ARTESIAN WELLS AND THE GREAT SAHARA. 531 



blacks of Sooclan and the vai'ious peoples of white race driven into the 

 desert by successive invaders. Diodorus, a priest of Tarsus in the 

 fourth century, speaking of the great oasis in the desert forty leagues 

 from the Egyptian frontier, mentions it being irrigated, not by rivers 

 nor by rains, but by springs that issue from the ground not sponta- 

 neously, nor in consequence of the rains sinking into the ground, but 

 by great labor on the part of the inhabitants. Several Avells alluded 

 to by him have been cleared since 1849 by a French chemist, M. Ayme, 

 who established alum-factories in two Egyptian oases. These old wells 

 were fitted with a stone j)ear-shaped valve by which the issue could be 

 regulated. 



About the middle of the sixth century, Olympiodorus of Alexan- 

 dria speaks of wells five hundred cubits deep. Ai'abian writers in the 

 middle ages describe them in detail ; their great historian, Ibn Khal- 

 doun, speaking of the spouting wells of the Sahara, considers them " a 

 miraculous fact." 



The origin of these subterranean waters is now well known. The 

 streams flowing down the southern slopes of the Atlas Mountains of 

 Morocco, Algeria, and Tunis, and on all sides of the Tibesti, Hogar, 

 and other Saharan mountains, quickly disappear through the sands. 

 M. Vivien de Saint-Martin, in "Le Nord de I'Afrique dans I'anti- 

 quite," says that, " under the sandy crust through which the waters 

 necessarily sink, layers of clay have been found everywhere at various 

 depths underground, where sheets of water make actual rivers." 



The natural question then arises as to what causes these streams ; 

 how the parched desert furnishes rivers ? Rains are quite abundant 

 on the summits of the mountams — so much so that, in winter esi^ecially, 

 the streams attain considerable proportions. The Sahara experiences 

 at times tremendous storms and torrents of rain that in a few moments 

 cause violent freshets. Dr. Barth, in his " Reisen in Nord und Central 

 Afrika," cites among others a deluge that he witnessed at Tintagoda 

 in latitude 19°. In less than an hour after a heavy rainfall on the 

 mountain a sheet of water was rushing by with such force as to carry 

 away herds of cattle and uproot trees ; it covered to a considerable 

 depth the whole valley, over a mile broad. In "Les Touaregs du 

 ]^ord," M. Duveyrier says : " I had occasion on the 30th of January, 

 1861, while at Oursel, at the foot of the Tasli Mountains, to observe 

 the overflowing of one of the numerous torrents that descend from 

 that mountain. The rapidity of the stream was a metre a second, and 

 the water brought down such alluvia that afterward the Touaregs 

 could sow cereals where before there had been no arable ground." 

 Further on the same traveler says : " In the spring of 1862 a storm of 

 rain falling on the southern slopes of the Ahaggar brought such quan- 

 tities of water into the valleys of Idjeloudjal and Tarhit that a portion 

 of the mountain was carried away. The action of the water was so 

 rapid as to sweep away and destroy an entire tribe encamped at the 



