MICROBIOLOGY OF WATER. 211 



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 considerable depth 

 of soil, and undergo purification by means of the aggregations 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. 



