1888.] 



On the Plasticity of Glacier and other Ice, 



357 



section gradually diminished, and the tension consequently increased r 

 as given in the seventh column. The mean section was about 

 4*1 sq. cm. The ice was under tension for twenty-four hours previous 

 to the observations given below, but during this time the indicators 

 seem to have slipped. 



The weight was removed for twelve hours on the 14th owing to 

 the thaw. It is curious to notice how irregularly the extension is 

 divided between the two sides ; the ice bends first one way then the 

 other. The fluctuations in the mean rate of extension seem mainly 

 due to the temperature. During thirteen hours at a temperature 

 between — 1*5° and — 3"0° the rate was 0"0028, while daring thirty- 

 eight hours at a temperature above —0*7° the rate was 0*0034. The 

 ice was full of minute bubbles, though not in sufficient quantity to 

 make it quite opaque. The component crystals were very small, less 

 than a millimetre in diameter, and with optic axes arranged quite 

 irregularly. 



Experiment No. 8. Bough Apparatus. — The specimen was a piece of 

 glacier ice (D). The wire indicators and connecting fibres were 

 hooked through wire loops firmly fastened to the glass needles em- 

 bedded in the ice, so there was no possibility of slipping. The 

 multiplication was about 22, so the small extensions are accurate to 

 0*02 mm. The area of section was 6*3 sq. cm. The table is arranged 

 as in the last experiment (6). 



Thus the whole extension in three and a half days was more than 

 4 per cent, of the length. At 20 h. 15 m. the inner indicator had 

 moved off: the scale against a stop, so the extension was probably 

 rather greater, certainly not less than that given. The extension at a 

 particularly low temperature, mentioned in the general summary, was 

 between February 18th, 21 h., when the temperature was — 9"0°, and 

 February 19th, 9h. 15 m. There was a contraction on the outer side 

 during this interval of 0*01 mm., and an extension on the inner side of 

 0*23 mm., so the mean rate per hour per 10 cm. was 0065 mm. 



It should be mentioned that the points on the glass needles, where 

 the indicators were attached, were not quite close to the ice, but at 

 the distance of a centimetre perhaps. Hence, while the mean rate is 

 correctly given, the extension on the inner side of the bar is ex- 

 aggerated, and that on the outer side made too small. Taking the 

 ice as 2*5 cm. thick, this consideration leads to the result that the 

 total extension of the outer face of the bar was 2*9 mm., of the inner 

 face 9' 7 mm. 



This experiment shows how completely the plasticity depends on 

 changes in the internal structure of the ice. Thus, for the first two 

 days we find, under a slight stress, a moderate rate showing some 

 tendency to decrease more rapidly than can be easily attributed to 

 the fall of temperature. An increased tension produces as usual a 



