264 SAND AFFECTED BY WINDS. 



use to'carry us out of this danger, and thg full conviction of 

 this riveted me to the spot." A similar account of these 

 moving pillars of sand is given by M. Adanson, who had 

 an opportunity of observing one of them crossing the rivei 

 Gambia f/om the Great Desert. It passed within eighteen 

 or twenty fathoms of the stem of the vessel, and seemed to 

 measure ten or twelve feet in circumference, and about 250 

 feet in height. Its heat was sensibly felt at the distance of 

 100 feet, and it left a strong smell, more like that given out 

 by saltpetre than sulphur, and which remained a long time. 



Sand-wind. — The overpowering effects of a sudden 

 sand-ioind, when nearly at the border of the Desert, often 

 destroy a whole kafila, already weakened by fatigue. 

 " Indeed," says Denham, " the sand-storm we had the 

 misfortune to encounter in crossing the Desert gave us a 

 pretty correct idea of the dreaded effects of these hurri- 

 canes. The wind raised the fine sand, with which the ex- 

 tensive Desert was covered, so as to fill the atmosphere, 

 and render the immense space before us impenetrable to 

 the eye beyond a few yards. The sun and clouds were 

 entirely obscured, and a suffocating and oppressive weight 

 accompanied the flakes and masses of sand which, I had 

 almost said, we had to penetrate at every step. At times 

 we completely lost sight of the camels, though only a few 

 yards before us. The horses hung their tongues out of 

 their mouths, and refused to face the clouds of sand. A 

 parching thirst oppressed us, which nothing alleviated." 



How the prevailing Winds affect the Sand of the Desert. 

 — The prevailing winds in the Sahara are the easterly and 

 westerly, — the first blows nine months, the second but three 

 months. This circumstance is intimately connected with 

 the motions and distribution of the sand of the Desert. In 

 the eastern half of the Sahara the sand is more gravelly, 

 and the general cover of sand shallower than in the western 

 half; so that, in travelling towards the west, the depth of 

 the sand and the completeness of the sandy covers in- 

 creases. This distribution of the sand is probably owing 

 to the easterly wind, which blows «o much longer than the 

 westerly, carrying the sand before it from the East Sahara. 

 To the same cause we may refer the less frequent appear- 

 ance of rocks, the gradual diminution in magnitude and of 



