8o4 through ASIA 
the dune sends out a horn or wing, which gradually tapers 
off, becoming lower and more pointed by degrees. When 
the dunes run into one another continuously, their original 
form is departed from ; they merge into each other, and 
coalesce with their neighbours. In spite of that however 
they always preserve the same structural formation. 
Daily and hourly during a long desert march you cannot 
detect any deviation in this respect. On the sheltered 
side their angle of inclination is always thirty - three 
degrees, and from this they vary very little indeed. On 
the windward side, on the contrary, they incline at all 
angles from twenty degrees to one ; sometimes they are 
even vertical, or overhang the foot, sloping as much as 
ten degrees inside the top outer edge. If you disturb the 
sand at the base of the dune, on the leeward side, fresh 
sand trickles down from the top, and a groove gradually 
shows itself running all the way up from the bottom. 
This marks the line at which the sand is packed closest, 
owing to its being exposed to the greatest wind-pressure. 
Hence, as soon as the groove has worked its way to the 
summit of the dune, it is easy to discern a double inner 
structure — namely, two series of strata, one parallel to the 
windward side, the other to the leeward. This structure, 
which corresponds to the lines of cleavage in a crystal, 
runs right through the dune from top to bottom. The 
fine delicate rippling of the surface is merely secondary 
and superficial, quite independent of the inner structure. 
It travels at much greater speed than the sand-dune itself. 
With a moderately strong wind, it will move inches 
in the hour. These ripplings owe their formation to the 
drift-sand, which, swept along by the wind, impinges 
upon the summit of the dune, and trickles down on the 
leeward side, where the sand is always loosest, since it 
settles through its own inherent weight, and is not con- 
solidated by the pressure of the wind. It is impossible 
for camels to climb up the leeward side of a dune. When 
such an obstacle confronts you, you have no choice but 
to go round it. In the level spaces between the dunes 
the sand is also loose, so that the camels sink in to a 
