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XIII. Illustrations of the Viscous Theory of Glacier Motion. 
Part II. An attempt to establish by observation the Plasticity of Glacier Ice. 
By James D. Forbes, Esq., F.R.S.S. L. and E., Corresponding Member of the Institute 
of France, and Professor of Natural Philosophy in the University of Edinburgh. 
Received July 28, 1845, — Read January 15, 1846. 
§ 3. De Saussure’s Theory. 
§ 4. Modifications of De Saussure’s Theory. 
^ 5. Experiments at Chamouni on the Plasticity of Ice. 
§ 3. De Saussure’s Theory. 
When Gruner proposed the explanation of glacier motion by the sliding of the 
ice over its bed, and De Saussure illustrated and confirmed it by considerations 
drawn from the lubricating action of the earth’s heat melting the ice in contact with 
the soil*, there is no reason to suppose that either of them thought it necessary to 
take into account the varying form of the channel through which the glacier had to 
pass, and the consequently invincible barrier presented to the passage of a rigid 
cake of ice through a strait or narrow aperture when it occurred. This is the more 
remarkable, because he conceives that the inequalities of the bed or bottom may be 
overcome by the hydrostatic pressure of the water, which he supposes may be impri- 
soned between the rock and the ice, so as absolutely to heave the latter over the 
resisting obstacles. 
I believe that in no part of De Saussure’s writings will there be found any, the 
slightest reference to the possibility of the glacier when fairly formed moulding itself 
to the inequalities of the surfaces over which gravity urges it ; nor is there any trace 
of the correlative fact of an unequal motion of the sides and centre of the ice, which 
may in some sense be considered as the geometrical statement of the preceding 
physical fact. The fact of plasticity was suspected by Basil Hall, and more distinctly 
announced by Rendu, as shown in the first part of this paper ; but it could not be 
proved until the geometrical fact of the swifter motion of the centre of the glacier 
relatively to the sides was established in 1842 f-. The contrary opinion at that time 
* To do Gruner justice, he appears to have been aware of the effects of the earth’s heat and the lubricating 
action of the water thawed from the glacier : “ Lorsque les cotes de l’amas [de glace] qui touchent la montagne, 
fondent en entier, toute la masse entrainee par son poids glisse sur son fond et s’avance dans la vallee,” French 
translation, p. 333 . . . “ il est vraisemblable que leur surface inferieure [i. e. des glaciers] se liquefie autant, et 
peut-Stre plus que la superieure,” ib. p. 289. 
f Edinburgh Philosophical Journal, October 1842, and Travels in the Alps of Savoy, p. 134. 
