50 



DEPTHS OF THE OCEAN 



Centrifuge. 



the vessel is able to tow through the water, and by comparing 

 the catches in the manner described in Chapter IX. one can 

 ascertain the depths at which the animals lived. It is really a 

 development of the plan adopted by the "Challenger," which 

 towed its small nets along at different depths, or else attached 

 them to the sounding-line (see above, p. 34). 



The pelagic investigations of recent years have shown 

 that a great many marine organisms are so small that they pass 

 through the meshes of all nets — even the finest silk nets (see 



Fig. 35. — Centrifuge dri\e.\ by Electric Motor. (From a catalogue.) 



Chapter VI., where these organisms and their occurrence are 

 described). To catch them in greater quantities we employed 

 a large centrifuge (Fig. 35) as used by physiologists, which 

 could centrifuge 1200 cubic centimetres at a time. The centri- 

 fuge was driven by one of the small steam-winches usually 

 for a period of seven minutes and at a speed of 500 to 700 

 revolutions per minute. 



This short description of the outfit of the "Michael Sars " 

 does not claim to be exhaustive. During past years probably 

 most kinds of fishing gear and scientific instruments available 

 for the investigation of the sea have been made use of by us. 

 When undertaking a definite limited cruise, however, a pro- 

 gramme of the researches contemplated must necessarily be 

 drawn up in advance and the gear selected accordingly. 



Our Atlantic cruise proved that a greater number of 

 appliances could hardly have been employed during a cruise 



