478 MK. CffAS. DAVISON ON SNOWDKIFT DEPOSITS. [Aug. 1 894, 



Dr. Rae suggests, by the friction of hard particles of wind-driven 

 snow. Of these, the first is probably by far the more efficient. 



Dust due to Evaporation of Interstitial Ice. — Evaporation takes 

 place from ice and snow at the lowest temperatures experienced 

 in high latitudes. " When a shirt, after being washed," says 

 llichardson, "is exposed in the open air to a temperature of 40° or 



50° below zero, it is instantly rigidly frozen In an hour or 



two, however, or nearly as quickly as it would do if exposed to the 

 sun in the moist climate of England, it dries and becomes limber." 

 The joints in houses built of ice-blocks are gradually enlarged by 

 evaporation, so that drift-snow can enter ; and a freely-suspended 

 cube of ice, according to Payer, lost one-hundredth of its weight 

 daily from the same cause during the latter half of March. 1 It is 

 important to notice that the evaporation occurs without any ap- 

 parent wetting of the surface on which the snow or ice has been 

 resting 2 ; so that, when the interstitial ice of frozen soil evaporates, 

 the surface is left dry and dusty. In England I have frequently 

 noticed that the roads become dusty after a few days of dry frosty 

 weather 8 ; and the same phenomenon has been observed by Sir G. 

 Nares, but to a far greater extent, in lat. 82° 26' N. " Along the 

 borders of the old lake-bottoms," he remarks (on May 7th, 1876), 

 " the mud, which was frozen as hard as any rock during the winter, 

 is now pulverized ; where a month ago it was difficult to dig out stones 

 and shells with a metal instrument, a stick or the finger can now 

 easily be forced an inch deep into the softened earth ; this must be 

 entirely due to evaporation." And again (Aug. I3th) he writes : — 

 " At this season the ground was evidently hardening for the winter. 

 During the spring, long before the temperature of the air was above 

 freezing-point, the earth became pulverized to the depth of two or 

 three inches, all the moisture which had rendered it hard through- 

 out the winter having evaporated. During the latter part of the 

 summer the moisture again collects as dew and the earth hardens 

 completely." 4 While dust must be formed in this way, though in 

 small quantity, all through the winter, it will be noticed that it is 

 greatest in amount at a time of j*ear when snowdrifting is of fre- 

 quent occurrence. 



Dust due to Friction of Hard Particles of Wind-driven Snoiv. — 

 At very low temperatures the fine grains of snow become intensely 

 hard. If rubbed upon the face in cases of frost-bite, they cut 

 through the skin. Sledges no longer glide, but drag heavily as if 

 over a surface of sand or sandstone, unless the runners are wet and 

 so coated with a thin layer of ice. When driven by the wind 

 these hard particles act as a sand-blast, wearing down blocks of 

 snow and ice ; and, in a passage already quoted, Rae states that 



1 Hayes, pp. 218-19; Nordenskiold, vol. i. p. 509; Payer, vol. i. pp. 244, 

 258, 276, vol. ii. p. 114 ; Richardson, vol. ii. p. 100. 



2 Hooker, vol. i. p. 245 ; Nares, vol. i. pp. 272, 276, 311-12, 318. 



3 I may add that I have never observed any deposits from snowdrift when 

 the weather preceding the fall of snow was comparatively mild and damp. 



4 Nares, vol. i. pp. 315-16, vol. ii. pp. 134-35. 



