188 WAVES OF SAND AND SNOW 



gravel nor dust collects thus, for the gravel is not 

 caught up by the eddy, and the dust which the eddy 

 catches up cannot escape. 



On the prairies of Manitoba in mid-winter 1 

 found that a projecting lump of hardened snow or 

 a post in the ground had two tails of drifted snow 

 behind it, one on either side of the central line, 

 each having a gently convex surface. The drifting 

 material of which these tails were formed was a 

 fine powder or dust to which much of the snow had 

 been reduced. This conforms so precisely to the 

 movement of the eddying air that it can only deposit 

 during lulls or calm. The rapid setting of finely 

 powdered snow when lying in a heap helps to make 

 these two-tailed drifts permanent. 



White steam issuing in large quantity from a 

 factory chimney forms a two -tailed streamer, and 

 the spiral motion of the condensed steam in each 

 half can be easily seen. As it leaves the mouth 

 of the chimney it rolls downwards and inwards 

 from either side. When once this dual motion has 

 been seen in white steam it is not difficult to detect 

 the double whirl in the single streamer of black 

 smoke issuing from the funnel of a steamer. 



The action of a transverse barrier upon the drift 

 and deposition of dust is quite different from that 

 by which it arrests the superficial travel of heavier 



