CHAPTER I 

 DESERT SAND-DUNES 



Sand-waves near Helwan, their height and length — Formation 

 of peaks on the ridges — Course of the wind as it blows 

 over a series of the ridges, direct current and eddy — 

 Note on the wind-screen of motor vehicles and the return 

 current — Height of wave limited by velocity of wind — 

 Significance of constant ratio of length to height — The 

 eddy-curve ichthyomorphic, with blunt head and fine tail — 

 Longitudinal bank to leeward of a peak — On the forma- 

 tion of the pits called fuljes — Travelling mounds of sand 

 of crescentic shape, called harchans — Rate of advance of 

 the sand-waves — Mode by which the undulations originate 

 in a level deposit of sand — On a limit to the size of level 

 ridges due to fineness of the sand — Large dunes east of 

 the Suez Canal — Illusion of mountainous size — Formation 

 of dew upon the sands — Dunes west of Suez Canal ; 

 influence of moist ground on their formation — Effect of 

 electrification upon the mobiUty of sand — On the sizes of 

 the sand-grains. 



On May 7, 1899, when staying at Helv^an, near 

 Cairo, I crossed the Nile one morning in order 

 to see the desert on the west bank near the 



