SNOW-WAVES AND SNOW-RIPPLES 195 



The next observation relates to the action of 

 the eddy with a vertical axis upon the drifting 

 leaves of the plane-tree. The place was the broad 

 pavement in front of the Imperial Institute. The 

 building projects at the principal entrance, on the 

 west side of which there are two high walls facing 

 south and west respectively. For some time in 

 the autumn I noticed every day a large circular 

 heap of leaves of the plane-tree within this angle, 

 the outer edge of the heap being several feet from 

 both walls. As other leaves arrived, gliding over 

 the pavement, they circled round nearer to the 

 walls, but, after a time, approaching the heap, they 

 were either deposited against its side or were lifted 

 and dropped more centrally upon it. The course 

 of events was clear. The leaves continued to travel 

 until they approached the centre of the eddy, where 

 the air, in order to escape, had to travel upwards. 

 Here the horizontal motion of the air ceased, and 

 the leaves, even if momentarily lifted by the upward 

 current during a gust, quickly settled. 



This accumulation of heavy material is in a 

 position similar to that in which eddies of water 

 scour out circular depressions in a sandy bottom, 

 of which the particles are readily suspended. On 

 the pavement in Cromwell Road I found all the 

 plane-leaves banking against the high wall, but 



