56 GENERAL SCIENCE 



liberated within the cistern waters every time the pump is 

 used. 



There is always danger that surface waters and sewage 

 may seep into a cistern through cracks in its upper walls. 

 These walls should always be so laid as to prevent leakage 

 into the cistern as well as leakage out from it. This can be 

 accomplished by use of non-porous brick laid in cement for 

 the walls, and by having the inner side of the walls plastered 

 with cement. 



The aeration of the waters of city systems is accomplished 

 in various ways. In cases where water is pumped directly 

 into the mains enough air may enter with the water to keep 

 it thoroughly aerated. In storage reservoirs arranged on 

 different levels above one another, the water is allowed to 

 flow from the higher to the lower levels in long thin sheets, 

 falling with more or less of spray into the lower reservoirs. 

 Very often, too, provision is made for filtering these waters 

 through layers of sand and gravel. 



In dwellings where no city water supply is available, water 

 from cisterns or wells is sometimes forced into large steel 

 cylinders partly filled with air. By reason of the pressure 

 of the compressed air, the water flows through pipes to all 

 parts of the house so long as the air pressure is maintained by 

 frequent pumpings. 



SUMMARY 



The provision of an abundance of pure water for household and 

 general uses is indispensable to the welfare of people whether living 

 in towns or in rural districts. Sanitary conditions require an ample 

 supply of water for the removal as sewage of the waste matter of towns 

 and cities. The large use of water for fire protection, for street clean- 

 ing, for homes, for public laundries, for mills and factories and various 

 other industrial plants, makes necessary large outlays for the construc- 

 tion and maintenance of water systems. 



The utmost vigilance is at all times necessary to safeguard the water 



