1888.] 



On the Plasticity of Glacier and other Ice. 



351 



experimentalists is owing to an unequal expansion of ice in different 

 directions. At any rate, taken together, they are sufficient to explain 

 our rough results, on the supposition that the extension of the last 

 five days was entirely due to the rise of temperature. 



The experiment was brought to a close by the bar breaking at a 

 point above the upper needle, where it was not protected by gutta- 

 percha tissue, and had become very thin through evaporation. The 

 thickness had been reduced by this cause in three weeks from 

 2*85 cm. to 2*2 cm. The temperature, at which the fracture 

 •occurred, was between — 0*5° and — 1*0°, certainly not above the former. 

 The breaking tension was 8*35 kilos, per sq. cm. There was a groove 

 running right round the bar near the middle of its length, but 

 no sign of a crack could be seen in the interior of the ice. This 

 groove may have been caused by the outer layer cooling more rapidly 

 than the interior. Under the polariscope no break in the continuity 

 of the crystalline structure could 'be detected. The rings and cross 

 were seen very plainly, and the direction of the optic axis appeared to 

 be the same on both sides of the crack. It was perpendicular to the 

 length of the bar and also to the needles. By a rough measure 

 of the rings we found the difference between the two indices of 

 refraction to be 0*0018. In quartz it is 0*0094 ; in Iceland spar 0*172. 



'Experiment No. 5. Main's Apparatus. — The specimen was a piece of 

 glacier ice (B). The measurements were taken with the cathetometer. 

 We had already found, in the other apparatus, that glacier ice would 

 stretch, but we thought it desirable to confirm the fact with a 

 different mode of measurement, So in this one case we used the 

 cathetometer again, in spite of its disadvantages for this kind of 

 work. The length between the needles was about 20 cm., the area of 

 section 7*3 sq. cm., and the tension 2*7 kilos, per sq. cm. The second 



<Glacier Ice B. Length between Needles 20 cm. Tension 2*7 kilos. 



per sq. cm. 



Date. 



Tempera- 

 ture at the 

 time. 



Interval. 



Extension. 



Kate 

 per hour 

 per 10 cm. 



Temp 

 Max. 



erature. 

 Mean. 



-3-5° 

 -4-5 

 -3 



„ io, „ 



„ „ 16 h. 30 in. 

 „ 11, 8 h. 45 m. 

 » 12, 



„ 13, 16 h. 45 m. 

 Total 



-2-5° 

 -2-5 



-3 

 -4 

 -0*5 



hours. 

 24 



16*25 



32 



mm. 

 0*78 



0-44 



0*53 



mm. 

 -0160 



-0135 



-0083 



-2-5° 



-2-5 



-0-5 





72 25 



1-75 



-0116 







