264 THE SEAS 



sampler that collects a known volume of the detritus 

 overlying the sea floor with all the microscopic life con- 

 tained in it and in this case the method of effecting suction 

 is very ingenious and no pump is required. The receptacle 

 into which the sample is to be sucked is sealed with a glass 

 disc. As this is done on the deck of the ship the pressure 

 inside must be equal to that of one atmosphere. At the 

 sea bottom the pressure inside the cylinder is still one 

 atmosphere, but the pressure of the surrounding water is 

 considerably greater. On striking bottom the glass disc 

 is broken by a sliding tube with a point on it and, because 

 of the difference in pressure inside and outside, a sample is 

 forced into the receptacle. 



It is a more dif&cult matter to find out how many fish 

 there are in the sea. Fish can swim very fast and so avoid 

 nets, therefore it is not safe to argue, from a catch of fish, 

 that all those present over the area scoured by the net 

 have been captured. 



More indirect methods have to be employed and the 

 chief of these is to mark the fish themselves, let them loose, 

 and then find out how many are recaptured. More will 

 be said about the marking of fish under the chapter on 

 fishery research. 



There remains the problem of catching the members of 

 that huge drifting community, many of which are so 

 extremely minute and delicate. The instrument par 

 excellence of the plankton research worker is the tow net. 

 This is simply a cone-shaped bag made of silk or other 

 material attached to a ring (Plate 96). On the size of the 

 meshes of the material naturally depends the size of the 

 organisms which will be caught. 



If only the larger animals are required, a coarse material 

 will be used so that the smaller creatures will filter through 

 and the catch will consist merely of the kinds of animals 



