Underground Water in South- Eastern Bechuanaland. 261 



off in its yield. This may be got over by sinking fresh wells, but 

 they should be so far distant from one another that they do not 

 interfere. 



The effects of extensive and prolonged draining of the soil are 

 becoming very marked in several countries, sometimes so much so 

 as to give reasonable cause for alarm. 



In Southern California * in the last ten years the level of the 

 ground water has been lowered at various places from 30 to 90 feet 

 over extensive areas. 



This is worthy of special attention, because this portion of North 

 America in certain respects resembles Bechuanaland, as the region 

 is one of low rainfall and the flow in the rivers is lost in the sands 

 of the plains which extend along the foot of the Sierra Nevada. 



In London,! during the last 30 years, the level of the water in the 

 chalk and lower tertiary sands has fallen a distance of from 40 to 

 60 feet ; in this case, however, the drain upon the resources of the 

 basin has been excessive. 



Conclusion. — In the foregoing I have endeavoured to point out the 

 various influences — some beneficial, others adverse — that affect the 

 underground water supply, and though I may have given rather 

 greater prominence to the latter, it is chiefly with the object of 

 drawing attention to a matter of no small importance. 



There is much need for extended study of the conditions for and 

 against the accumulation of water, and of experiments in order to 

 determine how great a proportion of the annual rainfall is actually 

 available, and how this amount is affected by periods of abnormal 

 rainfall or of unusual drought. Not only does this apply to 

 Bechuanaland but to all parts of South Africa, though each district 

 will have influences that will modify these problems. 



As regards supplies in this area I think that we can only rely 

 upon shallow wells and boreholes of no great depth. I have only 

 come across a few spots where the conditions are apparently favour- 

 able for artesian supplies ; but such occurrences are strictly limited, 

 and the areas so favoured are of but small extent. 



Wells have the disadvantage over boreholes in the matter of depth 

 and cost, while in places where the soil is sandy and deep the walls 

 may require supporting by means of cribs with linings of wood or 

 sheet-iron ; this has been done in some of the wells beyond Genesa. 



Towards the Kalahari this difficulty may perhaps be overcome by 

 the employment of drive- wells, e.g., tubes perforated at their lower 



* United States Geological Survey, Monograph xlvii., p. 427. 

 f Prestwich, " Water Supply of London," p. 189, 1895. 



