8 VREDENBURG : RECENT ARTESIAN EXPERIMENTS IN INDIA. 



it has the disadvantage of leaving outside of any well-defined catc 

 gory all those non-flowing wells which nevertheless are not ordinary 

 percolation wells, but which derive their supply from an under- 

 ground reservoir of water under pressure. At the same time, 

 adopting Mr. Medlicott's definition of artesian action as the tendency 

 of water to percolate more easily along the planes of stratification 

 than across them, the separation from an ordinary percolation well 

 is, especially for practical purposes, not always very distinct. The 

 «Sp r ing-weiis" of the "spring wells " of the Gangetic alluvium, and 

 Gangetic alluvium. some very similar ones in Gujdr^t, are in- 

 stances of this. The alluvium of these regions contains thick beds 

 of impervious clay known as "mota" in the North-West Provinces. 

 These beds may occur at a depth coinciding with the level of 

 permanent saturation, or else ten, twenty feet or more beneath it. 

 When such a clay-bed occurs at an accessible depth at some locality 

 where a well is needed, the well is first sunk up to the surface of 

 this clay-bed. A narrow shaft is then excavated into the clay, and 

 when the clay-bed is pierced, water charged with sand rushes up- 

 wards with such violence that the workmen may have barely time 

 to escape. Captain Clibborn has given a most interesting account of 

 such wells in a treatise on well-irrigation in the Gangetic plain, 

 in which their peculiar action is fully explained. 1 Briefly stated, it 

 is thus. The stratum of indurated clay that supports the well may 

 not, and in most cases does not, extend laterally to any great dis- 

 tance, As therefore the water below the clay-bed is in free com- 

 munication round the edges of this impervious stratum w|th that 

 above it, the pressure underneath the clay stratum depends merely 

 on its depth below the level of saturation of the ground directly 

 above it. When the well is sunk as far as the clay-bed, the water 

 from the saturated permeable sands above the clay is prevented by 

 the masonry from percolating into the shaft. What may have 



1 Report on Well-irrigation in the North-Western Provinces and Oudh, by Captain 

 Clibborn, B.S.C. Roorkee, 18S3. 



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