FACTORS INFLUENCING RESULTS. 135 



namely, the bacterial content of the water entering the well 

 Again, if the sediment of the well has been stirred up a high 

 value is obtained. Ordinary wells of medium depth contain 

 from 100 to 2000 per c.c. With regard to rivers very varied re- 

 sults are obtained. In any one river the numbers present vary 

 at different seasons of the year, whilst the prevailing tempera- 

 ture, the presence or absence of decaying vegetation, or of 

 washings from land, and dilution with large quantities of pure 

 spring water, are other important features. Thus the Frank- 

 lands found the rivers Thames and Lea purest in summer, and 

 this they attributed to the fact that in this season there is most 

 spring water entering, and very little water as washings off 

 land. In the case of other rivers the bacteria have been found 

 to be fewest in winter. Thus a great many circumstances 

 must be taken into account in dealing with mere enumerations 

 of water bacteria. Such enumerations are only useful when 

 they are taken simultaneously over a stretch of river, with 

 special reference to the sources of the water entering the river. 

 Thus it is usually found that immediately below a sewage 

 effluent the bacterial content rises, though in a comparatively 

 short distance the numbers may decrease enormously, and it 

 may be that the river as far as numbers are concerned may 

 appear to return to its previous bacterial content. The numbers 

 of bacteria present in rivers vary so enormously that there is 

 little use in quoting figures, most information being obtainable 

 by comparative enumerations before and after a given event has 

 occurred to a particular water. Such a method is thus of great 

 use in estimating the efficacy of the filter beds of a town water 

 supply. These usually remove from 95 to 98 per cent of the 

 bacteria present. Again, it is found that the storage of water 

 diminishes the number of bacteria present. The highest counts 

 of bacteria per c.c. are observed with sewage ; for example, in the 

 London sewage the numbers range from six to twelve millions. 

 Much more important than the mere enumeration of the bac- 

 teria present in a water is the question whether these include 

 forms pathogenic to man. The chief interest here, so far as 

 Europe is concerned, lies in the fact that typhoid fever is so 

 frequently water-borne, but cholera and certain other intestinal 

 diseases have a similar source. The search in waters for the 

 organisms concerned in these diseases is a mattej^j^he greatest 



