178 Concluding Remarks 



this important test, but the great obstacles should not blind us to the 

 necessity of undertaking it. Perhaps the most promising technique for 

 direct deep -current measurements underneath the Gulf Stream is a method 

 which is now being refined by Dr J. C. Swallow at the British National 

 Institute of Oceanography and was recently described at a London con- 

 ference on the deep ocean circulation. A small float, so ballasted as to 

 reach and remain at a fixed predetermined depth, carries a battery- 

 powered ultrasonic noisemaker. A ship with a directional hydrophone 

 array can track the drift of the float, and hence obtain direct measures of 

 the drift of deep currents. Whether this technique can be reflned to 

 measure currents as accurately (say to 0-5 cm. /sec.) as will be needed to 

 determine the level of no motion below the Gulf Stream, remains to be 

 seen; but it is important that this test should be made.^ 



Another example of an untested hypothesis is the entire conception that 

 large-scale lateral mixing occurs in the ocean. As yet, there does not seem 

 to be any obvious way to test this idea. 



I should like to make it clear, finally, that I am not beHttHng the survey 

 type of oceanography, nor even purely theoretical speculation. I am 

 pleading that more attention be given to a difficult middle ground: the 

 testing of hypotheses. I have not explored this middle ground very 

 thoroughly, and the few examples given in this book may not even be the 

 important ones ; but perhaps they are illustrative of the point of view in 

 which attention is directed not toward a purely descriptive art, nor toward 

 analytical refinements of idealized oceans, but toward an understanding of 

 the physical processes which control the hydrodynamics of oceanic circu- 

 lation. Too much of the theory of oceanography has depended upon purely 

 hjrpothetical physical processes. Many of the h3^otheses suggested have 

 a pecuhar dreamlike quality, and it behooves us to submit them to especial 

 scrutiny and to test them by observation. 



* For a report of very recent work of this kind see Chapter XI, footnote 2. 



