CHAPTER X. 



THE SIGNIFICANCE AND APPLICABILITY OF THE BAC- 

 TERIOLOGICAL EXAMINATION. 



THE first attempt of the expert called in to pronounce 

 upon the character of a potable water should be to make 

 a thorough sanitary inspection of the pond, stream, or well 

 from which it is derived. Study of the possible sources 

 of pollution on a watershed, of the direction and velocity 

 of currents above and below ground, of the character of 

 soil and the liability to contamination by surface-wash 

 are conceded to yield evidence of the greatest value. 

 Often, however, some opinion must be formed upon the 

 quality of water sent from a distance without the oppor- 

 tunity of examining its surroundings; and even when 

 sanitary inspection can be made, its results are by no 

 means conclusive. No reconnoissance can show cer- 

 tainly whether unpurified drainage from a cesspool does 

 or does not reach a given well; whether sewage discharged 

 into a lake does or does not find its way to a neighboring 

 intake; whether pollution of a stream has or has not been 

 removed by a certain period of flow. Evidence upon 



these points must be obtained from a careful study of the 



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