500 



Dr. J. F. Main. 



during that interval of the temperature. Owing to the rapid evapo- 

 ration from ice, even at low temperatures, when the surface is never 

 liquefied, at such an elevation (6100 feet above sea level) as St. 

 Moritz, in the Engadine, where the observations were carried on, the 

 specimen is subjected to a continually increasing stress. This varia- 

 tion in the stress can no doubt be diminished by clothing the speci- 

 men in flannel. The change of temperature I expect to diminish by 

 filling the interval between the two surrounding wooden boxes with 

 some non-conducting material. In this way I have good hope that, 

 on resuming the experiments next winter at St. Moritz, I may be able 

 to determine more nearly the law of extension, That there is such 

 extension, and that it goes on continuously with all stresses above 



I kilo, per square centimetre, and at all temperatures between 

 — 6° C. and freezing point, is shown by the above experiments. When 

 ice is in a condition such that pressure with 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, although not by so great an 

 amount, as when it will permit the passage through it of the same 

 needle without showing the least trace of flaw or scar. 



In the discussions, for the most part a priori, on the extensibility of 

 ice, sufficient importance has not usually been assigned to the neces- 

 sity of distinguishing between the effect of even a small blow or jar 

 and that of a much greater force applied gTadually and steadily 

 during a long interval. A bar of ice may bear a stress of 4 and 

 5 kilos, per square centimetre if the load is steady, which would 

 fracture at once with a much smaller sudden stress, especially if not 

 uniformly distributed. 



In the first experiment we notice a total extension in nine days of 



II mm. In No. 2 there is an extension of 1*8 mm. in five days, and 

 in No. 3 of 1*7 mm. in three days. If we assume for the moment an 

 extension proportional to the time, we should thus get a mean daily 

 extension in the three experiments of 1'2 mm., 0"36 mm., and 0*56 mm. 

 respectively. To account for the discrepancy we remark that the 

 stress in No. 1 is much greater than in 2 or 3, and the temperature 

 not so very low during the day, although low at night. The effect of 

 increased stress is well shown in Experiment No. 2, on February 23rd 

 and 24th, where, on increasing the stress from 2'28 to 3'65 kilos. , 

 the extension in a day rises at once from 0'08 to 0*69 mm. In No. 2 y 

 for three out of the five days, the temperatures were below — 6° C, 

 whilst in No. 3 there was a low stress but comparatively high tem- 

 peratures. 



In Experiment No. 2 the large numbers obtained at first probably 

 arose from the fact that in preparing the specimen its conical ex- 

 pansion had frozen to the collar C. When the water run into the 

 collar B began to freeze it expanded, and thrust the ice upwards. 



