204 Introduction to the Study of Science 



ments of the given areas. The conditions necessary in dams to insure 

 safety in emergencies should also be studied. (Reference : Reclama- 

 tion Bureau, Washington, D. C. Current periodicals, as World's 

 Work, National Geographic Magazine, Review of Reviews, etc.) 



It is estimated that the amount of land which may be reclaimed 

 by irrigation runs from 40,000,000 to 45,000,000 acres. Ten million 

 acres have been brought under cultivation and are very productive. 



In what sort of places along river courses are dams usually built? 

 Study the Shoshone Dam, Salt River project, and others. From 

 what source does the government obtain money for reclamation 

 projects ? 



What plans are suggested for the reclamation of large areas of the 

 Sahara Desert? 



SUMMARY 



The average daily consumption of water per individual is 32 gal- 

 lons in small communities ; more than 100 gallons in manufacturing 

 cities. 



For many reasons communities have found it advisable to own 

 and control their water supply. 



An ideal water supply comes from a region high enough to give 

 sufficient gravity pressure, where the rainfall is abundant and pollution 

 improbable. 



Typical examples of provident and safe supplies are the water 

 systems of Los Angeles, New York, and London. 



The Ganges and Jumna rivers have exceptional powers of ster- 

 ilization. 



Deep wells, as in Leipzig, Berlin, Pittsburgh, St. Louis, and Galves- 

 ton, furnish a safe supply. 



Artesian wells may be obtained where the rock formations are 

 favorable. 



Ground water found in most places is slowly moving toward 

 springs, river courses, or the sea. 



Water has great solvent power and takes up in some quantity 

 nearly every mineral with which it comes in contact. 



Cisterns for storage of rain water are necessary in some localities. 

 They furnish soft water, but are liable to contamination. 



In nature the process of evaporation gives pure water vapor. Rain 

 formed in condensation contains impurities from the air. 



Man improves nature's process by controlling the conditions and 

 obtains pure " distilled water." 



