GEOLOGY. 705 



pests, to the least violent of which the name khamsin is 

 given in Egypt: "It comes towards one," he says, "growing, 

 spreading, and advancing as if on wheels. Its overhanging 

 siimmit is of a brick colour, its base deep red and almost 

 black. In proportion as it approaches it drives before 

 it burning effluvia, like the bi-eath of a limed^iln. Be- 

 fore it reaches us we are covered with its shadow. The 

 sound it makes is like that of a wind passing through a 

 pine-forest. So soon as we are in the midst of this hurri- 

 cane the camels halt, turn their backs, throw themselves 

 down, and lay their heads upon the sand. After the cloud 

 of dust comes a rain of imperceptible stones, violently 

 hurled about by the wind, and which, if it lasted long, would 

 quickly flay the skin from those parts of the body unpro- 

 tected by the clothes. This lasted five or six minutes, 

 and was frightful. Then the sky became clear again, and 

 gave the same feeling of sudden change to the eye as a 

 light suddenly brought into a dark place." 



It is in the sand deserts that the phenomenon of the 

 mirage takes place most frequently. I was enabled to see 

 it once in all its splendour. 



The cai^tain or reis of our escort had asked leave to 

 stay at a part of the Nile where stood one of his harems, 

 in order to pass the day there with his wives and family. 

 I say one, for he had several, ingeniously established along 

 the river, the scene of the continual voyages which he 

 made. He stopped by successive stages at each of his 

 establishments, in such a way as not to excite the jealousy 

 of any of the sultanas whom he maintained. 



I had profited by this halt to make an excursion into 

 the desert, and had started when the reis, seeing me in 

 the distance, came with some Arabs of his tribe to beg I 

 would accept the hospitality of his roof. In the East, to 



