52 SIXTH LETTER ON GLACIERS. [1844. 



pleted its course, or arrived in the lower valley, whilst the 

 other, which was its companion, has advanced only three-fifths 



f 



c e 



d 



Fig. 9. 



of the distance, or remains perhaps several miles behind. 

 Thus it has been shewn from multiplied measurements of the 

 most precise and accordant kind, that a series of stones or 

 marks being supposed to be laid across a glacier in the line 

 ABCDEFG; they will be found, after a certain time, in the 

 position abcdefg, after other equal intervals at a'b'cd'e'fg', and 

 at a%"c'd"e'f"g"t by which time it will be seen that the neigh- 

 bour particles have entirely changed their relative positions, 

 and that the mass can have no pretension to be called rigid, 

 but moulds itself after the manner that a fluid or semifluid body 

 does in like circumstances the centre advancing fastest, and, 

 for some space in the centre, nearly uniformly, whilst the retar- 

 dation produced by the friction of the banks is most intense in 

 their neighbourhood, which is conformable to what we know of 

 the movement of viscous fluids. It is, therefore, no hypothesis, 

 but a simple statement of a demonstrated fact, that the, manner 

 of movement of the surface of a glacier is not such as is consistent 

 with the continuity of a rigid body, but that it coincides with the 

 manner of motion of a viscous or semifluid body. Whatever may 

 be the difficulty of conceiving the glacier to be a body thus 

 constituted, the fact admits of no doubt ; the effects of forces 



