DESERT SAND-DUNES 69 



date-palms grew and where water could be obtained 

 by digging. The day had been very hot, there 

 was no evidence of recent rain, neither could I 

 hear of rain having fallen recently in the neigh- 

 bourhood. The air was so dry that when I tried 

 to wash my hands the soap was left as a solid 

 film upon the skin owing to evaporation of the 

 water. Yet the sand 3 inches below the surface 

 was sufficiently damp to stick to the sides of the 

 glass sample-bottle. I slept that night in a tent 

 which had been pitched on the same spot before 

 sunset. The interior remained dry and very hot 

 all night, but on going out before sunrise I found 

 that the camp equipage outside was wringing wet 

 with dew. The heat of the sun evidently does 

 not penetrate far into the sand, but the size of 

 the interstices between the sand-grains permits of 

 interior distillation. Rain falling on the porous 

 surface cannot collect in superficial channels, but 

 sinks through the sand. Thus moisture lies at the 

 foot of the sand-dune and provides dew. 



At Bir Abu Ballah, south-west of Ismailia, I 

 saw a large mass of sand on the west of a culti- 

 vated tract, which had formerly been occupied by 

 a salt marsh. The sand was piled in a steep bank, 

 which had a very sinuous frontage, following appar- 

 ently the margin of the former salt marsh, and 



