376 Professor Forbes's Eighth Letter on Glaciers. 



surements to a still greater degree of minuteness, and with 

 results which shew that the methods I have employed are 

 trustworthy, and are able to afford the direct solution of ques- 

 tions which at first appeared to admit of only indirect or in- 

 ductive proof. 



Of this class by far the most important appeared to be the 

 manner in which the glacier alters its form in such a way, 

 and to such a degree, as to suffer its central portion to de- 

 scend towards the valley with double or treble the velocity of 

 its lateral parts. Such, for instance, I have found to be the 

 case in the middle region of the great glacier of Aletsch, 

 where its inclination is small (about 4°), and where the con- 

 tinuity of the ice with the side wall is preserved without the 

 interference of large fissures. I there found that, whilst the 

 velocity of the ice at 1300 feet, or about a quarter of a mile, 

 from the side, is 14 inches in 24 hours ; at 300 feet distant 

 from the side it was but 3 inches in the same time ; and, close 

 to the side, it had nearly, if not entirely vanished. Facts like 

 this seem to shew, with evidence, what intelligent men, such as 

 Bishop Rendu, had only supposed, previously to the first exact 

 measures in 1842, that the ice of glaciers, rigid as it appears, 

 has in fact a certain '' ductility" or "viscosity," which per- 

 mits it to model itself to the ground over which it is forced 

 by gravity, — and that^ retaining its compact and apparently 

 solid texture, unless the inequalities be so abrupt as to force 

 a separation of the mass into dislocated fragments, such as it 

 is well known that every glacier presents, when the strain 

 upon its parts reaches a certain amount, — as when it has to 

 turn a sharp angle, or to descend upon a rapid or convex 

 slope. 



The mutual action of the parts of the glacier, the drag 

 which the centre exerts upon the sides (and, by an exact 

 parity of reasoning, the top upon the bottom), seemed to me 

 so obvious, after measurement had proved their variable velo- 

 city, and observation had shewn that this was not necessarily 

 accompanied by a general dislocation of the mass, — that I 

 should scarcely have thought of attempting a direct proof of 

 the yielding and ductile nature of glacier ice, had I not been 

 favoured by Mr Hopkins with copies of his two ingenious 



