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DISCOVERY REPORTS 



Water masses in nature may not often be moving in opposite directions except at con- 

 siderable intervals of depth, but they may often, in fact usually, be going at different 

 speeds in the same direction, or very nearly the same direction, at quite small intervals 

 of depth. The principle holds 

 good for currents of different 

 speeds in the same direction 

 as for currents in the opposite 

 direction. We have so far 

 taken the simple but artificial 

 instance of two sharply de- 

 fined water masses separated 



Fig. 178. 



by the line ZZ' . Actually the change of speed will gradually decrease or increase with 

 the depth, more usually the former. 



Mr Deacon has very kindly worked out for me the varying speeds of water masses at 

 different depths between Sts. 137, 138 and WS 38 of our survey. They lie to the east 

 of South Georgia, and are not far distant from the position at which the consecutive net 

 experiment was made at St. 150. The speeds are as follows: 



At o m. 5-5 miles per day to the north 



.. 25 „ 5-4 

 ., 5° .. 5'3 

 ,. 75 » 5-2 

 „ 100 „ 5-0 



„ 15° .. 47 

 „ 200 ,, 4-3 



300 



3-4 



„ 4 00 » 2 '3 



Since the majority of the zooplankton organisms in this area are normally migrating 

 vertically through some 100-200 m. of water, these figures give an adequate basis for 

 the propositions which follow. Mr Deacon writes regarding these calculations as follows : 

 " I chose these stations because I had to make use of three stations made close together 

 at about the same time. As it happens the choice is not fortunate because the deep 

 current and the surface current seem to flow in the same direction towards the north. 

 The effect of a strong wind might increase the surface velocity to 10 miles a day towards 

 the NE, that at 25 m. to 7-8 miles a day towards the north, and the movement below 

 75 m. might be speeded up a little towards the east". 



Although we know that actually under the influence of wind different layers within 

 the depth of frictional influence (Ekman, 1928) will be travelling with decreasing speeds 

 at gradually changing angles to one another, we will first regard the different layers as 

 travelling in the same direction. Later we shall take the extreme case of currents flowing 

 at right angles to one another, and show how the results of the former proposition would 

 be modified. Let us consider four water layers W, X, Y and Z moving as in Fig. 179 

 from right to left with speeds of 5, 4, 3, 2 units per day respectively in relation to the 

 sea bottom at P. Now in considering an animal migrating between these layers we want 

 to understand its path within the system; i.e. in relation to the water layers rather than 



