80 WAVES OF SAND AND SNOW 



developed in breadth much more rapidly than in 

 length, for when the wave-length had attained 

 2^ inches the breadth, or lateral extension, of the 

 ridges was no less than 3 feet 1 1 inches . 



The newly deposited accumulation of loose sand 

 began to ripple when its depth was certainly not 

 more than | of an inch. The size of the sand- 

 grains was such that, on an average, I was able to 

 arrange fifty-four of them, touching each other, in 

 the space of i inch. The best method of measuring 

 the height of the little ridges is to stretch a thread 

 over a group of them and measure down to the 

 surface at the crest and trough respectively. In 

 the earlier observations which I am now describing, 

 however, I used to make a longitudinal section of 

 the ridge with the blade of a knife, and, lying 

 flat on the sand, measure the height of the ridge 

 above the trough directly. I used also to count 

 the number of sand -grains which were piled one 

 upon another between the uppermost continuous 

 layer of sand, which is that at the level of the 

 trough, and the summit of the ridge. By count- 

 ing the number of sand -grains which could be 

 placed side by side in the space of i inch I 

 obtained an indirect measure of the height in one 

 or two cases when I was not provided with a 

 measure suitable for taking the height of very 



