12 Illinois State Laboratory of Natural History. 



tion. For example, at one time Cladocera predominate and 

 at another diatoms are present in vast numbers, and at still 

 another rotifers constitute practically the whole of the plank- 

 ton. Observations upon the operation of the net in the midst 

 of these fluctuations awakened the suspicion that the amount 

 of water actually strained was sul)ject to considerable varia- 

 tion, dependent, among other causes, upon the amount and, 

 more especially, the composition of the plankton. If the 

 plankton were constant in quantity, kind, and distribution, 

 the error arising from the progressive clogging of the net as it 

 traverses the thirty meters would be distributed alike in all of 

 the catches, and they would still l)e comparable ; but the exist- 

 ence of the fluctuations in the plankton just noted and the 

 consequent variation in the amount of water strained, con- 

 stitute an important source of error in any deductions based 

 upon comparisons of catches made under these variable con- 

 ditions. This source of error is present in the vertical as 

 well as in the oblique haul. Furthermore, change in the silk 

 itself consequent upon use adds to the errors due to the fact 

 that the collection is "made by drawing the net tJirongh the 

 water. A series of field experiments (to be described in a 

 later paper) upon the progressive clogging of the net and the 

 coefficient of various plankton nets, in a wide range of season 

 and situation, have abundantly justified our abandonment of 

 the system of collection in which the net is draivn through the 

 water for one in which a known quantity of water is put 

 through the net. 



II. THE PUMPING METHOD. 



For many years the biological examination of potable water 

 has been conducted by straining or filtering water delivered 

 through service pipes at the faucet by pressure due to the use 

 of a pump. 



Giesbrecht ('96) describes the collection of Copepoda in the 

 Eed Sea by Krjimer, who strained the water delivered by the 

 ship's pump to the bath-tub of an ocean steamer. 



Cleve ('96), at the suggestion of Dr. John Murray, collected 

 plankton on board a steamer in the North Sea by attaching a 

 silk net to the pump when the deck was washed. 



