METHODS OF OCEANOGRAPHICAL RESEARCH 265 



that are wanted. The large drifting animals also are more 

 widely distributed in the water so that a larger net will be 

 required than for small plankton. In towing it is essential 

 that as much of the water as possible will be filtered in 

 the net's journey through the water. For this purpose 

 the net itself is made very long, a length three times the 

 diameter of the mouth being the general rule. For the 

 larger animals the net generally used is the ring trawl 

 which has an opening of two yards diameter and is about 

 six yards long ; there is thus a tremendous area available 

 for filtering the water (Plate 96) The material in these 

 big nets is hemp. 



For the smaller animals the nets are usually about a 

 foot and a half in diameter at the mouth and the material 

 is silk. The coarseness of the silk depends on the size 

 of the organisms required, the very finest being used for 

 those minute drifting plants, the diatoms. The best silk 

 is that used by millers for grading their flour through, 

 known as bolting silk, and it has the advantage that the 

 required size mesh can at once be obtained and that these 

 meshes are so constructed that they keep a very constant 

 size. 



Much time has been spent in working out how much 

 water the various nets will filter at different speeds of tow- 

 ing, because thereby it becomes possible to calculate how 

 many creatures are present in a given volume of water. 



In studying the distribution of plankton organisms in 

 the different water layers it is necessary that the net should 

 fish only at the required depth and not at all on its journey 

 up and down. For this purpose nets have been con* 

 structed that can be sent down with their mouths shut ; 

 on arrival at the required depth the net can be opened, 

 and when it has fished for long enough it can again be 

 closed so that it catches nothing as it is being hauled up 



