Bacteriological Examination. 183 



supplies or of the effluents from municipal filtration 

 plants. 



In the commoner case, when normal values obtained 

 by such routine analyses are not at hand, the problem of 

 the interpretation of any sanitary analysis is a more diffi- 

 cult one. The conditions which surround a source of 

 water-supply may be constantly changing. No engineer 

 can measure the flow of a stream in July and deduce the 

 amount of water which will pass in February; yet the 

 July gauging has its own value and significance. So a 

 single analysis of any sort is not sufficient for all past and 

 future time. If it gives a correct picture of the hygienic 

 condition of the water at the moment of examination it 

 has fulfilled its task, and this the bacteriological analysis 

 can do. The evidence furnished by inspection and by 

 chemical analysis should be sought for and welcomed 

 whenever it can be obtained, yet we are of the opinion 

 that, on account of their directness, their delicacy, and 

 their certainty, the bacteriological methods should least 

 of all be omitted, and, if necessary, they alone may fur- 

 nish conclusive testimony as to the safety of a potable 

 water. 



