. 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 „ 400 » 2'3 Since the majorit
. 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 „ 400 » 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 consid
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