80 FRESH-WATER BIOLOGY 



of bolting cloth may be attached outside the wire gauze filter, and 

 the whole instrument is then adapted for the various work of the 

 ordinary net. 



On account of its cheapness and portability a pump of this form 

 is probably best adapted for work not carried on from a station 

 especially equipped for aquatic biology. 



(b) The clock pump has been used for some years at the Uni- 

 versity of Wisconsin (Juday, 1904). At Wisconsin the pump is 

 fixed to the bottom of the boat and the water, drawn through 

 a half-inch garden hose, is pumped into a submerged plankton 

 net of No. 20 bolting cloth. 



FIG. 23. Thresher tame-pump m use. i'he water reaches the pump through the hose at the left and is 

 delivered to the net through the hose at the right. The net cone is seen supported by a rectangular 

 wooden float. (After Kofoid.) 



(c) The thresher tank-pump, a double-acting force pump with 

 two cylinders each six by nine inches, has been used by Kofoid 

 (1897). The mode of using the pump is shown (Fig. 23). This 

 pump is fastened to the boat and is too heavy to be carried or 

 to be used apart from a permanent mounting. 



3. The Water Bottle. To obtain small samples of water for the 

 study of the nannoplankton a water bottle may be used. Many 

 complicated and expensive forms of these bottles have been devised 

 (see Helland-Hansen) for use at all depths in the sea. The bottle 

 described by Theiler appears to be the simplest and least expensive 

 of them. For use in fresh water a Meyer's bottle (Fig. 24) serves 

 fairly well and is easily made. A stout glass bottle of one or 



