MICROORGANISMS IN WATER 329 



water is limited, a large area is needed, and a stone or brick well is 

 necessary. 



Reference to the illustrations will show that every endeavor is made 

 to prevent surface water from entering directly into the well. The walls 

 are impervious; and the earth or clay is well rammed against the outer 

 side of the wall. The curb is carried well above the surface of the 

 ground. The waste water is conducted by means of a sloping platform, 

 trap, and drain, away from the well; and the well opening is properly 

 covered. All water entering such a well must percolate through a con- 

 siderable depth of soil, and undergo purification by means of the aggre- 

 gations of living bacteria in the soil spaces. Thus the soil around a well 

 fulfils the same function in purifying the surface water as the scum 

 layer that forms on the surface of gravel filters. 



