32 Elements of Water Bacteriology. 



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 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 removing the 

 stopper and allowing 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 forward 

 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, 

 some method of removing the stopper when that depth 



