22 CONTENTS 



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, Hquid, and granular, and 

 suggestion that an emulsion 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 photo- 

 graphs — The positive and negative compared — The 

 former shows the white clouds, the latter the blue sky 

 — It is the latter which has the form of ripple-mark in 

 sand — The clouds correspond, not to ripple-mark but 

 to the eddies of water which make the ripple-mark. 



APPENDIX ....... 373 



INDEX ........ 379 



