1853.] NOISES PRODUCED BY ICE-CRACKS. 197 



also shifted her bed. The great masses of snow on the 

 ice, hourly consolidating from evaporation from beneath, 

 probably press the floe downwards, and produce these 

 very sharp sounds, very similar to the snapping of heavy 

 bolts of iron. The prevailing impression, I believe, is 

 that it arises from the latter cause. However, this is 

 not satisfactory : no bolt or channel plate, exposed, has 

 been known to part ; and if all these sounds proceeded 

 from such causes, I fear there would be none left to 

 trust to, next summer. 



The mercurial thermometers having indicated tempe- 

 ratures much below the freezing-point of mercury, and 

 this affording an opportunity of examining this metal 

 but seldom presented to quiet and careful research, I 

 determined to avail myself of it. 240 grains were 

 weighed in my cabin, contained in a fine porcelain mor- 

 tar ; the mortar and mercury having been previously 

 subjected to a continued heat on my stove, to expel any 

 possible moisture. The mercury was pure, being part 

 of that which escaped from the standard barometer on 

 my cabin sofa, and carefully preserved in a stoppered 

 phial for such experiments. (Temperature 40.) It was 

 then carefully removed to the thermometer-house, and 

 the balance found correct. Exposed for twenty minutes 

 to a temperature of 47'7 the mercury began to cry- 

 stallize : the circumference became very convex at its 

 edges of contact with the conical vessel in which it was 

 contained, and the centre raised to a point when it had 

 actually congealed ; it lost 1'5 grains. At the first fif- 

 teen minutes' exposure, the scale in which the mercury 

 was contained rose very perceptibly as it diminished in 



