36 WAVES OF SAND AND SNOW 



is to say, their " amplitude " or height from trough 

 to crest was greater. The actual level of the 

 troughs had been lowered, as was evident from 

 the fact that a relatively hard floor of a compact 

 material was exposed, whereas in the morning there 

 was some depth of loose sand even in the troughs. 

 Whether the absolute level of the crests had been 

 raised or whether the increased amplitude was 

 accompanied by a lowering of the crests less than 

 the lowering of the troughs I did not ascertain. 

 The average level of the surface was certainly 

 lower, for the sandbank was obviously being 

 depleted by the wind. The quantity of sand now 

 flying in permanent suspension was great, and it 

 formed a thick haze 20 or 30 feet high. It was 

 necessary to protect the eyes, nose, and ears from 

 the penetrating particles, and even then prolonged 

 observations were attended by considerable discom- 

 fort. Wandering over the sandbank, I found that 

 in some parts the crest of each ridge varied 

 considerably in height, presenting a series of peaks 

 and saddles. In order to determine the relation 

 of height to length in the waves I measured next 

 day. May 8th, a continuous series of 24. The 

 first ridge was 40 yards from the Nile, and 

 the measurements were taken proceeding down 

 wind and away from the Nile to the end of the 



