CHAPTER VII 



ON THE SMALLEST DELTA, ON THE COMPOSITION 

 OF QUICKSAND, AND ON "MACKEREL SKY" 



On the Transverse and Longitudinal Ridges formed in the Flow of 

 Watery Sand^ and on the Smallest Delta. 



Settlement of sand through water and subsidence of water 

 through sand — Transverse and longitudinal ridges formed 

 during the latter process — Rapid deposition of the small 

 longitudinal ridges in radiating branches — Their diameter 

 equal to that of a fully grown drop of water. 



On the Composition of Quicksand. 



Observations of Mr. C. Carus-Wilson on presence of marsh 

 gas — Experiments of Mr. C. E. S. Phillips on effect of 

 included gas — Author's observations on effect of imprisoned 

 air — On the effect of some finer earthy particles to prevent 

 the escape of air from sand — The three kinds of fluid : 

 gaseous, liquid, and granular, and suggestion that an emul- 

 sion of all three produces a quicksand. 



On the Rippled Clouds called " Mackerel Sky." 



Ruskin's remark — Mathematical investigations by Helmholtz 

 and others — Sir G. H. Darwin's description — The author's 

 observations — Shape of the clouds — Their wave-length — 

 Description of author's photographs — The positive and 



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