1888.J 



On the Plasticity of Glacier and other Ice. 



359 



to say, impressed a gradual retardation in spite of a rising tempera- 

 ture. It would thus appear that in this case, while a rapid extension 

 increased the plasticity, a gradual extension had the effect of diminish- 

 ing it. This is an anomalous result, but it must be remembered 

 that we are measuring the sum of a large number of independent 

 actions. The behaviour of the whole is probably much more com- 

 plicated than that of any one of the individuals. 



Being curious to see the effect of great tension, we applied 4*2 kilos, 

 per sq. cm. This brought the experiment to an end, for after half 

 a minute the ice gave way. It was found broken both at the lower 

 collar and at a point below the upper needle, where we had previously 

 noticed a crack extending part of the way across the bar. At which 

 point it broke first we cannot say. The bar was examined at the 

 end of the experiment. It was nearly straight in spite of one side 

 having extended so much more than the other. It contained 

 several large bubbles, one perhaps 2 cm. long, drawn out into very 

 irregular shapes, which seemed to show this piece had suffered great 

 distortion while it still formed part of the glacier. It contained part 

 of a very large crystal which composed, perhaps, one third of the 

 whole bar, and ran three quarters of the length between the needles. 

 This crystal occupied one of the angles adjacent to the inner face, 

 which extended so much. Its optic axis was inclined at perhaps 70° 

 to the length of the bar. 



Experiment No. 9. Bough Apparatus. — The specimen was a bar of 

 lake ice, with the crystalline columns parallel to the length of the 

 bar. The section was 8 sq. cm. in area. The arrangements were the 

 same as in the last experiment (8). The extensions are so small 

 that the deduced rate during each interval would be very inaccurate. 

 We have therefore given, in the second, third, and fourth columns of 

 the table, the extensions measured from the length at the time of the 

 first reading and reduced to the proportionate value for a bar 

 10 cm. long. They are probably correct to 0*01 mm. The fifth 

 column gives the temperature shown by the thermometer at each 

 reading ; and the next three the maximum, minimum, and mean 

 temperatures of the ice during each interval, estimated from a large 

 number of observations not quoted. 



Previously to 15 h., February 28th, the ice must have been thawing, 

 probably for about an hour. The weight was removed for the next 

 three hours. The total extension during 208 hours per length of 

 10 cm. was 0*145 mm. on the outer side, and 0*048 mm. on the inner, 

 giving a mean rate per hour of 0*00046 mm. The mean rate during 

 the first 168 hours was 0*00039 mm., and during the last 40 with the 

 heavier weight 0*00076 mm., notwithstanding a slightly lower mean 

 temperature. But these rates were so small as to be beyond our 

 means of accurate measurement. 



