Professor Forbes's Eighth Letter on Glaciers, Zll 



papers on the subject of glaciers, read to the Cambridge 

 Philosophical Society on the 1st May and 11th December 

 1843, which were put into my hands here less than a month 

 ago, by his friend Mr Williamson. I there found it stated that 

 there is " a necessity of proving, by independent experimental 

 evidence, that glacier ice does possess this property of semU 

 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 me« 

 chanical explanation of the fact. This he does by assuming 

 the existence of vertical fissures, parallel to the sides of 

 the glacier, dividing it into a series of longitudinal stripes, 

 whose adjacent surfaces, according to him, slide over 

 one another, and, in the case of a glacier forcing its way 

 through a gorge, the lateral portions are altogether arrested, 

 whilst the central parts slip down between them. 



These parallel stripes of ice are supposed by Mr Hop- 

 kins to be of considerable breadth, and to have no sort of 

 analogy with the ribboned structure, to which the readers of 

 my earlier letters will recollect that I have ascribed a similar 

 origin, being lines of discontinuity arising from the crushing 

 of one portion of the semi-rigid glacier past another. This 

 Mr Hopkins regards as " no more possible than that a mass 

 should permanently maintain a position of unstable equili- 

 brium.'' The veined structure of glaciers he considers to be 

 unexplained, and, in the present state of science, inexpli- 

 cable. 



Although the general absence of such a system of longitu- 

 dinal fissures as Mr Hopkins has figured in page 14 of his 

 First Memoir, and the regularity and continuity of motion of 

 the glacier and of its parts, wholly inconsistent with the jost- 

 ling of huge masses of dislocated ice, might be considered as 

 a sufficient answer to this modification of the theory of De 

 Saussure, the consideration of this demand for a direct proof 

 of the flexibility of glacier ice led me to think of its practica- 

 bility ; and I shall now state what I have succeeded in doing, 

 towai'ds the solution of this practical question in the only way 



