336 dw the Strttciwre of Ice. 



Lake usually remains open till late in December, but by the 

 middle of October the bays and straits are frozen across. 

 As the structure of ice has of late years attracted the atten- 

 tion of speculative geologists, principally in connection with 

 the movements of glaciers, I am induced to mention here a 

 iew facts which intinided themselves on ,roy.Qbsery9-tiQB, , dur- 

 ing my residences in the fur countries.^fli»i^mi I ,9^r«ff ,(i<yx^ 



The first step in the freezing of rivers in this rigorous cli- 

 mate, after the water has been cooled down to 32° by a suc- 

 cession of cold weather; is the formation of somewhat circu- 

 lar plates of ice, six or eight inches in diameter. These 

 drift for a time wdth the current, until they have become 

 numerous enough to cover the surface of the water, when 

 they are arrested in a narrow part of the river, or by any 

 slight obstacle, and speedily adhere to each other, after 

 which the interstices between the circles fill rapidly with 

 crystals that bind all firmly together. The sheet of ice thus 

 produced is at first nearly opaque ; but when, in the course 

 of a day or two, it has acquired the thickness of a few inches, 

 it becomes transparent, and remains so until a fall of snow 

 has obscured the surface. In unsheltered lakes the wind 

 drifts the snow to the beach, and would perhaps keep the ice 

 clean for great part of the winter, were it not that in certain 

 hygrometric conditions of the atmosphere small starry tufts of 

 most beautiful tabular and latticed crystals are deposited at 

 short intervals on the ice, and freeze firmly to it. In a dry 

 atmosphere, these crystals evaporate again, but should a fall 

 take place of the fine dust-like snow, which is the most com- 

 mon kind in high latitudes, they serve to detain it until it 

 consolidates, so as to resist the wind. It is rare, however, 

 for the snow to lie more than a foot deep on any of the large 

 lakes, unless where it has drifted under the lee of piled-up 

 slabs of ice, or of rocks, islands, or other shelt^^frsifo n^^d 



During winter, the ice receives an increase of thickness 

 from beneath, and at the same time evaporates above ; the 

 latter process going on with a rapidity that would scarcely 

 be credible to one ignorant of the extreme dryness of the air 

 in an arctic winter. The ice acquires a thickness of from 4 to 

 8 feet, according to the severity of the season, the depth of 



