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Dr. J. F. Main. On the Viscosity of Ice. [May 5, 



free from air, water was boiled and then frozen. It was then melted 

 and again frozen in a mould. Some difficulty was found in holding 

 the ice-bars in the testing machine. The mode which answered best 

 was to freeze the ends of the ice bar into conical metal collars, which 

 fitted the shackles of the machine. Extensions were measured by 

 vernier callipers reading to one-fiftieth of a millimetre between marked 

 points on the metal collars. To determine if any appreciable effect i 

 was due to distortion of the enlarged ends of the bars in the metal 

 collars, pieces of paper were gummed on the ice, and the extensions 

 also measured between fine pencil marks on these pieces of paper. It 

 was found that nearly all the stretching observed in measuring 

 between the metal collars was due to stretching of the bar of ice, and 

 only a very small part to shearing action in the collars. In conse- 

 quence of rapid evaporation from the surface of the ice bar, the stress 

 with a fixed load on the lever increased from day to day. 



Three experiments are given on bars initially about 234 mm. in 

 length, loaded to stresses of from 4*3 to 2 - kilos, per square cm., 

 and lasting from four to nine days. 



The three experiments show that ice subjected to tension stretches 

 continuously by amounts which depend on the temperature and the 

 tensile stress. When the stress is great and the temperature not very 

 low, there are extensions amounting to 1 per cent, of the length per 

 day. So continuous and definite is the extension, that it can even be 

 measured from hour to hour. These extensions took place at tem- 

 peratures which preclude the possibility of melting and regelation. 



The author hopes that on resuming the experiments next winter 

 at St. Moritz, he may be able to determine more exactly the 

 law of the extension. He has shown already that the extension 

 increases continuously with all stresses above 1 kilo, per square cm., 

 and at all temperatures between — 6° C. and freezing. When ice 

 is in a condition such that the point of a needle will cause a set of 

 radiating fractures to pass from the point of contact in all directions, 

 it stretches as certainly, though not by so great an amount, as when 

 it will permit the passage through it of the same needle without 

 showing flaw or scar. 



In the first experiment there was a total extension of 11 mm. in 

 nine days; in the second of 1*8 mm. in five days; in the third of 

 1*7 mm. in three days. If we assume the extension proportional to 

 the time, there was a mean daily extension of 1*2 mm., 0*36 mm., and 

 0'56 mm. respectively. The stress in No. 1 was greater than in 

 Nos. 2 and 3, and the temperature not so very low in the day, though 

 low at night. In No. 3 there was a low stress, but comparatively 

 high temperature. 



