METHODS OF RESEARCH 17 



intervals, so that the approximate depth of 

 each haul was known (see Plate I.). The 

 results obtained were then controlled by using 

 vertical closing nets. 



Centrifuge. — Recent pelagic investigations 

 have shown that a great many marine organ- 

 isms are so small that they pass through the 

 meshes of even the finest silk nets. These 

 minute organisms may be studied by centri- 

 fuging samples of sea-water taken by the 

 water-bottle from various depths. The 

 " Michael Sars " used a large centrifuge driven 

 by one of the small steam winches on board. 

 This had six glasses, and as much as 1200 

 cubic centimetres of sea-water could be 

 centrifuged at one time. It made 700 to 800 

 revolutions in one minute, and after eight 

 minutes the organisms were all collected at 

 the bottom of the glasses. The clear water 

 was then poured away, and the deposit after 

 being rinsed was put into a smaller glass with 

 a tapering bottom, where it was subjected to 

 the action of a small hand-centrifuge, such as 

 is used by physiologists. In this way all the 

 contents of say 300 cc. of sea-water were col- 

 lected in one drop, which was examined in a 

 counting chamber beneath the microscope. 



Deep-Sea Thermometers. — Six's maximum 

 and minimum thermometer was one of the 

 earlier forms used in taking deep-sea tempera- 



