BRITISH ASSOCIATION. 235 
was not greater than nine or ten degrees) in nearly tlie same 
ratio ; and (3) the rate of movement was of the same order 
of magnitude as in actual glacial motion^ which may be stated 
generally, in cases yet observed, never to exceed two feet a 
day. The extremely small friction between the plane and the 
ice, indicated by the small inclination necessary to produce mo- 
tion, was manifestly due to the circumstance of the lower surface 
of the ice being in a state of gradual disintegration, which how- 
ever, was extremely slow, as proved by the small quantity of 
water proceeding from it. In the application, therefore, of these 
results to the case of actual glaciers, it was necessary to show 
that the temperature of their lower surfaces could not generally 
be less than 32° Pahr. Such, the author stated must necessarily 
be the case, unless the conductive power of ice was greater than 
it was deemed possible that it could be. He considered the sub- 
glacial currents as powerful agents in the disintegration of the 
lower surfaces of glaciers, especially near their lower extremities. 
The results of Prof. Forbe^s observations ou the motion of the 
Mer de Glace of Mont Blanc, afforded, as regards that glacier 
(and, by inference, as regards all other glaciers) a complete refu- 
tation of the theories which attribute glacial movements to any 
expansion or dilation of the ice. The Professor had, however, 
put forth a new theory, which agreed with that offered by Mr. 
Hopkins in attributing glacial motion to the action of gravity, 
but differed from it entirely as a mechanical theory, in other 
respects. The Professor appeared to reject the sliding theory of 
De Saussure, on account of the difficulties already mentioned 
(which were now removed by the above experiments),, and as- 
signed to the mass of a glacier the property oi plasticity ^ or semi- 
fluidity , in a degree sufficient to account for the fact of its de- 
scending down surfaces of such small inclination. According to 
this theory, the motion was due to the small cohesion of one par- 
ticle of glacial ice to another. Mr. Hopkins stated his convic- 
tion that the internal cohesion of the mass was immensely 
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