CISTERNS AND HOUSE TANKS 147 



Amount of water falling annually on roofs of varying slopes at different rates of rainfall. 



Location of Cistern. - - Theoretically the location of a cistern 

 makes little difference in its liability to pollution, but practically 

 it is of the greatest importance. As has been indicated, cracks 

 in the walls are of common occurrence, in some cisterns being 

 sufficiently open to admit enough outside water to make the 

 supply noticeably hard, and where such water can enter pollution 

 can enter also. Bacteria or disease germs can develop and enter 

 the cistern through cement walls even when there are no cracks. 

 It is therefore highly desirable that the cistern should not be 

 located near a sewage drain, barnyard or other source of contami- 

 nation, the same precautions being observed as have been indi- 

 cated for wells. The site selected should be in firm ground, as 

 otherwise there will be danger of the cistern settling and cracking. 

 Roots of trees are also a frequent cause of injury, and cisterns 

 should be located as far away from them as possible. 



Construction and Equipment. - - The excavation should be 

 made large enough and deep enough to permit the laying of 

 proper foundations and adequate walls. For curbing either 

 cement or stone or brick laid in mortar and lined on the inside 

 with a thick coating of hydraulic cement may be used. It is pref- 

 erable that the top be arched over with brick or stone and lined 

 with cement. An opening, provided with an air-tight COM r. 

 through which a person may enter the cistern for purposes of ex- 

 amination, cleaning and repair, should be left at the top. After 

 the completion of the cistern, it should be frequently examined, 

 in order to detect and remedy any cracks or other defects due to 



