GENERAL CONSIDERATION. 13 



proportion of the available rainfall, which renders possible agricul- 

 tural operations which would be out of the question if attempted 

 over the entire surface of the collecting area. As already pointed 

 out, the rain water that escapes evaporation, partly flows along the 

 surface, and partly sinks into the ground. It is this latter portion 

 which in some instances becomes available in the shape of artesian 

 water. Some of the recent alluvial formations peculiar to desert 

 areas are particularly suited to the production of artesian conditions. 

 Of late years, complete success has been achieved in the case of 

 artesian wells sunk through such formations at Quetta, but these 

 same reservoirs have been drawn upon since time immemorial by 

 means of the underground channels so well known as " karez " 

 in Baluchistan, and " kanat" in Persia, a feature which is forcibly 

 brought to the attention of any one travelling in those regions, 

 which would be otherwise uninhabitable. The water which supplies 

 the underground reservoirs represents but a fraction of the amount 

 of water that percolates into the ground, this amount itself being 

 but a fraction of an already scanty rainfall ; but compared with the 

 supply from rivers, it has the advantage of being much less subject 

 to variations. Such are then the capabilities and the limitations 

 of artesian water in a desert : its yield may be locally abundant 

 and constant enough to allow of successful irrigation, but this yield 

 is derived from a source so scanty that under the most favourable 

 circumstances, the area cultivated sinks into complete insignificance 

 compared with the immensity of the surrounding desert. One of 

 the artesian districts that have most attracted the attention of the 

 world is that of the Oued Voir and neighbouring oases in the 

 Algerian Sahara. According to some statistics collected in a pub- 

 lication issued in connection with the Paris exhibition of 1889/ the 

 total yield of the wells was then 8,475 cubic feet, or 63,400 gallons per 

 minute. The area under cultivation was not more than 4,000 acres, 

 that is a few thousand acres amongst many thousand square miles 



Societe Agricole et Industrielle de Batna et du Sud Algerien. 



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