428 Notices of Memoirs — On the Viscosity of Ice. 



local ice-floes, the dispersion and extensive distribution of flint so con- 

 spicuous in all southern beaches, and extending even as far as the 

 Scilly Isles, can alone be accounted for." 



III. — 0^' THE ViscosiTr of Ice. (Abstract.) By R. M. Deeley, F.G.S.' 



niHE movement of glaciers has excited a great deal of interest, and 

 J_ the facts recorded concerning their motion have shown that the flow 

 is such as would result if ice obeyed the laws of viscous flow. The 

 viscosity of a liquid is measured by the tangential force per unit area 

 of either of two horizontal planes at unit distance apart, one of which 

 is fixed, while the other moves with unit velocity, the space between 

 the planes being filled with the viscous substance. Taking such, 

 figures as are available, and estimating others as nearly as may be, it 

 is possible to calculate roughly the viscosity of several glaciers. 

 Stated in dynes per square centimetre by 10'", the results obtained 

 are roughly as follows: — The Mer de Glace 27, Morteratsch 143, 

 Lower Grindelwald 3, and Great Aletsch 126. It seems probable that 

 these discrepancies arise rather from differences in the actual viscosity 

 of the glacier ice, due to its varying granular structure, than to errors 

 in the data. Further measurements are required, however, before this 

 can be regarded as proved. 



It is also shown that the viscous motion of a glacier such as the 

 Great Aletsch must exercise a drag on the floor upon which it rests 

 amounting to two and a half tons per square foot, and that owing to 

 the ability of the ice to transmit thrust, this force may be greatly 

 exceeded at points where much resistance to motion is caused by 

 inequalities in the floor upon which the ice rests. 



McConnel made a number of very careful experiments on the 

 shearing motion which can be produced by even very small forces in 

 directions at right angles to. the optic axes of ice caystals. A careful 

 consideration of the experimental data shows that the rate at which 

 the motion is produced is proportional, very nearly, to the stress, and 

 that the resistance to shear increases very rapidly with rise of 

 temperature. The flow at right angles to the optic axis is such as 

 would be the case if ice were viscous (liquid) in a direction at right 

 angles to the optic axis, the viscosity at 0° C. being about 3x10'" 

 dynes per square centimetre. 



McConnel showed that when the load was taken off a bar of ice 

 which had been yielding viscously, there was a slow partial recovery 

 of the original form. Experiments with highly brittle pitch also 

 showed that when the load was taken off a weighted bar there was an 

 immediate elastic recovery, and also an additional slow recovery. 

 When this slow recovery ceased, the pitch bar again began to bend 

 under its own weight. 



» Koyal Society, June 4th, 1908. Commuaicated by Dr. H. Woodward, F.R.S. 



