Methods in Plankton Investigations. 13 



<b ' 



Previous to this, Hensen ('87) used the steam pump for 

 putting known amounts of surface water through the filtering 

 net. Hensen's quantitative work was, however, based upon 

 collections made by vertical hauls of the plankton net. Peck 

 ('96), in his work upon the marine plankton of Buzzard's 

 Bay, obtained water for examination by the Sedgwick -Rafter 

 method (see Rafter, '9*2, and Twenty-third Annual Report of 

 the State Board of Health of Massachusetts for 1891, pp. 

 395-421) by means of a steam-pump connected with a two- 

 inch hose which was lowered to the desired depth. Beyond 

 these instances no other applications of the pump to the col- 

 lection of plankton have come to my notice, and there appears 

 to be no record of its use in quantitative work by the Hensen 

 method. 



The impossibility of using the vertical haul in shallow 

 waters, the difficulties in the operation of the oblique haul, 

 and especially the error involved in the variable coefficient of 

 the net, have led to the adoption of the pumping method in 

 the plankton work at Havana. 



1. The Pnmp (Plates Y. and VI. ). — The pump we use is 

 a double acting force-pump, known in the trade as a "Thresher 

 Tank Pump." It is worked by an upright handle, and has 

 two cylinders, each 6x9 inches, and throws an almost contin- 

 uous stream. Its capacity is one cubic meter of water per six 

 hundred strokes, provided that the water is delivered to the net 

 without elevation. The stroke is of definite length and its 

 action is regular, the rate employed being one stroke per sec- 

 ond. The pump is provided with 20 feet of 2 -inch spiral- 

 wound suction hose, terminating in a funnel 20 cm. in diam- 

 eter. The mouth of the funnel is covered with a linen net 

 of ^-inch mesh to prevent the entrance of stray bits of vegeta- 

 tion, and the end of the hose is weighted to insure its sink- 

 ing readily. It was found necessary to paint all exposed iron 

 in the water chambers of the pump with a thin coat of asphal- 

 tum to prevent the formation of rust scales. 



Before the pump was put in use for regular plankton col- 

 lections, tests were made of the straining capacity of the silk 

 under the impact of the current from the discbarge hose. It 



