148 Mr. H. H. Howorth on 



at a rate of from 0.013 mm. to 0*022 mm. per hour per length 

 of 10 cm., the variation in speed being attributable to the 

 temperature. The second piece began at a rate of 0'0i6 

 mm. and gradually slowed down till it reached, at the same 

 temperature, a rate 0*0029 mm., at which point it remained 

 tolerably constant, except for temperature variations, till a 

 greater tension was applied. The third piece, on the con^ 

 trary, began at the rate of o*oi2 mm., increased its speed 

 with greater tension to 0*026 mm., and stretched faster and 

 faster with unaltered tension till it reached the extraordinary 

 speed of i*88 per hour per length of 10 cm. We put on a 

 check by reducing the tension slightly, whereupon the speed 

 fell at once to 0*35 mm., and gradually declined to 0*043. 

 mm. . , . During twelve hours, with a maximum tem- 

 perature — 9° and a mean temperature probably — 10*5, the 

 rate under the light tension of 1*45 kilo, per sq. cm. was 

 0*0065 mm." . , , . "We tried further experiments 

 on compression of ice, the pressure being applied to 

 three nearly cubical pieces at once. Of three pieces of 

 glacier ice, under a pressure of 3*2 kilos per sq. cm., the 

 mean rates of contraction during five days were respectively 

 0*035 mni., 0*056 mm., and 0*007 f^^i^- P^^ hour, per length 

 of 10 cm. These figures show that while the plasticity 

 varies enormously in different specimens, the rate of dis- 

 tortion is of the same order of magnitude, whether the force 



applied be a pull or a thrust We have now 



shewn by direct experiment that ordinary ice, con- 

 sisting of an irregular aggregation of crystals, exhibits 

 plasticity both under pressure and under tension, at 

 temperatures far below the freezing point — in the case of 

 tension at any rate down to —9° at least, and probably 



much lower It will be interesting to make some 



comparison between the figures we have given and the 

 plasticity actually observed in the motion of glaciers. 

 Perhaps the most striking proof of the existence of plasticity 



