62 EIGHTH LETTER ON GLACIERS. [1844. 



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 show, 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 permits it to model itself to 

 the ground over which it is forced by gravity and that, re- 

 taining 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 ob- 

 vious, after measurement had proved their variable velocity, and 

 observation had shown 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 papers on the subject 

 of glaciers, read to the Cambridge Philosophical Society on the 

 1st May and llth December 1843, which were put into my 

 hands here less than a month ago, by his friend Mr. Williamson. 

 1 there found it stated that there is " a necessity of proving, by 

 independent experimental evidence, that glacier ice does possess 

 this property of semi-fluidity or viscosity, if we would attribute 

 to that property the effectiveness of gravity, in setting a glacier 

 in motion." First Memoir, p. 3. 



Since Mr. Hopkins admits the fact of the swifter central 

 motion of the glacier, he must have recourse to some mechani- 



