24 ELEMENTS OF WATER BACTERIOLOGY. 



often great, since if the flooring about the pump is not 

 tight, as is usually the case, continued pumping may wash 

 in an unusual amount of surface pollution. 



In sampling surface-waters, the greatest precautions 

 must be observed to prevent contamination from the 

 fingers. In still waters the fairest sample is one taken 

 from several inches down, as the surface itself is likely to 

 have numerous dust particles floating upon it. The 

 method most frequently recommended is to plunge the 

 bottle beneath the surface to a depth of a foot or so, then 

 remove the stopper and allow the bottle to fill. 



Another method which is comparatively free from 

 objection and which has been employed by the writers is 

 to remove the stopper first and then, holding the bottle by 

 the base, plunge it mouth downward into the water, turn- 

 ing it at the desired depth so as to replace the enclosed 

 air by the water. Whenever any current exists the 

 mouth of the bottle should be directed against it in 

 order to carry away any bacteria from the fingers. If 

 there is no current, a similar effect can be produced by 

 turning the bottle under water and giving it a quick for- 

 ward motion. In rapidly flowing streams it is only 

 necessary to hold the bottle at the surface with the mouth 

 pointed up-stream. 



For taking samples of water at greater depths, a num- 

 ber of devices have been employed, all of which are fairly 

 satisfactory. The essentials are, first, a weight to carry 

 the bottle down to the desired depth, and, second, a device 



