SNOW-WAVES AND SNOW-RIPPLES 191 



bodies. The single track of the Canadian Pacific 

 Railway is slightly raised above the level of the 

 prairies. When ,1 crossed these great plains they 

 were covered to a depth of some inches with snow, 

 but the railway line was clear. There was a rise 

 of the air in passing over the obstruction, and this 

 was evidently sufficient to carry the snow-dust clear 

 of the line. There must have been an eddy, with 

 horizontal axis and vertical whirling formed by 

 the obstruction, and it seems as if the snow-dust 

 must have travelled over it, borne along in the un- 

 broken sweeping curve of the main current of air. 



Particles less fine, whose rate of subsidence was 

 the same as that of sand, would certainly have 

 formed some deposit on the lee side of the obstruc- 

 tion, for some of them would have subsided through 

 the eddy and been held back by its return draught, 

 which flows along the ground. 



I also observed that the fine snow-dust of the 

 prairie used to fly quite over the city of Winnipeg. 

 Looking out from the suburbs to the prairie on 

 windy days, the snow was seen to be drifting there 

 so as to obscure the lower story of the farmhouses, 

 the upper story and roof standing out clearly. In 

 the streets of the city there were no flying particles 

 of snow, but high above the air was hazy with the 

 poudrette, or cloud of minute snow particles, which 



