicit state- 

 ment of my own views, combined with a free, and, I trust, a 

 candid criticism on those of Prof. Forbes. I proceed in the 

 first place to the consideration of the sliding theory, to which 

 this first part of my communication will be restricted. 



T/ie Sliding Theoty. — One of the first consequences of the 

 recent researches in glacial phaenomena has been to cast great 

 doubt on the adequacy 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 gla- 

 ciers should not only descend, but overcome powerful obsta- 

 cles to their descent, if there were 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 inclination 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 con- 

 sideration of the great weight of the moving mass, or of the 

 extent of its surface in contact with that over which it moves; 

 for, according to the observed laws of sliding bodies, the mo- 



* The following results are given by Prof. Whewell in his Mechanics of 

 Engineering, on the authority of Morin. If 6* be the angle of the plane 

 down which sliding will just take place, and ji the coefficient of friction, we 

 have for 



